1
INDEX
PAGE NO.
1. INTRODUCTION
3
2. SINGLE L INE DIAGRAM
4-5
3. MAIN EQUIPMENTS
6
4. COMMON TERMINOLOGY
7
5. LIGHTNING ARRESTER
8-9
6. INSTRUMENT TRANSFORMER
10 - 17
7. ISOLA TORS
18 - 19
8. CIRCUIT BREAKER
20 - 23
9. CAPACITORS
24 - 25
10. TRANSFORMER AND ITS TESTING
26 - 46
11. CONTROL A ND RELA Y PANEL
47 - 50
12. STATION TRANSFORMER & BATTERY
51 - 52
13. MEASURING INSTRUMENTS
53 – 57
14. NUMBER NOTATION
58
15. TROUBLE SHOOTING
59 – 62
16. SAFETY ELECTRICAL CLEARA NCES
63 – 65
17. EARTHING OF SUB-STATION
66 – 70
18. PROTECTION RELAY
71 – 75
19. RELAY SETTINGS CALCULA TIONS
76 – 81
20. OPERATIONAL DUTY AT SUBSTATION
82 - 85
21. MAINTENANCE SCHEDULE
86 - 92
22. FIRE & FIRE EXTINGUISHERS
93 - 94
2
INTRODUCTION:The Substation may be defined as assembly of apparatus which transforms the characteristics of electrical energy from one form to another, say for example, from A.C. to D.C. and from one voltage to another. A.C. electrical energy is generated at low voltage but for transmission the voltage is stepped up. Higher the voltage, lesser is the current and lesser is the power loss (I²R) and lesser is the voltage drop (IR). Similarly the consumers do not use high voltage and so the same must be stepped down to low voltage. The stepping up and stepping down of voltage is done in the substations. There are various bays for Incoming Lines, Outgoing Lines, Transformer, Bus-coupler and Bus transfer etc. Each bay has equipments such as CT, PT, Circuit Breaker, Isolators, Post Insulators, etc. Substation up to 11 KV is generally Indoor substation with metal-clad draw out type switchgear. In such switchgear, circuit breaker is mounted on a carriage. After opening of the breaker, the same can be lowered and the carriage can be pulled out. For voltage of 33 KV and above, outdoor substation is generally preferred. In such substation, circuit breakers, isolators, CT, PT, transformers, are installed outside. Bus bars and connectors can be seen by naked eye. The bus bars are supported on post insulators or strain insulators. The substation has galvanised steel structures for supporting the equipment, insulators and the incoming or outgoing lines.
Classific ation o f Voltage Le vels Low Voltage
:
Below 1000 Volts …..LV
Medium high voltage
:
Between 1000 Volts and 33 KV …..MV
High Voltage
:
Above 33 KV and up to 132 KV …..HV
Extra High Voltage
:
220 KV, 400 KV …..EHV
Ultra High Voltage
:
760 KV …..UHV 3
SINGLE LINE DIAGRAM 132 / 33 K V SUBSTATION 132 KV I/C LINE 1 LINE PT
LA
ISOLATOR WITH E/SW LINE CT (CT1) LINE CB
BUS ISOLATOR 132 KV BUS BUS ISOLATOR BUS PT HV SIDE CB HV CT OF TRAFO. (CT2)
POWER TRANSFORMER 132 / 33 KV
LV CT OF TRAFO. (CT3) LV SIDE CB BUS ISOLATOR 33 KV BUS
BUS ISOLATOR FEEDER CT (CT4)
STATION TRANSFORMER
BUS PT FEEDER CB
ISOLATOR WITH E/SW
33 KV O/G LINE FEEDER # 1
4
Above-mentioned Single Line Diagram is General layout of Switchyard Equipments. Only Single Incoming 132 KV Line and Single 33 KV Outgoing Feeder are shown. There may be ‘n’ number of Feeders as per design. It depends upon the rating of installed Switchyard Equipments. The CTs in circuit are used for different purpose as mentioned below: CT1 – 132 KV Line Metering and Distance, Directional O/C & E/F Protection CT2 – Transformer HV Metering & Differential, REF & Non-Directional O/C & E/F Protection CT3 – Transformer LV Metering & Differential, REF & Non-Directional O/C & E/F Protection CT4 – 33 KV Feeder Metering & Non-Directional O/C & E/F Protection
5
Substation Ma in Equipm ents a nd Its Function s 1) Power Transformer : To step up or step down voltage and transfer power from one A.C. Voltage to another at the same frequency 2) Circuit Breaker : Automatic switching during normal or abnormal condition
3) Current Transformer : To step down the current for measurement / protection 4) Potential Transformer : To step down the voltage for measurement / protection 5) Isolator : Disconnection of circuit under no load condition 6) Earthing Switch : To discharge the voltage on dead lines to earth 7) Bus Section : For connecting Incoming / Outgoing Circuits 8) Lightning Arre ster : To discharge lightning & switching over voltages to earth 9) Capacitor Bank : To Improve the power factor of the system & provide compensation to reactive power absorbed by inductive loads, reduce the over loading of the cables, transmission lines & transformers for the same load to be handled 10) Protecti ve Rela y: To sound an alarm or to close the trip circuit of breaker so as to disconnect a component during abnormal conditions (over load, under voltage, unbalanced load, short circuits) 11) Battery Banks : To maintain the D.C. supply continuity during A.C. supply failure for keeping equipment in operation for normal & abnormal conditions 12) Station Transformer : To supply of A.C. power for charging the batteries and provide D.C. control supply for station equipments operations, for Illumination, for spring charging motors of breakers, for cooling system of transformer 6
Some Common Terms used wit h meanings 1) ONAN
-
Oil Natural, Air Natural (For Transformer Cooling)
2) ONAF
-
Oil Natural, Air Forced (For Transformer Cooling)
3) OFAF
-
Oil Forced, Air Forced (For Transformer Cooling)
4) WTI
-
Winding Temperature Indicator
5) OTI
-
Oil Temperature Indicator
6) PRV
-
Pressure Relief Valve
7) OSR
-
Oil Surge Relay
8) OLTC
-
On Load Tap Changer
9) RTCC
-
Remote Tap Change Control
10) MOG
-
Magnetic Oil Level Gauge
11) IDMT
-
Inverse Definite Minimum Time (For Relay)
12) NO
-
Normally Open Contact
13) NC
-
Normally Closed Contact
14) LILO
-
Loop In Loop Out (Used for defining Substation)
15) CRP
-
Control Relay Panel
16) TTB
-
Test Terminal Block
17) ACDB –
A.C. Distribution Board
18) DCDB –
D.C. Distribution Board
19) MB
–
Marshalling Box (For Breaker, Transformer control)
20) AVR
–
Automatic Voltage Control (For Tap Changing on RTCC Panel)
7
Detail descript ion of Each Equipments, Its Te sting & Maintenance 1) LIGHTNING ARRESTER Lightning arrester gives protection to substation equipments by discharging lightning & switching over voltages to earth. It consists of a series of spark gaps and several non-linear resistances like thyrite, metrosil, etc. A non-linear resistor is one whose resistance is not constant but inversely proportional to applied voltage, it decreases rapidly as the voltage across it is increased, i.e. it has an extremely low value when the high surge voltage appears & allows the flow of heavy currents of the order of thousands of amperes & dissipates energy quickly & recovers again, presents a high resistance value to the normal line voltage as soon as surge has disappeared, so that any tendency of the arc to continue is immediately suppressed. In a system which has its neutral solidly earthed, the rated voltage of the arrester is usually taken as 80% of its maximum line to line voltage. In an unearthed system it is taken as 100% of line-to-line voltage since under fault conditions when one line is earthed, the arrester connected to the other two lines would be subjected to full line-to-line potential.
Surge Counter
Lightning Arrester 8
Surge Counter
Testing : 1) IR Testing between Stack to stack & between each Stack to earth by suitable megger. 2) Surge Counter Test - Apply 230V AC supply across the counter & check pointer movement in clockwise direction. Maintenance : 1) Insulator cleaning 2) Connections tightness 3) Checking of Earthing connections 4) Reading of leakage current on daily basis to be taken. If current shoots in red zone, then that particular LA is to be replaced as early as possible.
9
2) INSTRUMENT TRANSFORMERS These transformers are minimum oil type & hermetically sealed. They are expected to be maintenance free during their service life. They transform the high current or high voltage connected to their primary windings to the standard low values in the secondary that feed the metering and protection apparatus. It also isolates the secondary circuits from very high voltages of power system. From the application point of view, these are divided into mainly two categories 1) Metering 2) Protection type. Metering Type – The specified performance of CT is to be maintained in the range normally 5% to 120% of the rated current. The CT cores should be such that it saturate at its instrument security factor (ISF) for safeguarding the instrument from getting damaged under fault condition. The VT designed for metering is required to perform as specified within the voltage range near to the rated voltage normally 80% to 120% of the rated voltage. Protection Type
– Main requirement of performance of protection class CTs is
that its cores should not get saturated below its Accuracy Limiting Factor (ALF) up to, which the primary current should be faithfully transformed to the secondary, maintaining the specified accuracy. During fault conditions, the primary of CT carry very high current and the first few cycles of wave have the D.C. component, which may sature the core. Behaviour of the cores in such condition should be such as to avoid getting magnetized & to come to normalcy (demagnetised stage) soon after clearing fault. Outdoor Type Instrument Transformer
– These are used in Substations and
Power stations where high voltages are employed. While designing for their performance, following factors should be considered. A) Effect of atmosphere environment:-Use of porcelain insulators for external isolation between Live and Ground. These insulators provide outer casing for all the atmospheric conditions like rain, dust, chemical contamination, wind, sun, etc. 10
B) The insulation between primary & secondary windings has to be suitable for withstanding the disturbance in the network system such as switching surges, lightning surges, temporary over voltages, fault currents, over load currents, etc. C) These transformers are normally oil filled with paper insulation and are hermetically sealed to avoid ingression of moisture.
Instrument Transformer has the following major components:1) Primary Winding 2) Secondary Winding 3) Major Insulation 4) Insulator 5) Transformer Oil 6) Metal Tank
PT
CT
CT is connected in series with the supply line & PT is connected across the supply line. The CT secondary should never be open circuited and no fuse should be inserted. In a PT the secondary should never be short-circuited and a fuse is used in PT secondary circuit.
11
Curre nt Transfor mer: Types a) Window CT : - This is constructed with no primary winding and is installed around the primary conductor. b) Bushing CT : - This is window CT specially constructed to fit around a bushing and it cannot be accessed. c) Bar CT : - It is window CT but has a permanent bar installed as a primary conductor. d) Wound CT : - This CT has a primary & secondary winding like a normal transformer. This CT is rare and is used at very low ratios and currents, typically in CT secondary circuits to compensate for low currents, to match different CT ratios in summing applications, or to isolate different CT circuits. The type of primary winding depends upon the type of CT insulation i.e. whether Dead tank or Live tank (Inverted Type) Design. Dead Tank : - In this design, the secondary core windings are housed in metallic tank, which is lower part of the CT and solidly earthed. The leads of the primary winding are brought at top chamber for termination. The primary winding in the shape of ‘hair pin’ or ‘bolt’ is passed through the secondary cores and full insulation is provided on primary windings. Live Tank (Inverted) : - In such design, the secondary cores and the primary windings are assembled in the metal tank located at the top of the Current Transformer. Here the secondary cores assembly is insulated fully for high system voltage & primary winding is looped through the core assembly. The primary winding can be single bar primary or multi-turn primary.
12
Hermetically Sealing : - The Instrument Transformer is supposed to be maintenance free and hence there is no scope of filtering or change of oil during its life. This makes it essential to hermetically seal the transformer to avoid breathing of atmospheric air. 1) Sealing wi th Metallic Bellows : - It is fitted in expansion chamber mounted at the top of the Instrument Transformer, which separates oil with any external environment. This allows the expansion and contraction of oil volume, as the bellow is free to expand and contract. 2) Sealing by Nitrogen Cushion
: - Expansion chamber on top of the CT is
evacuated first applying vacuum and then vacuum is filled with dry Nitrogen. The chamber is then sealed thus avoiding breathing with outer atmosphere. If CT and Protective devices located within same switchgear, 5 Amp secondary current is used. If CT lead goes out of the switchgear, 1 Amp secondary current is preferred. Ac cu rac y Class: - It is the rated ratio accuracy in percent. Ac cu rac y Lim it Factor (AL F): - It is the ratio of largest value of current to CT rated current up to which CT must retain the specified accuracy. Example: - CT - 5P20, 5 VA, ALF = 20 It means error < 5 % up to 20 times rated current for burden of 5 VA Accuracy class 1% means max. Ratio error < 1% at rated current & burden.
CT Core Identif icati on as per class : 1) Class - 0.2s, 0.5s, and 1.0s: - Metering Core 2) Class - 5P10, 5P20, etc.: - Backup protection core (O/C & E/F Protection) 3) Class - PS: - Primary protection core (Differential, Distance, REF etc.)
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CT Testing : 1) IR Testing – a) Primary to earth by 5 KV megger b) Secondary each core to earth by 500 V megger c) Primary to secondary by 5 KV megger d) Secondary core to core by 500 V megger 2) Polarity Test
- For carrying out this test, we require one 1.5 V cell, DC
analogue ammeter.
P1
P2
S1 +
− S2 Analogue Ammeter
+
−
CELL By making above connection, if there is positive deflection of ammeter, then polarity is confirmed. 3) Ratio Test - Inject current in primary winding & measure induced secondary current for different current readings and verify with CT Ratio. 4) Knee point check
for PS class core - Inject 230 V variable AC voltage in
secondary core with ammeter in series. At certain stage, with 10% increase in voltage, current shoots up almost 50%. This is the Knee point voltage. After performing this test, Voltage is gradually reduced to Zero to demagnetise the CT. 5) Winding Resistance Test
- Measure secondary winding resistance by micro-
ohm meter.
