GENERATOR
PROTECTION
CONTENTS
GENERATOR
PROTECTION...........................................................................
INSULATION FAILURE......................................................................................
EARTHING BY RESISTOR..................................................................................
EARTHING BY TRANSFORMER.......................................................................
STATOR PROTECTION........................................................................................
EARTH-FAULT PROTECTION............................................................................
ROTOR EARTH-FAULT PROTECTION.............................................................
UNSATISFACTORY OPERATING CONDITIONS.......................................
UNBALANCED LOADING.................................................................................
OVERCURRENT PROTECTION.........................................................................
OVERLOAD...........................................................................................................
FAILURE OF PRIME MOVER.............................................................................
LOSS OF FIELD...................................................................................................
OVERSPEED........................................................................................................
OVERVOLTAGE.................................................................................................
PROTECTION OF GENERATOR/TRANSFORMER UNITS....................
GENERATOR PROTECTION
The a.c. generator needs protection against
a number of conditions some of which require immediate disconnection and some
that rnay be allowed to continue for some time.
In broad terms the former are connected with insulation failure whilst
the latter are generally associated with unsatisfactory operating conditions.
Of all the items of equipment which make up
a power system the generator is uniquc in that it is usually installed in an
attended station and is therefore subject to more or less constant
observation. The point here is that some
of the unsatisfactory operating conditions could be dealt with by an operator
whereas if the generator was not attended tripping would be the only course of
action.
INSULATION FAILURE
Stator faults are caused by the breakdown
of the insulation between the armature conductor and earth; between conductors
of different phases or between conductors of the same phase.
The most likely place for an earth fault to
occur is in the stator slots. Arcing
will probably occur resulting in the burning of the iron at the point of fault
and welding the laminations together.
Replacement of the faulty conductor may not be very difficult but the
damage to the core cannot be ignored as the fused laminations could give rise
to local heating. In severe cases it may
be necessary to dismantle and rebuild the core which is a lengthy and costly
process.
To reduce the possibility of damage
earth-fault current is usually limited by earthing the generator neutral point
via a resistor, reactor or transformer.
Practice varies as to the value to which the current is limited. From rated current in some cases to very low
values in others.
EARTHING BY RESISTOR
Earthing by means of reactors is uncommon
and earthing by transformer is usually limited to large machines. In an industrial system the generator, which
is usually directly connected to the power system without a transformer, is
earthed by a resistor which has a fairly low value. The earth-fault current is usually limited to
between 50% and 200% of the rated current.
In cases where the generator is connected
to the distribution system via a generator transformer a resistor designed to
allow an earth-fault current of about 300 A is used irrespective of generator
rating.
EARTHING BY TRANSFORMER
The other approach to earthing is to limit
the current to a level where burning does not readily occur. This level is said to be 5A. To achieve this high-impedance transformers
have been used. Initially voltage
transformers were used operating at a fairly low flux density but overvoltage
problems arising from the capacitance of the stator windings has resulted in
the general use of distribution transformers.
The secondary winding is loaded with a resistor so that under
earth-fault conditions a maximum of 5A will flow.
Phase-to-phase faults are far less likely
than earth faults and, as they are easily detected, damage caused can be
limited by rapid disconnection. On the
other hand, interturn faults, which are also uncommon, are very difficult to
detect and are generally only detected and cleared when they have developed
into an earth fault.
STATOR PROTECTION
Differential protection using
high-impedance relays is usual for stator protection and is applied on a
phase-by-phase basis. As the leads
between the two sets of current transformers may be long the resistance will be
fairly high but as the maximum through-fault current will be less than 10 times
full-load current a reasonably low voltage setting can be applied. This means that the CT magnetising current
will be low and therefore a low overall current setting can be expected.
The overall setting has a direct bearing on
the amount of the generator winding which is protected. This can be calculated as follows:
Max. fault current-say 5 x CT rating.
Overall
protection setting say 6%.
Amount of
winding protected
|
100% - = 98.8%.
This would be for a phase-phase fault. For an earth fault where the current is
limited to the full-load value only 94% of the winding would be protected. In fact slightly less as the full-load
current of the generator is usually less than the CT rating.
