generator protection by bhushan kumbhalkar

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 SelectivitySpeed Simplicity Economics

PROTECTION ZONES 5

GE Consumer & Industrial

Multilin

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 zone

Bus zone

Transformer zone Transformer zone

Bus zone

Generator

~

XFMR Bus Line Bus XFMR Bus Motor

Motor zone

MAIN EQUIPMENT FOR SWITCHGEAR OPERATION

Current transformer Potential transformer Relay Circuit breaker

7GE Consum

er & IndustrialM

ultilinVVPP

VVSS

Relay

• Voltage (potential) transformers are used to isolate and step down and accurately reproduce the scaled voltage for the protective device or relay

• VT ratios are typically expressed as primary to secondary; 14400:120, 7200:120

• A 4160:120 VT has a “VTR” of 34.66

Voltage Transformers

8GE Consum

er & IndustrialM

ultilin

• Current transformers are used to step primary system currents to values usable by relays, meters, SCADA, transducers, etc.

• CT ratios are expressed as primary to secondary; 2000:5, 1200:5, 600:5, 300:5

• A 2000:5 CT has a “CTR” of 400

Current Transformers

Alarm Act at Abnormal condition. Disconnect. Fast operation. Use system supply.

Simple electromechanical relay

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

•It like a fuse•It is a switch•Interrupt the faulty part•Operation

1.Voltage class2.Current rating3.Type of circuit breaker

1.Air breaker circuit breaker2.Miniature circuit breaker3.Air blast circuit breaker4.SF6 circuit breaker5.Low oil circuit breaker 6.Vaccum circuit breaker

SF6 CIRCUIT BREAKER

PLANT LAYOUT

GEN

UAT

AVR

220 kv bus

220 kv HVCB

6.6 KV CB

NGT

10.5 KV

220 KV

GT

EXT TR

SER TR

415 V AC

LA

Single line Diagram of generator connection

GENERATOR THEORY GENERAL OVERVIEW AND TYPICAL SYSTEM

500 MW TG ON TEST BED

NATURE OF FAULTS IN GENERATOR

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.

Fault Occur In Generator • 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

PROTECTION APPLIED TO GENERATOR Relays to detect faults outside generator Relays to detect faults in side generator Over speed protections. Temp measuring device for bearings, stator

winding, Oil temp.

EQUIPMENT GROUNDING

Prevents shock exposure of personnel Provides current carrying capability for the ground-fault

current Grounding includes design and construction of

substation ground mat and CT and VT safety grounding

SYSTEM GROUNDING

Limits overvoltages Limits difference in electric potential through local area

conducting objects Several methods

Ungrounded Reactance Coil Grounded High Z Grounded Low Z Grounded Solidly Grounded

SYSTEM GROUNDING25

GE Consumer & Industrial

Multilin

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

SYSTEM GROUNDING26

GE Consumer & Industrial

Multilin

3. High Resistance Grounded: Limits ground fault current to 10A-20A. Used to limit transient overvoltages due to arcing ground faults. R0 <= X0C/3, X0C is capacitive

zero sequence reactance4. Low Resistance Grounded: To

limit current to 25-400AR0 >= 2X0

SYSTEM GROUNDING27

GE Consumer & Industrial

Multilin

5. Solidly Grounded: There is a connection of transformer or generator neutral directly to station ground.Effectively Grounded: R0 <= X1,

X0 <= 3X1, where R is the system fault resistance

generator

NGT NGR RELAY

GENERATOR EARTHING

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

lesscommon ,these usually develop into an earth

faults.This causes-• Arcing to core• Damage of conductor and insulation

INTER-TURN FAULT PROTECTION

Stator inter-turn fault protection:

• 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.

mmmmmm

mmmmmm

mmmmmm

mm

mm

mm

mm

mm

mm

Loading resistor Over voltage relay

With time delay

STATOR EARTHFAULT RELAY

exciter

P.B

Field wdg

Voltage relay

ROTOR E/F RELAY

Rotor earth fault 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.

Rotor earth fault protection

AC Injection method

GENERATOR PROTECTION

1 ST ROTOR E/F PROTECTION (64R1)

D.C. INJECTION METHOD.

Rotor temperature alarm

• It is provided in large generators.

• It indicates the level of temperature but not the actual hot spot temperature.

• The relay measures the temperature by measuring the resistance .(as shown in fig)

GENERATOR PROTECTION

• Abnormal Operating Conditions

The "Wild"PowerSystem

G

Exciter

Loss of FieldLoss of Field

Overexcitation

Overexcitation

Overexcitation

OpenCircuits

Loss ofSynchronism

InadvertentEnergizing,Pole Flashover

AbnormalFrequency

AbnormalFrequency

BreakerFailure

ReversePower

OverPower

Loss of excitation protection:When the excitation of generator is lost it operate as aInduction generator. It derives excitation from thesystem and supply power at leading power factor. Which may cause- A fall in voltage & so loss of synchronism & system

instability. Over heating of rotor due to induction current on it.A protection having MHO characteristicis used to detect loss of field.

Differential protection of generator:

Differential protection using balancing resistor:

Modified differential protection

Modified differential protection:

• Generally protection is made for 80 to 85% of the winding.

• If any fault occurs near the neutral point then the fault current is very small and relay does not operate.

• Modified differential protection scheme is used to over come this.

• Two phase elements (PC and PA) and balancing resistor(BR) is connected in star and the earth relay(ER) is connected between the star point and neutral pilot wire.

External fault back-up protection

External fault back up protection:

• Over-current and earth-fault protection is provided for back-up protection of large sized generators protected by differential protection.

• Induction type IDMT relay is used for this purpose.

STEAM VALVE

C.B TRIP

Protective relay

Reverse power relay

Reverse power relay scheme

REVERSE POWER PROTECTION

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%.

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

Negative phase sequenceprotection:

• Unbalance may cause due to single phase fault or unbalanced loading and it gives rise to negative sequence current .

• This current in rotor causes rotor overheating and damage to the rotor.

• This can be protected by negative sequence current filter with over current relay.

Negative phase sequence protection:

Exciter

FUSE T1

T2

FUSE

TRIP

SHUNT

FILED WDG

Field failure protection

FIELD FAILURE PROTECTION

Loss of generator field excitation under normal running conditions may arise due to any of the following condition.1. Failure of brush gear.2. unintentional opening of the field circuit breaker.3. Failure of AVR.

When generator on load loses it’s excitation , it starts to operate as an induction generator, running above synchronous speed. cylindrical rotor generators are not suited to such operation , because they do not have damper windings able to carry the induced currents, consequently this type of rotor will overheat rather quickly.

Over voltage protection:

Overvoltage 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 IDMTCharacteristic

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.

GENERATOR PROTECTIONName Input Protecting to

Differential protection

Differential Current Stator core and winding

Stator earth fault Voltage Stator core and windingOver current Current Stator core and windingOver voltage Voltage Stator core and winding

Interturn short circuit Current Stator core and windingRotor Earth Fault Current Rotor windingOver and under

frequencyFrequency Turbine protection

Reverse power flow Voltage and current

Turbine protection

Loss of excitation Voltage and current

Power System Protection

Back up protection for lines

Voltage and current

Generator protection