GENERATOR PROTECTION

Prepared ByPriten Vasa

Introduction:In a generating station the generator and transformer are the most

expensive equipments and hence it is desirable to employ a protective system to isolate the faulty equipment as quickly as possible to keep the healthy section in normal operation and to ensure uninterruptable power supply.

The basic electrical quantities those are likely to change during abnormal fault conditions are current, voltage, phase angle and frequency . Protective relays utilizes one or more of these quantities to detect abnormal conditions in a power system.

Protective system cost is 4-5%of the total cost

Switchgear • Switchgear is a general term covering a wide

range of equipments concerned with switching and protection.

• Eg: Circuit breaker, Isolator, Earth switch etc.

•Desirable Protection Attributes• Reliability• Selectivity• Speed • Simplicity

RELAYS & CIRCUIT BREAKERS

• TYPES OF RELAYS1. Reed relay2. Latching relay3. Solid state relay4. Solid state contact relay5. Ratchet relay6. Coaxial relay7. Overload protection relay8. Forced guided contact relay9. Buchholz relay

• CIRCUIT BREAKERS TYPES BASED ON VOLTAGE CLASS AND CURRENT RATING1.Air breaker circuit breaker2.Miniature circuit breaker3.Air blast circuit breaker4.SF6 circuit breaker5.Low oil circuit breaker 6.Vaccum circuit breaker

Advantages Of Digital Relay

MultifunctionalCompatibility withdigital integrated

systems

Low maintenance(self-supervision)

Highly sensitive,secure, and

selectiveAdaptive

Highly reliable(self-supervision)

Reduced burden on

CTs and VTs

ProgrammableVersatile

Low Cost

GEN

UAT

AVR

220 kv HVCB

6.6 KV CB

NGT

10.5 KV

220 KV

GT

EXT TR

SER TR

440 V AC

LA

Single line Diagram of generator connection

Protection Zones Protection Zones

7GE Consumer & Industrial

Multilin

1. Generator or Generator-Transformer Units

2. Transformers

3. Buses

4. Lines (transmission and distribution)

5. Utilization equipment (motors, static loads, etc.)

6. Capacitor or reactor (when separately protected)

Unit Generator-Tx zone

Bus zone

Line zone

Bus zone

Transformer zoneTransformer zone

Bus zone

Generator

~

XFMR Bus Line Bus XFMR Bus Motor

Motor zone

SCHEME OF GENERATOR PROTECTION• CLASS A TRIPPING This is adopted for those electrical faults of Generator and Generator transformer unit

auxiliary transformer for which tripping can not be delayed.·         This leads to simultaneous tripping of      -  Generator Transformer HV side CB      -  Field Circuit Breaker      -  LV side incomer breakers of UAT      -  Auto changeover from unit to station for unit auxiliaries and tripping of turbine

• CLASS B TRIPPING                             This is adopted for all turbine faults (Mechanical) and for some Electrical faults of Generator,

Generator transformer and unit Auxiliary transformer for which it is safe to trip the turbine. ·Subsequently the Generator is tripped through low forward powerInterlock Ensures that unit does not over speed due to trapped steam in the Turbine during the

shutdown and also the loss of power to the grid From the generator is not sudden.   

CLASS C TRIPPING

• This is adopted for all faults beyond the Generator system which can be cleared by tripping of Generator transformer HV side CB alone.

• In this case the TG set runs with HP-LP bypass system in operation and the Generator continues to feed the unit auxiliary load through unit auxiliary transformers.

The "Wild"PowerSystem

G

Exciter

Loss of FieldLoss of Field

Overexcitation

Overexcitation

Overexcitation

OpenCircuits

Loss ofSynchronism

InadvertentEnergizing,Pole Flashover

AbnormalFrequency

AbnormalFrequency

BreakerFailure

ReversePower

OverPower

Abnormal Operating Conditions

FAULT OCURRENCE & FAULT CLASSIFICATION

• Insulation failure.

