Metallic bellows : **CT is hermetically sealed by the bellows. Oil filled metallic bellows compensates for the oil volume variation due to ambient temperature changes Nitrogen gas filled dome. **The space above the oil is filled with nitrogen which acts as a cushion for changes in oil volume due to temperature and load current variations. **The positive pressure of nitrogen inside prevents contamination from outside atmosphere, thereby provide effective hermetical sealing.
FUNCTION OF CT :
Converts primary current in to low magnitude secondary current for application of metering & protection purposes.
Five Cores are present in CT i) Core 1 & 2 for bus bar protection. ii) Core 3 for metering purpose. iii) Core 4 & 5 for distance protection.
CURRENT TRANSFORMERS RATINGS RATED CURRENT WITH EXTENDED CURRENT LIMITS RATED SHORT TIME CURRENT INSULATION LEVEL. NO. OF SECONDARIES. BURDEN & ACCURACY CLASS
CT ACCURACIES As per IEC-60044(1) Metering Core ± ±0.2 or 0.5%at rated Currents
Protection Cores
1% at rated current
METERING CORE Class
5% of rated I
20% of 100% 120% rated I of rated of rated I I
0.2
0.75
0.35
0.2
0.2
0.5
1.5
0.75
0.5
0.5
PROTECTION COR ES Class
Current Error at Composite Error at rated Primary rated accuracy limit Current Primary Current
5P
±1%
±5%
10P
±3%
±10%
RATIO & PHASE ERROR R atio Error = (KnIs- Ip)*100/ Ip Kn = R ated transformation ratio Ip = Actual primary current Is = Actual secondary current
The difference in Phase between the Primary and Secondary current vectors is phase angle error.
Knee Point Voltage 10% increase in Voltage will lead to 50% or
Induction B [T]
Current Transformers Saturation curve 18500 gauss Pr otection CT
8000 gauss Metering CT
10
Primary curr ent Ip/In 20
30
40
INSULATION LEVEL For Windings having Um greater than 300k V, the rated insulation level is determined by rated switching and lightning impulse withstand voltage For voltages < 300k V, insulation levels are decided by lightning impulse and power-frequency withstand voltages
INSULATION LEVEL
System Voltage
1 min Power Freq. Voltage
Switching Impulse withstand Voltage
Lightning Impulse withstand Voltage
220kV
395kV 460kV
-
950kV 1050kV
400kV
630kV
1050kV
1425kV
Checks on CT
Check for any transit damage Check the nitrogen pressure on the top chamber before commissioning. Check oil level in CT. Attend leakages on bushings immediately. Examine the porcelain for dirt deposits & clean periodically. Prior to erection confirm the position of P1 & P2 terminal of primary.
Checks on CT .(Contd) Ensure that the terminals of unloaded secondary are shorted Check that tank is earthed positively and permanently. Use the lifting lugs provided on the bottom tank for lifting the CT by slings Check the level of bellow. Check earthing of star point of secondary at relay panel before commissioning of CT. Check earthing of Tan delta point. Secondary box cover is closed tightly.
Checks on CT Contd.. Testing of oil after one month of Charging. All the CTs secondary TBs must be properly tighten at CTs JB,MB & at relay panel. Do not allow dirt and deposits on bushings. Clean periodically Do not leave terminals of unloaded secondary open Do not lift the CT by primary terminals or by porcelain housing. Carry out thermovision scanning after
CURRENT TRANSFORMERS PRECOMMISSIONING TESTS
INSULATION RESISTANCE MEASUREMENT CONTINUITY TEST SECONDARY WINDING RESISTANCE POLARITY TEST TAN DELTA AND CAPACITANCE MEASUREMENT CURRENT RATIO TESTS MAGENTISING CURVES PERFORMANCE CONTACT RESISTANCE MEASUREMENT
CURRENT TRANSFORMERS MAINTENANCE MONTHLY
CHECK OIL LEAKAGE FROM CT
CHECKING BELLOW FOR EXPANSION
CHECK OIL LEAKAGE FROM SECONDARY TERMINALS CHECKING HEALTHINESS OF GASKETS
CURRENT TRANSFORMERS MAINTENANCE YEARLY
CLEANING
TIGHTENING OF TERMINAL CONNECTORS AND SECONDARY TERMINALS
THERMOVISION SCANNING
CURRENT TRANSFORMERS MAINTENANCE 1 YEARLY
1. TAN DELTA & CAPACITANCE MEASUREMENT ACCEPTABLE LIMIT FOR TAN DELTA 0.007 PRE-COMM -COMM VALUE AT 0.001 PER INCREASE IN PRE YEAR UPTO 0.007 IF VALUE IS BETWEEN 0.007 TO 0.01 THEN HALF YEARLY MONITORING ABOVE 0.01 REPLACE THE CT
NITROGEN PRESSURE CHECKING (3 Yearly )
CAPACITIVE VOLTAGE TRANSFORMER
CVT (Measurement of HV)
Capacitor Voltage Transformers Line protection HF transmission CVT are used to convert higher primary voltage to lower secondary voltage for the protection & metering purpose. CVT provide least resistance path to higher frequency signals for PLCC functions.