If the required voltage setting was high
because of, say, long CT leads or if the CT magnetising current was high then
the overall current setting may be much higher than 5%. This means that the amount of generator winding
protected is also reduced maybe to an unacceptable level for earth faults. In this case a biased differential relay
would alleviate the position.
The use of a biased relay means that the
relay-coil circuit impedance can be reduced to about a twentieth of the
impedance of the relay coil in the unbiased scheme. This naturally reduces the voltage setting
and the CT magnetising current at setting resulting in an overall setting of
about 5%.
FIGURE
7.1 BIASED DIFFERENTIAL PROTECTION. ONE PHASE ONLY SHOWN
The biased differential scheme is shown in
Figure 7.1 and the value of the stabilising resistor, Rs, can be calculated
from
|
RS =
where B
is the ratio of bias coil turns to operate coil turns and is known as the
bias ratio and RB is the resistance of the bias coil.
EARTH-FAULT PROTECTION
Where the maximum earth-fault current is
restricted to a fraction of the generator rating earth-fault protection is
essential to compliment the differential protection scheme.
This earth-fault protection frequently
comprises an instantaneous relay having a setting of 10% to 20% and the IDMT
relay with a setting of 5% to 10%. Both
relays would be connected to a simple current transformer having a primary
current rating equal to that of the earthing resistor. Earth faults will be detected in 90% to 95%
of the generator winding even though the maximum earth-fault current may be as
low as 5% of the generator rating.
Even where the main differential protection
scheme is expected to provide adequate protection for earth faults an IDMT
relay, connected to a current transformer in the generator neutral-earth
connection, is used to provide back-up protection. Where the generator is directly connected to
the power system, i.e. without a
generator transformer, it provides back-up protection for the busbars and the
whole system. In this case it should
have a very long time delay and should be thought of as the last line of
defence.
ROTOR EARTH-FAULT PROTECTION
The field system of a generator is not
normally connected to earth and so an earth-fault does not cause any current to
flow to earth and does not, therefore, constitute a dangerous condition. If a second earth-fault occurs a portion of
the field winding may be short-circuited resulting in an unbalanced magnetic
pull on the rotor. This force can cause
excessive pressure on the bearings and consequent failure or even displacement
of the rotor sufficient to cause fouling of the stator. The overheating in the rotor can cause
deformation of the winding which could lead to the development of
short-circuits.
Two main methods are used for detecting
earth-faults in the rotor circuit. In
the first method a high-resistance potentiometer is connected across the rotor
circuit the centre point of which is connected to earth through a sensitive
relay (see Figure 7.2). The relay will
respond to earth faults occurring over most of the rotor circuit.
Figure
7.2 -
ROTOR EARTH FAULT DETECTION - POTENTIOMETER METHOD
Figure
7.3 -
ROTOR EARTH FAULT DETECTION - NEGATIVE BIASING METHOD
There is, however, a blind spot at the
centre point of the field winding which is at the same potential as the
mid-point of the potentiometer. This
blind spot can be examined by arranging a tapping switch which when operated
shifts the earth point from the phase rotation, produce a magnetic field which
induces currents in the rotor at twice the system frequency. This causes considerable heating in the rotor
and would cause damage if allowed to persist.
UNSATISFACTORY OPERATING CONDITIONS
These conditions in general do not require
immediate disconnection and, it could be argued that, in an attended station
the operator could take the necessary action to remove the condition. Undoubtedly this is possible in some cases
but on no account should protection be omitted on this basis.
UNBALANCED LOADING
Unbalanced loading of the generator phases
results in the production of negative phase sequence (NPS) currents. These currents, which have a phase rotation
in the opposite direction to the normal
Each generator will have a negative
phase-sequence rating which can exist continuously without damage, typically
0.15 p.u. of generator FL current, and an I2t rating when the
current exceeds the continuous value, typically I2t = 20.
Where I is per unit NPS current and tis the
time in seconds e.g. the generator would
carry a NPS of current 15% full-load current continuously and NPS current of
30% full-load current for a time of
|
0.32t = 20 t = = 222s.
