Generator Protection - Scribd

GENERATOR PROTECTION

S.M.Abbas

2GE Consumer & Industrial

Multilin


Multilin


Multilin


Multilin


Multilin


Multilin


Multilin


Multilin


Multilin


Multilin


Multilin


Multilin


Multilin


Multilin

• Three phase faults have the highest fault current.

• Single phase faults have the lowest fault current.

• The fault current is determined by the impedance of the fault path.

• Fault paths closer to the source will have less impedance.

• Faults caused by trees will have higher impedance.

Fault Data

Desirable Protection Attributes

• Reliability: System operate properly– Security: Don’t trip when you shouldn’t– Dependability: Trip when you should

• Selectivity: Trip the minimal amount to clear the fault or abnormal operating condition

• Speed: Usually the faster the better in terms of minimizing equipment damage and maintaining system integrity

• Simplicity: Less components simple wiring• Economics: Don’t break the bank


Multilin

Selection of protective relays requires compromises:• Maximum and Reliable protection at minimum

equipment cost• High Sensitivity to faults and insensitivity to

maximum load currents• High-speed fault clearance with correct selectivity• Selectivity in isolating small faulty area• Ability to operate correctly under all predictable

power system conditions • Primary objectives is to have faulted zone’s

primary protection operate first, but if there are protective relays failures, some form of backup protection is provided.

Art & Science of Protection


Multilin

Primary Equipment & Components

• Transformers - to step up or step down voltage level

• Breakers - to energize equipment and interrupt fault current to isolate faulted equipment

• Insulators - to insulate equipment from ground and other phases

• Isolators (switches) - to create a visible and permanent isolation of primary equipment for maintenance purposes and route power flow over certain buses.

• Bus - to allow multiple connections (feeders) to the same source of power (transformer).


Multilin

Primary Equipment & Components• Grounding - to operate and maintain equipment safely

• Arrester - to protect primary equipment of sudden overvoltage (lightning strike).

• Switchgear – integrated components to switch, protect, meter and control power flow

• Reactors - to limit fault current (series) or compensate for charge current (shunt)

• VT and CT - to measure primary current and voltage and supply scaled down values to P&C, metering, SCADA, etc.

• Regulators - voltage, current, VAR, phase angle, etc.

Types of ProtectionOvercurrent

• Uses current to determine magnitude of fault– Simple– May employ definite time or inverse time

curves– May be slow– Selectivity at the cost of speed (coordination

stacks)– Inexpensive– May use various polarizing voltages or ground

current for directionality


Multilin

Instantaneous Overcurrent Protection (IOC) & Definite Time Overcurrent

(DTOC)

t

I

CTI

50+2

50+2

CTI • Relay closest to fault operates first

• Relays closer to source operate slower

• Time between operating for same current is called CTI (Clearing Time Interval)

Distribution Substation

(TOC) Coordination

t

I

CTI

• Relay closest to fault operates first

• Relays closer to source operate slower

• Time between operating for same current is called CTI

Distribution Substation

• Selection of the curves uses what is termed as a “ time multiplier” or “time dial” to effectively shift the curve up or down on the time axis

• Operate region lies above selected curve, while no-operate region lies below it

• Inverse curves can approximate fuse curve shapes

Time Overcurrent Protection (TOC)

Types of ProtectionDifferential

– current in = current out– Simple– Very fast– Very defined clearing area– Expensive– Practical distance limitations

• Line differential systems overcome this using digital communications

Differential

• Note CT polarity dots

• This is a through-current representation

• Perfect waveforms, no saturation

I P

I S

I R-X

I P

I S

I R-Y

Relay

CT-X CT-Y

1 + (-1) = 0

+1

-1

0

Cur

rent

, pu

DIFF CURRENT

1 pu

Differential

• Note CT polarity dots

• This is an internal fault representation

• Perfect waveforms, no saturation

FaultI P

I S

I R-X

I P

I S

I R-Y

Relay

2 + (+2) = 4

+2

-2

0

Cur

rent

, pu

X

2 pu 2 pu

CT-X CT-Y

DIFF CURRENT

Types of ProtectionVoltage• Uses voltage to infer fault or abnormal

condition• May employ definite time or inverse time

curves• May also be used for undervoltage load

shedding– Simple– May be slow– Selectivity at the cost of speed (coordination

stacks)– Inexpensive

Types of ProtectionFrequency• Uses frequency of voltage to detect

power balance condition• May employ definite time or inverse time

curves• Used for load shedding & machinery

under/overspeed protection– Simple– May be slow– Selectivity at the cost of speed can be

