Generator Protection SystemDavinder kumarAssistant Engineer (EMC)

Topics for DiscussionBasics of Electrical Protection systemSynchronous machine Concept of reactive power and real power sharingDifferent Protections used for generatorAdvanced Protection Relays(Microprocessor based)Case studies (analysis of electrical faults)Synchronization

Different Protections used for GeneratorGenerator Differential Protection.Generator Inter Turn Protection.Generator Negative Sequence Protection.Generator Loss of Excitation Protection.Generator Over Voltage Protection.Frequency ProtectionGenerator Over Load/ Over Current ProtectionGenerator Rotor Earth Fault ProtectionGenerator Back Up Impedance protectionGenerator Low Forward power InterlockGenerator reverse power protection.95% stator Earth Fault Protection

Instrument TransformersCurrent TransformersTurns ratio is inversely proportional to the ratio specified Secondary current is according to primary current onlyShould not be left open circuitedPotential Transformers Turns ratio is directly proportional to the ratio specifiedSecondary current is according to burden connected Should not be short circuited

Flow Diagram of Tripping Process

Protection ClassesClass A ProtectionThis master trip will operate when the fault needs to isolate the generator immediately This gives command to open the generator breaker and field breaker.Class B ProtectionThis master trip will operate when the fault is not serious in view of generator or it is not directly harmful to generatorThis gives the command to trip turinePrime Mover TrippedThis indicates (gives feed back)prime mover is tripped.

Basics of Electrical Protection system

The Protection System Protection ClassesInputs to the protection system.Instrument TransformersContact InputElements of protection system.

Generator Differential ProtectionCTR 10000/5Relay Setting 5%Stabilizing ResistanceHigh impedance Relay Sensitivity to the type of fault

Elements of protection systemCurrent relaysVoltage relaysPower relaysImpedance relaysFrequency relaysSpecial function relaysAuxiliary RelaysTimers

Protection SystemCause of Operation of Protection is the the Fault. It Is Basically Designed, To detect the fault at its earliest possible stage.To give protection to the equipment against secondary damage.To avoid the equipment to run out side the capability specified.

Specifications of GeneratorMake: BHELMVA: 247Connection: Three Phase Double Star Connected.Voltage: 15750 VoltsCurrent: 9050 AmpFrequency: 50 HzSpeed: 3000 RPMNo. of Poles: 2

Synchronous Impedance: 222%Sub Transient Impedance: 30.5%Transient Impedance: 21.4%Cooling: water/ Hydrogen CooledNeutral Grounding: High Impedance (Through NGT)

Generator Inter Turn ProtectionDifference of sharing between split phasesReasons of diff. In currents If there is any inter-turn short with in the wingingIf gen is supplying through fault and small circulating currents gets proportionately increasedTripping provided at 5% of rated current

Generator Negative Sequence Protection.Balanced and unbalanced systemsSequence componentsPositive sequence componentI1 = 1/3 (IR +a2 *IY + a * IB)Negative sequence componentI2 = 1/3 (IR +a *IY + a2 * IB)Zero sequence componentI0 = 1/3 (IR +IY + IB) Effect of negative sequence component on synchronous machine.Generator Capability for I2Continuous I2 capability 5% of ImI22 * t = 8

Balanced Phasers

Generator Loss of Excitation Protection.

Study of Generator capability curveSetting criteriaXa1 = 0.5*Xd*CTR/PTRXd = 30.5% = 0.305 pu sub-transient reactance of generator Xd in ohms = Xd (pu)*kv2/MVA = 0.305*15.752/247 =0.306311993Xa1 = 2.139 ohm Xb1 = Xd * CTR/PTRXd (ohmic) = Xd(pu)*kv2/MVAXb1 = 2.22(15.752/247)(10000/5)(110/15750) = 31.14 ohm

Generator capability curve

Generator Over Voltage Protection.