In fact the time would be longer than the
calculated value as there would be some heat dissipation. An I2t value assumes no heat
dissipation and therefore the longer the time the more inaccurate the
result. The result will be fairly
accurate up to 2 minutes.
The actual negative phase-sequence current
is difficult to determine from the ammeters measuring the load current in each
phase. It is not greater than 65% of the
unbalanced current.
Relays to detect the condition usually have
an IDMT characteristic matched to the I2t value. The relay is connected to a network which
separates the positive and negative phase-sequence currents. The basis of the network is to produce a
phase shift of 60° in some components of the phase currents such that when the
phase rotation is positIve, i.e. r, y,
b, r, the net current in the relay is zero.
When the phase rotation is negative, i.e. r, b, y, r, a proportion of the current flows
in the relay. Any current which flows in
the generator neutral is known as zero-sequence current and this must be
eliminated if the network is to function correctly. Where the generator is connected to the
system via a delta/star transformer any zero-sequence current means that there
is a fault on the generator circuit and this will be cleared by earth-fault
protection. If the generator is directly
connected then zero sequence is eliminated by connecting in delta the current
transformers which feed the NPS network.
In this case the relay setting is related to the CT current x 1.73.
There is sometimes a reluctance to apply
NPS protection as all generators will be subject to the same conditions and
could lead to all generators tripping at the same time. An early warning of the condition can be
provided by an instantaneous relay connected to the NPS network to provide an
alarm after a short fixed time delay.
OVERCURRENT PROTECTION
An IDMT relay is generally used as back-up
protection but the operation of this relay is complicated because of the
current decrement in the generator during fault conditions. In some cases a setting is chosen, such that
the relay will not eperate for a system fault but will only respond when fault
current is fed into the generator, in this way it only acts as a back-up to the
main generator protection.
In most industrial installations the relay
is required to act as back-up to the system protection and settings must be
chosen to ensure positive operation.
The operation of IDMT relays under
generator decrement conditions can be calculated by dividing the decrement
curve into a number of zones of width, say 0.1 s. The mid-ordinate is the current level which
is converted to a multiple of the relay setting and the time for full travel determined
The difficulty in application arises from
the variation in the current decrement depending on generator conditions prior
to the fault. From a no-load condition
the current will decay to less than full-load current whereas from the
full-load condition the final current will be greater than full-load current
because the field current is higher. The
former case will be modified if there is a voltage regulator as this will
attempt to boost the field with a consequent increase in final current. This would have a large effect on the relay
and therefore a normal IDMT relay is generally unsatisfactory. However, this method can be used to determine
settings of feeder and transformer IDMT relays in finite busbar systems. For example, in off-shore installations or
any location where the only supply is local generation. The multiples of setting current in this case
will be much greater because the feeder and transformer rated current will only
be a fraction of that of the generator.
The higher multiples of setting means that the effect of the difference
in generator decrement between no load and full load will be small.
It may be that the current will decay to a
level where it is insufficient to cause the overcurrent relay to trip. In these circumstances it is necessary to
provide a relay which not only responds to current but also to the level of
voltage.
The principle of operation is that an IDMT
relay with a setting much less than the full-load current of the generator has
a feature added which increases the setting to above full-load current when
full system voltage is present.
By this means the longer operating times,
for discrimination with system protection when the fault is remote, will be
attainable as the voltage is high.
Close-up faults will remove the voltage restraint to enable the relay to
operate in the relatively fast time appropriate to the lower setting.
The relays for this type of protection can
be either voltage restrained, where the voltage element operates as a restraint
on the same disc as the overcurrent element, or voltage controlled, where the
setting of the overcurrent relay is changed by means of a voltageoperated
attracted-armature relay.
OVERLOAD
Overload protection is not generally
provided for continuously supervised machines but on large machines resistance
thermometers or thermocouples are embedded in the stator winding. There is some possibility of overload in
terms of MVA for, although the governors will restrict MW, the AVR may cause
the machine to deliver a disproportionate share of the MVAr. In cases where overload protection is to be
provided this would probably be of the thermal type with a characteristic to
match the generator thermal capacity.
Overload and overcurrent relays should not
be confused as they perform completely different functions. An overload relay operates in the hundreds to
thousands of seconds range whereas an overcurrent relay operates in the one- to
ten-second range.