CAPACITOR VOLTAGE TRANSFORMER
Capacitance potential divider connected between phase and gr ound of power cir cuit.
CAPACITOR VOLATGE TRANSFORMER
HV Power line
Capacitor ST ACK (or stack) Assembly of elements to r each higher voltage levels : ± 800 kV In general, the term CAPACITOR stands for a capacitor element as well as a capacitor stack. Ground
HV Power line
Capacitor Voltage Divider ( or CVT ) Connecting an MV inductive voltage transformer to a tap.
Ground
P1 C1
Intermediate Voltage : 10 to 20 kV/3
S1
C2
P2
S2
CVT CIRCUIT
Oil expansion device Capacitor elements Capacitor column Insulating oil Insulator f lange
CVT SECONDARY VOLATGE CVT Secondary Voltage v = k * V * C1/ (C1+C2) V ± Primary Voltage k ± Secondary Transformation ratio There are 280 ± 300 elements in C1 & C2 C1 will be about 260 to 280 elements C2 will be 15 to 20 elements NOTE: Puncturing of C1 ± Secondary Voltage will increase Puncturing of C2 ± Secondary Voltage will decrease
SECONDARY VOLATGE Periodic measurement to be carried out. In case of doubt, simultaneous measurement to be carried out with another feeder/ Bus CVT. For 400k V CVTs puncturing of one Capacitor element in C1 side is likely to increase Secondary Voltage by about 0.35 ± 0.45% (0.22 ± 0.28V) Failure of one Capacitor element in C2 side is likely to decrease Secondary Voltage by 5 ± 6% (3.2 3.8V)
CVT RATING
RATED PRIMARY VOLTAGE
RATED SECONDARY VOLTAGE
RATED BURDEN OF CORES
CAPACITANCE IN pF (FOR CVT ONLY)
CLASS OF ACCURACY
CVT REQUIRMENTS
3 SECONDARIES (2 FOR PROTECTION AND ONE FOR METERING) OUTPUT BURDEN 50 VA MINIMUM FOR EACH SECONDARY 3P ACCURACY CLASS FOR PROTECTION CORES 0.2 ACCURACY CLASS FOR METERING CORE
CVT PRECOMMISSIONING TESTS
INSULATION RESISTANCE MEASUREMENT
CONTINUITY OF WINDING
SECONDARY WINDING RESISTANCE
VOLTAGE RATIO TEST TAN DELTA AND CAPACITANCE MEASUREMENT
CVT MAINTENANCE MONTHLY:
LEAKAGE OF OIL. QUARTERLY: SECONDARY VOLTAGE YEARLY: CHECKING OF EARTHING OF HF POINT
DAMAGE TO BUSHING OF HF POINT CLEANING OF PORCELAIN INSULATOR STACKS AND TIGHTNESS OF TERMINAL CONNECTOR THERMOVISION SCANNING
CHECKING OF NEUTRAL EARTHING 3 YEARLY
CHECKS FOR CVT Check for any transit damage. Check oil level in EMU Unit. Check leakages on CVT . Examine the capacitor porcelain for dirt deposits and clean periodically on de de-energized and safely earthed CVT. Check that the primary and secondary terminal box covers are in position and sealed properly. I
CHECKS FOR CVT Check NHF point is earth when not used for PLCC. Check all the secondary wiring connections are tightly secured at the terminal box. Units must be short circuited with a bare wire between head & bottom flange until erection and connections are completed. The Tank of CVT should be earthed properly. Regularly monitor secondary voltage of CVTs w.r.t each other to identify any
CAUTIONS FOR CVT Never keep the units horizontally. Do not energized CVT from the secondary side. Discharge CVT prior to climbing. While removing connector from the CVT ensure local earthing on conductor as well as on CVT.
