GENERATOR AND AUXILIARIES DESCRIPTION The 200MW generator is a three phase, horizontally mounted two-pole cylindrical rotor type, synchronous machine driven by steam turbine. The stator winding is cooled by de-mineralised water flowing through the hollow conductor while the rotor winding is cooled by hydrogen gas maintained inside the machine. m achine. Fans mounted on the generator rotor facilitate circulation of hydrogen inside the machine. Four coolers mounted inside the machine cool the hydrogen gas. The generator winding is provided with epoxy thermo-setting type insulation. The machine is provided with completely static thyristor controlled excitation system, fed from terminals of the machine. Hydrogen being a light gas with good heat carry away capacity is used for cooling the rotor winding, rotor and stator core. Two hydrogen driers are provided to facilitate moisture removal. Hydrogen is circulated through them via the fans in dry condition. Normally one drier is kept in service and other is on regeneration. Four hydrogen coolers are provided to cool the hot gas to maintain the cold gas temperature at 40oC. Liquid Level Detectors (LLDs) are provided to indicate liquid in generator casing. This provision is to indicate leakage of oil or water inside the generator. It can be drained through drain dr ain valve. H2 gas purity is to be maintained of very high order i.e. more than 97%.
STATOR WATER-COOLING SYSTEM The stator winding of the generator is cooled by de-mineralised water circulating through hollow conductors of stator winding bars in a closed loop. The cooling water system consists of 2x100% duty AC motor driven pumps, 2x100% duty water coolers, 2x100% duty mechanical filters, 1x100% duty magnetic filter, expansion tank, polishing unit and ejector system. The stator water pump drive the water through coolers, filters and winding and finally discharges into the expansion tank situated at a height of about 5m above the TG floor. It is maintained maintained at a vacuum of about 250 mm Hg by using using water ejectors. A gas trap is provided p rovided in the system to detect any traces of hydrogen gas leaking into the stator water system.
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WATER PATH OF STATOR WINDING AND TERMINALS TERMINALS
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SEAL OIL SYSTEM To prevent leakage of hydrogen from generator housing, ring type shell seals are provided at both ends of the generator. During normal operation the AC seal oil pump draws the seal oil from the seal oil tank and feeds it into the shaft seals via 2x100% capacity coolers and 2x100% capacity filters. The differential pressure 2
regulator maintains seal oil pressure differential of 1.3 Kg/cm over the hydrogen pressure irrespective of the value of hydrogen pressure. pr essure. The seal oil is supplied to the shaft seals into the annular groove of seal ring via the passage in the seal ring carrier. The clearance between shaft and seal ring is such that frictional losses and seal oil temperature rise are minimum. Oil film is of sufficient thickness to provide proper sealing. Higher-pressure ring relief oil is fed in the annular groove in the airside seal ring carrier. Thus gas g as and oil pressure acting on the seal ring are balanced and friction between seal ring and seal ring carrier is minimized. The seal ring is free to adjust its position according to shaft position. Airside seal oil is directly returned to the seal oil tank via a float valve. The oil drained towards the hydrogen side is first collected in pre-chamber and then passed to the intermediate oil tank in order to separate any trace of hydrogen present in seal oil. The oil from this tank also is returned to the seal oil tank via a float valve. Any possible traces of gases or vapour etc. are removed by vacuum pump from top of the seal oil tank. In case of failure of DPRV-A or AC as well as DC seal oil pumps failure, DPRV-B will come into service and governing oil is used as seal oil.
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M E T S Y S L I O L A E S R O T A R E N E G
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SPECIFICATIONS OF THE GENERATOR Rated parameters: Maximum continuous KVA rating
247, 000 KVA
Maximum continuous KW rating
210, 000 KW
Rated Terminal Voltage
15, 750 V
Rated Stator Current
9050 A
Rated Power Factor
0.85 Lag
Excitation voltage at MCR condition
310 V
Excitation current at MCR condition
2600 A
Excitation voltage at no load
102 V
Excitation current at no load
917 A
Rated speed
3000 RPM
Rated frequency
50 Hz.
Stator winding resistance per phase at 20 oC. Rotor winding resistance per phase at 20 oC.
0.00155
Efficiency at MCR condition
98.49 %
Short circuit ratio
0.49
Rise in voltage with 100% load throw off
22.40 KV (without AVR)
Negative phase sequence current capability
1
Direction of rotation when viewed from slip ring
Anti clockwise
Phase connection
Double star
No of terminal brought out
9 (6 neutral, 3 phase)
Generator gas volume
56 m
Nominal pressure of hydrogen
3.5 kg/cm
Permissible variation of gas Pressure Nominal temperature of cold gas
0.2 kg/cm 40 oC. (Alarm)
Purity of hydrogen
> 97 %
Relative humidity of H 2 at nominal pressure
0.0895
2 2
Ω
Ω
t <8
3 2
±
2
60 %
Max temperature of cooling water inlet
36 oC.
Max temperature of cooling water outlet Hot gas temperature
43 oC. 75 oC (Alarm)
Nominal gauge pressure at winding inlet
3.09 Kg/cm
Max temperature of Stator Water at winding inlet
36 oC
2
Max temperature of Stator Water at winding outlet 70 oC
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Stator water flow Normal
27
±
3
3 m /hr 3
Alarm
21 m /hr
Trip
13 m /hr
3
Stator water conductivity Normal
< 5.0 micro mho/cm
High
13.3 micro mho/cm
Trip Stator water expansion tank Vacuum Auto start of standby SW pump
20.0 micro mho/cm 200-300 mm of WCL 2.4 Kg/cm
2
Nominal consumption of cooling water At 35 oC. At 37 oC.
110 m /hr
At 40 oC.