14
6) Tan Delta Measurement
– For getting concept of Tan Delta (Tan δ), we
consider the insulation of equipment as Capacitor. Ideal Ic = V/Xc Actual Ic
Loss Angle
δ
Xc
V
∼
φ Phase Angle V
If the capacitor is good or perfect, it will pass only capacitive or charging current on application of voltage. Ideal capacitive current Ic leads voltage by 90°. But in practice, insulation has impurities & actual charging current vector departs from the ideal Ic vector by a small angle (δ) called the loss angle. The loss angle (δ) = 90 – Power factor angle (φ) Higher tan δ produces high dielectric loss that causes increase in temperature of paper insulation. Increased value of Tan δ can be due to any of the following: a) Moisture in the insulation. b) Contamination of oil. c) Internal partial discharge. CT Maintenance : a) Checking of Oil level & leakage, rectify the same immediately. b) Checking of Insulation Resistance. c) Power connection tightness. d) Secondary connection tightness. e) Cleaning of Bushings / Insulators. f) Check the proper earthing of Body connection. g) Check the earthing of CT Secondary core star points. k) Check the working of stainless steel bellows. l) Check the nitrogen pressure in case of Nitrogen filled CT. 15
Potential Transformer : - There are two types of PTs as mentioned below: 1) Electromagnetic Voltage Transformer
– Its construction largely depends on
the rated primary voltage. Primary & secondary windings are wound on magnetic core like in usual transformer. For voltages up to 3.3 KV, dry type transformer with varnish impregnated taped winding is quite satisfactory. For higher voltages, it is a practice to immerse the core and winding in oil. It is used up to 66 KV level. 2) Capacitor Voltage Transformer
– For voltages above 66 KV, CVT is used. It
consists of a capacitive potential divider & inductive medium voltage circuit. Primary voltage is applied to a series capacitor group. The voltage across intermediate capacitor is taken to primary of auxiliary voltage transformer. The secondary of auxiliary voltage transformer is taken for measurement or protection. The inductive part is immersed in oil and sealed with an air cushion inside a steel tank. Fuses are provided in secondary box. Voltage Factor of PT is maximum system voltage, PT can withstand & is expressed in % i.e.120% continuous & 150% for 30 seconds. PT Testing : 1) IR Testing – a) Primary to earth by 5 KV megger b) Secondary each core to earth by 500 V megger c) Primary to secondary by 5 KV megger d) Secondary core to core by 500 V megger. 2) Ratio Test - Inject A.C. variable voltage in primary winding & measure induced secondary voltage at different voltages & verify the same with PTR.
16
PT Maintenance : a) Checking of oil level & leakage, rectify the same immediately. b) Checking of Insulation Resistance. c) Power connection tightness. d) Secondary connection tightness. e) Cleaning of Bushings / Insulators. f) Check the proper earthing of Body connection. g) Check the secondary fuse condition & replace if required by proper rating. h) Check the working of stainless steel bellows. i) Check the nitrogen pressure in case of Nitrogen filled PT.
17
3) ISOLATOR AND EARTH SWITCH Isolator is the device, which makes & breaks circuits in no load condition. Types of Isolators : a) Centre Break Rotating Type Isolator. b) Double Break Rotating Type Isolator. c) Pantograph Type Isolator. d) Tandem Isolator. Earthing Switch
is provided for safety purpose to work on Dead Lines and is
electrically & mechanically interlocked with Isolator.
Isolator Testing : 1) IR Testing – Phase to phase & Phase to earth by 5 KV megger. 2) Contact Resistance check - Measure contact resistance by suitable micro-ohm meter.
18
Isolator Maintenance : 1) Checking of the male / female contacts for good condition and proper connections. 2) Checking proper alignment of male & female contacts & rectify if required. 3) Cleaning of Insulators. 4) Lubrication of all moving parts on regular basis. 5) Tightness of all earthing connections. 6) In case of Isolator with Earth switch, check electrical and mechanical interlock i.e. Isolator can be closed only when E/switch is in open condition & vice versa. 7) As Isolators are operated on No load, hence check the interlock with Circuit Breaker, if provided i.e. Isolators can be operated when Breaker is in OFF condition. 8) The motor operating mechanism box, in case of motor operated isolators, should be checked for inside wiring, terminal connectors, etc. 9) Check the Panel indications i.e. semaphore & bulbs if provided (Isolator and Earth switch - close and open condition) and rectify if required.
19
4) CIRCUIT BREAK ER Circuit Breaker is used to close or isolate the circuit in normal and abnormal condition and to protect the electrical equipment against the fault. The parts of a circuit breaker include – 1) Poles with interrupter, support porcelain, arc quenching medium, etc. 2) Operating mechanism 3) Support structure 4) Control circuit SF6 Circuit Breaker
The part of the breakers assembled in one phase is called a pole. A circuit breaker suitable for three-phase system is called a triple pole circuit breaker. All the three poles operate simultaneously. Each pole comprises one or more interrupters or arc quenching chambers. The interrupter is mounted on support insulators. The interrupter encloses a pair of fixed and moving contact. The moving contact can be drawn apart by means of the operating mechanism. The operating mechanism gives the necessary energy for opening and closing of contacts of the breakers. The arc produced by the separation of current carrying contacts is extinguished by a suitable medium. 20
When a fault occurs in the protected circuit, the relay connected to the CT actuates and closes its contacts. D.C. current flows from the source in the trip circuit. As the trip coil of the breaker is energized, the circuit breaker operating mechanism is actuated & it operates for the opening operation automatically. The spring in the operating mechanism is charged by electrically or manually. Breaker auxiliary switches are mechanically attached with the operating mechanism of breaker. The contact changeover takes place as per breaker operation. Auxiliary contacts are used for breaker operation circuit, indication circuit, and trip circuit supervision circuit. The Circuit breakers are classified on the basis of arc extinction medium: (i) Bulk Oil type (ii) Minimum Oil type (iii) Air Blast type (iv) SF6 Gas type (v) Vacuum type In short, difference of individual breaker is listed below: 1) Bulk Oil Circuit Breaker
– Contacts are separated inside a steel tank filled
with transformer oil used for arc quenching. 2) Minimum Oil Circuit Breaker
– Contacts are separated in an insulated
housing (interrupter) filled with transformer oil used for arc quenching. In the case of MOCBs after certain number of tripping, oil is to be replaced as recommended by the manufacturer. After 2 to 3 times of oil replacement, or after certain numbers of serious faults, it is necessary to overhaul the complete breaker. 3) Ai r Bl ast Cir cu it Br eaker – It utilizes high-pressure compressed air for arc extinction.
21
4) SF6 Gas Circ uit Breaker – Sulphur-Hexa-fluoride gas is used for arc extinction in this breaker. It is must to monitor the SF6 gas pressure inside the breaker pole and check periodically the contact resistance of each pole or the travel of each pole. This is helpful to prevent the problem of bursting of poles. The SF6 breaker has an advantage that the rate of restricting voltage is zero & hence the burning of male / female contacts is less. Operating mechanism is of two types: 1) Movement of contacts is controlled by spring mechanism. (Spring Operated) 2) Movement of contacts is controlled by air pressure. (Pneumatic operated) 5) Vacuum Circuit Breaker
– In this breaker, the contacts are housed inside a
permanently sealed vacuum interrupter. The arc is quenched as the contacts are separated in high vacuum. For VCBs, the vacuum bottle is hermetically sealed and as such no maintenance is required. However to ascertain the failure of vacuum bottle, it is necessary to check the contact resistance of each pole or the travel of each pole as specified by the manufacture. VCBs are generally used up to 33 KV voltage systems. Definit ion of Some C ommon Terms related with Circuit Breake r a) Fault clearing time
– It is the time elapsed between the instant of the
occurrence of a fault and the instant of final arc extinction in the circuit breaker. It is the sum of relay time and breaker time. b) Relay ti me – It is the time elapsed between the instant of occurrence of fault & the instant of closure of relay contacts, i.e. closure of trip circuit. c) Breaker time – It is the time elapsed between the instant of closure of trip circuit and the instant of final current zero. d) Anti Pumping
of a circuit breaker – It blocks the repeat closing pulse when
breaker is already in closed condition.
22
e) Auto- reclosin g of a circuit breaker – Auto-reclosing is provided to restore the supply after interrupting a transient fault on overhead lines. f) Rated short circuit breaking current –
It is the highest value of short circuit
current, which a circuit breaker is capable of breaking under specified conditions of recovery voltage and power frequency recovery voltage. g) Rated short circuit making current
– It may so happen that circuit breaker
may close on existing fault. The circuit breaker should be able to close without hesitation as contact touch. The rated short circuit making current should be at least 2.5 times the R.M.S. value of a.c. component of rated breaking current. h) Operating sequence of a circuit breaker
– The operating sequence denotes
the sequence of opening and closing operations, which the circuit breaker can perform under specified conditions. The operating mechanism experiences severe mechanical stresses during the auto-reclosure duty. 1) O-t-CO-T-CO
where O = opening operation, C = closing operation, CO =
closing followed by opening, t = 0.3 second for breaker to be used for rapid autoreclosure, T = 3 minute. 2) CO-t’-CO
where t’ = 15 seconds for breaker not to be used for rapid auto-
reclosure. Maintenance of Circuit Breaker
:-
a) Tightness of power connections & control wiring connections b) Cleaning of Insulators c) Lubrication of moving parts d) Checking of contact resistance, close-open timing, Insulation resistance e) Checking of gas pressure for SF6 circuit breaker (leakages if any) f) Checking of air pressure for pneumatic operated breaker (leakages if any) g) Checking of Controls, Interlocks & Protections like checking of pole discrepancy system i.e. whether all three poles are getting ON – OFF at the same time h) Cleaning of Auxiliary switches by CTC or CRC spray and checking its operation 23
5) CAPACITOR BANK In any power utility, maintaining stable power supply at proper voltage is always a problem. Due to lot of inductive load, the reactive power flow takes place in the system which results into lowering of system voltage and increase in Transmission & Distribution losses. The HT capacitor provides an interim solution in improving the power system stability, the voltage and power factor. HT capacitor bank also compensate the losses occurring in the transmission lines. Capacitor unit has one steel container, two bushings and several capacitor elements enclosed in the unit. A single HV Capacitor may have a capacitance of 5 KVAr to 200 KVAr. Several identical units are mounted on Insulator racks and connected in series parallel combination to obtain a High Voltage Capacitor Bank. Before commissioning, megger the capacitor bank between phases and earth. The megger reading for individual capacitor should not be less than 50 M Ω. For more than one unit in parallel, minimum acceptable megger value can be derived by dividing 50 MΩ by the number of units connected in parallel. Before switching on capacitor, bus voltage, system incoming load current and power factor can be noted. After energising, check that capacitor draws almost balance current in all the 3 phases and is near to its rated value. Note the change in bus voltage, load current and system power factor. Normally after capacitors are energised, there will be little rise in bus voltage and some reduction in system load current and improvement in power factor. In case load current increases instead of reducing, it shows that capacitors connected are more than required for the load and in this case the power factor shall be leading. When Residual voltage factor (RVT) is used for unbalance protection, measure open delta voltage, which should be negligible. In case, capacitors are connected in double star with neutral CT, the current on the secondary side of neutral CT can be measured, which should also be negligible.
24
Protection of Capacitor bank : 1) Fuse is provided for each capacitor in the bank. The fuses shall be external type for 11 KV capacitor bank. The capacitor unit together with external fuse shall be arranged in such a way to avoid bird faults by providing adequate clearance between the body and the line terminal. Capacitor bank of voltage level more than 11 KV is provided with internal fuse type. In case of fault, the faulty element will automatically go out of circuit. 2) Discharge resistors are provided within the capacitor unit to ensure safety after de-energisation of capacitor (To reduce the residual voltage from crest value of rated voltage to 50 volts or less within 5 minutes). The power loss in these resistors is negligible. 3) Each capacitor bank is protected against lightning by gapless zinc oxide arrester. 4) The capacitor protection equipment include over current, earth leakage and protection to detect unbalance loading due to abnormal conditions. Maint enance of C apacitor s : Capacitors should be allowed to discharge through the discharge device provided for the purpose before working on them. Never discharge capacitor by shortcircuiting its terminals, as it can get damaged this way. Following maintenance is carried out on capacitor bank: 1) Cleaning of bushings 2) Tightness of connections of capacitor bank, series reactor 3) Checking value of capacitance & discharge Resistors 4) Checking earthing connections and tightness 5) Checking of all protections (Relays) 6) Checking of capacitors units for any leakage. 7) Checking of oil BDV of series reactor and NCT/RVT.
25
6) POWER TRANSFORMER Transformer is one of the most important equipments in a power transmission and distribution system. It does stepping up or stepping down the voltage and transfer power from one A.C. voltage to another A.C. Voltage at the same frequency. Transformer has Primary & Secondary windings housed in main tank filled with insulated oil. Oil is used for providing insulation as well as cooling of windings. 1) The capacity of Transformer is expressed in Volt-ampere (KVA / MVA) 2) The transformation ratio K (constant) = Vs/Vp = Ns/Np Where Vp, Np denote primary voltage & turns respectively. And Vs, Ns denote secondary voltage & turns respectively. If K > 1, then transformer is called step-up transformer If K < 1, then transformer is called step-down transformer For an ideal transformer, Input VA = Output VA i.e. Vp x Ip = Vs x Is or Is/Ip = Vp/Vs = 1/K (where Ip & Is are Primary and secondary current respectively). Hence currents are in the inverse ratio of the (voltage) transformation ratio.
