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 4.4.2.3 Overspeed. A prime mover accelerates when the generator it drives becomes
separated from its load. The acceleration depends on the inertia, the rate and amount of load
loss, and the governor response. A generator overfrequency relay can be used to supplement the
prime mover electromechanical overspeed equipment.
4.4.2.4 Loss of Excitation. Field relays protect against varying degrees of abnormally low
excitation. The relay operates on an abnormally low value or failure of machine field current to
keep the generator from falling out of synchronism with the rest of the system.
4.4.2.5 Motoring. Generator motoring protection is designed to protect the prime mover
flow from the system into the generator. Time delay is normally included to prevent operation
during system transients.
4.4.3 Transformer Protection. Differential relays are the principal form of fault protection for
transformers rated at 10 MVA and above. These relays, however, cannot be as sensitive as the
differential relays used for generator protection. Overcurrent relays can also be used as
protection against external or internal faults. Directional distance relaying can be used for
transformer backup protection when the setting or coordination of the overcurrent relays is a
problem. Smaller transformers may be protected, entirely by primary and secondary overcurrent
devices. Transformer protection must provide the following:
4.4.3.1 Protection of the transformer from harmful conditions occurring on the connected
system.
4.4.3.2 Protection of the power system from the effects of transformer failure.
4.4.3.3 Detection and indication of conditions occurring within the transformer which
might cause damage or failure.
4.4.4 Bus Protection. Differential protection is the most sensitive and reliable method for
protecting buses. Differential protection provides quick action and permits complete overlapping
with the other power system relaying. Differential protection methods generally used, in
decreasing order of effectiveness, are as follows:
(a)
Voltage responsive and linear coupler methods.
(b)
Percentage differential.
(c)
Current responsive.
(d)
Partial differential.
4.4.4.1 Auxiliary Relay. Since the differential relay must trip all circuit breakers
connected to the bus, a multi-contact auxiliary relay is needed.
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