Apart from direct temperature measurement of the stator windings and the internal air, the protection of a generator is largely based on the sensing of current and voltage from CTs and VTs.
The number and type of protective relay functions increases with the generator
kV A rating and voltage level.
Protective relays are electromagnetic (traditional) or electronic (increasingly more common) which are mounted on the generator front panel of the main switchboard.
Some protective functions may be grouped together within a single relay case.
Settings for level and time-delay must be periodically checked by injecting currents and/or voltages directly into the relay (usually via a special multi-pole socket adjacent to the relay and internally wired to it).
OCIT (Over Current Inverse Time) protection of ship generator
The Over Current Inverse Time relay function monitors general balanced overloading and has current / time settings determined by the overall protective discrimination scheme
Typical setting ranges for current (I) and time (t) are:
I>:0.7-2.In, (In : normal or rated generator current)
and t: 1-10s
OC(INST.) – “Instantaneous” trip protection of ship generator
“Instantaneous” trip to protect against extremely high overcurrent caused by a short-circuit fault.
Typical setting ranges are:
I >>: 2 – 10 In, and t: 0.1 – 1s
NPS (Negative Phase Sequence relay) protection of ship generator
A Negative Phase Sequence relay determines the amount of unbalance in the stator currents which is an indirect measure of the generator stator and rotor temperature.
A relatively small degree of unbalance causes a significantly increased temperature rise so the NPS current setting is low at around 0.2.In.
DIFF (Differential Measurement) protection of ship generator
This is a differential measurement of current at each end of a stator phase winding.
This comparison of current is to detect an internal fault in the stator windings which may be caused by partially short-circuited coil turns and/or earth faults.
Current settings for this very serious fault are very low e.g. about 0.1.In.
EL (Earth Leakage) protection of ship generator
An Earth Leakage relay (sometimes called Zero Phase Sequence) detects an earth fault current returning back through the earthed neutral connection.
In a ship’s HV generator system the earth fault current is limited by a high impedance NER (neutral earthing resistor) or earthing transformer so the pick-up current setting is very low, e.g. 1-5 A with a time delay of 0.1- 0.5 s.
UV/OV (Under Voltage and Over Voltage) protection
Under Voltage and Over Voltage functions are monitored by these relays with
settings of around 0.8 Un and 1.2 Un respectively (Un : rated voltage) with time delays of about 2s.
An overvoltage function may not be required in many protection schemes.
UF/OF (Under and Over Frequency) protection of ship generator
Under and Over Frequency settings are typically 58 Hz and 62 Hz for a 60 Hz system.
LO (Lock Out) protection of ship generator
This is the master Lock Out or trip / hand-reset relay responsible for tripping the generator circuit breaker.
Its action is instantaneous when triggered by protective relay.
It can also be used to trip the generator prime-mover and initiate generator field suppression together with the signalling of an alarm.
ARP (Reverse Power Protection) of ship generator
Generators intended to operate in parallel must have reverse power protection (RP).
A reverse power relay monitors the direction of power flowing between the generator and the load.
If a prime-mover failure occurred the generator would act as a motor.
The reverse power relay detects this fault and acts to trip the generator circuit-breaker.
The pick-up power level setting and time-delay setting are adjustable and are pre-set to suit the prime-mover.
If the prime mover is a turbine, very little power is absorbed when motoring and a reverse-power pick-up setting of 2-3 % is usual.
If the prime mover is a diesel then a setting range of 5-15% is usually adopted. A time delay range of about 0.5 – 3 s is usual.
The RP (Reverse Power) relay operation is easily checked during a generator changeover.
The outgoing generator is gradually throttled down so that it motors causing the reverse power relay to trip its generator circuit-breaker.