SURGE ARRESTOR
SURGE ARRESTORS
PROTECTS SUBSTATION EQUIPMENTS FROM TRANSIENT OVERVOLTAGES CONNECTED BETWEEN PHASE AND EARTH PROVIDES LOW RESISTANCE PATH TO EXCESSIVE HIGH VOLTAGE. RATING IN PHASE TO EARTH VOLTAGE
SURGE ARRESTORS
GAPPED SILICON CARBIDE SURGE ARRESTORS GAPLESS ZINC OXIDE ARRESTORS (PREFERRED THESE DAYS DUE TO HIGHER ENERGY ABSORPTION CAPABILITIES) M ADE OF SM ALL DISCS OF NONNON-LINEAR RESISTANCE M ATERIAL ZnO WHICH PROVIDES LEAST RESISTANCE PATH DURING SWITCHING SURGES OR LIGHTENING STROKE
ARRESTER FUND AMENT AL Need
for High Resistance to limit Discharge Current
Need
Low Resistance during flow of lightning current to limit Surge Voltage
A
non-linear resistance material required
EQUIVALENT CIRCUIT OF SURGE ARRESTER
Concept of Leakage Current Resistive Current
Capacitive Current
In phase with voltage In 90 Deg phase with voltage Increase with temperature
Increase with aging
Insignificant change with temperature and voltage
does not affect due to aging
SURGE ARRESTORS PRE--COMMISSIONING TESTS PRE
INSULATION RESISTANCE MEASUREMENT
LEAKAGE CURRENT MEASUREMENT
SURGE COUNTER CHECK
SURGE ARRESTORS MAINTENANCE YEARLY
CLEANING
LEAKAGE CURRENT MEASUREMENT
SURGE COUNTER CHECKS
WAVE TRAP
LINE TRAP LINE TRAP OFFER HIGH RESISTANCE TO HIGH FREQUENCY PLCC SIGNAL AND LOW RESISTANCE TO POWER FREQUENCY SIGNAL. BLOCKING FREQUENCY RANGE WITH COMBINATION OF INDUCTANCE AND CAPACITANCE : 50Khz TO 500Khz. COMPRISES OF INDUCTOR,TUNNING CIRCUIT,LA.
LINE TRAPS RATINGS
INDUCTANCE IN Mh
RATED CURRENT
BLOCKING BANDWIDTH
Checks for Wave trap. Tuning circuit and arrestor are properly tightened. Connectors are properly tightened. Bird Guards are properly fixed. Check for any physical damage.
LINE TRAPS PRE--COMMISSIONING TESTS PRE
INSULATION RESISTANCE MEASUREMENT
CONTACT RESISTANCE MEASUREMENT
LINE TRAPS MAINTENANCE YEARLY CHECKS
TIGHTNESS & CLEANLINESS OF LINE TRAP INCLUDING TUNING UNIT TIGHTNESS OF TERMINAL CONNECTOR
ISOLATORS Operate under no load condition. Isolate the zone which is under maintenance. Three types of Isolators : A) Pantograph. B) Horizontal center break. C) Double Break.
TYPE OF ISOLATORS BASED ON LOCATION OF USE Line Isolator Bus Isolator Reactor Isolator Transformer Isolator
TYPE OF ISOLATORS BASED ON E/S Without Earthswitch (Used where earthing is not required) With one Earthswitch (Commonly used for earthing the equipment for which Isolator has been provided) With two earth switch (Generally used for Bus earthing)
Checks on Isolators Proper alignment. Proper fixing of limit switch. Proper closing of Isolators to avoid hot spot. Check mechanical/electrical interlock. Ensure twin earthing of Isolator MB. Proper closing of Isolators MB.
HCB ISOLATORS
HCB ISOLATORS ISOLATORS MAIN MAIN CONTACTS
DOUBLE BREAK ISOLATORS
DB ISOLATOR ISOLATOR-----CENTRAL CENTRAL ROTATING STACK
DB ISOLATOR ISOLATOR---CONTACTS CONTACTS
PANTOGRAPH ISOLATOR
PANTOGRAPH ISOLATOR-ISOLATOR-CONTACTS
Checks on Isolators Ensure proper position of NO/NC contact in closed & open condition of Isolators. Apply grease regularly in Gear box. Check pole discrepancy function is working properly.
