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.