• Tends to deteriate with rising temp.

• Insulation failure may cause inter-turn fault, ph to ph or earth fault.

• Bring winding in to direct contact with core plates.

• Any failure to restrict earth fault may result into core plate damage.

• Insulation of rotor winding is also important

• Stator Fault• Rotor fault• Abnormal Running Condition

1) Unbalanced Loading2) Over loading3) Over Speed4) Over Voltage5) Failure of Primer Mover6) Loss Of Excitation7) Excessive vibration8) Difference in expansion between

rotating and stationary parts9) Loss of synchronism

• Limits overvoltages• Limits difference in electric potential through local area

conducting objects• Several methods

i. Ungroundedii. Reactance Coil Grounded iii. High Z Groundediv. Low Z Groundedv. Solidly Grounded

System Grounding

1. Ungrounded: There is no intentional ground applied to the system-however it’s grounded through natural capacitance. Found in 2.4-15kV systems.

2.Reactance Grounded: Total system capacitance is cancelled by equal inductance. This decreases the current at the fault and limits voltage across the arc at the fault to decrease damage.

X0 <= 10 * X1

4.High Resistance Grounded: Limits ground fault current to 10A-20A. Used to limit transient overvoltages due to arcing ground faults.

5.Low Resistance Grounded: To limit current to

25-400A

3.Solidly Grounded: There is a connection of transformer or generator neutral directly to station ground

Protection Relay – ANSI Standards1.Current protection functions ANSI 50/51 – Phase overcurrent ANSI 50N/51N or 50G/51G – Earth fault or sensitive earth fault ANSI 50BF – Breaker failure ANSI 46 -Negative sequence / unbalance ANSI 49RMS – Thermal overload

2.Directional power protection functions• ANSI 32P – Directional active overpower• ANSI 32Q/40 – Directional reactive overpower

3.Voltage protection functions ANSI 79 – Reclose the circuit breaker after tripping

4.Machine protection functions ANSI 37 – Phase undercurrent ANSI 48/51LR/14 – Locked rotor / excessive starting time ANSI 66 – Starts per hour ANSI 50V/51V – Voltage-restrained overcurrent ANSI 26/63 – Thermostat, Buchholz, gas, pressure, temperature detection ANSI 38/49T – Temperature monitoring by RTD

Stator protectionStator faults include the following-i. Phase-to-earth faultsii. Phase-to-phase faultsiii. Inter-turn faultsFrom these phase faults and inter turn

faults are less common ,these usually develop into an earth faults.

This causes-I. Arcing to coreII. Damage of conductor and insulation

Generator Stator Ground Fault Protection

87N, 51N, 59N & 27-3N

Generator Stator Phase Fault Protection

87G

•Inter-turn fault on the same phase of the stator winding cannot be detected by transverse differential protection as it does not disturb the balance between the currents in neutral and high voltage CTs.•For protection against inter-turn faults the following protection schemes are used. (1)Cross differential protection. (2)Residual voltage protection.

Stator inter-turn fault protection

Stator Earth Fault Relay

Typical Example Stator Earth Fault

Rotor Earth Fault E/F Protection

•DC injection method or AC injection method.•The dc or ac voltage is impressed between the field circuit and ground through a sensitive overvoltage relay and current limiting resistor or capacitor(in case of ac).•But dc source is generally used as over-current relay in case of dc is more sensitive than ac.

•A single earth fault in rotor circuit will complete the path and the fault is sensed by the relay.

LOSS OF EXCITATION

CAUSES• Field open circuit• Field short circuit• AVR control Failure• Accidental tripping of field Breaker• Loss of supply to main Exciter• Poor Brush contact in Exciter• Field Current Breaker Latch Failure• Slip ring Flash Over

Consequences• With mechanical power remaining

intact and field excitation is loss it would attempt to remain in synchronization by acting as a INDUCTION GENERATOR

• This phenomenon can be explained by following energy flows

Exciter

FUSE T1

T2

FUSE

TRIP

SHUNT

FILED WDG

Field failure protection

Types of Differential Protection.

