Unit – III

Protection of AC Generators &Transformer

PROTECTION OF GENERATORS
The generators used in the power system are the alternators which produce very high A.C.
voltages. The protection of generators is very much complex due to the following reasons,
1. The generators are very large machines producing very high voltages and are connected to bus
bars.
2. Various other equipment are always associated with the generators. Such equipment’s are
prime movers, excitation systems, voltage regulators, cooling systems etc. Thus protection of
generators must consider the presence of these other equipment’s also.
3. The generators are very costly, expensive and very important factor in a power system. The
protection scheme must be such that it should not shut off the generators result in a power
shortage.
All these factors make the design of protection scheme for the generator, very much
complex.
GENERATOR FAULTS
The various faults which can occur associated with a generator can be classified as,
1. Stator faults : The faults associated with the stator of the generator.
2. Rotor faults : The faults associated with the rotor of the generator.
3. Abnormal running conditions : This includes number of abnormal conditions which may
occur in practice, from which the generator must be protected.
1. Stator Faults
The stator faults means faults associated with the three phase armature windings of the generator.
These faults are mainly due to the insulation failure of the armature windings. The main types of
stator faults are,
1. Phase to earth faults
2. Phase to phase faults
3. Inter-turn faults involving turns of same phase winding.
The most important and common fault is phase to earth fault. The other two are not very
common while inter-turn fault is very difficult to detect.
1. Phase to Earth Fault :
These faults mainly occur in the armature slots. The faults are dangerous and can cause severe
damage to the expensive machine. The fault currents less than 20 A cause negligible burning of
core if machine is tripped quickly. But if the fault currents are high, severe burning of stator core
can take place. This may lead to the requirement of replacing the laminations which is very
costly and time consuming. So to avoid the damage due to phase to earth faults, a separate,
sensitive earth fault protection is necessary for the generators along with the earthling resistance.
2 Phase to Phase Faults :
The phase to phase faults means short circuit between two phase windings. Such faults are
uncommon because the insulation used between the coils of different phases in a slot is large.
But once phase to earth fault occurs, due to the over heating phase to phase fault also may occur.
This fault is likely to occur at the end connections of the armature windings which are
overheating parts outside the slots. Such a fault cause severe arcing with very high temperatures.
This may lead to melting of copper and fire if the insulation is not fire resistant
3 Stator inter-turn Faults :
The coils used in the alternators are generally multi turn coils. So short circuit between the turns
of one coil may occur which is called an inter-turn fault. This fault occurs due to current surges
with high value (L di/dt) voltage across the turns. But if the coils used are single turn then this
fault can not occur. Hence for the large machines of the order of 50 MVA and more, it is a
normal practice to use single turn coils. But in some countries, multi turn coils are very
commonly used where protection against inter-turn faults is must.
2. Rotor Faults
The rotor of an alternator is generally a field winding as most of the alternators are of rotating
field type. The field winding is made up of number of turns. So the conductors to earth faults and
short circuit between the turns of the field winding, are the commonly occurring faults with
respect to a rotor.
These faults are caused due to the severe mechanical and thermal stresses, acting on the
field winding insulation. The field winding is generally not grounded and hence single line to
ground fault does not give any fault current. A second fault to earth will short circuit the part of
the field winding and may thereby produce unsymmetrical field system. Such an unsymmetrical
system gives rise to the unbalanced forces on the rotor and results in excess pressure on the
bearings and the shaft distortion, if such a fault is not cleared very early.
So it is very much necessary to know the existence of the first occurrence of the earth
fault so that corrective measures can be taken before second fault occurs. The unbalanced
loading on the generator is responsible to produce the negative sequence currents. These currents
produce a rotating magnetic field which rotates in opposite direction to that of rotor magnetic
field. Due to this field, there is induced e.m.f. in the rotor winding. This causes overheating of
the rotor.
Rotor earth fault protection and rotor temperature indicators are the essential and are
provided to large rating generators.
3. Abnormal Running Conditions
The protection must be provided against the abnormal conditions. These abnormal conditions
include,
1. Overloading
2. Over speeding
3. Unbalanced loading
4. Overvoltage
5. Failure of prime mover
6. Loss of excitation (Field failure)
7. Cooling system failure
Failure of prime mover : In the event of prim-mover failure the machine starting motoring
meaning thereby that it draws electrical power from the systems and drives the prime mover.
This condition imposes a balanced load on the system, which can be detected. by a power
relay with a directional characteristic.
Loss of Excitation (Field Failure) Protection :
Two distinct effects of loss of excitation are that the machine starts drawing magnetizing current
of large magnitude from the system, and the slip frequency emfs induced in the rotor circuit ;
both of them cause overheating of the rotor. Loss of excitation can be detected by measuring the
reactive component of stator current ; an excessive value of VAR import indicates either actual
or prospective loss of synchronism. To allow for system transients which may cause a
momentary reversal of VAR component it is usual to incorporate a fixed time delay of between
one and five seconds in the tripping sequence of the relay.