130 m /hr
3
95 m /hr 3 3
Safety Valve release (A.C. seal oil pump)
9 Kg/cm
2
Safety valve release (D.C. seal oil pump)
9 Kg/cm
2
Seal oil temperature after Seal oil cooler Normal Alarm
20 - 40 0C. 45 0C.
Seal oil outlet temperature Normal Alarm
40 0C. 65 0C.
Differential pressure across duplex filter Normal Alarm Seal oil pressure at Turbine and Slip ring end
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0.4 Kg/cm
2
0.6 Kg/cm
2
5.9 Kg/cm
2 2
(0.9 Kg/cm static head).
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Permissible temperature rating of Generator: High
V. High
1.
Generator bearing and seal Babbitt temperature
75 0C.
85 0C.
2.
Generator bearing oil outlet temperature
60 0C.
70 0C.
3.
Stator winding temperature
75 0C.
105 0C.
4.
Rotor winding temperature
110 0C.
115 0C.
5.
Stator core temperature
95 0C.
6.
Hot gas temperature
75 0C.
7.
Cold gas temperature
44 0C.
OPERATION LIMITS Capability of the Generator The generator is capable of delivering 247 MVA continuously at 15.75 KV terminal 2
voltage and stator current 9050 A, at a Hydrogen pressure (g) of 3.5 Kg/cm . The cold gas temperature not to exceed 44 0C and distillate temperature at inlet of stator winding not to exceed 45 0C. Output of the generator at various lagging and leading power factors are as per the generator capability curve. Variation of Terminal Vol tage
Generator can develop rated power at rated power factor when the terminal voltage changes within ± 5% of the rated value i.e. 14.96 KV to 16.54 KV. The stator current should accordingly be changed within limits of 5% ± i.e. 8600 A to 9500 A. Permissible voltage of operation and corresponding values of the MVA outputs of stator currents are given in Table - 1. During continuous operation of the generator at 110% of the rated value stator current should not increase beyond 9500 A corresponding to 105% of the rated value.
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TABLE – I Terminal 17.32 voltage in KV Output in 217 MVA Stator current 7.24 in KA
17.17
17.01
16.85
16.7
16.54
15.75
14.96
224.7
231
237
242
247
247
247
7.56
7.92
8.14
8.37
8.6
9.05
9.5
During continuous operation of the generator at 110% of the rated value stator current should not increase beyond 9500 A corresponding to 105% of the rated value.
Frequency Deviation The Generator can be operated continuously at rated output with a frequency variation of ±5% ±5% of the rated value i.e. 47.5 Hz. to 53.5 Hz. However, the performance performance of the generator with frequency variation is limited by t he turbine capability.
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Temperature of the Coolants If the temperature of the cooled hydrogen or inlet water to gas coolers increases beyond the rated value, the t he unloading of the generator has to be carried out as per the given curves. The operation of the generator ge nerator with cold gas temperature more than t han 0 55 C is not permitted. Operation of the generator with cold gas temperature below 20 0C is not recommended. Similarly, if cold distillate temperature at inlet of stator windings increases beyond the rated value, unloading of the generator has to be carried out as per the curves. The operation of the generator with cold distillate temperature more than 48 0 C is not permitted. Operation of the generator with cold distillate temperature below 35 0 C is not recommended.
UNLOADING SCHEDULE DUE TO HIGH DISTILLATE TEMPERATURE
Overloading Under normal conditions, the generator can be over loaded for short duration. Permissible values of short time over loads of Stator Current Vs Time in minutes and Rotor Current Vs Time in seconds are given in Table - II and Table - III respectively.
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TABLE – II Stator current in KA 13.57 Time in minutes
1
12.67
12.22
11.76
11.31
10.86
10.41
9.95
2
3
4
5
6
15
60
TABLE - III Rotor current in KA Time in seconds
5.2
3.9
3.12
2.73
20
60
240
360
Operation under Unbalanced Load The turbo-generator is capable of operating continuously on an unbalanced system loading provided that continuous negative phase sequence current during t his period shall not exceed 5% of the rated stator current i.e. 452.5 A. It implies that maximum difference between limit current is about 10% of the rated value. At the same time current of maximum loaded phase should not exceed the permissible value for normal conditions of operation of turbo-generator under balanced loading. If the unbalance exceeds the above permissible limits, measures shall be taken immediately to eliminate or reduce the extent of unbalance within 3 to 5 minutes. In case it is not possible to achieve this, the machine has to be r un down and tripped. If negative sequence current reaches a value of 20-25% of the rated value trip-relay will operate and the generator generator will be automatically tripped.
Asynchronous Operation Operation Asynchronous Operation of the generator on field failure is allowed depending upon the permissible degree of the voltage dip and acceptability of the system from the stability point of view. During field failure, the field suppression shall be cut out from the circuit and active load on the generator shall be decreased to 60% of the rated value within 30 seconds and to 40% in the following 1.5 minutes. The generator can operate at 40% rated load asynchronously for a total period of 15 minutes from the instant of excitation failure. Within this steps should be taken to establish the reasons of field failure to restore normalcy or the set should be switched over to reserve excitation.
Unsymmetrical Short Circuit Performance The duration d uration of unbalanced u nbalanced operation should be such that the product of square s quare of negative sequence component of current I
2 2
expressed in terms of per unit value of
stator current and its duration durati on in seconds does not exceeds 8 (I
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NEGATIVE SEQUENCE CURRENT CAPABILITY CURVE
The permissible value of negative sequence current and the t he corresponding durations are given in Table - IV.
TABLE – IV Duration of short circuit in seconds
1.2
5
10.9
Negative sequence Current
2.5
1.25
0.9
Operation at Reduced Hydrogen Pressure Continuous operation of the turbo-generator with lower hydrogen pressure than the 2
rated value of 3.5 Kg/cm is not permitted. However, during emergency, the generator can be run at reduced hydrogen pressure with reduced load for a short duration as given in Table - V.
