Power system Protection

2. INTRODUCTION
1. It is imperative need to install some protective system to protect the
expensive components of the protective system like generators,
transformers, transmission lines etc. from different types of faults which
occurs in power system.
2. In generating stations , continuous operation of generator is more
necessary so the faulty part needs to be cleared very quickly for
uninterruptable power supply

3. TYPES OF GENERATOR FAULTS:
i. Stator Winding Fault
ii. Rotor Winding Fault
iii. Abnormal Operating Conditions
DIFFERENT PROCESS OF PROTECTING THE GENERATOR:
i. Stator Protection
ii. Overcurrent Protection
iii. Overvoltage Protection
iv. Overspeed Protection
v. Rotor Fault Protection
vi. External Fault Backup Protection

4. STATOR FAULTS AND ITS PROTECTION
Stator Faults Include The Following:
i. Phase to earth faults
ii. Phase to phase faults
iii. Inter-turn faults
Phase to phase faults & Inter-turn faults are less common, these generally developed
into an earth fault. Inter-turn fault are more difficult to be detected.
Different Protection Schemes for Stator Protection:
Differential Protection
Biased Circulating Current Protection
Balance Earth-Fault Protection
Stator Overheating Protection

5. DIFFERENTIAL PROTECTION
Two sets of identical CTs, each mounted on either side of stator phase winding.
The secondaries of these current transformers sets are connected in star.
At normal operating conditions, the currents at two ends of protected section
remains same(I1=I2). So relay will not operate.
When the faults occurs the balance condition gets disturbed and hence
current(I1-I2) flows through the operating coil of the relay causing relay
operation and the trip circuit gets closed.
The relays employed in this protection system are generally of electro-magnetic
type & are arranged for instantaneous operation as faults are expected to be clear
as soon as possible

6. DIAGRAM

7. Balanced earth faults
In case of small generators the
neutral end of 3 phase winding is
not available because it is made
inside the generator and grounded
through some low resistance then
percentage differential relay for
ground fault is provided & is
known as balanced earth fault
This scheme can be used only for
ground faults but not for phase
faults

8. ROTOR FAULTS & Its protection
Faults in the rotor circuit may be either earth faults or between the turns of the field winding.
Field circuits are normally operated un-earthed. So a single fault will not affect its operation.
But when a second fault arises then field winding is short circuited & produce unsymmetrical field
system which leads to unbalanced forces on rotor and results in excess pressure and bearing and
shaft distortion
The Different Protection Schemes For Rotor Protection:
i. Rotor Earth Fault Protection
ii. Loss of Excitation Protection
iii. Rotor Temperature Alarm
iv. Automatic Field Suppression & Use Of Neutral Circuit Breaker

9. ROTOR EARTH FAULT
PROTECTION
A high resistance is connected across the rotor
circuit and its mid point is grounded through a
sensitive relay.
 The another method of rotor earth fault
protection is done by
i. DC injection method
ii. AC injection method
The dc or ac voltage is impressed between the
field circuit and ground through a sensitive
overvoltage relay and current limiting resistor
or capacitor(in case of ac).
 A single earth fault in rotor circuit will
complete the path and the fault is sensed by the
relay.
DC injection method is simple & has no
problems of leakage current.

10. LOSS OF EXCITATION PROTECTION
 When the excitation of generator is lost
it operates as an Induction generator. It
derives excitation from the system and
supply power at leading power factor
which may cause-
i. A fall in voltage so loss of
synchronism system instability.
ii. Over heating of rotor due to induction
current on it.
The relay mostly used for this type of
protection is Directional distance type
relay.

11. ROTOR TEMPERATURE ALARM
This type of protection scheme is
installed generally in large
alternators.
It indicates the level of
temperature but not the actual
hot spot temperature.
The relay measures the
temperature by measuring the
resistance(as shown in fig).
The relay measures the ratio of
voltage & current.

12. Automatic Field Suppression & Use Of Neutral Circuit
Breaker
In case of a fault in the generator and though the circuit breaker is tripped ,the
fault continues to fed as long as excitation will exist because emf is induced in the
generator itself.
Hence all protection system not only trip the generator circuit breaker but also trip
the “automatic field discharge switch “

13. NEGATIVE PHASE SEQUENCE PROTECTION
Unbalance may cause due to
single phase fault or unbalanced
loading and it gives rise to
negative sequence current.
This current in rotor causes rotor
overheating and damage to the
rotor.
This can be protected by negative
sequence current filter with over
current relay.

14. CONCLUSION
Switchgears & other protective devices are used in the electrical machines & in the power
system elements.
i. To detect the abnormal faulty condition.
ii. To notify this information to the maintenance officer of that factory or generation station.
iii. To disconnect the faulty elements from the healthy system or shutdown the faulted apparatus
for continuous operation of the machines, for safety of this expensive electrical devices as well
as for the safety of the workers.
The protective device does not prevent the cause of fault or occurrence of fault , it is used
after the occurrence of fault for quick action to remove those faulty section for further
normal operation .