Generator Operation and protection

GENERATOR OPERATION, PROTECTION & PROTECTION SYSTEM

IOCL, PARADIP REFINERY

Topics covered

Generator detailsChecks during normal operationDifferent capability curves of generatorControl & Monitoring ArchitectureOverview of Generation & Distribution busControl philosophy of GISBrief about DAVROverview of Generator & Generator Transformer ProtectionProtection chart of Generator & Generator TransformerWHY & HOW of each type of protectionGenerator CO2 system

GENERATOR MODULES

Air Cooled Turbogenerator : TARI Hydrogen Cooled Turbogenerator : THRI Hydrogen/Water Cooled TG : THDF

MODULE NOMENCLATURE (EXAMPLE)

TARI 108/46 Paradip Refinery : TARI 1080-36P for GTGTHRI 108/44 TARI 800-20P for STGTHDF 115/59

MW CAPACITY RANGE OF TURBOGENERATOR

Air Cooled Turbogenerator : 80MW – 240MWHydrogen Cooled Turbogenerator : 100MW – 350MWHydrogen and Water Cooled Turbogenerator : 450MW – 1000MW

GENERATOR DETAILS

PARAMETERS OPERATION VALUES ALARMS

Stator teeth temp. 50-100 deg. C 120 deg.CGenerator winding temp. 50-100 deg. C 120 deg. CStator core temp. 50-100 deg. C 120 deg. CCold air temp. < 55 deg. CHot air temp. <75 deg. C 80 deg. CGenerator front bearing temp. 75 deg. C 110 deg. C

Generator rear bearing temp. 75 deg. C 110 deg. C

Brushless exciter bearing temp. 75 deg. C 110 deg. C

CHECKS ON GENERATOR DURING NORMAL OPERATION :

During normal operation the following generation parameters shall be monitored.PARAMETERS•KV•MW•MVAR•PF•Hertz•STATOR CURRENT R.Y.B.•Field currentOperator should check from DCS panel the temperature profiles in accordance with operating values from the given table.

PARAMETER OPERATING VALUE ALARM TRIPBearing vib < 10 mm/sec 12.5 mm/sec. 25 mm/sec (peak)

Bearing temp < 85 deg. C 100 deg. C 120 deg. C

Load Limits : As per the capability curve

Rate of loading : Permissible rate of loading depends on the condition of the winding Insulation

Space Heater : when generator is not running space heater should be ON

Monitoring : a) Temperature monitoring of components (through temp scanner)b) Generator hot air temp c) Shaft grounding brushd) Rotor vibratione) Fuse of brushless exciter with the help of stroboscope

Generator IR value : Min. IR value = (KV rating +1) M Ohm @ 40 deg C with 5 KV IR testerAction required if IR value is less than min : Identification of low IR section , if found in winding drying out to be adopted.

CHECKS ON GENERATOR DURING NORMAL OPERATION :

PERMISSIBLE SYNCHRONISING CRITERIA

PMG

Main Exciter Rotor winding

Rectifier Wheel

Ring for rotor earth fault

EXCITER ROTOR

BRUSHLESS EXCITATION SYSTEM

ECS panel in CPP C/R

I/O panel

SCAP

Toshiba GIS

I/O panel

ECS panels in GIS

building

Operator work

station

EEP panel

Control & Monitoring Architecture

FO cable

66KV BUS 1

11KV

MRC

S

66KV GIS SLD CPP Generation Bus & S/S-303 Distribution Bus

66KV BUS 3

66KV BUS 4

11 /66 KV 10 MVA(ONAN)

YnYn0,Z=12%

66KV BUS 2

X

To Transformers at downstream distribution sub-stations

S

X

118.3 MW

GTG 1 GTG 2

S

X

XX

11 /69 KV 148/118 MVA(ONAF/ONAN)