TABLE - V H2 Pres. kg/cm 2 (g)
Generator Load (MW)
Duration of Operation
3.0
200
Continuous
2.0
115
Continuous
1.0
30
Continuous
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Within this time action should be taken to restore the t he hydrogen pressure to normal value. Operation of generator generator with air-cooling is NOT PERMITTED.
Capacity of the Generator with One Cooler out of Service The generator can deliver deliver 185 MW continuously when one gas cooler is out of service. service. The operation of the generator with more than one cooler out of service is not permitted. Refer to Fig
Motoring Action Motoring of the turbo-generator is permissible within the limitation of the turbine.
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STATIC EXCITATION SYSTEM Description Static Excitation System is used in most of the 200 MW Generator sets. The AC power is tapped off from the generator terminal, stepped down and rectified by fully controlled thyristor bridges and then fed to generator field as excitation power, to control the generator-output voltage. A high control speed is achieved by using an inertia free control and power electronic system. Any deviation in generator terminal voltage is sensed by an error detector and causes the voltage regulator to advance or retard the firing angle of thyristor thereby controlling the field excitation. The static excitation system system consists of: of: i.
Rectifier Transformer
ii.
Thyristor Converter
iii. Automatic Voltage Regulator iv. v.
Field flashing Circuit Field breaker and field discharge discharge equipment.
Rectifier Transformer The power transformer gets input supply from the generator output terminals. The secondary is connected to the Thyristor Bridge, which delivers a variable DC output to the generator field. Normally it is a dry type transformer.
Thyristor Converters The converter is assembled in one or more numbers of cubicles depending on the number of thyristor bridges connected in parallel. The number of bridges is so designed that in case one bridge fails during operation, the remaining bridges will have adequate capacity to feed the generator field for full load output. Fans mounted on the top of the cubicles cool the thyristor bridges.
Field Flashing Circuit Since it is difficult to start the excitation system with the residual voltage at nominal speed, a field flashing circuit is provided to overcome this problem. Initially the station auxiliary supply of 415 volts is stepped down by a small transformer and then rectified in a rectifier bridge and supplied to the generator field. As soon as the generator output builds up to 40%, the excitation system starts working smoothly. A back-up battery supply is given in parallel parallel to field flashing output. output.
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FIELD BREAKER CUBICLE For rapid de-excitation of synchronous machine and complete isolation of the field from the Thyristor Bridge, a field breaker is provided. In case of electrical faults, the field breaker provides protection by isolating DC source from the field. The magnetic field energy is dissipated through a field discharge d ischarge resistance.
AUTOMATIC VOLTAGE REGULATOR It is the heart of excitation system. It consists of the following components:
ERROR DETECTOR AND AMPLIFIER The generator terminal voltage is stepped down by three phases PTs and fed to the Automatic Voltage Regulator (AVR). The AC input thus obtained is rectified and compared against a highly stabilized reference value and any difference is amplified in different stages of amplification. For parallel running of generators compounding feature is provided. Three CTs sensing the current in the generator terminal feed proportional current across the variable resistors in the AVR. The voltage thus obtained across the resistor can be added vectorially either for compounding purpose or for transformer droop compensation.
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GRID CONTROL UNIT The output of the AVR is fed to a grid control unit. It gets its synchronous AC reference through a filter circuit and generates a row of pulses whose position depends on the DC input from the AVR i.e. the pulse position varies continuously as a function of the control voltage. The pulse limit for rectifier and inverter (deexcitation) operation can be adjusted independent of each other by potentiometer provided on the front side of unit. Six double pulses displaced by 60 o from one another are generated at this output. Two relays are provided, by exciting which, these pulses can be either blocked completely or shifted to inverter mode of operation.
PULSE AMPLIFIER The pulse output of this grid control unit is amplified further at an intermediate stage of amplification. This is also known as pulse coupling stage. It has also DC power supply unit which operates from a three-phase 380V supply and delivers + 15V, -15V, +5V and a coarse stabilised voltage UL. A built-in relay is provided which can be used for blocking a 6-pulse channel. In a two-channel system, energising and de-energising the relay affect the changeover.
PULSE FINAL STAGE The unit receives input pulse from the previous stage i.e. pulse amplifier (intermediate stage) and transmits them through pulse transformer to the gates of the thyristor. The step pulse at the output ensures simultaneous firing of several thyristor in parallel. A built- in power supply provides the required DC supply (+ 15V +5V & UL) to the final amplifiers.
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Each thyristor bridge has its own final pulse stage. Therefore, even if a thyristor bridge fails with its final pulse stage, the remaining thyristor bridges bri dges can continue to provide full load output and thereby ensure (n-2) operation. Pulses can be blocked with an internal relay provided in this unit. Pulses are blocked in case of Failure of one or more thyristor fuses. -
Failure of power supply of the final stage.
-
Failure of the converter fans.
MANUAL CONTROL CHANNEL A separate manual control channel is provided where the controlling DC signal is taken from a stabilized DC voltage through a motor operated potentiometer. The DC signal is fed to a separate grid control unit whose output pulses after being amplified at an intermediate stage can be fed to final pulse stage. When one channel is working generating the required pulses, the other remains blocked. Therefore, blocking or releasing the pulses of corresponding intermediate stage affects a changeover between auto and manual control. control. A pulse supervision unit detects spurious pulses or loss of pulses on the pulse bus bar and transfers control from from 'Auto' to 'Manual' channel. channel.