To calculate current of Primary & Secondary winding of 132 / 33 KV, 50 MVA Transformer:a) Primary Current in amp = Ip = VA / √3 x Vp, where Vp & Ip are primary voltage and current respectively. Hence Ip = (50 x 10*6) / (√3 x 132 x 10*3) = 218.69 Amp b) Secondary Current = Is = VA / √3 x Vs, where Vs & Is are secondary voltage and current respectively. Hence Is = (50 x 10*6) / (√3 x 33 x 10*3) = 874.77 Amp
26
General view of Power Transformer :-
Main fix tures of Power Tra nsfor mer and their funct ions are listed below
:-
a) Buchholz Relay - This relay is designed to detect transformer internal fault in the initial stage to avoid major breakdown. Internal fault in transformer generates gases by decomposition of oil due to heat & spark inside the tank. These gases pass upward towards the conservator tank, trapped in the housing of the relay, thereby causing the oil level to fall. The upper float rotates & switches contacts close & thus giving alarm. In case of a serious fault, gas generation is more, which causes operation of lower float & trips the circuit breaker. The gas can be collected from a small valve at the top of relay for Dissolved Gas Analysis (DGA).
27
Checking the floats operation manually
:-
a) Close the both valves. (From Transformer and main conservator side) b) Drain oil from the buchholz relay. c) Top float makes contact as the oil gets lowered and gives Alarm. d) If oil is further drained, bottom float makes contact and gives trip signal. After testing, both valves must be opened without fail and released the air from relay. Alarm & Trip circuit can also checked by shorting contacts externally by link. b) Oil Surge Relay - It is similar to Buchholz relay with some changes. It has only one float & operates when oil surges reach and strike the float of OSR. It is used with OLTC for detection of any damage or fault inside the tap changer and prevents tap changer from damages in case of low oil level in OLTC tank. Checking the float op eration manually: a) This relay can be checked by pressing test switch provided on top side. Here only one contact is provided which gives trip signal on operation of float. By shorting contact externally by link, trip circuit can also be checked.
c) Explosion Vent
- It consists of a bent pipe with bakelite diaphragm at both
ends. A protective wire mesh is fitted on the opening of transformer to prevent the pieces of ruptured diaphragm from entering the tank. The wire mesh is also provided at the upper end to protect upper diaphragm from any mechanical damages. At the lower end, there is a small oil level indicator. When the lower diaphragm ruptures due to excess internal pressure, the oil level rises in the vent pipe & is visible through the indicator. In case the internal pressure developed is not reduced to safe value after the bursting of lower diaphragm, upper diaphragm gives away throwing the gas and oil outside and prevents further mechanical damages.
28
d) Pressure Relief Valve
- When the pressure in the tank rises above pre-
determined safe limit, this valve operates & performs the following functions: 1) Allows the pressure to drop by instantaneously opening the port. 2) Gives visual indication of valve operation by raising a flag. 3) Operates a micro switch, which gives trip command to breaker.
Checking t he PRV operatio n manuall y: a) The operation of PRV can be done by lifting the plunger (Plunger operates switch). By shorting contact externally by link, trip circuit can also be checked. e) Oil Temperature Indicator
- It is dial type thermometer, works on the vapour
pressure principle. The bulb, which is known as ‘Probe’ is exposed to the temperature to be measured, is connected by a length of flexible tubing to a borden gauge tube, which is known as 'operating bellow'. This bellow is filled with a volatile liquid. The change in bulb temperature causes change in the vapour pressure of the liquid & pointer moving on a dial calibrated in degree centigrade indicates the consequent movement of the operating bellow. It has two pair of contacts, one for Alarm & another for Trip. In general, oil temperature alarm is set at 80°- 85° C and oil temperature tripping is set at 85°- 90° C. Checking th e OTI operatio n manually : a) The operation of OTI can be checked by tilting the float position. The first float S1 is used for alarm and another float S2 is for trip signal. Alarm & Trip circuit can also checked by shorting contacts externally by link. f) Winding Temperature Indicator
- It is also similar to OTI but has some
changes. It consists of a probe fitted with 2 capillaries. Capillaries are connected with two separate bellows (operating/compensating). These bellows are connected with temperature indicator. Operating bellow is surrounded by heater coil, which gets current from one WTI CT, when load on transformer increases, corresponding current passes to the heater coil mounted on operating bellow. The heater coil heats the operating bellow, which is filled with volatile liquid. 29
Due to this heat, vapour pressure of volatile liquid increases hence WTI shows more temperature as compared to OTI. There are four mercury switches, 1 contact for Alarm, 2 for Trip circuit and 3 for cooler control and 4 as a spare. In general, winding temperature alarm is set at 85°- 90° C and winding temperature tripping is set at 90°- 95 ° C. The fan Auto ON operation is set at 60 ° C and Fan auto OFF is set at 55° C. Checking the WTI operation manually: a) The operation of Winding Temperature Indicator can be done by tilting the float position. The first float S1 is used for alarm and another float S2 is for trip signal. Fan auto operation can also be checked by float movement. Alarm / Trip circuit can also be checked by shorting contacts externally by link. g) Conservator - As expansion and contraction occurs in transformer main tank, consequently the same phenomena takes place in conservator as it is connected to main tank through a pipe. Conservator communicates with the atmosphere through a breather, incorporating a dehydrator, which is connected to the breather pipe. Other end of this pipe opens at the top in the conservator, just below the conservator upper wall. h) Breather - This is a special air filter incorporating a dehydrating material, called, Silica Gel. It is used to prevent the ingress of moisture and contaminated air into conservator. It consists of an inner metal cylinder filled with silica gel. Both ends of this cylinder are enclosed by wire mesh screen. This cylinder is enclosed in an outer casing of cast iron. Casing has 2 parts. The upper part is cover; where as lower part is attached with an oil seal. When transformer breathes in, the air enters which passes into the oil seal. The contamination, if any, is observed in this oil. Then air passes through silica gel, where the moisture, if any, is observed by the silica gel and pure and dry air goes to conservator tank of transformer. Normal colour of Silica Gel is blue. If it turns to pink, then Silica Gel is to be reactivated / replaced by fresh Gel. 30
i) Oil Level Indicator
- It is also known as magnetic oil gauge (MOG). It has a pair
of magnet. The metallic wall of conservator tank separates magnets without any through hole. Magnetic field comes out and it is used for indication. This eliminates any chances of leakage. The driving magnet rotates and acquires the position corresponding to height of oil level, as it is linked with a float. The float is hinged & swings up and down with oil level. This rise or fall rotates driving magnet with the help of bevel gear and pinion. Follower (Driven) magnet moves accordingly and operates a pointer & a cam. The pointer reads oil level & cam is set to operate a mercury switch to give low oil alarm as per the oil position. Checking t he MOG operatio n manuall y: a) Operation of MOG can be done by tilting the float position which gives alarm signal. Alarm circuit can also checked by shorting contacts externally by link. j) Radiators - Small Transformers are provided with welded cooling tubes or pressed sheet steel radiators. But large transformers are provided with detachable radiators plus valves. For additional cooling, exhaust fans are provided on radiators. The hot oil in main tank goes up and enters in the radiators. After cooling in radiators, either by natural air or forced air, oil again goes to main tank from the lower valve and circulates continuously. k) Bushings – It comprises a central conductor surrounded by graded insulation. Bushing is necessary when a conductor is taken out through metallic tank or wall. Oil filled bushing is used for 33 KV applications. For making bushing compact, synthetic resin bonded condenser bushing is used for 33 and 66 KV applications. For 132 KV & above voltages, oil impregnated paper condenser bushing is used. It consists of a central conductor surrounded by alternate layers of insulating paper & tin foil. The capacitor formed by alternate layers of tin foil and paper insulation results in uniform electric stress distribution between conductor surface and earthed flange. The bushing core is coated with suitable resin.
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The assembly is enclosed in hollow porcelain and is provided with support flange and top hood. The porcelain is filled with oil. Creepage Distance (CD) – It is the shortest distance between two conductive parts along the surface of the insulating material. CD requirement depends upon rated phase to ground voltage and degree of atmospheric pollution.
Degree of Pollution
Recommended Min. CD
1) Clean area
16 mm / KV
2) Moderately polluted area
20 mm / KV
3) Industrial area
22 mm / KV
4) Heavily polluted/coastal area
25 mm / KV
l) Tap Changer - As load on the transformer increases, secondary terminal voltage decreases. To maintain the secondary voltage, tap changers are used. Tap changers are connected with H.V. winding (Primary winding). Therefore in tap changers transformer, there are two windings in H.V. side, 1) Main winding and 2) Tap winding. There are two types of tap changers. A) Off Load Tap Changer
- In this type, before moving the selector, transformer
is made OFF from both ends. Such tap changers have fixed brass contacts, where taps are terminated. The moving contacts are made of brass in the shape of either roller or segment. B) On Load Tap Changer
- In short we call it as OLTC. In this, taps can be
changed manually by mechanical or electrical operation without making off the transformer. For mechanical operation, interlocks are provided for non-operation of O.L.T.C. below lowest tap position and above highest tap position. Similarly for electrical operation, limit switches are provided in circuit for non-operation of tap change below lowest tap position and above highest tap position. For mechanical operation, one hand interlock switch is provided in the circuit. 32
As soon as we insert handle, hand interlock switch opens out the electrical circuit and no one can operate O.L.T.C. electrically. RTCC (Remote tap change control cubicle) is used for tap changing by manually or automatically through Automatic Voltage Relay (AVR) which is set +/- 5% of 110 Volt (Reference taken from secondary side PT voltage). During Auto tap changing, Bell / Hooter will ring up thus gives information to substation operator for tap changing. Transition resistances are used in OLTC for avoiding momentarily interruption of power supply during tap changing. At the time of tap changing, load current passes through the transition resistances & no power interruption occurs during tap changing. Transform er Tap: - Tapping is provided in Primary winding. Hence by changing the tapping, we can change secondary voltage as per requirement. The transformer equation is: - V2/V1 = N2/N1 i.e. V2 = (N2 x V1)/N1 There is an Inverse relationship exists between secondary voltage & primary turns. When primary turns are decreased i.e. Tap position is shifted from 3 to 4, secondary voltage gets increased and if primary turns are increased i.e. Tap position is shifted from 4 to 3, then secondary voltage gets decreased. Parallel Operation of Transformers
:
Before paralleling two or more transformers, the four principal characteristics of those transformers should match as given below: 1) Same voltage ratio 2) Same percentage impedance 3) Same polarity 4) Same vector group
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If two transformers of same output operating in parallel, the % impedance must be identical, if Transformers are to share equally. If % impedance is not identical, suppose T/F 'A' is having 4% impedance and T/F 'B' is having 2% impedance, then load sharing will be, Load A = L x ( Z2 / Z1+Z2 )
Where L is total combined output
Load B = L x ( Z1 / Z1+Z2 )
and Z is percentage impedance
So that A transformer will share only 1/3rd load & B transformer will share 2/3rd load. Hence operating transformers in parallel, the output of the smallest transformer should not be less than 1/3rd of the output of largest one. When operating two transformers in parallel, one of the RTCC panels is kept on Master mode and another one is kept on Follower mode so that simultaneously tap changing is possible on both transformers. If transformers are not running parallel, the control switch is kept on Independent Mode i.e. both transformers taps can be separately changed. Site Testing of Transformer : 1) Insul ation Resis tance Test – a) Between HV & Earth. b) Between LV & Earth. c) Between HV & LV by suitable range of megger. 2) Voltage Ratio Test - This test is essential to check the output or the secondary voltage on each tap position. By virtue of this test the problems in the OLTC can be easily detected. 3 Phase, 440 V LT supply is applied to the primary side of the transformer and the output volts at the secondary side for each tap position is measured. If any break in voltage reading is observed during change of tap position, then there is some problem in that particular tap.
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3) Magneti c B alance Te st - This test is carried out to check the balancing of the induced voltages in the windings & flux distribution. Transformer is kept on normal tap position and 3 Phase, 440 V LT supply is applied to the primary windings as given below: 1) YNyn0 Transformer: - First the voltage is applied between R & N. Voltage will be induced in between YN and BN. Voltages are noted & will be observed that: In Primary side: - V RN = V YN + V BN = 2/3 + 1/3 On Secondary side: - V rn = V yn + V bn = 2/3 + 1/3 If the voltage readings on secondary are observed as above, then it can be assumed that the flux distribution is balanced & proper. If the magnetic balance is not correct, readings will be different and typical noise will be observed. This will indicate that there is some problem in the core of the transformer.
Again apply voltage to YN, the result will be: In Primary side: - V YN = V BN + V RN = 1/2 + 1/2 On Secondary side: - V yn = V bn + V rn = 1/2 + 1/2 Similarly apply voltage to BN, the result will be: In Primary side: - V BN = V YN + V RN = 2/3 + 1/3 On Secondary side: - V bn = V yn + V rn = 2/3 + 1/3 Note: - In case of Dyn11 Transformer, voltage is applied on primary side between first R and Y terminals (R Phase winding), next Y and B terminals (Y Phase winding), and B and R terminals (B Phase winding). Result will be same as mentioned above for YNyno Transformer.
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4) Vector Group Test - This test is carried out to check correctness of windings connections. The Phase angle difference arises out of the internal connections of the windings. A star / star transformer having the similar vector diagram for primary and secondary side can be connected in two different ways internally. In the first case there is 0° displacement between primary and secondary whereas in the second case there is 180° displacement. In addition to this, a +30 or −30° displacement is possible in a 3 phase transformer when the vector diagram is different i.e. delta/star OR star/delta type. For parallel operation, secondaries must have same phase angle displacement with respect to their primaries so that there may be no phase difference between the terminals of the secondaries themselves. A three-digit vector symbol is adopted to designate the vector group. a) First letter in capital represents Primary winding connection - D: Delta & Y: Star b) Second letter in small represents Secondary winding connection - d: Delta and y: Star c) Third digit represents the phase displacement between the primary and secondary. The convention employed is to describe it by the hour in a clock in which the HT voltage is represented by the minute hand set to 12 o'clock position, and the LT voltage is represented by the hour hand. Since 12 hours represents 360 °of a full circle, each hour represents a 30 ° phase difference. Thus ‘0’ represents no phase difference, ‘1’ stands for minus 30°, ‘6’ for 180° and ‘11’ for plus 30° displacement as referred to the standard counter clockwise vector rotation.