Merz-price Differential protection:

In merz-price differential protection the primaries of the CTs are connected in series on
the both side of each phase winding of the generator. The secondaries of the CTs are connected
in additive manner to pass the circulating currents through a closed path. The differential relay
constantly checks on the secondary sides of CTs as to whether the incoming current of a phase
winding is equal to the respective outgoing current of the same winding.
The directions of currents passing through the secondary side of the CTs are shown in fig.
If the currents on the primary sides , that is, IA1 and IA2, have the same magnitude, then the
secondary side currents, ia1 and ia2 will also have the same magnitude, considering that CTs on
both sides have same turns ratio and have identical characteristics. If there is a significant
difference in currents (Id) on both sides of the windings, then this indicates a fault in the
protection zone of the stator winding of the generator. Hence the differential relay trips if the
value of Id exceeds a predetermined value (relay setting). On the other hand, during external
faults, the differential relay remains stable and does not initiate a trip signal.

If the CTs are identical in nature, then the functioning the differential relay is
straightforward. However, in practice, it is impossible to achieve CTs with identical saturation
characteristics. Hence, the secondary currents of the CTs are unequal even though the primary
currents are same. This current widely known as spill current. This spill current passes through
the relay and may mal-operate the relayif the value exceeds the setting of the relay. This is
possible particularly in case of heavy through fault conditions. Moreover, if thelength of the
connecting wires (pilot wires) is unequal , then the value of the spill current increases. In order to
avoid mal-operation of the differential relay in those situations, a stabilizing resistance is
connected in series with the realy. However , incorporation of the stabilizing resistance reduces
the sensitivity of the relay during an internal fault.
Biased or Percentage Differential Protection:
The main drawback of Merz-price differential protection is the reduction in sensitivity of the
relay due to the incorporation of the stabilising resistance. Hence, to minimise this effect and
also to increase the sensitivity of the differential relay, biased differential protection scheme is
used. Fig shows the principle of the biased percentage differential scheme where no additional
stabilizing resistance is connected in series with the relay.

The biased percentage differential relay has 2 settings namely basic setting and
bias setting. Basic setting is the difference between two CT secondary currents (i1-i2). Bias
setting is the ratio of the difference between two CT secondary currents to the average values of
two currents ().In the case of a normal condition or external fault condition some amount of spill
current (i1-i2) is already available because of the non-identical CTs and un equal lead length.
However at the same time average value also increases hence fault current will not cross the bias
setting of the relay even though the basic setting is exceeded therefore mal-operation of the relay
can be avoided.

