LESSON
LESSON
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25
25
GENERATOR PROTECTION
GENERATOR PROTECTION
OUTLINE OF THE LESSON
OUTLINE OF THE LESSON
1. STATOR WINDING PROTECTION 1. STATOR WINDING PROTECTION 2. OVERLOAD PROTECTION
2. OVERLOAD PROTECTION
3. OVER CURRENT PROTECTION 3. OVER CURRENT PROTECTION 4. OVER VOLTAGE PROTECTION 4. OVER VOLTAGE PROTECTION
OUTLINE OF THE LESSON
OUTLINE OF THE LESSON
1. STATOR WINDING PROTECTION 1. STATOR WINDING PROTECTION 2. OVERLOAD PROTECTION
2. OVERLOAD PROTECTION
3. OVER CURRENT PROTECTION 3. OVER CURRENT PROTECTION 4. OVER VOLTAGE PROTECTION 4. OVER VOLTAGE PROTECTION
STATOR WINDING PROTECTION
STATOR WINDING PROTECTION
The most satisfactory method ofThe most satisfactory method of
protecting an alternator stator is the protecting an alternator stator is the Merz-Price circulating current
Merz-Price circulating current technique
technique
Both longitudinal and transverseBoth longitudinal and transverse
differential; protection systems are differential; protection systems are used
LONGITUDINAL DIFFERENTIAL
LONGITUDINAL DIFFERENTIAL
PROTECTION OF DIRECT
PROTECTION OF DIRECT
CONNECTE
Phase and earth fault
Phase and earth fault protectionprotection system
PROTECTION SCHEME FOR
EARTH FAULTS ONLY
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
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
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
Two different schemes are available for complete protection of the stator
winding:
1. Low frequency injection 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
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.
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
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
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
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
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
TRANSIENT OVERVOLTAGE
Surge overvoltages originate largely in the transmission system because of switching and atmospheric
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
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.
