Page 540 - Industrial Power Engineering and Applications Handbook

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(f) Overvolts per Hertz protection (V/Hz) (Relay Code or transient instability or due to loss of excitation or
59-G) Per unit voltage divided by per unit frequency, out-of-phase synchronism. Out-of-step (phase)
called V/Hz, is proportional to flux in generator and operation can result in high peaks of current and
generator-transformer cores. (See also Section 1.2, frequency, and cause winding stresses, pulsating torque
equation (1.5) and Section 6.2.2). Excessive flux can and mechanical resonance. All these may endanger
cause serious overheating of metallic parts and melting the generator windings. Loss of excitation relay may
of generator core laminations. (See also Section 1.6.2. provide detection of such a situation, but it may not
(Aiv) eauations (1.12) and (1.13).) Excessive flux be reliable under all conditions. An MHO-type relay
~,
~
I,
(V/Hz) ;an be caused by voltage regulator failure, is generally used to protect the generator from such
load rejection or excessive excitation. Even though a situation.
V/Hz is more likely to occur when generator is off- 0) Abnormal frequency operation (Relay Code 81-
line, it can also happen when generator is on-line. G) For a generator connected to a power system,
Normally a two-stage protection is provided for abnormal frequency operation may be the result of a
V/Hz protection. If V/Hz exceeds 1.18 per unit, a severe system disturbance. An isolated unit, however,
protection relay trips the generator within two can operate at a low or high frequency, due to an
seconds (typical). For V/Hz 1.1 to 1.18 per unit, a incorrect speed control adjustment or a malfunctioning
protection relay trips the generator within 45 seconds of the speed control device.
(typical). The generator can tolerate underfrequency operation
Generator stator ground fault protection (Relay for long periods, provided that the loads and the
Codes 64-G and 51-GN) The neutral of the generator voltage are proportionately reduced. It can also tolerate
stator normally operates at a potential, close to ground, overfrequency operation, when the voltage is within
through a high-impedance single-phase distribution- the permissible limits.
type transformer the secondary of which is shunted If a system disturbance requires extra generating
through a resistor that has a voltage relay device 64- capacity, the generator may drop speed, reducing the
G connected across the resistor. In the event of a frequency proportionately. The under-frequency relay
ground fault, current flows through the resistor and 81-G will detect such a condition and operate a
the relay 64-G operates due to a voltage drop across systematic load shedding in a programmed manner
the resistor. An overcurrent relay, 5 1-GN, connected in order to meet the load demand. If the abnormal
to the current transformer in the secondary winding condition persists, the generator is taken off the main
of the grounding transformer is used as back-up for supply system.
64-G relay. (k) Differential protection (Relay Code 87-G) Diffe-
Exciter protection Protective devices are incorpo- rential protection to detect stator phase-to-phase fault.
rated, as required for protection of the specific (I) Surge protection This is similar to that for motors
excitation system components and are supplied by (Sections 17.10 and 18.8).
generator manufacturers as an integral part of the
excitation equipment. Some of the protective devices Protection of unit auxiliary transformer (UT)
are 49-UT RTD protection for warning of winding
Field ground protection (Relay Code 64-GF) overheating (see also Sections 12.7 and
The generator field winding is electrically isolated 12.8 ).
from ground. Therefore one ground fault in the 5 1 -UT Overcurrent protection
rotor windings will usually not damage the rotor. 5 1 N-UT Ground fault protection
However, two or more ground faults in the rotor 63-UT Gas protection through a Bucholz relay
windings will cause magnetic and thermal imba- 64R-UT Restricted ground fault protection
lances, which may result in localized heating and 87-UT Differential protection to detect a phase-
may damage the rotor. Protection provided in the to-phase fault.
excitation system is used to trip the generator in
the event of a ground fault in the rotor windings. Prntertion of generator transformer (GT)
Field overheating protection This is a part of 49-GT RTD protection for warning of winding
excitation system to prevent the field from prolon- overheating.
ged overcurrent through an alarm or a signal. It Overcurrent
will trip the generator only if it is absolutely protection For overcurrent protection, comments are
essential. The field winding can carry momentary the same as for generator protection, 49-G
overcurrents for a short period, expressed in terms item ‘e’
of field voltages, as noted below: 5 1 N-GT Ground fault protection
63-GT Gas protection through a Bucholz relay
Time (seconds) 10 30 60 120 64R-GT Restricted ground fault protection
Field voltage 208% 146% 125% 112% 87-GT Differential protection to detect a phase-
to-phase fault.
Pole slipping or out of step protection (Relay code Note
78-G) This is a protection against loss of synchro- Relay Code numbers are according to ANSI designations. For details
nism. It can occur as a result of steady-state, dynamic refer to Appendix 2.