OCTCYNd11,Z=14.8%

STG 2S

X

X

STG 1S

X

X

GTG 3S

X

X X

XX

X

3150 A 40 KA ,1 SEC 3150 A

11 /69 KV 37.5 MVA

(ONAN)OCTC

YNd11,Z=11.2%

118.3 MW 118.3 MW 30 MW 30 MW

BS 1

BS 2

X

x x

x

66KV BUS 2X Xx

x66KV BUS 1

SS 303 GIS -66kV,3150 A,40kA for 3 sec 3150 A 3150 A

X X X X XX X XXO/g FDR

O/g FDR

O/g FDR

O/g FDR

O/g FDR

O/g FDR

O/g FDR

O/g FDR

O/g FDR

O/g FDR

Generation Bus

Distribution Bus

X

x

BC 1

X

x

BC 2

GIS control philosophy

•Synchronisation facility is provided to generators, sectionalisers, bus couplers and Tie-transformer breakers

•Synchronisation can be performed from SCAP as well as ECS in Auto as well as in manual mode.• SCAP /ECS/OFF selection switch provided on SCAP

• All breakers can be controlled from SCAP as well as ECS with Remote in LCC

•All isolators can be controlled from SCAP as well ECS with Remote in LCC

•All earthing switch can be controlled from LCC only with Local selection in LCC

•All breakers can be controlled from LCC in maintenance mode (line & breaker side earth switch connected) with Local selection in LCC

•All isolators can be controlled from LCC with Local selection in LCC

A section of 66kV TOSHIBA GIS Generation Sub-Stations Bays Local Control Panels

CPP Synchronisation Control & Annunciation Panel (SCAP)

Block Diagram of DAVR with Dual Auto (inbuilt Manual) Channels & Dual PLC

21G,27G,32G,37G,40G,46G,50GDM,51G,59G,60G,64G(95%),64G(100%),64F,78G,81G,87G & 99GT

Y

G

x

P345 P345P127

P633

P633

P141

P127

66 Kv GIS Bus

87GT

51GT/ 64RGT

870A/ 51NGT

P345

51VG 51VG

3 nos VTs for AVR Ch1, Ch2, Met & Prot

To rotor E/F protection

GENERATOR & GENERATOR TRANSFORMER PROTECTION OVERVIEW

66/11 KV transformer

Integrated Numerical generator protection relay Areva Make Model P345 (two independent relay GR1 & GR2)

Protection code Description21G: Generator backup impedance protection27G: Under voltage protection32G: Reverse power protection37G: Low forward power protection40G: Field failure protection46G: Negative sequence current protection50GDM: Dead machine protection51G: Definite time overload protection59G: Overvoltage protection60G: Voltage balance relay (fuse failure)64G(95%): Stator earth fault protection(95% winding)64G(100%): Stator earth fault protection(100% winding)64F: Rotor earth fault protection78G: Out of step(pole slipping) protection81G: Under/Over frequency protection87G: Differential protection99GT: Gen & Gen Transformer Overfluxing protection

PROTECTION RELAYS

Protection Relay Protection Code

Description of protection

Areva Make P127 Numerical protection relay (two independent relay)

51VG Generator voltage restrained over current protection

Areva Make P141 Numerical protection relay

51GT/64RGT Generator Transformer HV side over current & restricted earth fault protection

Areva Make P633 Numerical protection relay (1st relay)

870A/51NGT Overall differential & Generator transformer backup earth fault protection

Areva Make P633 Numerical protection relay (2nd relay)

87GT Generator Transformer differential protection

Areva Make VAEM21 relay 64F1A 1st rotor earth fault relay

Areva Make CAEM33 relay 64F2 2nd rotor earth fault relay

PROTECTION RELAYS

Important Accessories for protection relays

Code Description of code

Areva Make P931 RGR2 low frequency square wave generator for rotor E/F

Areva Make PR5104 REP Repeater for rotor E/F protection signal

Siemens Make 7XT3300 G20Hz 20Hz generator for 100% stator E/F

Siemens Make 7XT3400 FGR2 Band pass filter with built in voltage divider for 100% stator E/F protection

PROTECTION RELAYS

OVERVIEW OF GENERATOR PROTECTION

GENERATOR PROTECTION

Electrical protection provided - To quickly detect & initiate shut down for major electrical faults associated with the generating plant.Abnormal electrical conditions arise as a result of some failure with the generating plant itself,but can also be externally imposed on the generator. Common categories of faults and abnormal conditions to be detected are:

1. Internal Faults• Phase and /or ground faults in the stator and associated protection zone• Ground faults in the rotor (field winding)2. Abnormal Operating Conditions. • a. Loss of field.• b. Overload.• c. Overvoltage.• d. Under and over frequency• e. Unbalanced Operation e.g. single phasing.• f. Loss motoring i.e. loss of prime mover. • g. Loss of synchronization (out of step).• h. Subsynchronous oscillation.

