generator protection

LESSON – 25

GENERATOR PROTECTION

OUTLINE OF THE LESSON 1. STATOR WINDING PROTECTION 2. OVERLOAD PROTECTION 3. OVER CURRENT PROTECTION 4. OVER VOLTAGE 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 CONNECTED GENERATORS 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 for complete 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 overcome by providing two protection system operating 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.

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 b) The zero sequence voltage at the terminals w.r.t. the neutral of the generator rather than w.r.t. earth

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

 Power

overvoltage

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.