FOLLOW-UP UNIT To ensure a smooth changeover from 'Auto' to 'Manual' control it is necessary that the position of the pulses in both the channels should be identical. A pulse comparison unit detects any difference in the position of the pulses and with the help of a follow-up unit in actuates motor operated potentiometer on the manual channel to turn in direction so as to eliminate the difference. However, while transferring control from manual to auto, any difference in the two control levels can be visually checked on the balance meter provided on the control panel and after matching the two signals the changeover can be done.
LIMIT CONTROLLERS When a generator is running in parallel with the power network it is essential to maintain it in synchronism without exceeding the maximum permissible load on the machine and also without the protection system tripping. So it is necessary to influence the voltage regulator by suitable means to limit the over-excitation and under-excitation. The following limiters are normally used in the the static excitation system. system. a.
Stator Current Limiter
b.
Rotor Current Limiter
c.
Rotor Angle Limiter
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Stator/Rotor current Limiters limit the control voltage to a value corresponding to the permissible excitation in over excited operation while rotor angle and stator current limiter limits the control voltage to value corresponding to permissible excitation in under excited operation.
STATOR CURRENT LIMITER This unit functions in conjunction with an integrator unit, which provides the necessary dead time and the gradient that can be adjusted by potentiometers. The regulator consists of a measuring converter, two comparator, two PID regulators and a DC power pack. A discriminator in the circuit differentiates between inductive and capacitive current. The positive and negative signals processed by two separate amplifiers are brought to the output stage and only that output which had to take care of the limitations is made effective. The inductive current limit is affected through the integrator while the capacitive current gets directly on the AVR output.
ROTOR CURRENT LIMITER The unit basically comprises an actual value converter, a limiter with adjustable PID characteristics, a reference value, dv/dt sensor and a signaling unit. The field current is measured as the AC input side of the thyristor converter and is converted into proportional DC voltage. The signal is compared with an adjustable reference, amplified, and with necessary time lapse, fed to the voltage regulator input. The limit is reduced during (n-2) operation through a relay switched circuit. Also, during a fault condition (when dv/dt is large and -ve), the limit is raised (field forcing limit).
ROTOR ANGLE LIMITER This unit limits the load angle i.e. the angle between the voltage of the network centre and the rotor voltage. The limiter is fed with generator terminal voltage & current and through a simulation circuit derives the rotor voltage & grid voltage & hence the load angle. It comprises an actual value converter, a limiting amplifier with adjustable PID characteristics and a reference value unit. The limiting regulator operates as soon as the DC value exceeds the t he reference value.
SLIP STABILISING UNIT The slip stabilisin stabi lising g unit is used for the suppressi supp ression on of rotor rot or oscill ations atio ns of the alternator through the additional influence of excitation. The slip as well as acceleration signals needed for the stabilisation are derived from active power delivered by the alternator. Both the signals are amplified and summed up to influence the excitation of the synchronous machine through AVR in a manner so as to suppress the rotor oscillation .
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EXCITATION SYSTEM PROTECTION Excitation Transformer Protection The prote ction unit for the excitatio excit ation n transforme trans formerr is normally norma lly mounted mount ed on the swing frame of the regulator cubicle. It consists of two over current relays, with adjustable ranges. The current supply for the relays is made through a DC converter, which receives its input supply from battery through a filter circuit. Besides over-current protection a dry type rectifier transformer is embedded with temperature dependent resistance at the low voltage windings. With rise in temperature the resistance value changes sharply after a certain level. This change with one resistor is used for 'warning' and with another for 'tripping'.
Converter Protection Fuses Each thyristor in the converter is connected with a fast acting semiconductor fuse to protect in case of over-current.
Resistor/Capacitor Network across to each thyristor for protection against hole storage effect.
Airflow Airf low Monito Mo nitoring ring Since converters are air-cooled by fans the airflow is monitored by airflow relays.
Redundancy The thyristor thyri stor bridges brid ges are designed desig ned such that in case of failure fail ure of one, the remaining bridges will be adequate to provide full load output with field flashing.
Isolator Isolators are provided on the input and output side of the converters to enable replacement of defective thyristor under running load.
AVR Prote P rotection ction All D.C. power supplies receive their input AC supply through Miniature Circuit Breaker with thermal overload relays. Failure of protection and control voltage , as also Regulation Supply, result in tripping of the Field Breaker. Failure of Auto Power Supply / Regulation supply result in to Manual channel change over.
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Power chart of a Turbogenerator (Solid rotor design)
SIGNIFICANCE OF MACHINE CAPABILITY DIAGRAM Capability diagram of the generator gives the safe operating domains. It gives the basic information regarding the limiting zones of the operation so that limiter can be set / commissioned suitably for safe operation of unit.
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EXCITATION SYSTEM PARAMETERS ( KORBA STAGE-1 ) GENERATOR MVA
MW
VOLTAGE
STATOR CURRENT
POWER FACTOR
SPEED
247
210
15.75 KV
9050 A
0.85 LAG
3000 RPM
EXCITATION SYSTEM Ifn
:
2512 A
V fn fn
: 300 V
Ifmax
:
3000 A (contin.)
Ifo
: 917A
:
120 V
V fo fo
Field resistance
:
0.0896
Field forcing
:
1.4 I fn
(n-2) limitation
:
0.65 I fn, 1.1 Ifn
Ω
( 3900 A, 620 V for for 10 sec )
α
min
:
30o
α
max
:
72o
Over voltage setting
:
2.5 KV
Rotor Earth fault
:
5 K Ohm, 2.2 K Ohm.
Range of control of AVR
:
Field flashing off
:
30% (85% - 115%) Blocking of Ch. pulses upto 30% voltage 70% V
Excitation transformer
:
15.75 KV/575 V, 2500 KVA,
CTs Generator
:
10,000/5 A
Excitation Transformer
:
200/1 A, 2500/5 A
DCCT
:
3000/2 A
PTs
:
15750/110 V
DCPT
:
600V/20 mA
Field flashing transformer
:
5 KVA, 415/35 V
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Field Breaker closing 1 GE2 Open 2
386GX Reset
Field Flashing
:
Up to 70% voltage within 20 Secs + RPM Relay (2950 rpm)
Field Breaker Tripping Protections 1
Class A 30Z → 86G→ 386 GX → Field Breaker Trip
a. b.