GENERATOR DIFFERENTIAL (87G)WHY

• For protection of the stator windings against internal faults- to detect stator winding multi phase and earth faults . Normally involves high fault current , so fast clearing required.

HOW• Based on the principle of circulating currents. The difference of two currents

of the two sets of CT’s (one set on the neutral side & other set on line side of generator) flow through the relay.

• Operates only with in the protected zone for internal faults.• Stability to be ensured for stability against out of zone fault

GENERATOR DIFFERENTIAL (87G)

VOLTAGE DEPENDENT OVERCURRENT (51VG)

WHY• A fault close to the generator will result in a fault current decrement since

the armature reaction of the generator significantly reduces the fault current.• Provides back up protection for uncleared downstream faults with time

delay.HOW

• Voltage restrained over current relay. When voltage is low, relay will operate on low current

STATOR EARTH FAULT PROTECTION (64G)WHY • Most faults in a generator are a consequence of insulation failure. They may lead

to turn – to – turn faults and ground faults. Required for the earth fault in generator stator windings, potential transformers, lightning arrestors, surge capacitors & neutral bus duct. Fault current must be low as it may damage the core , which is very costly affaire. Hence ground fault protection is very essential for generators.

HOW• Maximum resistive fault current limited to 7.8 Amp (under field forcing) by neutral

grounding transformer & secondary loading resistor.95% of stator winding is protected by sensing voltage (overvoltage) across secondary loading resistor.100% of stator winding is protected by low frequency injection method.

• 100% stator earth fault protection can be provided by injecting an external low frequency alternating voltage into the starpoint or the terminals of the machine. Under normal healthy conditions only a very small current flows via the stator earth capacitance due to the high impedance of this path at low frequencies (Xc = 1/2fc). In the event of an earth fault the measured current increases due to the smaller impedance of the earth fault path.

STATOR EARTH FAULT PROTECTION (64G)

ROTOR EARTH FAULT PROTECTION (64F)WHY • An earth fault in the rotor winding does not cause immediate damage;

however, if a second earth fault occurs it constitutes a winding short-circuit of the excitation circuit. The resulting magnetic unbalances can cause extreme mechanical forces which may cause damage to the machine.

HOW• The rotor earth fault protection injects a DC voltage into the rotor circuit; the

polarity of the voltage is reversed at low frequencies and the frequency is selectable by the user through a link selection.

• Every time the DC voltage is reversed in polarity, a charging current is applied due to the capacitance of the rotor windings to earth. Under no fault conditions, the charging current should be discharged to zero.

• When a rotor earth fault occurs, the steady state current will no longer be zero, the magnitude of which can then be used to calculate the fault resistance.

• Other method: -ve biased voltage injection(VAEM) for 1st rotor earth fault and potentiometer method(CAEM) for 2nd rotor earth fault.

ROTOR EARTH FAULT PROTECTION (64F)

REVERSE POWER PROTECTION (32G)WHY

• In the event of prime mover failure, a generator connected in parallel with a power system - will begin to run as ‘Motor’ . The active power is drawn from the power system to cover alternator & failed prime mover mechanical losses. HOW

• A time delay provided to reverse power protection tripping - to prevent false tripping during some system fault conditions & power system swings.

• A typical setting for reverse power protection - with 0.2% to 0.5% of the rated power of the generator.

• A time delay provided for operation without turbine trip.