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Vector Group Testing at the time of Commissioning or on repaired Job: 1) YNyn0 Transfo rmer : - Keep the transformer on normal tap position. Short R & r of windings. Apply 3 Phase L.T. voltage to primary windings. Measure voltages on the secondary side. R, r R
r n
n b
b B
y
Y
y N
B
Primary
Secondary
Y Vector Representation
Following conditions are to be satisfied: a) V RN = V Nn + V rn
b) V Bb = V Yy
2) Dyn11 Transformer : - Keep the transformer on normal tap position. Short R & r of windings.
R,r R
r
y n
n
b
b B
y
B
Y
Y Primary
Secondary
Vector Representation
Apply 3 Phase L.T. voltage to primary windings. Measure voltages on the secondary side. Following conditions are to be satisfied: a) V Bb < V By
b) V Yb = V Yy
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5) Magnetizing Current Test
- When a Power Transformer is charged, it is
generally presumed that it is to be charged on NO LOAD condition because it draws magnetizing current containing high harmonics. Transformer may trip on differential protection if it is not provided harmonic restraint protection. This current inrush is due to the iron losses of the transformer. This current should be equal in all three phases so that there would not be any spill current in the relay to trip the primary circuit breaker of the transformer. The test is carried out at normal tap position. Apply 3 Phase L.T. voltage to primary windings through ammeters (ma) connected in series of windings and keeping secondary winding open. It would be seen that the current drawn by all the three phases would be same. The current is drawn on account of the magnetizing of the core. (Iron loss) It can also be called as no load current when the transformer is charged with rated primary voltage applied across the primary, keeping secondary open.
6) Short Circuit Current Test
– Short circuit test is carried out to check the
healthiness of windings. Apply 3 Phase L.T. voltages to primary windings & secondary windings are shorted through ammeters of suitable range. If the readings are equal in all three phases, transformer is supposed to be healthy. Actually here the term “% Impedance" comes into picture. The reduced voltage required to be applied across the primary of a transformer to cause rated full load current to flow through this winding when secondary winding is shorted, is known as impedance voltage. It is expressed as a percentage of the rated voltage of former winding. In this case current flowing through secondary is the full load current & is indicative of the copper losses.
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7) Oil Test - The oil is used as insulation between windings & core and between windings & tank. Without oil, the paper insulation of the windings could be punctured early which in turn will result in failure of transformer. The oil facilitates cooling of the windings and magnetic circuits. The oil protects windings and core of transformer from the absorption of moisture. The test on oil is divided into two different categories. 1) Physico chemical Testing
:-
a) Density - It indicates the type of transformer oil whether paraffin base or naphtha base. b) Kinematics Viscosity
- The oil should circulate freely in the equipment to
maximize heat transfer. A low viscosity oil fulfils this need. Viscosity of oil increases because of oxidation taking place at all times. If viscosity increases by 15%, then oil needs replacement. c) Flash Point – Flash point is a minimum temperature at which oil will support instantaneous combustion (flash) but before it burns continuously. Flash point of new oil should be fairly high. d) Pour Point - It is the indicator of the ability of oil to flow at cold operating conditions. It is the lowest temperature at which the fluid will flow when cooled under prescribed conditions. e) Neutralization Value
- This indicates the presence of combined acids i.e.
organic & inorganic. The degradation of oil gives rise to acidic compounds and formation of sludge. The acidity is given by its neutralization value, which indicates the total acidity and is evaluated by milligrams of KOH per gram of oil. Acidity content in oil should be low.
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f) Water Contents - It is expressed in parts per million (ppm). Dielectric strength of oil is very high when water content is low. g) Inter Facial Te nsi on - It is a measure of the molecular attractive force between their unlike molecules at the interface. When oil oxidizes, the organic acid thus produced are concentrated at the placing a drop and used oil on water surface, water is spread rapidly over the surface in contrast to a new oil, which may float as a lens on the water. It is considered that IFT gives an indication of degree of slugging of oil as dissolved impurities in the oil tends to diffuse into the water, which lowers the IFT. 2) Electrical Testing : a) Dielectric Strength - The BDV of oil is its ability to withstand electric stresses without failure. b) Resistivity - It is the measure of electrical insulating properties of oil. High resistivity reflects low content of free flowing particles. c) Dielectr ic Dissip ation Facto r (Tan ) - It is a measure to the ratio of the power dissipated in the oil to the product of effective voltage and current. It is tangent of loss angle & expressed in unit or percent. It determines the cleanliness of oil & is related to aging characteristic of the oil.
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Standar d Values of New Oil as p er IS : 335-1983 S/No. Characteristics 1 Breakdown Value a) > 145 KV b) 72.5 to 145 KV c) < 72.5 KV 2 Dissipation Factor at 90°C (Tan ) 3 Specific Resistance at 90°C 4 Water content ppm a) > 145 KV b) 72.5 to 145 KV c) < 72.5 KV 5 Inter Facial Tension 6 Density 7 Flash Point 8 Pour Point 9 Total Acidity Test
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Standard Values 60 KV (min.) 50 KV (min.) 40 KV (min.) 0.05 1 X 10 * 12 ( Ω-cm) 15 ppm (max.) 20 ppm (max.) 25 ppm (max.) 0.03 N/m (min.) 0.89 g/cu.cm (min.) 140°C (max.) - 6°C (max.) 0.03 mg KOH/g (max.)
Dissolved Gas Analysis (DGA) : - Transformer, in operation, is subjected to various thermal and electrical stresses, resulting in liberation of gases from the oil which is used as insulation media and coolant. The solid insulating materials like paper, wooden support, pressboard, etc. cause degradation and form different gases, which get dissolved in the oil. The most significant gases generated are Hydrogen (H2), Methane (CH4), Ethylene (C2H4), Acetylene (C2H2), Propane (C3H8), Propylene (C3H6), Carbon Monoxide (CO), Carbon Dioxide (CO2), and Ethane (C2H6). The gas connected in the relay will help to identify the nature of the fault. The greater the rate of gas collection, the more severe is the nature of the developing fault. Colour of the gas helps in finding the affected material. Colour
------ Identifi cation
White
----- Gas of decomposed paper and cloth insulation
Yellow
----- Gas of decomposed wood insulation
Grey Black
----- Gas of overheated oil due to burning of iron portion ----- Gas of decomposed oil due to electric arc
Ratio Method used for Analysis of DGA results:-In this method three ratios of gases are used. They are methane / hydrogen, ethylene / ethane, acetylene / ethylene. If the ratio comes out more than one, it indicates abnormal deterioration and less than one indicates normal aging.
Particulars
C2H2/C2H4
CH4/H2
C2H4/C2H6
a) Less than 0.1 b) 0.1 to 1.0
0 1
1 0
0 0
c) 1.0 to 3.0
1
2
1
d) More than 3.0
2
2
2
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Interpre tation of the faults accordin g to the observed ratios of Characteristic Fault
Ratio Code C2H2/C2H4
No Fault Partial Discharge of low energy density Partial Discharge of high energy density Discharge of low energy
Gases
Diagnosis
CH4/H2 C2H4/C2H6
0 0
0 1
0 0
Normal Aging Discharge in gas filled cavities due to incomplete impregnation
1
1
0
As above but leading to tracking or perforation of solid insulation
1 to 2
0
1 to 2
Discharge of high energy
1
0
2
Thermal fault of low temp. less than 150°C Thermal fault temp. 150 to 300°C Thermal fault temp. 300 to 700°C Thermal fault temp. > 700°C
0
0
1
0
2
0
0
2
1
0
2
2
Continuous sparking in oil between bad connections of different potential or to floating potential. Breakdown of solid material Discharge of power follow through arcing, breakdown of oil between winding or between coils to earth General insulated conductor overheating Local overheating of core due to concentration of flux. Increasing hot spot temp.; varying from small hot spots in core, shorting links in core, overheating of copper due to eddy currents, bad contacts / joints (pyrolitic carbon formation) up to core and tank circulating currents.
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Ac ti ons du ri ng f ail ur e / tr ip pi ng of Transf or mer – The action to be taken depends upon the size of the transformer, operation of protective relay, whether tripping is accompanied by loud noise, smoke or expulsion of oil from the transformer, etc. Observe the transformer external condition; look for any damage to the bushings, leads or cable box. Note the temperature of oil & check if the oil level in the conservator is right. Take megger readings between primary and secondary and also of each to earth. If everything is right, proceed as noted below: - The failure may possibly be due to a sudden and heavy overload, or short-circuit. If a DO fuse has dropped out, check if its ampere rating is right. If incorrect, replace by the correct size and energise the transformer, after switching off the load. If everything is all right, close the secondary circuit; if fuse blows again, the fault is obviously in the outgoing lines, which should be traced and rectified; if on the other hand, the primary circuit fuse blows out, even when the load is disconnected, an internal fault is indicated. This also apply, if an over current relay alone has operated and tripped the breaker. - If a differential relay operates when a transformer is first switched on, it may be due to a switching surge. Check the harmonic restraint circuit and setting. If, on the other hand, relay operates when the transformer is in service, it is a sure indication of an internal fault. Any tripping of buchholz relay requires to be carefully looked into. If the lower assembly has tripped due to sudden evolution of large quantities of gas, a major internal fault is to be inferred especially if either over current or differential or earth fault relay has operated. If, on the other hand, the upper assembly has operated due to slow release of gas it is necessary to find out its composition before any conclusions can be drawn. If it is air only, there is no cause for worry, as air can enter into the transformer in many ways.
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When transformer is commissioned, it sometimes happens that the buchholz relay upper assembly operates, after a few hours of run, due to the release of air bubbles entrapped within the windings, such as when hand filling is employed for filling of oil into the tank. If the accumulated gas is not air, an incipient fault is indicated. DGA would help in identifying the nature of the fault, and this should be done as a routine measure. If the buchholz relay has tripped, without any gas being given out, it may be due to electrical fault in the wiring. - Thorough checking is required if the earth fault relay has tripped, or there is an evolution of smoke or oil, and also PRV has operated in case of large transformer. In such cases reclosing of breaker should not be permitted as it may cause further extensive damage. Detailed testing of transformer is to be carried out and compare the results with test certificate figures and consult the manufacturer. In most of the cases, the cause of the fault can be found out if you carefully observe the condition of windings by lifting the core and coil assembly. The following notes may be of help in identifying the cause: Lightning discharge or over voltage:
This is characterised by breakdown of the
end turns close to the line terminal. There may be a break in the turns or end lead, and also flash marks on the end coil and earthed parts close to it, but the rest of the coils will be found to be healthy. Sustaine d overloads: The windings in one or all the phases would show signs of overheating and charring; the insulation would be very brittle and have lost all its elasticity. Inter-turn short:
The same signs as for sustained overloads would be noticed,
but only on one coil, the rest of the coils being intact.
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Dead short-circuit:
This can be identified by the unmistakable, lateral or axial
displacement of the coils. The coils may be loose on the core; some turns on the outermost layer may have burst outwards and broken as if under tension. If, in addition to these signs, the windings are also completely charred, it is conclusive evidence that the short-circuit has continued for an appreciable period, not having been cleared quickly by the protective relays. Visual checking of
Transform er : -
- Check the colour of silica gel. If it is pink, reactivate or replace it. Also ensure proper quantity of oil in breather oil cup. - Check oil level in Conservator of Main Tank & OLTC. It should be > ½ level marking. - Check oil level in Bushings. - Check for any oil leakage. Arrest leakages, if any. - Check the working of OTI & WTI by taking hourly temperature readings. There should be changes in readings as per loading of transformer and atmospheric condition. - Check the cooling system by making fans / pumps operation by manually. - Check the tap position of RTCC panel and OLTC panel. It should have same position number. Check the humming noise & vibration of transformer. If any abnormality found, it is to be referred to concerned manufacturer.
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7) CONTROL & RELAY PANELS Control or relay boards are built up by using requisite number of self-contained sheet steel cubicles, comprising a front panel to carry the control apparatus & the hinged or removable back cover to give access to interior wiring, cable termination. This type is called as Simplex type panel. When panels are arranged back to back in corridor formation, and door is then fitted at each end, are called as Duplex panels. Depending upon the size of the substation the control and relay board may incorporate the followings: 1) Indicating and metering instruments mounted on front. 2) Relays mounted on the backside in Duplex panel, flush mounting on front in Simplex panel. 3) A mimic diagram representing main circuit connections is incorporated on the front panel. It is a single line diagram incorporated on the front side of the control panel. This diagram represents the actual physical position of various HT electrical equipments in the sub-station yard along with status of equipments, ON and OFF positions of various breakers and isolators through semaphore indication or lamp indication.
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4) The automatic semaphore indicators are used to denote position of switches.
5) Circuit Breaker control switch (TNC switch) is fitted on front. Normally switch is on Normal (Centre) position. Handle is moved to the right or left to initiate close or trip operations. 6) Indication lamps mounted for various purposes follow a standard colour code. Red - C.B. or switch CLOSED Green - C.B. or switch OPEN White - Trip circuit healthy Amber - Alarm indication i.e. CBs tripped on fault 7) Annunciation System – It gives alarm in case of any abnormality in the system. Alarm bell rings and appropriate facia lamp flashes ON & OFF. Substation operator has to ACCEPT the signal by pressing a button, which silences the bell and causes the lamp to show a steady light. After taking remedial action, the operator RESETS the alarm circuit by pressing another push button, the lamp being simultaneously extinguished.
