LESSON – 25

GENERATOR PROTECTION

OUTLINE OF THE LESSON

1. STATOR WINDING PROTECTION

2. OVERLOAD PROTECTION

3. OVER CURRENT PROTECTION

4. OVER VOLTAGE PROTECTION

STATOR WINDING PROTECTION The most satisfactory method of

protecting an alternator stator is the Merz-Price circulating current technique

Both longitudinal and transverse differential; protection systems are used

LONGITUDINAL DIFFERENTIAL

PROTECTION OF DIRECT

CONNECTED GENERATORS

Phase and earth fault protection system

PROTECTION SCHEME FOR

EARTH FAULTS ONLY

FIG

This arrangement is likely to be used

only when the individual phases are not

brought out at the neutral end.

Example: A 6600V, 4000KVA star connected alternator has a reactance of 2 ohms/phase and negligible resistance. It is protected by Merz-Price longitudinal differential protection which operates when out of balance current exceeds 30% of the full load current. If Rn= 7.5 ohms, Determine % of winding which remains unprotected. Show that the effect of the generator reactance can be ignored.

The portion of the stator winding

which remains unprotected following

earth fault depends on earthing

resistance and relay setting

Virtually the whole winding is protected against interphase faults since no limiting impedance is included in the fault circuit

Longitudinal differential protection

System does not detect interturn

faults

EARTH FAULT PROTECTION FOR THE COMPLETE STATOR WINDING

The earth fault protection schemes

(percentage bias differential protection

or neutral overcurrent relay or voltage

relay) protect a certain portion of the

winding leaving a part of winding at the

neutral end unprotected.

For large machines there is a

requirement for detection of earth

fault occurring anywhere in the

stator winding

Two different schemes are available forcomplete protection of the stator winding:

1. Low frequency injection scheme.

1. Third harmonic voltage scheme

LOW FREQUENCY INJECTION SCHEME

In this scheme a sub harmonic voltage is

applied via an injection transformer

connected in series with the neutral earthing

resistance.

A relay which monitors the sub

harmonic current is arranged to

operate when current increases due to

an earth fault on the stator winding.

This scheme provides effective

coverage of the complete stator

winding. However, the cost of the

implementation tends to be high

due to the cost of the injection

equipment.

THIRD HARMONIC VOLTAGE SCHEME

This scheme utilizes the third harmonic

voltage produced by non linearities

within the generator.

Under healthy conditions, this voltage

causes the circulation of third harmonic

capacitive charging currents resulting

in third harmonic voltage appearing

between the neutral of the generator

and ground.

The value of the voltage will depend on

1. The relative values of the impedance

of the earthing devices.

2. The capacitance to earth of the stator

windings, the capacitance to earth of the

busbars, cables and transformer windings

connected to the generator.

When fault occurs close to the

neutral of the generator, the third

harmonic voltage between the

neutral and ground will reduce

to near zero-value.

For high resistance earthed

generators, measurement of this

voltage provides a clear discrimination

between the faults in the neutral

region of the stator winding and

healthy conditions.

Fig given below shows the variation of

a) The third harmonic voltage during fault and

b) The pre-fault third harmonic voltage as

the function of earth fault position.

Fig.

It may be noted that the pre-fault third

harmonic voltage depends on the

power output of the machines.

Fig shows the band over which the

prefault voltage may vary.

The third harmonic voltage developed

by faults at a distance x to y from the

neutral of the generator lies in the

same range as produced by pre-fault

operating condition.

Thus the location of fault anywhere

from x to y represents a blind zone.

The relay operates if the magnitude

of the third harmonic voltage is

a) Less than OA/or

b) more than OB

Fig.

The problem of blind-zone is overcomeby providing two protection systemoperating simultaneously

1) The one system monitors the

fundamental component of the

neutral voltage.

2) Monitors the third harmonic

voltage of neutral

The fig. shows relative operation

zones of complementary stator

earth fault relay elements

Fig.

With the combined protection system,

each relay element covers the blind

zone of the other and the combined

protection system will detect earth

faults anywhere on stator winding

INTERTURN FAULT PROTECTION

OF THE

STATOR WINDING

INTER-TURN PROTECTION BY ZERO SEQUENCE VOLTAGE

MEASUREMENT

Interturn faults in a generator with a

single winding can be detected by

observing the zero-sequence

voltage across the machine

terminals.

Normally, no zero sequence voltage

should exist but a short circuit of

one or more turns on one phase will

cause the generated e.m.f. to contain

such a component

The zero-sequence voltage based

interturn fault protection must

discriminate against

1. External earth fault will also

produce a zero sequence voltage

on a directly connected generator.

b) The zero sequence voltage at

the terminals w.r.t. the neutral of

the generator rather than w.r.t.

earth

a) Most of the voltage will be expended

on the earthing resistor, the drop on

the generator winding being small

and the zero-sequence voltage being

limited to one or two percent

c) This is done by a voltage

transformer connected to the line

terminals, with the neutral point of

the primary windings connected

to the generator neutral, above

the earthing resistor

d)The voltage transformer has a broken

-delta connected secondary winding

that energizes a relay which therefore

receives a quantity proportional to the

zero-sequence component only

1. The third harmonic component of

the e.m.f. is of zero-sequence and is

likely to be of a magnitude

exceeding the required relay setting.

It is therefore necessary to provide a

filter to extract the third harmonic

component from the VT output and

apply it as a relay bias

a) With a direct connected machine it

is still possible that a close-up earth

fault will produce a zero-sequence

voltage drop greater than that produced

by the short-circuiting of one-turn.

It is therefore necessary to apply a

short-time delay to tripping outlet

b) An external earth fault cannot

draw zero-sequence current

through the generator-transformer

unit and hence will produce no

residual voltage from the voltage

transformer. NO TIME DELAY IS

REQUIRED IN THIS CASE

OVERLOAD PROTECTION

Overload in terms of current or MVA as

distinct from megawatts is possible.

It is desirable to provide an overload

relay having a suitable time

characteristic.

For monitoring the stator winding

temperature embedded thermocouples

or resistance thermometer elements

are provided.

The rotor winding temperature is

checked by measuring the resistance

of the field winding.

OVER CURRENT PROTECTION

It is usual to provide overcurrent

relays of the IDMT pattern to

generators, as a general ‘back-up’’

feature. These relays are in no way

related to the thermal characteristics

of the generator and are intended to

operate only under fault conditions.

OVER VOLTAGE PROTECTION

Transient overvoltage

Power frequency overvoltage

TRANSIENT OVERVOLTAGE

Surge overvoltages originate largely

in the transmission system because

of switching and atmospheric

disturbance (lightning)

Surge diverters are provided on the

incoming lines or the station bus

bars

Sometimes surge diverters are

connected also to the generator

terminals.

POWER FREQUENCY

OVERVOLTAGE

Overvoltages should not occur on a

machine fitted with a voltage

regulator.

Over voltage may be caused by the following contingencies:

1. Defective operation of the AVR

2. Operation under manual control

with the AVR out of service

3. Sudden loss of load (due to line

tripping) may cause the hydro

set to over-speed.

Overvoltage protection is not usually

provided on attended generators but

maybe required on unattended

automatic hydro stations.

Where applied, it is most effective to

use instantaneous relay with high

setting.