Differential Protection protection using Balancing Resistors

Biased Differential Protection

Over Voltage ProtectionOvervoltage protection is required in case of hydro-electric or gas

turbine generators but not in case of turbo generators.Over voltage may be caused due to- Transient over voltage in the transmission line due to lightening. Defective operation of the voltage regulator. Sudden loss of load due to line tripping.The protection is provided with an over voltage relay.It is usually of induction pattern with an IDMT(Inverse Definite Minimum

Time) Here current is Inversely proportional to timeMore the rise off current or voltage faster the relay will be operated

Overcurrent protection:

• Overloading of the machine causes overheating in the stator winding.

• This can be prevented by using over-current relay with time delay adjustment.

• But overheating not only depends on over-current but also the failure of the cooling system in the generator.

• So temperature detector coils such as thermistors or thermocouples are used at various points in stator winding for indication of the temperature.

Over /Under Frequency Protection 81 O/U

• Causes:Significant load additionSudden reduction in mechanical input

powerLoss of generationLoss of load•Underfrequency can cause:Higher generator load currentsOverexcitationTurbine blade fatigue•Overfrequency can cause: Overvoltage on hydro turbines

Abnormal frequency presents hazards to other parts of the plant such as:

Steam turbine vibrations and increased stresses on blades

Reduced capacity of auxiliary equipment

High temperatures caused by increased excitation current

Overexcitation of transformers

Out of step Protection(78)

• 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

Impedance Variation Detection

m

m

46

mm

Zc ZA

A

BC

Ia

IbIc

VZC

VZA

POSITIVE SEQ

Ia

IbIc

VZC

VZAVZA+VZC

X Y

NEGATIVE SEQUENCE

Negative phase sequence protection

Inadvertent Energization Protection(27, 50, 60, 81U, 62 and 86)

Protects against closing of the generator breaker while machine is not spinning / on turning gear

Caused by operator error, breaker flash-over, control circuit malfunction.

Two schemes illustrated: Frequency supervised overcurrent Voltage supervised overcurrent

Inadvertent Energization ProtectionFrequency Supervised Overcurrent

Frequency Supervised Overcurrent &

• Uses an underfrequency relay (81U) to enable a sensitive instantaneous overcurrent relay (50)

• Overcurrent relay picks up at 50% or less of expected inadvertent energizing current. • Frequency relay contacts must remain closed if sensing voltage goes to zero• Voltage balance relay (60) protects against loss of sensing• Time delay relay (62) protects against sudden application of nominal voltage during inadvertent energization,allowing overcurrent to trip lockout relay (86)• Lockout relay must be manually reset

Same illustration as frequency supervised overcurrent except 81U replaced by 27

• Undervoltage setpoint of 85% of the lowest expected emergency operating level

Voltage Supervised Overcurrent

Loss of Voltage Transformer Protection (60)

Common practice on large systems to use two or more VTs• One used for relays and metering• The other used for AVR• VTs normally fused• Most common cause of failure is fuse failure• Loss of VT protection blocks voltage based protectivefunctions (21, 32, 40 …etc)• Loss of VT protection measure voltage unbalance, typical setting is 15%

When prime-mover fails machine starts motoring and draws electrical power from the system and this is known as inverted operation .

The generator can be protected from inverted operation by using single-element directional power relay(reverse power relay) which senses the direction of power flow.

Failure of the prime mover of a generator set ,will keep the set running as a synchronous compensator, taking the necessary active power from the net work and could be detrimental to to the safety of the set, if maintained for any length of time. The amount of power taken will depend on the type of prime mover involved. It ranges from 5% to 25%.

Reverse Power Protection

STEAM VALVE

C.B TRIP

Protective relay

Reverse power relay

Reverse power relay scheme

Thank You