Transformer Over Current Instantaneous Transformer Over Current delayed delayed 2 nd stage
c.
Rotor Over Voltage
d.
48V supply fail
e. Three or more (≥/3) failed i.
Fan fail a. b.
ii.
Air Flow Relay Fan supply
Pulse final stage power supply failed
iii. Thyristor fuse failed iv.
2
Isolator open.
Class B 30F → UTR → LFPR(32GI) a. b
3
→
32GI→ 2/32G1
→
86G
→
386G → FB Trip
Rotor Earth Fault, stage-II Regulator supply fuse fail
c
Manual Channel fail
d
Thyristor Fan supply fuse fail
e
Transformer Temp. HI - HI
Direct Trip Trip the F.B. directly, Gen Gen trips through field failure failure relay. a b
Field Flashing disturbed Switch in Test Position Position
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4
Automatic Changeover to Manual a b
Supply A1 fail Supply A fail
c
Excitation transformer O/C delayed Stage-I
d
Excitation transformer temp HI
e
Auto Channel pulse failure
f
AVR PT Fuse Failure
Alarms a b
Protective change over to manual AVR Fault
c
FB Tripped due to AVR fail
d
Limiters in Action
e
Loss of Control voltage
f
Rotor E/F
Limiters a Stator Current Limiter b c
Rotor current limiter Rotor Angle Limiter
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GENERATOR PROTECTION Description The core of an electrical power system is the generator. During abnormal operating conditions certain components of the generator are subjected to increased stress and therefore, could fail, referred to as faults. It can be either internal fault or external fault depending upon whether they are inside or outside of the machine. The machine with fault must be tripped immediately. The corrective measures against generator's abnormal operation are taken care by stubborn protective system.
Task of the protective system: - Detect abnormal condition or defect. - Limit its scope by switching to isolate the defect. defect. - Alarm the operating staff. - Unload and/or trip the machine machine immediately.
Requirement of Protective Device: - Selectivity: Only that part of of the installation actually containing fault should be disconnected. - Safety against faulty tripping: There should be no trip when there is no fault. - Reliability: The device must always act within the required time. - Sensitivity: Lowest signal input value at which which the protective protective device must act. - Tripping time: There should be clearly a distinction between the tripping time of the device, considering the circumstances such as current and total tripping time for the fault. The total fault clearing time now now is of the order order of 100 (mill sec.)
Protective Devices The choice of the protective equipment for a generator requires precise knowledge of the stress to which the generator is subjected to during services in order that preventive measure may be devised for avoiding inadmissible stress. Important stresses include the electrical voltages to which insulation is exposed, the mechanical forces affecting various parts of the machine and effects of the temperature rise.
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Types of Protections The details of the protective circuits of a 200 MW turbo-generator are given in the above fig. The several faults occurring in generator can be either electrical or mechanical in nature. As such generator protection is broadly segregated into two parts i.e. electrical protection-mostly Class-A type and mechanical protection-mostly Class-B type.
ELECTRICAL PROTECTION 1.
Differential Protection: a.
Generator Differential
b. UAT Differential c.
Overhead Line Differential
d. G.T. Restricted Earth Fault, Main e. 2.
Overall Differential
Earth fault protection: a.
Stator Earth Fault
b. Stator Earth Fault, stand by c.
Rotor Earth Fault
3.
Stator Interturn Fault
4.
Negative Phase Sequence Current
5.
Generator Backup Impedance
6.
Loss of Excitation
7.
Pole Slipping
8.
Over Voltage
9.
Over Fluxing
10.
Low Forward Power
11.
Reverse Power
12.
Generator Local Breaker Backup (LBB)
13.
Generator Transformer Protections a.
Buchholz Protection
b. PRD Protection c.
Winding Temperature High
d. Oil Temperature High e.
Fire Protection
14.
UAT Protection
15.
Bus Bar Protection
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DIFFERENTIAL PROTECTION a. GENERATOR DIFFERENTIAL A direct short circuit between different phases of the winding causes a severe fault current flow through the windings and results in extensive damages. As a result there is a distinct difference between the current at the neutral and terminal ends of the particular winding. This difference is detected by differential relay. The current entering and leaving the protected object are determined by current transformers and compared by relays by means of a differential circuit as shown in the figure. A fault f ault inside the protected zone is fed from either one side or both sides depending upon the current sources present, thus producing a difference current in the differential circuit. If this differential current exceeds a set percentage of the current flowing in the protected object, the relay picks up. The relays used is designated 87 G and is CAG 34.5 amp type. It is set to operate 10% (0.5 amp) relay current which corresponds to 1000 amp fault current.
b. UATS DIFFERENTIAL Since UATs are connected directly to the stator windings, it has been provided with a biased differential protection in a similar circulating current scheme. The relays are designated 87 UAT and 87 UTB are DTH 31 type. ty pe.
c. G.T. OVERHEAD LINE DIFFERENTIAL The 400 KV bushings of the generator transformer are connected to the switchyard double moose conductor overhead line. Any fault occurring on these lines is detected by overhead line differential differential protection. The relay designated 87 L is of CAG 34 type 1 amp.
