Page 533 - Industrial Power Engineering and Applications Handbook

Captive (emergency) power generation 161507
engine fails to start at three consecutive attempts, the (e) Low lube (lubricating) oil pressure
starting relay (SR) automatically locks out and emits no
further ignition impulse. This feature is essential to protect Electrical faults
the engine against a possible hunting and a drain of the (a) Overcurrent alarm
battery. (b) Battery charger problems
(c) Overcurrent and ground fault
Switching off (d) Reverse power supply
(e) A fault condition in the differential scheme
Bus voltage relay (Relay code 27) provides an impulse (f) Breaker trip
to the generator trip circuit as soon as normal supply is
restored. The generator falls out of the circuit automatically
after a pause of 2- 10-30 seconds, and the engine stops. Figure 16.11 illustrates a general arrangement of such an
The relay now also has a delayed feature as it had during AMF panel and the above alarm and annunciation
the start, to allow a pause to the main supply in case the provisions.
main supply fails quickly again.
16.8.2 Electrical protection and metering
Lubricating oil
The following protections and metering are considered
An interlock is provided through a centrifugal type of necessary for a DG set:
pressure switch (PS) to trip the engine in the case of low
lube (lubricating) oil pressure during a run. Since during
a start oil pressure has not built up, a timer, T, is introduced For small ratings (say, up to 150 kVA)
to bypass the trip interlock and avoid a false trip. A relay
(contactor), d,, is used to provide lube oil pressure Short-circuit protection ~ through HRC fuses
interlock. Overcurrent-cum-short-circuit protection, through
thermal overcurrent and built-in short-circuit releases.
Figure 16.12 illustrates a general protection and
Fuel oil solenoid valve (SV) metering scheme.
This is to cut off the fuel supply to the engine on a trip A kVAr meter to indicate the reactive power of the
or a normal stop of the engine. circuit and to take corrective steps. when necessary.

Speed and voltage control For medium ratings (say, up to 1000 kVA)
Push button Nos 22 and 23 may be provided on the AMF (a) Voltage-controlled overcurrent and ground fault
panel and also on the remote panel to raise and lower the protection (relay Code 51N/64) By a three-pole
speed and voltage, when required to control the speed voltage controlled definite time delay relay with a
cf;) and voltage (E,) of the generator in order for it to be current setting of 50-200% and a time delay range
synchronized with another generator or an infinite bus. of 0.1-1 second. The relay is typical with a
Figure 16.10 illustrates a typical scheme incorporating combination of an overcurrent-time relay and an
all these features and interlocks. undervoltage (u.v.) supervision unit. The purpose vf
the U.V. unit is to detect an U.V. due to a ground fault
Notr in the machine. During a fault the relay switches
The above controls can also be achieved more precisely and almost over instantaneously to a lower response current value
instantly by applying the same logic with the use of PLCs, as and effects faster tripping. Typical characteristics can
discussed in Section 16.13. be
- An overload characteristic say, 50-200%, matching
16.8 Protection of a DG set the thermal withstand characteristic of the
generator. This operates during an overlvad
condition when the generator voltage is normal.
16.8.1 Alarm and annunciation (Figure 16.10) - A ground fault characteristic determined by the
U.V. unit monitoring the generator voltage. During
Some or all of the following alarms and annunciations a ground fault, causing an u.v., the same
may be provided on the AMF panel, depending upon the overcurrent characteristic is reduced to another
size of the DC set and the type of load it has to feed to set value, resulting in a faster tripping, as desired.
forewarn the operator. and prevent a trip:

Mechanical faults Note
The relay can also be selected for a phase fault.
(a) Engine fails to start
(b) Engine overspeeds
(c) Engine high water temperature and high lube oil (b) Reverse power supply protection (Relay Code 32)
temperature To detect a motoring action by a single-pole reverse
(d) Low level of fuel power relay with a time setting of 2-10 seconds for