FAILURE OF PRIME MOVER
In the event of a prime mover failure the
generator continues to run but as a synchronous motor and this can cause a
dangerous condition in the prime mover.
In a steam turbine the turbulence of the steam In the turbine causes a
temperature rise which can quickly reach serious proportions in pass-out
sets. In condensing sets the temperature
rise is not as fast and therefore less urgent action is needed. In engine-driven sets the loss of motive
power is likely to be due to mechanical failure and the continued running of
the set is likely to cause damage.
The machine, as a synchronous motor, will
draw power from the system and it is this reverse power which is detected by
the protection. The power required is
usually small, about 10% in case of large turbo-alternators. The power factor depends on the excitation of
the machine and can be quite low and either leading or lagging. This means that the reverse power relay must
respond to a low value of power when the MVAr is high and consequently must be
sensitive and have only a small phase-angle error.
A single-element relay is used because the
power will be balanced in the three phases.
It is used in conjunction with a time-delay relay to prevent operation
during power swings and synchronising.
LOSS OF FIELD
Failure of the field system results in
acceleration of the rotor to above synchronous speed where it continues to
generate power as an induction generator the flux being provided by a large
magnetising component drawn from the system.
This condition can be tolerated for a short time but clearly there will
be increased heating of the rotor because of the slip-frequency currents which
flow.
Loss of field can be detected by a simple
undercurrent relay connected to a shunt in the field circuit. It must have a setting below minimum field
current and a time delay if field forcing is used. A time-delay relay is also required as the
undercurrent relay may respond to the slip-frequency circuit in the field
circuit. This relay would have an
instantaneous pick-up and a time-delayed drop off to maintain the circuit to
the main time-delayed relay.
The more up-to-date method is to detect the
loss of field on the a.c. side of the
generator by comparison of the stator voltage and current. By either a relay measuring the reactive
power (MVAr) which is being imported or by an impedance relay which has a
characteristic as shown in Figure 7.4.
As can be seen under normal operation the apparent impedance, as
measured by stator voltage and current is well away from the tripping
zone. When there is a loss of field the
impedance vector moves to the operation zone.
FIGURE
7.4 -
DETECTION OF GENERATOR FIELD FAILURE
OVERSPEED
The speed is very closely controlled by the
governer and is held constant as the generator is in parallel with others in an
interconnected system. If the
circuit-breaker is tripped the set will begin to accelerate and although the
governer is designed to prevent over- speed a further centrifugal switch is
arranged to close the fuel valve.
There is still a risk, however, that the
fuel valve will not close completely and even a small gap can cause overspeed
and so where urgent tripping is not required it is usual to lower the
electrical output to about 1% before tripping the circuit-breaker. A sensitive under-power relay is used to
detect when this value is reached.
OVERVOLTAGE
Voltage is generally controlled by a
high-speed voltage regulator and therefore overvoltages sbould not occur and
overvoltage protection is not generally provided for continuously supervised
machines. On unattended machines an
instantaneous relay set at, say 150% is used to cater for defective operation
of the voltage regulator.
PROTECTION OF GENERATOR/TRANSFORMER UNITS
Where a generator is connected to the power
system by means of a generator transformer it is usual to protect the generator
and transformer as a single unit using biased differential protection.
The current transformer balance is produced
in terms of both phase and magnitude, i.e.
in the arrangement shown in Figure 7.5 there is an overall phase change
of 30° which is corrected by connecting a set
of auxiliary current transformers in delta.
Because of the difference in current transformer ratios the settings of
the generator transformer protection has to be somewhat higher than the
settings of the generator protection.
Because of this the generator is sometimes protected separately but is
also included within the zone of the generator-transformer protection as an
extra insurance. The transformer is
connected directly to the generator and so no harmonic restraint circuit is
required in the protection as no switching can occur. There is a low level of magnetising inrush
current following a fault when the voltage is restored from being depressed but
this is usually insufficient to unbalance the protection. Figure 7.5 shows a complete protection system
for a generator.
FIGURE
7.5 GENERATOR PROTECTION
No comments:
Post a Comment