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Colours for Interna l Wiring Red
- Phase connection, either directly connected to the primary circuit or Connected to secondary circuit of CT and PT
Yellow
- Phase connection, either directly connected to the primary circuit or Connected to secondary circuit of CT and PT
Blue
- Phase connection, either directly connected to the primary circuit or Connected to secondary circuit of CT and PT
Black
- A.C. neutral connection, Star point connections of secondary circuit of CT and PT, and connections in A.C. and D.C. circuit
Green
- Connections to earth
Grey
- Connections in D.C.circuit
Each wire should have a letter to denote its function. D.C. supply from +ve source should bear odd number & from -ve source should bear even number.
CT Secondary Terminal – S2 of all protection & metering cores are shorted in CT junction box. Only one common wire of S2 along with S1 wires of all 3 phases CTs are brought to CRP. Earthing of S2 wires is done at one end. (preferably at CRP end) In substation, various drawings are available namely: a) Wiring Dra wing: The routing of wires from various equipments in a control and relay panel is shown in this drawing. The route of the particular wire as per its purpose of application can be traced easily while attending any faults in the particular circuit. For reading of drawing it should be kept in mind that drawing is prepared when isolator & breaker positions are OFF & spring of the breaker mechanism is in deenergised condition.
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b) Schematic Drawing: This drawing is a representation of various circuits such as metering, protection, control, indication, annunciation, etc. in a control and relay panels. c) Layout Drawing:
This drawing shows arrangement of various indoor and
outdoor equipments in a particular installation in a sequential order.
Common Fe rrule Numbers used in wirin gs A: CT secondary connection for primary protection like Differential, Distance, REF Relay). Small “a” used for PT secondary connection in PT terminal box. B: Bus bar Protection (CT secondary connection). B for B phase indication. C: Back up Protection (CT secondary connection for O/C & E/F Relay). D: Metering (CT secondary connection). E: Metering & Protection (PT secondary connection). H: A.C. connection. J: D.C. connection (Before Fuse). K: D.C. connection for control (After Fuse). L: D.C. connection for Indication (After Fuse). M: Motor Supply (Spring charging Motor in Circuit Breaker). N: RTCC (Tap Changer) connection. Also for denoting A.C. Neutral connection. P: PT primary connection & DC circuit of Bus bar protection scheme. R: R Phase Indication. S: CT secondary connection in Terminal Box. U: Circuit Breaker auxiliary contacts. X: TB Numbering. Y: Y Phase Indication.
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8) STATION TRANSFORMER This is a small distribution transformer located in the substation premises. It has given protection through proper rating of D.O.Fuse. Incoming HT supply to the transformer is tapped from LT bus of substation through Isolator. The output voltage 440 Volt is terminated to ACDB through LT cables. The main purpose of station transformer in substation is to provide auxiliary supply to various equipments through A.C. Distribution Board (ACDB) via MCBs or Switch Fuse units: 1) A.C. supply is used for battery charger, which converts A.C. to D.C. supply for charging the batteries and parallel provides D.C. source for various controls of substation equipments. In case of A.C. supply failure, batteries will take care of D.C. supply continuity for equipment’s controls.
2) A.C. supply is used for OLTC for tap changing operation of transformer and also cooling arrangement of transformer. 3) A.C. supply is used for spring charging mechanism of breakers. 4) A.C. supply is used for Office and Yard Illumination. 5) A.C. supply is used for Oil filtration, some miscellaneous welding work, and Test supply for carrying out testing of various equipments in switchyard.
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9) BA TTERIES & B ATTERY CHARGER For controlling various operations of substation equipments, suitable D.C. supply is required. In battery charger panel, A.C. 1 phase or 3 phases is given, which converts A.C. to D.C. supply. This D.C. supply is given to various control panels of substation and for charging the batteries through D.C.Distribution Board. (DCDB) In case of A.C. supply failure, batteries provide D.C. supply for controlling the operations of substation equipments in normal or abnormal conditions. Battery capacity is expressed in ‘Ampere Hours’ which is the useful quantity of electricity that can be taken from a battery at the specified rate of discharge before its cell voltage falls to the specified value, which is equal to 1.75 volts multiplied by the number of cells. Ampere hours is equal to the product of the specified discharge current in amperes multiplied by the number of hours before the battery discharges to the specified extent. Precauti ons / Maintenance: - Batteries should be cleaned regularly. - Cell voltages & Specific gravity is to be recorded as per schedule. - Batteries should be charged in a well-ventilated place, so that the gases and the acid fumes are blown away. - Do not disturb any connection with charger on, as there is risk of sparking. - If acid or electrolyte gets spattered into the eyes, wash them immediately with large quantity of clean, cold water. - Tighten connections periodically. Apply petroleum jelly to terminals to prevent corrosion. - Maintain level of the electrolyte – Add only the distilled water. Add electrolyte only if some of the electrolyte spills out.
52
10) MEASURING INSTRUMENTS a) Voltmeter : - Voltage in an AC circuit is measured by voltmeter. The voltmeter is connected across the load or winding. For high voltage, voltage transformer is necessary to step down the voltage for measurement. Voltmeter is connected across the secondary circuit of PT. Voltmeter can be replaced on line by removing fuses or keeping voltmeter selector switch in OFF position. b) Am met er : - Current in a circuit is measured by ammeter connected in series of current path. If current is high, suitable current transformer (CT) is necessary to step down current for measurement. Ammeter is connected in series of secondary circuit of CT. Ammeter can be replaced by shorting CT secondary wires or keeping ammeter selector switch in OFF position. c) Energy Meter : -The Power in electrical circuit is measured by energy meter. Energy is the total power consumed over a certain period and is measured in kilowatt-hour (KWH). One kilowatt-hour is equal to the energy consumed when power is utilized at the rate of one kilowatt for one hour. The term ‘unit’ used for expressing consumption of electrical energy is equal to one kilowatt-hour, and all tariffs for energy consumption are based on this unit. A registering mechanism in the energy meter indicates the total energy consumption. Energy meters will record correctly, if connections are made with due care to the polarity and the terminal markings. Energy meters can be changed or replaced while in service by use of T.T.B. (Test terminal block). In TTB, CT secondary can be shorted during removal of Meter (avoiding open circuit of CT secondary) & PT supply can be made OFF by disconnecting type arrangement or by removing fuses. Energy meter records Import / Export energy parameters. Import parameters are displayed by arrow in in
direction.
53
direction and Export parameters
POWER TRIANGLE
Apparent Power KVA (S)
Reactive Power KVAr (Q)
φ Active Power KW (P) Power Factor = Cos φ = (Active Power) / (Apparent Power) Ac ti ve Pow er : The actual amount of power that produces the effective work is called active or real power. It is measured in Watts. Reactive Power : The power drawn by reactive load such as Capacitors and Inductors in a system is called reactive power. It is measured in VAr (Volt amp reactive). Ap paren t Power : The total power demanded by the load is the product of current and voltage. This power is referred as apparent power. It is measured in VA (Volt amp).
Multiplication Factor of Energy Meter : M.F. = [(Feeder CTR x PTR) / (Meter CTR x PTR)] Case 1: - a) Feeder CTR = 600/1 A, Feeder PTR = 33000/110 V b) Meter CTR = 300/1 A, Meter PTR = 33000/110 V [(600/1) x (33000/110)] M.F. of Energy Meter = ----------------------------- = 2 [(300/1) x (33000/110)]
54
Case 2: - a) Feeder CTR = 100/1 A, Feeder PTR = 33000/110 V b) Meter CTR = 400/1 A, Meter PTR = 33000/110 V [(100/1) x (33000/110)] M.F. of Energy Meter = ----------------------------- = 0.25 [(400/1) x (33000/110)]
A Ty pi cal SECURE Energy Meter c on nec ti on is sh ow n bel ow
SECURE MAKE 3 Phas e 4 Wire Energ y Meter
1
2
3
M
1R
L1
4
5
6
7
M2
YL2M
3B
8
9
10
L3 N
R
R
R
Y
Y
Y
B
B
B
P H A S E
P
P H A S E
P H A S E
P
P H A S E
P H A S E
P
E
P H A S E
C T
C T
C T
C T
P T
C T
s 2
s 1
s 2
s 1
H A S E
P
C T s 1
T
H A S E
H A S
P T
55
s 2
N E U T R A L
d) Earth Tester : - Resistance of the earth pit ‘E’ in following figure can be measured directly with the help of an ‘earth tester’ Earth Tester P1
C1
P2
C2
E
P Potential Electrode
Electrode (Earth Pit)
C Current Electrode
E is the earth pit electrode under measurement; P & C are two auxiliary electrodes of 15-20 mm diameter and 40 cm long bars. The electrode C1 is planted at a distance of approx. 25 metres from E and P1 is fixed centrally between E and C1. One reading of Pit resistance is taken by rotating handle of earth tester. Two more readings are taken by shifting P1 a distance of 3 metres on either side of its central position. The value is the resistance of Electrode E to the earth. e) Insulation Tester (Megger): -
Insulation resistance between an insulated
conductor (part) and earth is checked by megger. Phase conductor is connected to the terminal marked ‘Line’ on the megger and the terminal marked ‘Earth’ is connected to the earth continuity conductor or an efficient earth. The handle is turned to indicate a steady reading on the instrument. A megger, with its handle being turned gently, should record zero when its two leads are touched together, and read infinity when its leads are held apart.
56
f) Oil Tester (BDV Tester) : - Dielectric breakdown strength of transformer oil is one of the most reliable tests for proving the condition of oil. Oil sampling is done by taking due care. The glass bottle into which oil is drawn should be perfectly clean, clear, transparent and dry. It should then be thoroughly rinsed with oil known to be good. The sample of oil should drawn preferably be drawn from the bottom of the transformer tank. As water is heavier than oil, it settles down at the bottom. The first sample or two may be thrown away if it contains sludge or droplets of water. The gap between two electrodes is to be maintained / checked at 2.5 mm by gauge and the test cup is cleaned properly. The cup is then filled with the sample oil to be tested up to 1 cm above the electrodes. The cup top should then be covered with a clean glass plate and allowed to rest for at least 5 minutes so that all air bubbles may disappear. Any bubbles still standing on the surface may be removed with a clean glass rod. Use thin rubber gloves if you can, so that the sweat on your fingers may not cause any contamination of the oil. Carry out test as per procedure until there is positive and final breakdown of the oil. The test is carried out for six times on the same sample after a gap of at least 5 minutes. The average of all six readings is the dielectric strength of oil under test.
