Generator Protection

• Generators are the most expensive equipment in an ac power system.

A 210 MW turbo-generator which includes an alternator, a steam
turbine, a boiler and other auxiliaries costs more than hundred crores
in Indian rupees.

• The generator also represents the most complicated unit demanding

an extensive protection system comprising a large variety of
protective relays.
• The protective system of a generator must be carefully chosen since
an inadvertent operation of the relay is almost as serious as a failure
of operation.

• This is because the disconnection of a large generator may overload

the rest of the system and cause power oscillations resulting in an
unstable power system.
• On the other hand, failure to clear a fault promptly may cause
extensive damage to the generator and may again lead to disruption
of the whole system.

• Another difficulty with the generator protection system is the fact

that, unlike other equipment, opening a breaker to isolate the
defective generator is not enough to prevent further damage, since
the generator will continue to supply power to its own fault until its
field excitation has been suppressed.
• It is, therefore, necessary to remove the field supply, shut off the
steam, water or fuel supply to the prime mover, trip the boiler and
shut off all the auxiliaries of the generator.

• Further, carbon dioxide is pumped into some large machines to

extinguish any burning of insulation, which could have been
initiated by the rotor movement
 Major faults and abnormal conditions in case of generators
1. Failure of insulation of the stator winding
2. Failure of insulation of the rotor winding
3. Unbalanced loading
4. Field failure
5. Overload
6. Overvoltage
7. Failure of prime-mover
8. Loss of synchronism
9. Over-speed
10. Under-frequency
11. Over-heating
Protective schemes employed for generator protection
• 1. Differential protection
• 2. Inter-turn fault protection
• 3. Stator earth-fault protection
• 4. Overcurrent and earth-fault protection
• 5. Rotor earth-fault protection
• 6. Negative phase-sequence protection
• 7. Field failure protection
• 8. Overload protection
• 9. overvoltage protection
• 10. Reverse power protection
• 11. Pole-slipping protection
• 12. Back-up impedance protection
• 13. Under-frequency protection
Class A, Class B and Class C Protections

• If a fault is of a very serious nature and impacts the

generator, generator-transformer, prime-mover or
boiler (i.e., the fault is likely to cause a direct and
critical damage to the unit even after isolating the unit
from the infinite bus),

• the protective scheme that operates is known as a

Class A protection.
 Actions initiated when Class A protection operates
(i) Generator breaker is tripped
(ii) Generator field breaker is tripped
(iii) Incomer breakers of unit auxiliary transformer are tripped
(iv) Tie breakers between the auxiliary station bus and auxiliary
unit bus are closed
(v) Boiler trips
(vi) Prime-mover trips
(vii) All unit auxiliaries are tripped
(viii) ‘Class A Trip’ annunciation appears
• The consequences of certain faults are such that the generator is not
required to be isolated from the infinite bus immediately; but prime-
mover and boiler are tripped immediately.
• Because of this tripping, the generator will lose input and hence the
power output will gradually reduce.
• Because of this action, the generator does not speed up and the
stored kinetic energy is utilized.
• The protective scheme, which initiates the sequence as depicted
above, is said to be a Class B protection.
Actions initiated when Class B protection operates
(i) Boiler is tripped
(ii) Turbine is tripped
(iii) ‘Class B Trip’ annunciation will appear.
(iv) Class A protection will operate through low forward power relay.
Low forward power relay is a time-delayed relay