expensive

Types of ProtectionPower• Uses voltage and current to

determine power flow magnitude and direction

• Typically definite time– Complex– May be slow– Accuracy important for many

applications – Can be expensive

Types of ProtectionDistance (Impedance)– Uses voltage and current to determine

impedance of fault– Set on impedance [R-X] plane– Uses definite time – Impedance related to distance from relay– Complicated– Fast– Somewhat defined clearing area with

reasonable accuracy– Expensive– Communication aided schemes make more

selective

1. Generator or Generator-Transformer Units2. Transformers3. Buses4. Lines (transmission and distribution)5. Utilization equipment (motors, static loads, etc.)6. Capacitor or reactor (when separately protected)

Unit Generator-Tx zoneBus zone

Line zoneBus zone

Transformer zone Transformer zone

Bus zone

Generator

~

XFMR Bus Line Bus XFMR Bus Motor

Motor zone

Protection Zones

1. One-line diagram of the system or area involved2. Impedances and connections of power equipment,

system frequency, voltage level and phase sequence3. Existing schemes 4. Operating procedures and practices affecting

protection5. Importance of protection required and maximum

allowed clearance times6. System fault studies7. Maximum load and system swing limits8. CTs and VTs locations, connections and ratios9. Future expansion expectance10.Any special considerations for application.

What Info is Required to Apply Protection

Abnormal Operating Conditions

• Latest developments reflected in:– Std. 242: Buff Book– C37.102: IEEE Guide for Generator Protection– C37.101: IEEE Guide for AC Generator Ground

Protection– C37.106: IEEE Guide for Abnormal Frequency

Protection for Power Generating Plants

These are created/maintained by the IEEE PSRC & IASThey are updated every 5 years

ANSI / IEEE Standards Latest developments reflected in:

– Std. 242: Buff Book– C37.102: IEEE Guide for Generator

Protection– C37.101: IEEE Guide for AC Generator

Ground Protection– C37.106: IEEE Guide for Abnormal

Frequency Protection for Power Generating Plants

Small Machine Protection IEEE Buff Book

• Small – up to 1 MW to 600V, 500 kVA if >600V

32 Reverse Power 40 Loss of Excitation

51V voltage restraint 51G Ground O/C

87 Differential


• Medium – up to 12.5 MW

32 Reverse Power 40 Loss of Excitation 46 Negative Sequence51V voltage restraint

51G Ground O/C87 Differential


• Large – up to 50 MW

32 Reverse Power 40 Loss of Excitation 46 Negative Sequence49 Thermal Overload51V voltage restraint

51G Ground overcurrent64 Ground Relay

87 Differential

• Unit Connected, High Z Grounded

Large Machine ProtectionIEEE C37.102

32 Reverse Power 40 Loss of Excitation 46 Negative Sequence49 Thermal Overload51V voltage restraint

51G Ground overcurrent64 Ground Relay

87 Differential

Under & Over Voltage Protection

• Protects against a severe overload condition (27)

• Initiates the starting of an emergency standby genset (27)

• Load shed shut down in the event of AVR failure (27)

• Protect against dangerous over-voltages (59)

• Backup to internal V/Hz limiters

• Commonly combined 27/59

Devices27 / 59

Reverse Power Protection• Provides backup

protection for the prime mover.

• It detects reverse power flow (kW) should the prime mover lose it’s input energy without tripping its generator feeder breaker

• Prevents motoring, drawing real power from the system

Device32

Loss of Field Protection

Device40

• Loss of excitation can occur:

• Loss of field to the main exciter.• Accidental tripping of the field breaker.

• Short circuits in the field circuits.• Poor brush contact in the exciter.• Field circuit-breaker latch failure.

• Loss of ac supply to the excitation system.

• Reduced-frequency operation when the regulator is out of service.