LOW FORWARD POWER PROTECTION(37G)WHY

• To avoid over speed damage to Large turbo alternators, with slender, low inertia rotor designs like that of steam turbines - do not have high over speed tolerance , non urgent tripping of the generator breaker & the excitation system can be interlocked with a low forward power function.HOW

• Measurement of the low power - done similar to that of reverse power function

• A typical under power setting : 0.5% of rated power.• A time delay provided for operation without turbine trip.• In gas turbine driven generators - Low forward power protection not required.

NEGATIVE PHASE SEQUENCE PROTECTION(46G)WHY

• Negative sequence currents create an mmf wave in opposite direction to the direction of rotation of rotor. This cuts the rotor at twice the rotational speed, and induces a 100 Hz eddy current flows in the outside skin of the rotor body, on the wedges & in the top winding conductors and cause heating which can cause severe over heating and ultimately, the melting of the wedges in the air gap. HOW

• Negative sequence current is measured by the relay. An inverse-time overcurrent relay excited by negative sequence current can be used for this protection. The machine designer establishes constant k. It can be in the range of 5 – 50.

• 8% continuous negative sequence current can tolerated.

UNBALANCED LOAD-TIME CURVE

FIELD FAILURE PROTECTION (40G)WHY

• When the excitation of a generator fails - its internal e.m.f. will decay. This results in fall of active power output (accelerate to super synchronous speed) & increasing level of reactive power being drawn from the system.

• In the lagging power factor-operating region, limits are determined either by rotor field heating limit or by stator armature heating limit. During the leading power factor-operating region, it is the iron end region-heating limit due to eddy currents that is detrimental to the machine. Turbo-alternators may not have adequate reactive power absorption capability. Hence, they are seldom operated with leading power factor.

HOW• This protection function - measures the impedance at the terminals of a

generator to detect failure of the generator’s excitation. • During loss of excitation - the terminal impedance of the generator undergoes

a transition from the first quadrant to the fourth quadrant.• Offset Mho relay Xa= 0.5 Xd’ , Xb= Xd• Time delay used. But in case of U/V no time delay.

X

R-Xa

Xb

Load Point

Machine terminalloss of fieldlocus

Field Failure protection function characteristic with typical machine impedance

TRIPNO TRIP

UNDER VOLTAGE PROTECTION (27G)

WHY• Under voltage - can be used to detect abnormal operating conditions, AVR failure

or an un-cleared power system fault by other generator protection.• It can be interlocked with the field failure protection - to prevent its operation

during stable power swings.• Under voltage protection - not a commonly specified requirement for generator

protection.

HOW• Operates when the three phase voltages fall below the common set point. An

adjustable timer is available .• Under voltage threshold ( V< ) setting - set below the steady state phase-phase

voltage seen by the relay for a three-phase fault at the remote end of any feeder connected to the generator bus.

• 70% of normal voltage.

OVER VOLTAGE PROTECTION (59G)

WHY• Over voltage protection - set to prevent possible damage to generator insulation,

prolonged over fluxing of the generating plant or damage to isolated power system loads.

HOW• Over voltage operates when the three phase voltages are above their common

threshold setting.• At 105% voltage , alarm with 2 sec delay• At 140% voltage, Trip without delay

UNDER FREQUENCY PROTECTION (81GUF)

WHY• Under frequency operation of a generator - occurs when the power system

load exceeds the prime mover capability of the generator. • Under frequency running at nominal voltage - will result in over fluxing of the

generator.• Unsafe for turbine

HOW• The under frequency protection function of the relay - utilises the AC voltage

input signals as the frequency measurand. • Two independent time-delayed stages of under frequency protections.• First stage – 48.5 Hz, Alarm with 2.5 Sec delay• Second stage – 47.5 Hz, Trip with 2 sec delay.

OVER FREQUENCY PROTECTION (81GOF)

WHY• Over frequency running of a generating set arises when the mechanical

power in put to the generator is in excess of the electrical load & mechanical losses.

• Over frequency protection - a back up protection function to cater governor or throttle control failure following loss of load & prevent over speeding.