G.T. RESTRICTED EARTH FAULT The H.V. winding of the generator transformer is star connected and the neutral is solidly earthed. This protection meant for complete protection of H.V. winding of generator transformer. The delta side of the generator transformer is considered as a part of the generator and its earth fault would cause the earth fault current to flow toward the generator neutral and be detected as generator earth fault. The relay is designated as 64 GT and is CAG 14 type one amp. and a high impedance definite current attracted armature type.
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d. G.T. OVERALL DIFFERENTIAL Since Generator Transformer is directly connected to the stator winding, it would be proper to include the transformer windings associated bus ducts including those for UAT HV side and conductors in a similar circulating current protection scheme. The relay is designated 87 GT and is of DTH 32 type 5 amp which is a biased differential type relay. Biased setting of 30% is used to prevent the relay operation in case of a through fault when the current transformer may saturate and produce an erroneous secondary current.
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EARTH FAULT PROTECTIONS a. STATOR EARTH FAULT (Main) The generator neutral is earthed through the primary winding of neutral grounding transformer of the rating 50 KVA, 15.75/0.24 KV ratios. The secondary winding of the transformer is shorted through loading resistance of 0. 42 Ω. For an earth fault in the generator the E/F current flows in the primary of the neutral grounding transformer. As a result a voltage across the resistor is developed which activates stator E/F sensing relay. The reason for this kind of protection is due to mechanical damages resulting from the insulation fatigue, creepage of the conductor bases, vibration of the conductors or other fittings of the cooling systems. The earth fault relay designated is VDG 14 type 64 G1. The relay has a inverse definite minimum time characteristics. Generally 5% Generator winding starting from neutral point remains unprotected because a fault in these these portions will generate too low a voltage for relay operation.
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Stator Earth Fault (Main)
b. STATOR STANDBY EARTH EARTH FAULT The relay is connected across an open delta of the generator PT P T secondary windings. When there is no E/F, the sum of the t he phase voltages of the generator and hence the voltage across the relay is zero. The voltage across the point a & b will assume a positive value when one phase voltage of the generator drops because of earth fault on that phase. The relay is 64 G2, VDG 14 type. It has a inverse inverse time voltage characteristics. characteristics.
c. ROTOR EARTH FAULT Ground leakage in the rotor circuit c ircuit of a generator does not adversely affect operation, if it occurs only at one point. Danger arises if a second fault occurs causing the current to be diverted in part at least, from the intervening turns, which can burn the conductor causing severe damage to rotor. If a large portion of winding is shorted, the field flux pattern may change causing the flux concentration at one pole and wide dispensation at the other. The attractive force, which is proportional to the square of the flux density, will be stronger at one pole than the other which will cause high vibration and may damage the bearings and may sufficiently displace rotor thereby fouling the stator. Rotor E/F protection is provided by monitoring m onitoring the I.R value of the rotor winding I.R value < 5.5 K Ω : Alarm , I.R value < 2.2 K Ω : Trip
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STATOR INTER-TURN FAULT When leakage occurs between the turns in the same phase of a winding the t he induced voltage is reduced and there will be a voltage difference between the centre of the terminal voltage triangle and the neutral of the machine. Therefore, in a generator having one winding per phase, a voltage transformer is connected between each phase terminal and the neutral of the winding, the secondary transformer leads being connected in open-delta, when inter-turn leakage occurs at the ends of the open delta, it is detected by a polarised voltage relay. For generators having several parallel windings per phase, the neutral ends are connected together to form, as many neutrals as there are parallel windings per phase. These neutral are then joined through current transformer to current relays, or through voltage transformer to voltage relay. If an inter-turn fault occurs in the machine, the current transformer carries a transient current or alternatively voltage transformer produce a voltage thereby picking up relay and tripping the Generator. The relay is designated 50 GI, is a CAG 14 type 5 amp attracted armature, definite current operated type.
NEGATIVE PHASE SEQUENCE A three phase balanced load produces a reaction field, which is constant and rotates synchronously with the rotor field system. Any unbalanced condition could be resolved into positive, negative and zero sequence components. The positive sequence component is similar to the balanced load. The zero sequence components do not produce armature reaction. The negative sequence component is similar to that of
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positive sequence but the resulting reaction field rotates in the opposite direction. Hence the flux produced by negative phase sequence current cuts the rotor at d ouble the rotational speed thereby inducing double frequency currents. As a result eddy currents produced are very large and cause severe heating of the rotor windings particularly damper windings. For any current conditions in the three phases the amount of unbalance can be determined from the values of negative sequence components I 2 of current by the method of symmetrical components. The degree of unbalance is taken to the value of the negative sequence current component expressed, as percentage of rated current. The losses in the rotor are 2
proportional to the square of the degree of unbalance. This generator has I 2 t = 8 2
characteristics indicating that within I 2 t < 8, the generator is capable of withstanding but beyond it there is time delay. The time delay has to be matched to the machine negative sequence current withstands capability. The relay used is designated 46G and is of solid-state design and CTNM C TNM type.
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GENERATOR BACK-UP IMPEDANCE PROTECTION Three-phase zone impedance is provided for the back-up protection of generator against external three phases and phase to phase faults in 400 KV systems. The zone of impedance relay should be extended beyond 400 KV switchyard and it should be connected to trip the generator after a time delay of 1 to 1.5 seconds so that the generator is tripped only when 400 KV protections has not cleared the fault even in the second zone. The relay used is designated 21G.