57
Some I mport ant Numbers used with 2:
Time Delay Relay or Timer
21:
Distance Protection Relay
27:
Under Voltage Relay
49:
Winding Temperature Indicator
their meanings
50/51: IDMT Over Current Relay with Instantaneous element 50/51N: IDMT Earth Fault Relay with Instantaneous element 52:
AC Circuit Breaker
59:
Over Voltage Relay
62:
Pole Discrepancy Relay with timer
63:
Gas Operated Relay (Buchholz Relay)
64R: 67: 67N:
Restricted Earth Fault Relay Directional Over Current Relay Directional Earth Fault Relay
75:
P.T. selection Relay
80:
DC Supervision Relay
86:
Master Trip / Locking Out Relay
87:
Differential Relay
89:
Line Switch / Isolator (Electrically Operated)
94:
Anti-pumping Relay (For Breaker Control)
95:
Trip Circuit Supervision Relay
96:
Gas Pressure Relay (For Breaker Control)
58
Trouble S hoot ing Works : -
S/No. 1
Probable Trouble CT Circuits Noise in CT
2
Ammeter is not recording
1
PT Circuits Voltmeter not showing correct reading
2
Cause / Works to be attended - Open circuit of secondary circuit - Loose connections in secondary circuit - Ammeter switch fault causing open circuit in CT secondary - Ammeter may be faulty - Ammeter switch is not making contact
- Check fuses - If above is OK, voltmeter may be faulty - Loose connections in PT circuit - CT secondary circuit may be in
Energy meters recording on lesser side
1
shorting position for one or two phases - PT circuit fuse may be blown - Loose connections - Energy meter may be faulty
D.C. Protection Circuits Non working of trip healthy indication
- Fusing of bulb or bulb may be fitted loose - Loose connections - Resistance may be open circuited - Misalignment of auxiliary contacts of breaker
- D.C. fuse may be loose or blown off - Trip coil is open - Push button may be faulty
59
S/No. 2
3
Probable Trouble Cause / Works to be attended D.C. Protection Circuits Non tripping of breaker - D.C. fuse may be loose or blown off - Loose connections - Trip coil is open or burnt - Misalignment of auxiliary contacts of breaker - No free movement of plunger of trip coil - Mechanical trouble in breaker - Trip switch or Push button may be faulty - Air pressure may be low in case of pneumatic operated breaker. If low, correct it - Check the circuit as per circuit diagram Non closing of breaker - D.C. fuse may be loose or blown off - Loose connections - Close coil is open or burnt - Misalignment of auxiliary contacts of breaker - No free movement of plunger of close coil - Mechanical trouble in breaker - Spring might not have been charged - Non resetting of Master / Trip relay - Non closing from remote end, if Local / Remote switch on local position - Closing switch or Push button may be faulty - Check the circuit as per circuit diagram
4
Tripping of breaker without Indication
- Due to shorting of D.C Positive - D.C. leakage
60
S/No. 5
Probable Trouble D.C. Protection Circuits Mal-operation of Relay
Cause / Works to be attended
6
Relay Flag not resetting
7
Spring charging motor does not - Either loose fitting of fuse and link start or blowing of fuse - Loose connections - Failure of A.C. or D.C. supply - Misalignment or defective limit switch of Spring charging mechanism - Defective motor
- Defect in relay or setting, If relay is defective, relay needs to be replaced - Wiring connection problem - Mechanical defect in the flag mechanism
1
Annunciation Circuits Non working of Bell
2
Continuous ringing of Bell
- D.C. leakage - Disturbance in aux. relay contacts adjustment
3
Non resetting of Bell
- Accept push button faulty
- Either loose fitting of fuse and link or blowing of fuse - Loose connections - Burning of Bell coil - Disturbance in bell adjustment - Aux. relay provided may not working - Sealing (hold on) supply getting to the aux. relay through ‘accept’ push button might have disconnected due to faulty ‘accept’ push button
- D.C. leakage - Aux. relay faulty 4
Non resetting of Indication
- Reset push button faulty - D.C. leakage
61
S/No. 1
2
Probable Trouble Indication Circuits Lamp not indicating for breaker ON-OFF position
Semaphore not working
Cause / Works to be attended - Either loose fitting of fuse and link or blowing of fuse - Lamp may be loose fitted or fusing of lamps - Loose connections - Defective aux. switch contacts of breaker - Either loose fitting of fuse and link or blowing of fuse - Semaphore coil might have burnt - Loose connections - Defective aux. switch contacts of breaker or Isolator/Earth switch, as applicable
62
Safety Electric al Cleara nces : Most of the equipments in a substation are provided with sufficient insulation from breaking down. There is a need for assurance that the breakdown or flashover will not occur to the operating personnel and some safe distance is to be maintained. Clearances are broadly categorized as below. a) Phase to earth clearance : - Equipment phase to earth clearance depends on the type of insulation material used. While equipment bushings take care of external clearances, the insulating material inside the equipment like oil, SF6, and vacuum take care of earth clearance internally. b) Phase to phase clearance : - It is the clearance between two conductors charged electrically. Sufficient phase-to-phase clearance has to be provided in air to prevent flash over & breakdown of air insulation. This clearance is one of the factors in deciding bay width in substations. c) Section clearance : - This clearance is required from point of safety to operating personnel. It is distance between two sections of a substation that enables a person to work on one section of a substation, in a safe manner, that the phase to earth clearance is maintained between the live point and the approach of the working personnel with sufficient margin. d) Ground clearance : - It is a distance between ground level and bottom of any insulator in an outdoor substation. Standard Safe Clearances Voltage (KV) BIL (KVp) Ph-Earth Clearance (Cm) Ph-Ph Clearance (Cm) Ph-Ground Clearance (Mtr) Section Clearance (Mtr)
33 KV 170 32 40 3.7
66 KV
110 KV
325 63 75 4.0
550 115 135 4.6
1050 240 210 5.5
1425 350 410 8.0
2.8
3.0
3.5
4.3
6.5
63
220 KV 400 KV
Indian Electri cit y Rule – 7 7 A) Clearance from Ground to lowest cond uctor of Electric Overhe ad Line : a) Ac ro ss any s treet :1. Low and medium voltage lines
- 5.795 Metres
2. High voltage lines
- 6.100 Metres
b) Al on g any street :1. Low and medium voltage lines 2. High voltage lines
- 5.490 Metres - 5.795 Metres
c) Elsewhere than along o r across street :1. Below and up to 11 KV (Bare conductor)
- 4.575 Metres
2. As above for insulated conductor
- 3.965 Metres
3. Above 11 KV
- 5.185 Metres
4. EHV line
- 5.185 Metres + 0.3 Metres for every additional 33 KV or part thereof by which the voltage of the line exceed 33 KV. Maximum – 6.096 Metres
Indian Electri cit y Rule – 8 7 Where an overhead line crosses or is in proximity to another overhead line, guarding arrangement shall be provided so as to guard against the possibility of their coming into contact with each other. Minimum c learances in metres betwee n lin es when crossi ng each other:
S/No. Nominal System Voltage 1 Low & Medium 2 11 – 66 KV 3 110 – 132 KV 4 220 KV 5 400 KV
11KV66 2.44 2.44 3.05 4.58 5.49
110KV – 132 3.05 3.05 3.05 4.58 5.49
64
220 KV 4.58 4.58 4.58 4.58 5.49
400 KV 5.49 5.49 5.49 5.49 5.49
Techn ical Da ta of som e common used Li ne Cond uct ors (ACSR ) Conductor
Area (Sq.Inch)
Rabbit Dog Coyote Panther Moose
0.050 0.100 0.125 0.200 0.500
Stranding & wire diameter (mm) Aluminium Steel No. Dia. No. Dia.
6 6 26 30 54
3.35 4.72 2.54 3.00 3.53
65
1 7 7 7 7
3.35 1.57 1.90 3.00 3.53
Calculated
Current
D.C. Resistance (Ohm/Km)
Rating with 1 mph Wind Speed
0.54490 0.27450 0.22140 0.13750 0.05517
200 305 380 510 880
Earthing for EHV Substation : One of the important aspects in the operation of the protective equipment is proper earthing. By earthing, it means making a connection to the general mass of the earth. Earthing also increases the reliability of the supply service as it helps to provide stability of voltage conditions, prevent excessive voltage peaks during disturbances and also as a means of providing a measure of protection against lightning. For outdoor substation, a main earthing ring should be provided round the substation which should be connected to all earth electrodes. The ring should be laid so as to have shortest connection from transformers, circuit breakers etc. Types o f Earthing : - It can be divided into Neutral earthing & Equipment earthing. a) Neutral Earthing
deals with the earthing of system neutral to ensure that
neutral points are held at earth potential and return path is available to neutral current. Points to be earthed: Transformer neutral is to be earthed to two separate and distinct earth electrodes interconnected with substation earth mat. b) Equipment Earthing
deals with earthing of non-current carrying parts of
equipments to ensure safety to personnel & protection against lightning. Points to be earthed: All non-current carrying metallic parts of equipments, structures, enclosures, overhead shielding wires, flanges of bushings, cores of transformer, cable sheaths, earthed screens, pipes, portable appliances, fences, doors, screens.
Common Earth System for Low and High Voltage S ystems : There should be common earth bus for both high and low voltage systems. If the low voltage neutral is not connected to the common earth system but has a separate earth bus, then there will be a difference of potential between the high voltage and low voltage neutrals and there can exist a dangerous potential gradient across earth surface which can endanger life. 66
With a low resistance earth bus and the neutrals connected to a common earth system, there will be no danger to the low voltage system and advantages in keeping everything in the station at a common potential above earth will outweigh the disadvantages. a) LA Earthing – The earthing lead for any LA shall not pass through any iron or steel pipe, but shall be taken as directly as possible from the LA to a separate earth electrode interconnected with substation earth mat. Individual earth electrodes should be provided for each station type lightning arrester, while for distribution type lightning arrester, one electrode may be provided for a set of lightning arresters. b) Coupling Capacitors Earthing –
A separate earth electrode, generally a
driven rod or pipe, should be provided immediately adjacent to the structure supporting the coupling capacitors of carrier current equipment. This earth should be used for the high frequency equipment only. c) Overhead Lin es Earth ing – Overhead lines are earthed: a) to eliminate danger from broken line conductors and insulators by ensuring the operation of the protective control-gear under such conditions. b) to discharge lightning strokes to earth. c) to minimize inductive interference with the communication circuits. One or more earth wires of G.I. are run along the power line (above the conductors)
Some C ommon Definit ions : a) Earth Electrode : Any plate, pipe or rod embedded in the earth to obtain effective electrical connection with general mass of the earth is known as Earth Electrode.
67
b) Touch Potential : If a person standing on substation floor touches a faulted structures by raised fingers, potential between his raised fingers and the feet is called touch potential. c) Step Potential : If fault current flowing through the ground of the sub-station, a potential between two steps of a person standing on the ground is called step potential. A person moving in the switchyard and touching an earthed metallic structure should not get a shock. Hence touch potential should be below 45 Volt. Also step potential should be below 45 Volt so that a person walking on substation floor does not get shock due to high step potential. Factors to be cons idere d for design of Earth Mat for a Substation : 1) Soil re sisti vity: Before designing earth mat, it is necessary to determine the soil resistivity of the area in which substation is to be located. Resistivity of the earth varies considerably from 10 to 10,000 Ω-m depending on the types of the soil. Also resistivity varies at different depth depending upon the type of soil, moisture content and temperature etc., at various depths, which affects the flow of current due to the fact that the earth fault current is likely to take its path through various layers. 2) Tolerable limits o f body cu rrent: Effect of current passing through vital organs of human body depends on magnitude, duration and frequency of current. Current in the range of 1-8 mA are known as 'let go current' because these currents, though unpleasant, impair the ability of a person, holding an energised object to release it. Currents in the range of 9-25 mA may be painful and impair the ability to release energised object. Still higher currents make breathing difficult. However, if the current is less than about 60 mA, the effects are not permanent & disappear when current is interrupted. Currents higher than 60 mA may lead to ventricular fibrillation, injury & death. 68
3) Fault cu rrent: As the earthing system has to carry the earth currents, the maximum earth fault current likely to flow in the system is considered for designing of earthing. A good earthing system for substation can be designed using an earth mat which is formed by a grid of horizontally buried conductors which serves to dissipate the earth fault currents to earth, also as an equipotential bonding conductor system, along with required number of vertical earth electrodes which are connected to the points of earthing of various equipments, structures and also interconnected with the horizontal earth mat. M.S. rods are generally used for the earthing of substation.
Total Earth resistance of the station system must be below 3 ohms for low voltage domestic system, below 0.5 ohms for low voltage and medium voltage substation, and below 0.1 ohm for 220 KV and 400 KV sub-station and power plants. If value of Earth Pit resistance is found high, then it is to be treated to bring back the value within the normal range. Electric Shock:The effect of electric shock may be death – a) due to fibrillation of heart. i.e. damaging the heart to small pieces causing stopping of breathing; b) due to stopping of breathing action caused by blockade in the nervous system causing respiration; c) due to local overheating or burning of body. The fibrillation of the heart is the most serious cause of death and there is no cure, although there is possibility of rescuing a man who has suffered by the latter two causes.
69
Cure of Shocks: When anyone gets a shock, the first and foremost duty of the observer is to break the contact of the live mains and body either by switching off the main supply or the body should be rolled away with a dry wooden stick. If a stick etc. is not at hand, a dry piece of cloth should be used to detach the body from the live mains or if that is not available the loose cloth such as coat or shirt of the victim should be pulled with care without touching his body. In most of the electric shocks, it is momentary and the contact with live wire is imperfect, in such cases breath stops momentarily. But due to the shock the victim becomes unconscious, stops breathing and his heart still beats, the most urgent and immediate cure for this victim is that he should be given immediate artificial respiration and it should be continued until the victim starts breathing normally. It should be borne in mind that if the artificial respiration is stopped just after the victim recovers, he is liable to become unconscious again. In such cases the artificial respiration is to be continued for 6 to 8 hours. Method of artificial respiration is displayed at every electric control room and substation. Precautions against Shock :- Prevention is better than cure. 1) Try to avoid work on live mains which should be switched off before working. If it is not possible to switch off the mains, be sure before working that your hands or feet are not wet. 2) When working on high voltages, be sure that the floor is not conductor. Concrete floors are dangerously conductive. When working on high voltage, try to keep your left hand in the pocket i.e. avoid your left hand to get in contact with any live conductor or metallic casing of an apparatus or metal pole or cross arms. 3) Do not work in such a place where your head is liable to touch the live mains before making the circuit dead. 4) In order to rescue a person who has got an electric shock if there is no other insulator available for rescuing, use your feet rather than hands.
70
PROTECTIVE RELAYS: Relay is a device by which electrical circuit is indirectly controlled during a fault condition. The purpose of relay is to operate the correct circuit breaker, so as to disconnect only the faulty equipment from the system as quickly as possible, thus minimising the trouble and damage caused by faults when they do occur. Essential Qualiti es of Protect ion (Relay) : 1) Reliability : - Protection scheme must operate, when the system condition calls upon to do so. Failure in the trip and control circuit of the breaker can be determined by continuous supervision arrangement (Trip circuit healthy lamps in the panel) 2) Selectivity : - Protective system must be such that it should correctly select the faulty section and cut off the same from the system without disturbing other healthy sections. 3) Speed : - To avoid unnecessary damage to plant, protection must operate quickly. 4) Stability : - The protection system should be stable and it must actuate from the concerned signal only and not from any other similar signal. Back Up Relaying : If due to some reason the primary relaying system fails to operate, the back up relays must operate and isolate the faulty equipments. Au to Recl osi ng Relays: These relays are used to reconnect the circuit so that if the fault is of transient nature, the system is returned to normal operation. This system is used mostly on overhead lines where 80 to 90% faults are of transient nature. (lightning, birds passing near or through lines, tree branches, etc.)
71
Three typ es of Relays: 1) Relay back up - To trip same breaker by other relay if main relay fails. 2) Local Breaker back up (LBB) – To trip next breaker on the same bus. 3) Remote back up – To trip breaker at upper station. EHV Line Protecti on : 1) Distance Relay : - This Relay is directional type & works on the principal of Impedance rather than current. Generally there are 3 Zones in forward direction and 1 Zone in reverse direction. Protected Zone - X KM Station C
Sta tioAn
Sta tioBn
Sta tioDn
Suppose Distance between Station A & Station B is X km. Distance between Station B & Station C, which is the nearest Station from Station B, is Y km. Distance between Station B & Station D, which is the far away Station from Station B, is Z km. Setting of relays is done on the impedance parameters of Overhead Line conductor (Given by the conductor Manufacturer). Setting of Relay at Station A is given below: Zone 1 : - Zone 1 mostly covers protected line. Setting of Zone 1 is taken as 80% of protected line length. So any fault in this zone, Station A will trip first and if fails to trip, then Station B trips in Zone 2 at it's end. Zone 2 : - Setting of Zone 2 is 100% of X + 50% of Y Zone 3 : - Setting of Zone 3 is 100% of X + 100% of Z Zone 4 : - Setting of Zone 4 is 10% of Zone 1 (Reverse Zone) 72
2) Over current & Earth Fault Rela y : This Relay is made directional type & is a backup for Distance Relay.