HOW• Over frequency protection function of the relay - utilises the AC input signals

as the frequency measurand.• A single time delayed stage of over frequency protection , with an over

frequency threshold setting ( F> ) and a time delay setting ( t ).• 51.5 Hz, Alarm with 2 sec delay.

VOLTAGE BALANCE FUNCTION (60G)

WHY• Voltage balance function - provided to detect PT fuse failure so that an alarm

can be raised & unwanted generator shut down by the voltage sensitive protection function can be prevented.

HOW• The voltage balance protection function - operates from signal derived from

the relay’s two main PT secondary inputs and signals derived from an additional pair of reference PT secondary inputs.

• The level of voltage difference is determined between each of the two main & reference voltage inputs.

• When a voltage difference in excess of an adjustable threshold ( Vs ) is detected - an alarm is raised.

GENERATOR BACKUP IMPEDANCE PROTECTION(21G)

WHY• Back-up protection must be applied at the generator so that faults are cleared

in the event of downstream protection/circuit breakers failing to operate. Also current will come down with time.HOW

• Under impedance protection. This element is set to monitor the system impedance at the terminals of the machine. If the impedance measured falls below a set threshold then the element will operate.

• System back-up protection must operate quickly during a fault and must not operate for load conditions.

GENERATOR DEAD MACHINE PROTECTION(50GDM)

WHY• To provide fast protection for accidental energization of a generator when the

machine is not running condition.

HOW• Instantaneous overcurrent element that is gated with a three-phase

undervoltage detector and is blocked by the VT supervision element.

GENERATOR OUT OF STEP PROTECTION(78G)

WHY• A generator might pole slip, or fall out-of-step with other power system

sources, in the even of failed or abnormally weak excitation or as a result of delayed system fault clearance.HOW

• To detect this condition, distance relay looking into the generator (or into the transformer-generator unit) should be installed. Even a distance relay used for loss-of-field protection will pick-up on such power swing.

• If the swing moves out of the relay characteristic, before the timer runs down, then, no trip action will be initiated. However, if the swing persists for sufficient time, the loss-of-excitation distance relay will operate on power swing.

GENERATOR DEFINITE TIME OVERLOAD PROTECTION(51G)

WHY• To protect the generator from going out of the capability and safe operation

limit.HOW

• Thermal modeling of the generator as per the given data and recommendation of supplier.

GENERATOR & GENERATOR TRANSFORMER OVERFLUXING PROTECTION PROTECTION (99GT)

WHY• High voltage or low frequency, causing a rise in the V/Hz ratio, will produce high

flux densities in the magnetic core of the machine or transformer. This could cause the core of the generator or transformer to saturate and stray flux to be induced in un-laminated components that have not been designed to carry flux. The resulting eddy currents in solid components (e.g. core bolts & clamps) and end of core laminations can cause rapid overheating and damage.HOW

• V/f element of the relay set as per the overfluxing withstand capability of the generator & generator transformer.

• Time delayed Alarm is used to take action• Inverse characteristics is used for trip

GENERATOR STATOR FRAME OVERTEMPERATURE PROTECTION (CO2 SYSTEM)

WHY• To provide protection against fire/ hot spots inside generator enclosure.

HOW• 80 deg C & 100 deg C fire detectors installed at equal distance along the

periphery of generator frame /enclosure (Turbine end & Exciter End).• Logic formed to avoid mal-operation of detector for release CO2 in the

generator enclosure and class A tripping.• CO2 is also released in case of actuation of generator differential protection.

Fire Detector arrangement inside generator enclosure(Turbine side)

Fire Detector arrangement inside generator enclosure (Exciter side)

GENERATOR CLASS-A PROTECTION TABLE

OR logic for class

A trip

GEN WINDING DIFFERENTIAL 87G

GEN TR DIFFERENTIAL 87GT

STATOR EARTH FAULT 64G

GEN TR REF 64RGT

OVERALL DIFFERENTIAL 870A

GEN FRAME TEMPERATURE 100 DEG CELSIUS

ROTOR E/F 64F

Tripping of both generator breaker and turbine occurs for any of the following conditions

EMERGENCY PUSH BUTTON

THANK YOU