LOSS OF EXCITATION Failure of the field system leads to losing of Synchronism and resulting in running above synchronous speed. It acts as an induction generator, the main flux being produced by wattless stator current drawn from the system. Operation as an induction generator necessitates the flow of slip frequency current in the rotor; damper winding, and slot wedges, excitation under these conditions requires a large reactive component which approaches the value of rated output of the machine. Since rotor would get over heated due to slip frequency current, the machine should not run more than a few seconds without excitation. Also, it could overload the grid, which may not be able to supply the required excitation MVAR. When loss of excitation is accompanied by under voltage it will initiate Class-A trip. Otherwise Class-B trip if the grid is able to sustain the voltage dip. The relay used is designated 40 G YCGF type.
POLE SLIPPING The asynchronous operation of the machine m achine while the excitation is still intact unlike loss of excitation causes severe shock to both machine and grid due to violent oscillations in both active power and reactive power. Because of this the machine may fall out of step or usually known as pole slipping trip. The oscillation may disappear in few seconds; in that case c ase it is not desirable to trip the machine. If however the angular displacement of the rotor exceeds the stability limit the rotor will slip a pole pitch. If this disturbance has been sufficiently reduced by the time this has occurred, the machine may regain synchronism, but if it does not, it must be isolated from the system. The swing curves can be detected by an impedance relay. The relay has two measuring elements set at two values near the independence as seen by the relay. As the impedance seen by the relay changes it comes in the operating zone of the two relays one after the other. The sequential operation is observed by auxiliary relays. Since the system faults would suddenly change the system impedance both the relays shall operate within 55 ms. however, during pole slipping, the two elements would operate sequentially and a trip command command is given when when both have operated. The relay can be set to be in operation for swings up to +90 O corresponding to the stability limits of the unit. The relay used is 98G and is of solid-state design of ZTO type. In order to discriminate against swings on the grid, the tripping is through an impedance relay (98 GY) set with a reach up to the 400 KV yard.
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OVER VOLTAGE The generator winding is rated for 15.75 KV at terminal, sustained over voltage would unduly stress the winding winding insulation and may lead to failure. To protect the machine against over voltage the protection relay senses the voltage at the secondary of the bus duct PTs. The relay is set to operate at 10% rise in the terminal voltage. A time delay of 3 seconds is provided to take care of transient over voltage arising from line charging, switching capacitive faults etc. The relay used is designated 59 G & is VTU 12 type 110V A.C.
G.T. OVER FLUXING The iron core of the generator transformer carries the the flux to produce required emf. If the flux increases unduly, the magnetic circuits of the generator and G.T become over saturated resulting in high magnetising current. This in turn leads to higher iron losses, which will increase the winding temp. of the transformer. Since core can be damaged because of this overheating, protection has to be provided against it. The flux is dependent on ratio of voltage & frequency. The condition of over fluxing could arise in case the voltage at the machine terminal rises or its frequency drops or both occurring simultaneously. Practically this condition will arise if the machine AVR misbehaves thereby unduly increasing the voltage even when the grid frequency is low. The relay used is 99 GT and is GTT21 type ty pe which senses v/f ratio r atio at the secondary of the bus duct P.T. and gives alarm and trip signals at different time delay. The adopted setting for relay is v/f = 1.2 P.U. i.e. 20% higher than rated r ated v/f ratio. Alarm is at 0.5 - 1 sec & trip at 12 sec in v/f v/f relay & generates AVR 'Raise' 'Raise' block. Surge voltage originating from lines because of switching or atmospheric disturbance disturb ance is dealt with directly by lighting l ighting arrestor and surge diverters.
LOW FORWARD POWER PROTECTION When a generator, synchronised synchronised with the grid, loses its driving force the generator remains in synchronism. The generator should be isolated from the grid after the steam flow ceases and the flow of power to grid reduces to minimum i.e. the point when the generator starts drawing power from the grid and acts as motor. When the load on generator drops to less than 0.5 percent, generator low forward power relay gets energised and with turbine tripped or stop valves closed, trips the generator with a time delay of 2 seconds. This is a protection to trip generator on other than electrical faults. Also this protection is used for a few electrical faults where generator trip can be delayed. However, provision for time lag unit is there to prevent undesired operation from transient power reversal. The power relay used is designated designated 32 G1 and is a WCD type.
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REVERSE POWER PROTECTION The generator must be disconnected from the grid as soon as turbine stop/control valves have closed, completely shutting off the steam. Continued full speed turbine rotation causes lot of turbulence of the trapped steam, which results in increase of temperature. Thus turbine will be subjected to excessive thermal overstress, vibration and distortion. So there is a back-up arrangement to trip the generator if it does not trip within 2 seconds i.e. on L.F.P. protection. This is known as Reverse power Protection which acts in two stages. 1st stage reverse power relay operates after 5 seconds time delay and includes stop valve closing/turbine trip. 2nd stage Reverse Power Relay acts after 60 seconds time delay which trips the generator irrespective of either stop valve closing or turbine trip. This acts as a final back up to L.F.P. protection. The power relay designated is 32 G2 and is also WCD type.
LOCAL BREAKER BACK-UP PROTECTION (LBB) This is a protection against against the main Gen. C.B. C.B. failure, which may occur due to (i)
Mechanical failure
(ii)
Trip circuits not healthy.
Hence, this protection acts as a back up to the main generator by tripping all the breakers connected to that particular bus.
Relay sensing •
D.C. to the relay extended through trip command (either 86 G or 286 G or B/B protection trip).
•
Over current element senses actual fault persisting.
When both the above conditions are satisfied LBB protection acts with a timer (0.2 secs.) to trip all other breakers connected to that bus. The LBB protection initiates bus bar protection. (Refer to the bus-bar protection scheme). The relay used is designated as 50Z.