Transform er Protection : 1) Differential Relay : - This Relay compares the currents in the windings of the transformer through CTs whose ratios are such as to make their currents normally equal. The polarities of the CTs are such as to make the current circulate without going through the relay during load conditions and external faults. During internal faults, the balance condition is disturbed and relay operates
DIFFERENTIAL PROTECTION OF POWER TRANSFORMER
P1
s1
P2
P2
s2
s2
P1
s1
PROTECTED ZONE
RELAY
GENERAL WIRING OF DIFFERENTIAL RELAY
73
2) Restricted Earth Fault Relay : -This protection helps easy and quick detection of fault in the star connected winding of power transformer. The relay operates whenever there is a fault in the tap changer or the star connected winding. Normally the balance three-phase loads are feed through the transformer. There is no current flow through the star point neutral to the earth in this normal situation. One CT of similar ratio and the protection class is provided in neutral side of the transformer which is used for matching / balancing the circulating current through the main CTs in case of external faults. HV REF PROTECTION OF POWER TRANSFORMER
P1
P2
R R
s1
s2
s1
s2
s1
s2
Y N B
Y
B
s2
P2
s1
P1
Relay
3) Over current & Earth Fault Rela y : This Relay is Non directional type having IDMT characteristic.
74
Feeder Protectio n: 1) Over cur rent & Earth faul t Relay : These Relays work on IDMT characteristic & are made directional and nondirectional as per requirement. Wiring Diagram of combined over current & earth fault relay:COMBINED 3 OVER CURRENT ELEMENT + 1 EARTH FAULT ELEMENT
R Ph
Y Ph
B Ph
P1 s1
C11 P1s1
P2
C31
s2 C51 P2 s2
C73
P1 s1
P2
s2 C71
COMBIN ED 2 OVER CURRENT ELEMENT + 1 EARTH FAULT ELEMENT
R Ph
Y Ph
B Ph
P1 s1
C11
P1s1 P2
C31
s2 C51 P2
s2
C31
P1 s1
P2
s2 C71
75
Relay Settings Calculations : Some common Formulae regarding Relay setting calculations: A) Plug Setting = It is the tapping connection of CT secondary winding If CTR is 400 / 1 Amp & PS selected is 50%, then allowable current in CT secondary is 200 Amp. B) Plug Setting Multipli
er = (Fault current flowing in CT secondary)/ (CTR x PS)
C) Time Se tting Multiplier = (Actual time of operation)/(Time as per curve)
A Sample calculations of Relay settings is given below 250/1 A
R1 132 / 33 KV, 25MVA, 10%Imp. 400/A 1
150/A 1
R5
Load - 190 A
600/A 1
R4
R3
13KB 2Vus
3KB 3Vus
FaulLteve5 -l80 MVA
250/ 1 A
R2 Load - 170 A
Fault Level of 132 KV Bus = 580 MVA Consider Base MVA = 100 % Source Imp. = [(Base MVA)/ (Fault MVA)] x 100 = (100/580) x 100 = 17.24% % Trafo. Imp. at Base MVA = [(% Imp. at 25 MVA x Base MVA)/(Trafo.MVA)] = (10 x 100/25) = 40% 76
Total % Imp. at 33 KV Bus = [Source Imp. + Trafo. Imp.] = 17.24 + 40 = 57.24% Fault MVA (33 KV Bus) = [(Base MVA)/ (Total Imp.)] x 100 = (100/57.24) x 100 = 175 MVA Fault Current (I) = [(Fault MVA x 1000) / (√3 X 33)] = 3061 Amp Part A: - 33 KV Feeder Relay Setti ng (R1) Feeder CTR = 250/1 Amp, Connected load on Feeder # 1 = 190 Amp Allow 10% overload, Current = 209 Amp, Say 210 Amp Select Plug Setting 85% for over current Relay and 20% for Earth Fault Relay Current allowed for O/C Relay = 212 A ; for E/F Relay = 0.2 x 250 = 50 A Time in Sec. @ TMS 1.0 for Nor mal Invers e = {(0.14) / [(PSM
0.02
) – 1]} and
Ab ov e 20 PSM, cu rv e bec om es fl at, Ti me i n s eco nd is 2.2
A) 33 KV Bus Fault current = 3061 Amp (as calculated above) Secondary current in over current relay = [(Fault Current)/ (CTR X PS)] = (3061) / (250 x 0.85) = 14.4 A For 14.4 PSM, Operating time = 2.55 Sec @ TMS 1.0 (Normal Inverse graph) Say, Fault clearing time = 110 ms i.e. operation time = 0.11 = (TMS x 2.55) Therefore TMS = (0.11) / (2.55) = 0.04313 Select TMS = 0.05, Operation of Relay = 0.05 x 2.55 = 127 ms for O/C Relay. B) Secondary current in Relay for Earth Fault = [(Fault Current)/ (CTR x PS)] = (3061)/ (250 x 0.2) = 61.22 A For 61 PSM, Operating time = 2.2 Sec @ TMS 1.0 Say, Fault clearing time = 110 ms i.e. operation time = 0.11 = (TMS x 2.2) Therefore TMS = (0.11) / (2.2) = 0.05 Hence Operation of Relay = 0.05 x 2.2 = 110 ms for Earth Fault Relay 77
Part B: - 33 KV Feeder Re lay Sett in g (R2) Feeder CTR = 250/1 Amp, Connected load on Feeder # 2 = 170 Amp Allow 10% overload, Current = 187 Amp Select Plug Setting 75% for over current Relay and 20% for Earth Fault Relay Current allowed for O/C Relay = 187.5 Amp Current allowed for E/F Relay = 50 Amp A) 33 KV Bus Fault current = 3061 Amp (as calculated above) Secondary current in Relay for over current = [(Fault Current)/ (CTR x PS)] = (3061) / (250 X 0.75) = 16.3 A For 16.3 PSM, Operating time = 2.43 Sec @ TMS 1.0 Say, Fault clearing time = 110 ms i.e. operation time = 0.11 = (TMS x 2.43) Therefore TMS = (0.11) / (2.43) = 0.04526 Select TMS = 0.05, Operation of Relay = 0.05 x 2.43 = 121 ms for O/C Relay. B) Secondary current in Relay for Earth Fault = [(Fault Current)/ (CTR x PS)] = (3061)/ (250 X 0.2) = 61.22 A For 61.22 PSM, Operating time = 2.2 Sec @ TMS 1.0 Say, Fault clearing time = 110 ms i.e. operation time = 0.11 = (TMS x 2.2) Therefore TMS = (0.11) / (2.2) = 0.05 Operation of Relay = 0.05 x 2.2 = 110 ms for Earth Fault Relay.
78
Part C: - 33 KV I nc om er Feeder Relay Setti ng (R3) Feeder CTR = 600/1 Amp Full load current of Transformer = [(Trafo. MVA x 1000)/ (√3 x 33 KV)] = 437 Amp Plug setting for Over Current Relay of R3 is kept at 70% Plug setting for Earth Fault Relay is kept 20% A) 33 KV Bus Fault current = 3061 Amp (as calculated above) Secondary current in Relay for over current = [(Fault Current) / (CTR X PS)] = (3061) / (600 X 0.7) = 7.28 A For 7.28 PSM, Operating time = 3.45 Sec @ TMS 1.0 For discrimination, we can give 240 ms delay between O/G Feeder & Incomer Time of Operation = 0.11 + 0.24 = 0.35 Sec Time of operation of O/C Relay = 0.35 = (TMS x 3.45) Therefore TMS = (0.35) / (3.45) = 0.1014. Select TMS at 0.11 Hence Operation of Relay = 0.11 x 3.45 = 380 ms for Over current Relay. B) Secondary current in Relay for Earth Fault = [(Fault Current)/ (CTR x PS)] = (3061) / (600 X 0.2) = 25.5 A For 25 PSM, Operating time = 2.2 Sec @ TMS 1.0 For discrimination, we can give 240 ms delay between O/G Feeder & Incomer Time of Operation = 0.11 + 0.24 = 0.35 Sec Time of operation of E/F Relay = 0.35 = (TMS x 2.2) Therefore TMS = (0.35) / (2.2) = 0.159. Select TMS at 0.16 Operation of Relay = 0.16 x 2.2 = 352 ms for E/Fault Relay.
79
Part D: - 132 KV HT Trans fo rm er Feeder Relay Setti ng ( R4 ) Feeder CTR = 150/1 Amp Full load amp of Transformer = [(Trafo. MVA x 1000)/ ( √3 x 132 KV)] = 109 Amp Plug setting for Over Current Relay of R4 is kept at 80% & Earth Fault is kept 20% A) Reflected 33 KV Bus Fault current on 132 KV side = (3061 / 4) = 765 Amp Secondary current in Relay for Over current = [(Fault Current) / (CTR x PS)] = (765) / (150 X 0.80) = 6.375 A For 6.8 PSM, Operating time = 3.58 Sec @ TMS 1.0 For discrimination, we can give 150 ms delay between Incomer & HT Side Relay Time of Operation = 0.35 + 0.15 = 0.50 Sec Time of operation of O/C Relay = 0.50 = (TMS x 3.58) Therefore TMS = (0.5) / (3.58) = 0.1396, Say 0.14 Hence Operation of Relay = 0.14 x 3.58 = 501 ms for Over current Relay
B) Secondary current in Relay for Earth Fault = [(Fault Current) / (CTR x PS)] = (765) / (150 X 0.2) = 25.5 A For 25 PSM, Operating time = 2.2 Sec @ TMS 1.0 For discrimination, we can give 150 ms delay between Incomer & HT Side Relay Time of Operation = 0.35 + 0.15 = 0.50 Sec Time of operation of E/F Relay = 0.5 = (TMS x 2.2) Therefore TMS = (0.5) / (2.2) = 0.227 Select TMS = 0.23, Operation of Relay = 0.23 x 2.2 = 506 ms for E/Fault Relay.
80
Part E: - 132 KV Li ne Feeder Re lay Sett in g (R5) Feeder CTR = 400/1 Amp Plug setting for Over Current Relay of R4 is kept at 100% & TMS is 0.1 Plug setting for Earth Fault Relay is kept 20% & TMS is 0.20 A) Reflected 33 KV Bus Fault current on 132 KV side = (3061 / 4) = 765 Amp Secondary current in Relay for Over current = [(Fault Current) / (CTR x PS)] = (765) / (400 X 1.0) = 1.91 A For 1.91 PSM, Operating time = 10.74 Sec @ TMS 1.0 Operation of Relay = 10.74 x TSM = 10.74 x 0.10 = 1074 ms for O/C Relay. B) Secondary current in Relay for Earth Fault = [(Fault Current) / (CTR X PS)] = (765) / (400 X 0.2) = 9.56 A For 9.56 PSM, Operating time = 3.03 Sec @ TMS 1.0 Hence Operation of Relay = 3.03 x TSM = 3.1 X 0.20 = 606 ms for E/Fault Relay
Summary of Relay Time for above case study S/No.
a) b) c)
33 KV Outgoing Feeder # 1 O/Current Relay PS - 0.85 TMS – 0.05 O/T – 127 ms
2 a) b) c)
E/F Relay PS - 0.20 TMS – 0.05 O/T – 110 ms
1
33 KV Incomer 132 KV Trafo. Feeder
132 KV Line Feeder
PS - 0.70 TMS – 0.11 O/T – 380 ms
PS - 0.75 TMS – 0.14 O/T – 501 ms
PS - 1.0 TMS – 0.10 O/T – 1074 ms
PS - 0.20 TMS – 0.16 O/T – 352 ms
PS - 0.20 TMS – 0.23 O/T – 506 ms
PS - 0.20 TMS – 0.20 O/T – 606 ms
81
Daily Operational Duties at SUBSTATION: The supervisor or operator on duty is responsible for following: Daily Operational Watch : 1) Watch the hourly transformer temperature, load, etc. and they do not exceed the permissible limit or the rating of all the equipment involved or connected in the circuit. 2) Note down the hourly consumption / generation on the feeders as applicable. 3) When a feeder trips, its indication should be noted and entered in the relevant register. VCB or SF6 CB controlling overhead lines can be charged after a 2-3 minutes of tripping as most of the problems are of transient nature. If the breaker trips again, the feeder should be declared as faulty and message sent to concerned person and patrolling arranged. In case of tripping of Transformer, cause of the same is to be cleared before charging. 4) Always check and ensure that proper D.C. supply is available on the trip circuit. For checking Trip circuit healthiness, Push button is provided on the CRP. Also on Trip circuit supervision relay, Green LED shows the healthiness, and Red LED shows unhealthiness. Take corrective actions accordingly if required. 5) Operate O.L.T.C. of Transformer and maintain voltage as required. 6) Ensure that batteries are in proper state of charge and have correct voltage and charge rates and the same are to be checked every day. Operating Instructio ns : These shall be displayed in the substation at a suitable place. The operator will perform the sequence of operation in accordance with these instructions. These instructions should be either in English or Local language.