Phase Balance Current Protection

• Unbalanced loads• Unbalanced system

faults• Open conductors• Unbalanced I2

currents induce 2X system frequency currents in the rotor causing overheating

Device46

Backup Overcurrent Protection

• The function of generator backup protection is to disconnect the generator if a system has not been cleared by the primary protective device

• Time delays

Device51V/21

Ground Overcurrent Protection

• Provides backup protection for all ground relays in the system at the generator voltage level

• Provides protection against internal generator ground faults

• Commonly provided as GF alarm.

Device51G

Voltage Balance Relay

• Monitors the availability of PT voltage.• Blocks improper operation of protective

relays and control devices in the event of a blown PT fuse

Device60

Device78

Out of Step Protection

•High peak currents and off-frequency operation can occur when a generator losses

synchronism.

•Causes winding stress, high rotor iron currents, pulsating torques and mechanical

resonances.

•Conventional relaying approach – analyzing variations in apparent impedance as viewed at

generator terminals.

•Variation in impedance can be detected by impedance relaying and generator separated

before the completion of one slip cycle

Differential Protection• For rapid detection

of generator Φ to Φ or Φ-G faults.

• When NGR’s are used, 87G should be used.

• Used for protection of larger generators

• Zone protection

Device87

Phase Fault Protection (87G)

Differential Protection (87)• A key point to remember is that differential

relays don’t prevent damage, they LIMIT damage.

• If a relay is properly operating it won’t trip until there is actually a line to ground fault somewhere in its zone of protection.

• By limiting the duration of a fault, it is often possible to limit damage, but there is STILL damage.

• Eventually, you will have to deal with it.

REF Protection (87GN / 64GN)

Significant load additionSudden reduction in mechanical

input powerLoss of generation / Loss of load

Underfrequency can cause:Higher generator load currents

OverexcitationTurbine blade fatigueDevice

81

Frequency Protection

Temperature Protection• Resistance

temperature detectors are used to sense winding temperatures.

• A long term monitoring philosophy that is not readily detected by other protective devices

RTD’s

FFBL GENERATOR PROTECTION LAYOUT

Digital Generator Protection System (DGP)

• Microprocessor Based Protection, Control and Monitoring System

• Waveform Sampling• User Friendly• GE/Hydro Quebec Joint

Development

Tripping MethodsFactors of selection includes severity of fault, probability of Fault spreading & overspeeding, time required to resynchronize, effect on power system etc.

• SIMULTANEOUS TRIP

• GENERATOR TRIP

• BREAKER TRIP

• SEQUENTIAL TRIP

• MANUAL TRIP

• MANUAL RUNBACK & TRIP

• AUTOMATIC RUNBACK

• MANUAL RUNBACK

Generator Faults (GE) • STATOR OVERCURRENT• STATOR GROUND FAULT• STATOR PHASE TO PHASE

FAULT• OVER VOLTAGE• VOLT PER HERTZ• FIELD OVEREXCITATION• FIELD GROUND• LOSS OF EXCITATION• UNBALANCED ARMATURE

CURRENT• STATOR

OVERTEMPERATURE

• LOSS OF SYNCHRONISM• ABNORMAL FREQUENCY

OPERATION• BREAKER FAILURE• HIGH SPEED RECLOSING• SUBSYNCHRONOUS

RESONANCE• INADVERTENT

ENERGIZATION• SYSTEM BACK UP• VOLTAGE SURGES• BEARING VIBRATION• SYNCHRONIZING ERRORS• MOTORING

Digital Generator Protection System (DGP)

The DGP is a digital system which provides a wide range of protection, monitoring, control and recording functions for AC generators.

•It can be used on generators driven by steam, gas and hydraulic turbine.

•Any size of generator can be protected with the DGP.

• A high degree of dependability and security is achieved by extensive self diagnostic routines and an optional redundant power supply.