Generally, in large generators, a low forward power relay is used to

sense the power output. When the power output reduces to around
0.5% of the rated power, low forward power relay trips and hence Class
A protection operates.
• The consequences of certain faults are such that the generator is only
required to be isolated from the infinite bus.
• The generator thus will feed its auxiliaries only (i.e., the generator
will feed house load only).
• Once the cause of the fault is found and the fault is cleared by a
relevant breaker, the generator can once again be synchronized with
the system.
• The process of synchronizing does not take much time. The protective
scheme, which thus trips the generator breaker only, is known as
Class C protection.
Class A Protections
• 1. Differential protection
• 2. Stator earth-fault protection
• 3. Inter-turn fault protection
• 4. Overcurrent and earth-fault protection (for small generators)
• 5. Rotor second earth-fault protection
• 6. Overvoltage protection
• 7. Reverse power protection
• 8. Pole-slipping protection
• 9. Generator-transformer overall differential protection (refer Chapter 6)
• 10. Local breaker back-up protection (refer Chapter 10)
• 11. Generator transformer restricted earth-fault protection
• 12. Differential protections of unit auxiliary transformers
• 13. Generator–transformer over-fluxing protection
• 14. Generator–transformer Buchholz trip
• 15. Generator–transformer pressure relief device operated
• 16. Unit auxiliary transformer Buchholz trip
• 17. Unit auxiliary transformer pressure relief device operated
• 18. Unit auxiliary transformer OLTC Buchholz trip
• 19. Unit auxiliary transformer instantaneous overcurrent protection
• 20. Overcurrent protection of excitation transformer (of static excitation system, if
installed)
• 21. Generator rotor overvoltage relay
• 22. Thyristor block failure (of static excitation system)
• 23. Generator–transformer and unit auxiliary transformer mulsifire protection
• 24. Automatic voltage regulator failure relay operated
• 25. Vacuum failure in outlet of LP turbine
Class B Protections
• 1. Negative phase sequence protection
• 2. Field failure protection
• 3. Back-up impedance protection
• 4. Under-frequency protection
• 5. Generator–transformer oil and winding temperature very high
• 6. Unit auxiliary transformer back-up overcurrent protection
• 7. Unit auxiliary transformer winding and oil temperature very high
• 8. Very high excitation transformer temperature
• 9. Thyristor (excitation system) fan supply failure
• 10. Very high stator water conductivity
• 11. Very high stator water flow
• 12. Master fuel trip relay of boiler has operated
• 13. Very high LP/HP heater water level
• 14. Thrust bearing (turbine) failure
• 15. Very low lubrication oil (turbine bearing) pressure
• 16. Very high and very low boiler drum level
• 17. Very high turbine bearing temperature
• 18. HP turbine inlet pressure low
• 19. Loss of boiler water
• 20. Loss of ID fans
• 21. Loss of FD fans
• 22. High condenser water level
• 23. Excessive over-firing in boiler
Class C Protections
• 1. Generator–transformer back-up earth-fault protection
• 2. Generator–transformer back-up overcurrent protection
• Many of these protections, particularly those related to boilers and
turbines are not dealt with here as they are not within the scope of
this discussion.
Information Required for Designing Protection Scheme for
Generator and for Relay Settings
Current Transformers
• 1. Class of CT
• 2. Knee point voltage
• 3. Magnetizing current
• 4. CT secondary resistance
• 5. CT ratio Actually, the CT requirements are engineered based on
generator particulars and type of protections employed. CT
requirements for different types of protections are already discussed
in CT chapter.
III. Pilot Lead Resistance
IV. Types of Relays Used for Different Types of Protections and their Technical
Particulars
V. Neutral Grounding Transformer
• 1. kVA rating
• 2. Voltage ratings of primary and secondary
• 3. Continuous rating/intermittent rating
VI. Type of Earthing of the Neutral
• 1. Resistance/reactance or any other type of earthing
• 2. Continuous duty/intermittent duty
• 3. Voltage rating
• 4. Current rating/power rating
VII. Potential Transformer
1. Voltage ratio
2. VA rating Once again, this is based on requirements of the relays
Differential Protection
Requirements of Generator Differential Protection
• The following are the requirements of generator differential
protection:
• The current transformers CT1 and CT2 need to be connected with
correct polarity.
• The differential protection shall operate sensitively for internal faults
and it shall remain stable against external faults.
• CTs on both the sides of the generator should have identical
saturation characteristics.
• The nonidentical CTs may not cause mal-operation for normal
conditions, but can cause inadvertent tripping of the generator for
very high through fault currents.
• The relay coil should be connected to the points, which are
equipotential [see points A and B, under normal conditions.
• The CTs and the machine to be protected are located at the turbine
floor and the relay is located in the control room.
• Hence, normally, it is not possible to connect the relay coil to the
equipotential points.
• If the connections are not at equipotential points then the burdens
on the two CTs are unequal, although the currents in the two CT
primaries are equal.
• This may cause the heavily burdened CT to saturate during through
fault conditions.
• This results in dissimilarity of ratio and phase angle errors of the CTs
producing an out-of-balance (spill) current in the relay coil, which
causes spurious operation of the relay.
• Differential relay should be immune to harmonics.
Voltage distribution in CT secondary during external fault

Voltage distribution in CT secondary during internal fault

• ideally identical CTs and equal lead lengths [from CT1 to relay and
from CT2 to relay in Figure cannot be obtained in practice.

• If the lead lengths are not equal, adjustable extra resistances can be
connected in series with pilot wires so that the relay coil is connected
to the equipotential points.

• For tackling the problem of non-identical CTs, a biased differential

relay can be used.

• The current through the bias (restraining) coil is made proportional to

the through current, thus making the relay stable with negligible
• If the lead lengths are not equal, adjustable extra resistances can be
connected in series with pilot wires so that the relay coil is connected
to the equipotential points.
• For tackling the problem of non-identical CTs, a biased differential
relay can be used.
• The current through the bias (restraining) coil is made proportional to
the through current, thus making the relay stable with negligible loss
of sensitivity on light faults.
• The biased differential relay can be set to pick-up at 5% of CT rating
and the percentage bias setting is usually about 10%.
CT and relay connections for differential
protection of generator
• Another way of getting rid of the problems of
non-identical CTs and unequal lead lengths is
the use of a stabilising resistance in series
with the relay coil.
• The value of the stabilising resistance can be
found out by considering the worst case; i.e.,
absolute saturation of one of the CTs while
the other is working in its linear range.
• This is the simplest way of assessing the
criteria of stability against through faults,
since if the relay setting is greater than the
spill current calculated by this method,
stability is assured.
• Referring to Fig. 5.2 and for the worst
condition assumed, Lm1 is infinite and Lm2 is
zero.
• An 11 kV, 3-phase, 50 Hz, 50 MVA, star-connected generator is protected by the
simple Merz-price protection. The CTs used are 3000/5 A. The relay is set to
operate for a current of 150 milli-amperes. Under direct through-fault condition
of 14 times full load, the CTs at one end will have a voltage that is 85% of that at
the other end. The relay having a resistive impedance of 100 ohms is connected
to the physical midpoint of the pilots. The pilot wire has a resistance of 0.54 ohm
per 100 metres. The distance between the two sets of CTs is 250 metres.
Determine the extra resistance required to be connected in series with the relay
to have a stability factor of 3 for this fault condition.