On the other hand during an internal fault two currents on the primary sides of the
CTs become unequal a spill current (i1-i2) is produced on the secondary side of the 2 CTs this
crosses not only the basic setting limit of the relay but also the bias setting limit because of the
increase in spill current and decrease in the average of the two currents. Finally the relay
operates and trips the generator as per the requirement. The fundamental advantage of this
scheme is that it makes the scheme more permissive towards CT mis-match, thus reducing the
relays sensitivity to external faults. More over it provides more sensitivity in case of internal
faults.
Rotor Earth Fault Protection:
The rotor circuit of the alternator is not earthed and d.c. voltage is imposed on it. And hence
single ground fault in rotor does not cause circulating current to flow through the rotor circuit.
Hence single ground fault in rotor does not cause any damage to it. But single ground fault
causes an increase in the stress to ground at other points in the field winding when voltages are
induced in the rotor due to transients. Thus the probability of second ground fault increases.
If the second ground fault occurs then part of the rotor winding is bypassed and the currents
in the remaining portion increases abruptly. This causes the unbalance of rotor circuit and hence
the mechanical and thermal stresses on the rotor. Due to this, rotor may get damaged. Sometimes
damage of bearings and bending of rotor shaft take place due to the vibrations. Hence the rotor
must be protected against earth fault.
The modern method of providing earth fault protection includes d.c. injection or a.c.
injection. The scheme is shown in the Fig.

A small DC power supply is connected to the field circuit. A fault detecting sensitive relay and
the resistance are also connected in series with the circuit. This high resistance limits the current
through the circuit.
A fault at any point on the field circuit will pass a current of sufficient magnitude through
the relay to cause its operation. The d.c. supply is preferred and simple to use and it has no
problem of the leakage currents. In case of a.c. injection, the high resistance is replaced by a
capacitor.
 The earth fault relays are instantaneous in operation and are connected to an alarm circuit for
indication and to take the proper action. This is because; a single ground fault does not require an
immediate action of isolating the generator.
Restricted Earth Fault Protection:

When the neutral is solidly grounded, it is possible to provide protection to complete winding
of the generator against ground faults. However, the neutral is grounded through resistance to limit
ground fault currents. With resistance grounding it is not possible to protect the complete winding
against ground faults. The percentage of winding protected depends on the value of the neutral
grounding resistor and the relay setting. The usual practice is to protect 80 to 85% of the generator
winding against ground fault. The remaining 15-20% from neutral end is left unprotected. The relay
setting for the differential protection is determined by the value of the neutral grounding resistor and
the percentage of winding to be protected. If the ground fault occurs at the point F of the generator
winding, the voltage VFN is available to drive the ground-fault current If through the neutral to

ground connection. If the fault point F is nearer to the neutral point N, the forcing voltage VFN, will

be relatively less. Hence, ground fault current If will reduce. It is not practicable to keep the relay
setting too sensitive to sense the ground fault currents of small magnitudes. Because, if the relay is
made too sensitive, it may respond during through faults. The percentage of winding that is left
unprotected;
Stator Inter-Turn Protection:
Merz-price circulating-current system protects against phase-to-ground and phase-to-
phase faults. It does not protect against turn-to-turn fault on the same phase winding of the stator.
It is because the current that this type of fault produces flows in a local circuit between the turns
involved and does not create a difference between the currents entering and leaving the winding
at its two ends where current transformers are applied. However, it is usually considered
unnecessary to provide protection for inter-turn faults because they invariably develop into earth-
faults.

In single turn generator there is no necessity of protection against inter-turn faults. However,
inter-turn protection is provided for multi-turn generators such as hydro-electric generators.
These generators have double-winding armatures
as they carry very heavy currents. The relays
Rcprovide protection against phase-to-ground and

phase-to-phase faults whereas relays R1 provide

protection against inter-turn faults.

Two current transformers are connected on the

circulating-current principle. Under normal
conditions, the currents in the stator windings S1
and S2 are equal and so will be the currents in

the secondaries of the two CTs. The secondary current round the loop then is the same at all
points and no current flows through the relay RI. If a short-circuit develops between adjacent
turns, say on S1, the currents in the stator windings S1 and S2 will no longer be equal. Therefore,
unequal currents will be induced in the secondaries of CTs and the difference of these two
currents flows through the relay RI. The relay then closes its contacts to clear the generator from
the system.