GENERATOR TRANSFORMER PROTECTIONS G.T. BUCHHOLZ OPERATION Any internal fault in generator transformer will result into rapid increase in the winding temperature resulting in vaporisation of oil (dissociation of oil) accompanied by generation of gas. The generated gas is utilised utilised for relay operation. operation. The relay is a gas-operated device arranged in the pipeline p ipeline between the transformer tank and separate oil conservator. The vessel is full of oil. It contains two floats b 1 and b2 which are to be hinged and to be pressed by their buoyancy against two stops. If gas bubbles are generated in the transformer due to fault, they will rise and
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get trapped in the upper part of the relay chamber thereby displacing the oil and lowering the float b 1. This sinks and eventually closes an external contact, which operates an alarm.
If the rate of generation of gas is small the lower float b 2 is unaffected. When the fault is dangerous and gas production is violent the sudden displacement of oil along with the pipe tilts the float b 2 and causes a second contact to be closed and making the trip-circuit and operating the main switches on both HV & LV sides. Gas is not produced until temperature exceeds about 150 oC, so momentary overload of transformer does not affect the relay unless the transformer is really hot. Also insufficient oil level in Buchholz relay could lead to same operation.
THERMAL OVERLOAD PROTECTION PROTECTION Vapour pressure thermometers or resistance temperature detectors are used for this purpose. The transformer winding temperature and the oil temperature are continuously monitored; when the temperature reaches a certain value it will give indication. Then the load on the transformer is to be reduced. If the temperature rises still further tripping will take place. p lace.
FIRE PROTECTION Sprinkler system is utilised to protect the transformers from fire hazards. Sprinkler installation comprising of a system of interconnected pipes into which sprinkler heads are fitted on a definite basis of distribution. Sprinkler heads are so constructed that the heat arising from fire will cause them to rupture. Generally the sprinkler/system consists of a compressed air line and a water line. Sprinkler heads are provided in the compressed air line.
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The compressed air line will always be kept in charged condition. When the sprinkler head ruptures, the pressure in the water header will open to send the water into the water header, from his water water will be sprinkled on to the the transformer.
BUS BAR PROTECTION This is a protection against 400 KV bus faults. This protection trips all the feeders connected to the faulted bus zone; it must be reliable and discriminatory so as to (i)
trip only the faulted bus section (zone)
(ii)
not to operate for external faults.
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The current differential senses the fault through high impedance impe dance voltage relay (Type (Ty pe FTG) to reduce chances of mal-operation on external faults due to CT saturation. All CTs in that particular zone are parallel with proper polarity to obtain the current differential, which is fed to the relay. Sensing of the particular zone is made through isolator contact status relay (type VAJC). 400 KV yard y ard has 6 bus zones (4 main m ain and 2 transfer zone) as shown in figure below. Section I Bus 1 (A)
X
Bus 3 (D)
Bus 2 (B)
X
Bus 4 (E)
Section II Trans. Bus 1(C)
X
Bus 3 (D)
Each feeder has one common CT for main zone bus protection. The current is switched into appropriate zone (zone A, B, C etc.) through isolator status relay contacts. The operation of the relays is used to energize B/B pr otection trip buses. The bus bar protection trip D.C. relays (97) are connected to these trip tr ip buses again through respective isolator status contacts. In addition to each of the main zones there is an overall check zone to increase reliability of the whole system. This zone covers the whole of 400 KV yard and uses a separate CT core to reduce chances of mal-operation due to CT saturation, loose connection, shorting etc. The A.C A. C current scheme is similar to main m ain zone except it i t is not routed through any isolator selection. The D.C trip circuit is not complete unless the check protection also operates i.e. for any Bus bar protection trip to occur, both or any one of the main zones and check zone must operate. Also a supervision relay '95' connected in the A.C. scheme is provided. This is set at a lower value so that it can sense shorting / opening of one CT circuit current at normal operating value. This provides an alarm and also isolates the B/B protection pr otection scheme.
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GENERATOR PROTECTION GT Fire Protection, 30G GT Pressure Release Valve, 30 B GT Buchholz Protection, 30C Bus Bar Zone ‘C’ Protection, 30 D LBB Protection, 30 E Excitation System Fault, 30 Z Generator Diff Protection, 87 GX Gen. Over head feeder Diff Protection, 87 LX UAT A Diff Protection, 87 UTX UAT B Diff Protection, 87UT BX GEN. Over voltage, 59 GC GEN. Inter-Turn Fault, 50 GIX GEN. Pole Slipping protection, 98 GX Stator E/F Protection, 64 GIX GT Restricted E/F Protection, 64 GTX Low Forward Power Protection, 32 G1
UTR-A UTR-B
GT Block Overall Diff Protection 87 GTX GEN. Stator stand-by E/F Protection 64 G2X UT-A Buchholz Trip 30 K UT-A Buchholz Trip 30 N UT-A Fire Protection 30 L UT-B Fire Protection 30 P Gen Negative Sequence Protection 46 T LBB Protection 50 ZX From Relay 86 G UAT-A Back-Up Over current Protection 51 UTAX UAT-B Back-Up Over current Protection 51 UTBX GT Over-Fluxing, 99 GTTX Gen. Back-up Impedance Protection, 21G Gen. Reverse Power Protection 32G Low Forward Power Protection, 32 G1
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GENERATOR PROTECTION Gen. Backup Impedance Protection, 21G Trip Relay 86 G Trip Relay 286 G GT E/F Protection
TRIP RELAY 386 G
GEN CIRCUIT BREAKER TRIP TRANSFER BUS COUPLER BREAKER TRIP
UAT-A WINDING TEMPERATURE 2 nd STAGE UAT-A OIL TEMPERATURE 2 nd STAGE
UAT-B WINDING TEMPERATURE 2nd STAGE UAT-B OIL TEMPERATURE 2 nd STAGE TRIP RELAY 86 G TRIP RELAY 286G
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6.6 KV UNIT BUA-B TRIP
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MEASUREMENTS AND CONTROL
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