Generator high voltage is harm full to the insulation of generator, generator transformer, excitation transformer, UAT, etcGenerator Over Voltage Capability 10%Setting AdoptedStage 1 = 10%, 5 Sec delayStage 2 = 15%, 500msec delay

Frequency ProtectionOver frequency protectionIt indicates excess generation than demand hence tripping should graded among the units Under frequency protectionUnder frequency is harmful to turbineAdopted settings

Adopted Frequency settings

UNIT NO. CAPACITY UNDER FREQ. TRIP SETTING OVER FREQUENCY TRIP SETTING

130 MW 47 HZ, 0.5 SEC DELAY54.5 HZ, 21.15 SEC DELAY230 MW 3210 MW 52.5 HZ, 1 SEC DELAY 4210 MW 52.0 HZ, 2 SEC DELAY 5210 MW 52.0 HZ, 2.0 SEC DELAY

Generator Over Load/ Over Current Protection1

% O/CGen CapabilityRelay Char.1.136001.15900652.81.2360362.41.25300252.61.3240192.61.35180155.41.4120129.31.56090

Generator Over Load/ Over Current Protection2

Chart1

900652.8

360362.4

300252.6

240192.6

180155.4

120129.3

6090

Generator Capability

Relay characristic

% over current

time in Sec.

Sheet1

1.13600

1.15900652.810.88

1.2360362.46.04

1.25300252.64.21

1.3240192.63.21

1.35180155.42.59

1.4120129.32.155

1.560901.5

Sheet1

Sheet2

Generator Capability

Relay characristic

% over current

time in Sec.

Sheet3

Rotor E/F

Generator Back Up Impedance protectionImpedance protection to be covered up to 220 KV bus bar that is Z = Z(G.T.) 12.5%Ohmic value of Z (G.T.) = Z(pu)*KV2/MVA = 0.125*15.752/250 = 0.124 ohm Z at C.T.,P.T. secondary level = Z*(CTR/PTR) = 0.124*(10000/5)*(110/15750)= 1.73 ohmFor 3 phase impedance 1.73*sqrt(3) = 2.996 ohm Time setting = 2.5 sec (considering the line under Impedance time setting that is to allow the line protections to clear the fault before tripping of generator)Z

95% stator Earth Fault Protection

Salient Features of the relay P343: Following Protections of generator are covered in the relay1.Generator Differential Protection: - High Impedance / Biased Differential2.Phase fault over current protection: - 3.Neutral voltage displacement protection: -4.100% Stator earth fault protection: - 5.Under/over voltage protection: - Two stage under and Over voltage protection.6.Under/over frequency protection: - Four stage under frequency and two stage over frequency protection.7.Reverse power: - Protection against loss of prime mover.8.Low forward power: - Provides an interlock for non-urgent tripping.9.Field failure: - Two stage element for protection against loss of excitation.1 Negative phase sequence protection: - Provides protection against unbalanced loading, which can cause overheating of the generator rotor.1Over fluxing: - Provides protection for the generator/transformer against unusual voltage or frequency conditions.1 Unintentional energization at standstill (dead machine) protection: - Protection against inadvertent closing of the generator circuit breaker when machine is not running.

Other non-protective features of the relay:

Voltage transformer supervision: - To prevent mal-operation of voltage dependent protection elements upon loss of a VT input signal.Measurements: - Various measurement of value for display on the relay or accessed from the serial communications e.g. Currents, voltages, Power etc.Fault / event / disturbance records: - Available from the serial communication or on the relay display (fault and event records only on relay display.Four setting groups: - Independent setting groups to cater for alternative arrangements or user specific application.Continuous self monitoring: - Power on diagnostics and self checking routine to provide maximum relay reliability and availability.Circuit breaker state monitoring: - Provides indication of discrepancy between circuit breaker auxiliary contacts.

P343

Case studies (analysis of electrical faults)Tripping of unit on Loss of excitation protection.Tripping of unit No 4 on over voltage protection during external disturbance.Local AC failure due to theft attempt in switch yardUnit 4 tripped on stator earth fault.Tripping of unit on Negative sequence protectionTripping of units while supplying external faults

Synchronization ProcedureKeep Excitation on Manual Channel.Make the FF Supply ON.Close the field Breaker and observeFF and FB both breakers ONVoltage start building upAt @ 70% of rated voltage FF breaker gets OFFVoltage stops rising at @ 14 KVRaise the voltage to @15.75KV with the help of Manual Control.Match the Null meter with the help of Auto Pot.Put the excitation on AUTOProceed for Synchronization.

Thank You

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