DGP Digital Generator Protection

Generator Trip Scheme • BREAKER TRIP

• 46 UNBALANACE• 32 REVERSE POWER• 51 V OVERCURRENT WITH VOLTAGE RESTRAINT• 81 U UNDER FREQUENCY

• TURBINE TRIP• 87 G DIFFERENTIAL• 40 LOSS OF EXCITATION• 24 OVER EXCITATION• 59 OVERVOLTAGE• 51 GN GROUND OVERCURRENT

• ALARM ONLY• 27 UNDER VOLTAGE• 81 O OVER FREQUENCY

• EXT VTFF• BLK # 9 (81, 32, 27, VTFF)

Applications• For Small, Medium and

Large Generator Protection

• Suitable for Variety of Prime-Movers - Gas, Steam, Hydro Turbines

• Most Commonly Used Protection Functions Packaged in a Standard Modular Case

THE DGP SYSTEM TAKES EIGHT CURRENT AND FOUR VOLTAGE SENSING INPUTS.

TYPICAL INPUT WIRING DIAGRAM OF DGP

The input currents in terminals BH1, BH3, and BH5 (IAS, IBS, and ICS) are used to process functions 46, 40, 32, and 51V.

These currents can be derived from system side or neutral side CTs as desired. Either the system or neutral side CTs can be used for these functions if the

Stator Differential (87G) function is enabled.

Current inputs INS and INR are derived from the residual connections of the respective phase CTs.

The current inputs INS and INR are derived from the residual connections of the respective phase CTs and do not require dedicated neutral CTs.

Zero-sequence current at system and/or neutral side of the generator statorwindings is calculated and then compared with the measured INS and/or INR

values by the DGP as a part of the background self-test.

The INR current is used to process the 51GN function DGP .If desired, a dedicated neutral CT can be used for the input INR.

The DGP phase voltage inputs can be wye or delta and are derived from the generator terminal voltage. VN is derived from the generator neutral grounding

transformer.

THE DGP SYSTEM INPUTS

DGP Monitoring

Present Values

• Currents• Voltages• Watts• Vars• Frequency• Negative Sequence

Current• 3rd Harmonic Voltage• Status of Digital Inputs

GEN Simulator DGP 0000PRESENT VALUES

Station ID:MALVERN

Generator ID:MODEL GENERATOR

10/28/93 14:37:23:446

IAS: 5696.0 A -014 DEGS VAN: 008.5 KV 000 DEGSIBS: 5488.0 A -142 DEGS VBN: 008.1 KV -118 DEGSICS: 4864.0 A 104 DEGS VCN: 008.2 KV 122 DEGS

IAR: 5680.0 A -014 DEGSIBR: 5456.0 A -142 DEGSICR: 4880.0 A 104 DEGSNEGATIVE SEQ CURRENT: 08.1 %

3RD HARM PH: 00.1 % 3RD HARM N: 03.7%

WATTS: +126.33 MWATT VARS: +041.95 MVAR

GEN OFF-LIN: OPEN INLET VLV: OPENDIG IN 3: OPEN DIG IN 4: OPENOSC TRIG: OPEN EXT VTFF: OPEN

FREQ: 60.00 SAMPLING FREQ: 720.0

Fault Report

Prefault– Currents– Voltages– Watts– Vars

– Frequency Post Fault

– Currents– Voltages

– Trip Targets– Operating Time

Selected Events Last 3 Faults Stored

Gen Simulator DGP 0000FAULT REPORT

Station ID:MALVERNGenerator ID:MODEL GENERATOR

FAULT#: 02FAULT DATE: 08/09/93 TRIP TIME: 05:10:37:829

FAULT TYPE: ABCTRIP TYPE: 87G SYSTEM OPERATING TIME: 000008

PREFAULT FAULT-------------------------------------- ------------------------------------------------------

IAS: 0128.0 A IAS: 014672 A IAR: 015664 AIBS: 0208.0 A IBS: 015264 A IBR: 016704 AICS: 0080.0 A ICS: 013600 A ICR: 014960 A

INS: 0048.0 A INR: 0384.0 AVAN: 010.2 KV

VBN: 010.2 KV VAN: 693.0 VVCN: 010.0 KV VBN: 693.0 V

VCN: 679.4 VFREQ: 60.00 VN: 047.0 V

WATTS: +1888.5 KWATTVARS: +3777.0 KVAR

05:10:37.834 87G PHASE A ON05:10:37.834 87G PHASE B ON

TYPICAL WIRING DIAGRAM OF DGP

END

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Generator Protection - Scribd