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 generators
- Failure of insulation of the stator winding
- Failure of insulation of the rotor winding
- Unbalanced loading
- Field failure
- Overload
- Overvoltage
- Failure of prime-mover
- Loss of synchronism
- Over-speed
- Under-frequency
- Over-heating
Protective schemes employed for generator protection
- Differential protection
- Inter-turn fault protection
- Stator earth-fault protection
- Overcurrent and earth-fault protection
- Rotor earth-fault protection
- Negative phase-sequence protection
- Field failure protection
- Overload protection
- overvoltage protection
- Reverse power protection
- Pole-slipping protection
- Back-up impedance protection
- Under-frequency protection
Class A, Class B and Class C Protections for Generator
Class A Protection
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
Class B Protection
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.
Class C Protection
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.
Types of protection in Class A, Class B and Class C Protections
Class A Protections
- Differential protection
- Stator earth-fault protection
- Inter-turn fault protection
- Overcurrent and earth-fault protection (for small generators)
- Rotor second earth-fault protection
- Overvoltage protection
- Reverse power protection
- Pole-slipping protection
- Generator-transformer overall differential protection (refer Chapter 6)
- Local breaker back-up protection (refer Chapter 10)
- Generator transformer restricted earth-fault protection
- Differential protections of unit auxiliary transformers
- Generator–transformer over-fluxing protection
- Generator–transformer Buchholz trip
- Generator–transformer pressure relief device operated
- Unit auxiliary transformer Buchholz trip
- Unit auxiliary transformer pressure relief device operated
- Unit auxiliary transformer OLTC Buchholz trip
- Unit auxiliary transformer instantaneous overcurrent protection
- Overcurrent protection of excitation transformer (of static excitation system, if installed)
- Generator rotor overvoltage relay
- Thyristor block failure (of static excitation system)
- Generator–transformer and unit auxiliary transformer mulsifire protection
- Automatic voltage regulator failure relay operated
- Vacuum failure in outlet of LP turbine
Class B Protections
- Negative phase sequence protection
- Field failure protection
- Back-up impedance protection
- Under-frequency protection
- Generator–transformer oil and winding temperature very high
- Unit auxiliary transformer back-up overcurrent protection
- Unit auxiliary transformer winding and oil temperature very high
- Very high excitation transformer temperature
- Thyristor (excitation system) fan supply failure
- Very high stator water conductivity
- Very high stator water flow
- Master fuel trip relay of boiler has operated
- Very high LP/HP heater water level
- Thrust bearing (turbine) failure
- Very low lubrication oil (turbine bearing) pressure
- Very high and very low boiler drum level
- Very high turbine bearing temperature
- HP turbine inlet pressure low
- Loss of boiler water
- Loss of ID fans
- Loss of FD fans
- High condenser water level
- Excessive over-firing in boiler
Class C Protections
- Generator–transformer back-up earth-fault protection
- Generator–transformer back-up overcurrent protection