Generator Protection

1. Dr. Ahmad Al-Subhi
SWITCHGEAR AND PROTECTION
Lecture 22
GENERATOR PROTECTION
Prepared by: Dr. Ahmad Al-Subhi

2. - Generator is the most important and expensive equipment in
power system.
- Synchronous generators are utilized in a wide range of sizes,
and different winding configurations, neutral grounding
connections, mechanical and cooling configurations,
connections to loads, and interconnections to utility grids.
- Sizes under consideration can range from a few hundred kilo-
Volt-Ampere (kVA) for standby duties in industrial facilities to
over 1000 MVA in large hydro or nuclear-generating facilities.
Prepared by: Dr. Ahmad Al-Subhi

3. - As the generator is accompanied by prime mover, excitation
system, voltage regulator, cooling system, etc., its protection
becomes very complex and elaborate.
- There are many different types of faults that synchronous
generators may experience.
- Hence, diverse types and complexities of protection may be
required.
- Faults occur less frequently than on transmission lines.
- But the damage caused by generator faults takes much more
time and money for repairing.
Prepared by: Dr. Ahmad Al-Subhi

4. - Since there are several types of failures, corresponding types
of relays are used to detect and address each type of failure.
- With the utilization of multi-function computerized relays,
multiple types of failures can be addressed by one relay
assembly.
Prepared by: Dr. Ahmad Al-Subhi

5. - The protection of the generator presents a very challenging
problem because of its system connections on three different
sides:
- It can be seen that on the one side, it is connected to the prime
mover and on the other side, it has to run in synchronism with
the grid because of its connection to the power system.
- On the third side, it is connected to the source of DC
excitation.
Prepared by: Dr. Ahmad Al-Subhi

6. - Unlike other apparatus, opening a breaker to isolate the faulty
generator is not sufficient to prevent further damage.
- This is because the generator will continue to supply power to
the stator winding fault until its field excitation is suppressed.
- It is therefore necessary to stop the field supply, shut off the
steam, water or fuel supply to the prime mover, trip the boiler
and shut off all auxiliaries of the generator.
- So, it is characterized by several protective devices that can be
more than 20 devices according to generator size.
Prepared by: Dr. Ahmad Al-Subhi

7. - Protection schematic for generators up to 5 MVA:
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8. - Protection schematic for generators over 5 MVA:
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9. - What can go wrong with generator?
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10. - Stator and rotor faults:
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11. Stator Faults:
- These are the faults associated with the stator of the generator.
- 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 phase faults: not common.
2. Phase to ground faults: common.
3. Inter-turn faults: not common and difficult to detect.
Prepared by: Dr. Ahmad Al-Subhi

12. • 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.
- Differential protection scheme discussed in transformer
protection is implemented to detect this type of faults.
- Most manufacturers recommend this type of protection for all
units larger than 1 MVA.
Prepared by: Dr. Ahmad Al-Subhi

13. Example 1: A generator stator winding protected by a percentage
differential relay is shown in the figure below. The relay has 15%
slope. The high resistance ground fault has occurred near the
grounded neutral end of the generator winding while the generator
is carrying load. The currents flowing at each end of the generator
winding are shown. Assuming CTs ratio are 500/5, will the relay
operate?
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14. Solution:
Prepared by: Dr. Ahmad Al-Subhi
- Id = 3.4 – 3 = 0.4 A.
- Ir = (3.4 + 3) / 2 = 3.2 A
- Point is (3.2, 0.4)
- Relay will not operate.

15. • Phase to Ground Faults:
- These faults mainly occur in the armature slots.
- The faults are dangerous and can cause severe damage to the
expensive machine.
- If the fault currents are high,
severe burning of stator core may occur.
- This may lead to the requirement of replacing the laminations
which is very costly and time consuming.
- The stator is protected from phase to earth faults using
differential and earth fault protection schemes discussed in
transformer protection.
Prepared by: Dr. Ahmad Al-Subhi

16. • Inter-turn Faults:
- Short circuits between the turns of one coil may occur if the
stator winding is made up of multiturn coils.
- This is common for hydro generators but not for large steam
turbine generators, which usually have only one turn windings.
- The inter-turn fault is a short circuit between the turns of the
same phase winding.
- Differential relaying, will not
respond to faults between turns
because there is no difference in the currents at the ends of a
winding with shorted turns.
Prepared by: Dr. Ahmad Al-Subhi

17. - Applicability of differential protection to stator faults:
Prepared by: Dr. Ahmad Al-Subhi
Differential protection
scheme provides very
fast protection to the
stator winding against
phase to phase faults
and phase to ground
faults only.

18. - A protection scheme called “Transverse differential protection”
is implemented to protect the stator from inter-turn faults.
- It can detect the unbalance between normally identical windings
caused by an inter-turn fault when the generator has two
windings per phase.
Prepared by: Dr. Ahmad Al-Subhi

19. Example 2: Application of transverse differential protection to
protect the stator from inter-turn faults:
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20. Rotor Faults:
- So the commonly occurring faults with respect to a rotor are:
1. Phase to ground faults.
2. Inter-turn faults.
- These faults are caused due to the severe mechanical and
thermal stresses, acting on the field winding insulation.
- Rotor earth fault protection and rotor temperature indicators are
essential and are provided to large rating generators.
Prepared by: Dr. Ahmad Al-Subhi

21. - The rotor is usually provided with the following protective
schemes:
1. Field ground fault protection.
2. Loss of excitation protection.
3. Protection against rotor overheating.
4. Rotor shorted turn protection.
Prepared by: Dr. Ahmad Al-Subhi

22. Abnormal conditions of generator:
- In practice, there are number of situations in which generator is
subjected to some abnormal running conditions.
- The protection must be provided against such conditions.
- These abnormal conditions include:
- Each of these conditions along with their protective schemes are
explained in details in various books.
Prepared by: Dr. Ahmad Al-Subhi
1. Overloading.
2. Over-speeding.
3. Unbalanced loading.
4. Over voltage.
5. Failure of prime mover.
6. Loss of excitation.
7. Overheating.

23. - Rotating machines are also complicated because of the fact
that important parts of the machine are in mechanical motion.
- This gives rise to problems related to vibration, bearing failure,
mechanical resonance, and other purely mechanical failure
modes that can cause the machine to be removed from service.
- Moreover, mechanical parts are subject to aging and wear that
require periodic monitoring in order to preserve the expected
life of the machine and maintain it in good working condition.
Prepared by: Dr. Ahmad Al-Subhi

24. - In general, following are some protective schemes applied to
generator protection:
Prepared by: Dr. Ahmad Al-Subhi
1. Differential protection.
2. Inter-turn fault protection.
3. Stator earth fault protection.
4. Rotor earth fault protection.
5. Vibration protection.
6. Field failure protection.
7. Overload protection.
8. Overvoltage protection.
9. Reverse power protection.
10. Pole slipping protection.
11. Back up impedance protection.
12. Under frequency protection.
13. Miscellaneous protection.
14. Negative phase sequence protection.
15. Bearing failure protection.
16. Coolant failure protection.
17. Fire protection.
18. Protection of power plant auxiliaries.

25. - This table gives data about the present practice of generator
protection:
Prepared by: Dr. Ahmad Al-Subhi

26. - For more detailed analysis and explanation about generator
protection, refer to the following books:
Prepared by: Dr. Ahmad Al-Subhi