82
Consider that in sub-station there is one Main Bus & one Transfer Bus as mentioned in following drawing: Main Bus (Live) Auxiliary Bus
89 A
89 C
52 B
89 A
89 C
52 T
BUS COUPLER
89 L
CT
TRANSFORMER
LOAD Carrying out Maintenance of Transformer HV side Breaker
without interruption
to load supply. One should follow the sequence as mentioned below: a) First ensure that breaker of auxiliary bay is in OFF condition. b) Close Isolators 89 A and 89 C of auxiliary bay. c) Now close the Isolator 89 C of transformer bay. d) Put control switch of transformer control panel on Intermediate position. e) Close the auxiliary bay breaker. 83
f) Put Off the Transformer bay breaker. Put control switch on Transfer position. Now auxiliary bay breaker will control all protections of transformer. g) Open Isolators 89 A and 89 L of transformer bay. Carry out maintenance of transformer bay breaker by taking shutdown permit & do the maintenance as per safety practices.
Restorin g the syst em without interruption to load supply. a) Remove all tools & tackles. Also remove temporary earthing if provided from working place. Return shutdown permit. b) Close Isolators 89 A and 89 L of transformer bay. c) Put control switch of transformer control panel on Intermediate position. d) Close the transformer bay breaker. e) Now open the auxiliary bay breaker. Then control switch is to be kept on normal position as srcinal. f) Open Isolators 89 C of transformer bay. g) Open Isolators 89 A and 89 C of auxiliary bay. Now auxiliary bay is dead. Statistical : Keep records of followings on daily basis: a) Maintenance logbook b) Tripping events c) OLTC operations of Transformers d) Counter operations of breakers & LA e) Recording of Gas pressure for SF6 circuit breaker and Air pressure for pneumatic operated breakers f) Oil leakage of Transformer, CT & PT, CB in case of OCB g) Colour of Silica Gel in Transformer h) Check the Batteries voltages and charger condition
84
Safety Rules : 1) Authorised persons of the substation should get themselves thoroughly familiar with the layout of the substation, Incoming feeders, Outgoing feeders, etc. The layout of the feeders should be displayed in the substation control room. 2) See that sufficient sets of earthing sets, hand gloves, ladders, etc. are always kept at the substation in proper working order & can be safely used for such work. 3) Only authorised men take permits to work as per formats. While issuing permits to others, proper earthing should be ensured. Danger notice ‘Do not operate, men at work’ should be affixed on the concerned dead feeder. 4) Fire fighting equipment, first aid box, etc. is maintained in proper condition. First aid chart should be displayed in control room in English or Local language. General : 1) All tripping at the substation should be reported. Also wrong tripping and nontripping should be reported to concerned superior. 2) Carry out maintenance as per ISO format or manufacturer’s standards. Ensure to maintain History Register of each equipment at the substation. It should contain information such as details of equipment, date of commissioning, preventative and breakdown maintenance done, spares used if any. 3) Single line diagram should show the incoming lines, isolators, breakers, transformers, LAs, CTs and PTs with their make, current rating, and rupturing capacity. 4) Schematic diagram indicates the scheme of protection with CT ratio, type of relays with setting available and actual settings.
85
Maintenance of Substation
Equipments: -
Maintenance is defined as a combination of actions carried out to return a equipment in or restore the equipment back to an acceptable condition. Different types of maintenance being done on equipment are: a) Breakdown maintenance b) Preventive maintenance c) Condition based monitoring d) Reliability centred maintenance a) Breakdown Maintenance:
This maintenance is carried out when the
equipment fails. This maintenance may be appropriate for low value items. However for costly substation equipments, it is not desirable to wait till the breakdown of the equipment, as it costs more to the utility as well as availability of power. b) Preventive Maintenance:
This maintenance is being mostly adopted by
almost all the utilities. In this type of maintenance, the equipments are inspected at a predetermined period, which is based on past experience and also guidance from the manufacturer of the equipment. This type of maintenance would require specific period of shutdown. Maintenance is carried out as per the formats. c) Condition Based Monitoring
: This type of maintenance technique is adopted
to assess the condition of the equipment by carrying out some tests. Some of the tests are done on on-line and some are done on off-line. However, this type of maintenance would need sophisticated testing equipments and skills for analysing the test results. d) Reliability Centred Maintenance: This is the recent technique being adopted in maintenance. Reliability centred maintenance policy is based on the life cycle cost concept and the decision for replacement of the equipment is taken based on techno-economic considerations. Its objective is to devise a system, which does not need periodic maintenance and at the same time predict in advance possible failures/problems of the equipments. 86
Recommended Maintenance Schedule of Power Transformer S/No.
Items to be inspected
Inspection notes
Action required
Hourly 1
Load Amps
2
Voltage
3
Winding and Oil Temperature
Check against rated figures Check against rated figures
If high, reduce the load An improper tap position can cause
Check that temperature rise is reasonable
excessive Shutdown core the loss transformer and investigate if either is persistently higher than normal
Daily 1 2
Oil level in Main & OLTC Conservator Oil level in bushings
Check oil level from oil Top up, if found low gauge Check oil level from glass Top up, if found low
3
Dehydrating Breather
4
Oil leakage
Check colour of silica gel, check that air passages are free, check oil level in oil cup Check for any leakages
Reactive/replace with new charge, if it is pink, make up oil in oil cup Arrest leakages, if any
Lubricate bearings, check gearbox oil level & inspect all moving parts, motor, etc. Check all circuits independently, check step by step operation including limit switches
Clean, adjust, or replace as required
Compare with earlier values Examine for cracks and dirt deposits
Take suitable actions if required Clean or replace if required
Monthly 1
OLTC driving mechanism
2
OLTC automatic control
If faulty, take suitable actions to set it right
Quarterly 1
Insulation Resistance
2
Bushings
87
S/No.
Items to be inspected
Inspection notes
Action required
Quarterly 3
Oil in transformer & tap changer
4
Cooler fan bearings, motors & mechanism
Check for BDV
Take suitable action to restore quality of oil Lubricate, check gear Replace burnt or box, examine contacts, worn contacts or check manual control and other parts interlocks
Half Yearly 1
Oil cooler
Test for pressure
Yearly 1
Oil in transformer
2
Gasket joints
3
Cable boxes
4
Surge diverter & gaps
5
Relays, protection circuits Earth resistance
6 7
Temperature Indication
8
Dial type oil gauge
9
Paint work
10
Diverter switches of OLTC after 10,000 operations
Check for BDV, acidity and sludge
Check for sealing arrangements Examine for cracks and dirt deposits Check for protection circuits Compare with earlier values Pockets holding thermometers should be checked Check pointer for freedom Should be inspected Check for worn out contacts
88
Filter or replace oil Tighten the bolts evenly to avoid uneven pressure Replace gasket if leaking Clean or replace Take suitable actions if required Take suitable actions if required Oil to be replenished, if required Adjust if required Touching up if required Replace worn out parts, filter oil
S/No.
Items to be inspected
Inspection notes
Action required
2 Yearly 1
Oil conservator
Internal inspection
2
Buchholz relay
Mechanical inspection
Should be thoroughly cleaned Adjust floats, switches, etc., as required
1
3 Yearly or after 15,000 operations of OLTC
Non-arcing selector switch of OLTC
Replace worn out parts, filter oil
1
1 to 3 MVA transformer (5 Yearly)
Overall inspection including lifting of core & coils
Wash by hosing down with clean dry oil, tighten all bolts, coil clamping screws
1
> 3 MVA transformer (7-10 Yearly)
Overall inspection including lifting of core & coils
Wash by hosing down with clean dry oil, tighten all bolts, coil clamping screws
Note: 1) All maintenance test results and observations should be specifically recorded. 2) In case of anything abnormal occurring during service, the matter should be reported to the manufacturer.
89
Recommended Maintenance Schedule of Circuit Breakers S/No. Points to be Inspected 1 Check gas pressure in case of SF6 CB 2 Check air pressure in case of ABCB or pneumatic operated breaker and check the operation of compressor by realising air pressure just below minimum value. Drain condensation from reservoir 3 4 5 6 7 8 9 10 11
12 13 14
Check oil level, its condition, any oil leakage in case of OCB and correct if required Check & tightness of all power connections and control wiring connections Check contact resistance Clean breaker insulator & check for any cracks Clean & lubricate of moving parts of operating mechanism Check condition of trip & close coil & its assembly for free movement. Replace if required Check breaker close-open timing Check insulation resistance Check all Controls, Interlocks & Protections like checking of pole discrepancy system i.e. whether all three poles are getting ON – OFF at the same time Clean auxiliary switches by CTC or CRC spray and check its operation Check structure supporting hardware & its tightness. Also check paint condition Carry out compressor maintenance in case of ABCB or pneumatic operated CB, replace oil in compressor if required
90
Periodicity Daily Daily
Daily Yearly Yearly Yearly Yearly Yearly Yearly Yearly Yearly
Yearly Yearly Yearly
Recommended Maintenance Schedule of Current Transformer S/No. Points to be Inspected 1 Check oil level & leakage, rectify the same 2 Check & tightness of all power connections and CT secondary connections 3 Clean bushings / insulators & check for any cracks 4 Check earthing connections and tightness 5 6 7 8 9 10
Check the working of stainless steel bellows Check the nitrogen pressure as per specification of manufacturer in case of Nitrogen filled CT Check insulation resistance Check and adjust the gap of arcing horn if CT is provided with arcing horn Check tan δ (Tan Delta) Check structure supporting hardware & its tightness. Also check paint condition of CT unit & structures, touch up if required
Periodicity Daily Yearly Yearly Yearly Yearly Yearly Yearly Yearly Yearly Yearly
Recommended Maintenance Schedule of Potential Transformer S/No. Points to be Inspected 1 Check oil level & leakage, rectify the same 2 Check & tightness of all power connections and PT secondary connections 3 Clean bushings / insulators & check for any cracks 4 Check earthing connections and tightness 5 Check the working of stainless steel bellows 6 Check the nitrogen pressure as per specification of manufacturer in case of Nitrogen filled PT 7 Check insulation resistance 8 Check the Secondary Fuse condition & replace if required by proper rating 9 Check structure supporting hardware & its tightness. Also check paint condition of PT unit & structures, touch up if required
91
Periodicity Daily Yearly Yearly Yearly Yearly Yearly Yearly Yearly Yearly
Recommended Maintenance Schedule of Isolator S/No. Points to be Inspected 1 Check & tightness of all power connections 2 Check proper alignment of contacts & rectify if required 3 Clean insulators & check for any cracks 4 Check earthing connections and tightness, clean earth blade contact 5 6
7 8
9
10 12
Periodicity Yearly Yearly Yearly Yearly
Lubrication of all moving parts In case of Isolator with Earth switch, check electrical and mechanical interlock i.e. Isolator can be closed only when E/switch is in open condition & vice versa Check insulation resistance As Isolators are operated on No load, hence check the interlock with Circuit Breaker, if provided i.e. Isolators can be operated when Breaker is in OFF condition The motor operating mechanism box, in case of motor operated isolators, should be checked for inside wiring, terminal connectors, etc
Yearly Yearly
Check the Panel indications i.e. Semaphore & bulbs if provided (Isolator Close and Open condition) and rectify if required Check structure supporting hardware & its tightness. Also check paint condition, touch up if required
Yearly
Yearly Yearly
Yearly
Yearly
Recommended Maintena nce Schedule of Light ning Arrester S/No. Points to be Inspected 1 Note down the recording of leakage current. If it is in red zone, replace that defective LA 2 Check & tightness of all connections 3 Clean insulators & check for any cracks 4 Check earthing connections and tightness 5 Check insulation resistance 6 Check structure supporting hardware & its tightness. Also check paint condition, touch up if required
92
Periodicity Daily Yearly Yearly Yearly Yearly Yearly
FIRE & FIRE EXTINGUISHERS: “Fire is a rapid, self sustaining oxidation process accompanied by the evolution of heat and light of varying intensity”. Fire results from the combination of fuel, heat and oxygen when a substance is heated to a certain critical temperature called the “Ignition Temperature”. The material will ignite & continue to burn as long as there is fuel, the proper temperature and a supply of oxygen (air). Classification of Fires is mentioned below: Class “A” Fires : These are fires involving solid materials (such as wood, cloth, paper, rubber, etc.), normally of an organic nature (compounds of carbon), in which combustion generally occurs with the formation of glowing ambers, where the cooling effect of water is essential for extinguishment of fire. Class “B” Fires : These are fires involving flammable liquids e.g. kerosene, naphtha, LDO, mix oil, gasoline, where blanketing effect (A layer of foam over the surface of burning liquid) is essential for extinguishing fire. Class “ C” Fires : These are fires involving gases e.g. LPG, Methane, Ethylene, Propylene, Hydrogen etc. Fire can be put out either by dry chemical powder or carbon dioxide gas. Here isolation of leaking source is essential. Class “ D” Fires : These are fires involving combustible metals, such as magnesium, titanium, sodium. These fires can be put out with the help of special dry powders. Ordinary DCP or Foam or Water is of no use on such fires. Electrical Fires : According to latest concept, electrical fires do not constitute a particular class. Any fire involving electrical equipment is a fire of class A or class B. The normal procedure in such fires is to cut off the electrical supply of the equipment and to use an extinguishing media appropriate to the burning material. Water in the form of hose stream should in no case be used in electrical fires unless positive isolation of electric supply has been ensured. 93
Classifi cation of Fire and Suitability o f Portable F ire Extinguish ers S/No.
Class of Fire
Foam Extinguisher
Carbon Dioxide Extinguisher
1
Class A Fire
Suitable
Not recommended except for small surface fire
2 3 4
Class B Fire Class C Fire Class D Fire
Suitable Not Suitable Not Suitable
Suitable Suitable Not Suitable
5
Electrical Fire Not Suitable
Suitable
Dry Chemical Powder Extinguisher Suitable
Suitable Suitable Special dry powder Suitable
Caution:1) Do not use Foam Fire Extinguisher on fires involving live electrical equipment and metal. 2) Do not use CO2 Fire Extinguisher on big size fire. It is also not to be used on metal fire. While extinguishing oil fire, precaution against flash back or re-ignition is to be taken.
94
