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10
A-C GENERATOR AND MOTOR PROTECTION
171
10
A-C GENERATOR AND MOTOR PROTECTION
The remaining chapters deal with the application of protective relays to each of the several
elements that make up the electric power system. Although there is quite good agreement
among protection engineers as to what constitutes the necessary protection and how to
provide it, there are still many differences of opinion in certain areas. This book describes
the general practice, giving the pros and cons where there are differences of opinion. Four
standard-practice publications deal with the application of protective relays.
1,2,3,4
Manufacturers publications are also available.
5,6,20
Bibliographies of relaying literature
prepared by an AIEE committee provide convenient reference to a wealth of information
for more detailed study.
7
Frequent reference will be made here to publications that have
been found most informative.
The fact that this book recognizes differences of opinion should not be interpreted as
complete approval of the various parallel practices. Although it is recognized that there
may sometimes be special economic and technical considerations, nevertheless, much can
still be done in the way of standardization.
GENERATOR PROTECTION
Except where specifically stated otherwise, the following will deal with generators in
attended stations, including the generators of frequency converters.
The protection of generators involves the consideration of more possible abnormal
operating conditions than the protection of any other system element. In unattended
stations, automatic protection against all harmful abnormal conditions should be
provided.
1
But much difference of opinion exists as to what constitutes sufficient protection
of generators in attended stations. Such difference of opinion is mostly concerning the
protection against abnormal operating conditions, other than short circuits, that do not
necessarily require the immediate removal from service of a machine and that might be
left to the control of an attendant.
The arguments that are advanced in favor of a minimum amount of automatic protective
equipment are as follows: (a) the more automatic equipment there is to maintain, the
poorer maintenance it will get, and hence it will be less reliable; (b) automatic equipment
might operate incorrectly and trip a generator undesirably; (c) an operator can sometimes
avoid removing a generator from service when its removal would be embarrassing. Most of
the objection to automatic protective equipment is not so much that a relay will fail to
operate when it should, but that it might remove a generator from service unnecessarily.
Part of the basis for this attitude is simply fear. Each additional device adds another 172
A-C GENERATOR AND MOTOR PROTECTION
contact that can trip the generator. The more such contacts there are, the greater is the
possibility that one might somehow close when it should not. There is some justification
for such fears. Relays have operated improperly. Such improper operation is most likely in
new installations before the installation kinks have been straightened out. Occasionally,
an abnormal operating condition arises that was not anticipated in the design or
application of the equipment, and a relay operates undesirably. Cases are on record where
cleaning or maintenance personnel accidentally caused a relay to trip a generator. But, if
something is known to be basically wrong with a protective relay so that it cannot be relied
on to operate properly, it should not be applied or it should be corrected one way or
another. Otherwise, fear alone is not a proper basis for omitting needed protection.
Admittedly, an alert and skillful operator can sometimes avoid removing a generator from
service. In general, however, and with all due respect to operators, the natural fear of
removing a machine from service unnecessarily could result in serious damage. Operators
have been known to make mistakes during emergencies and to trip generators
unnecessarily as well as to fail to trip when necessary.
8
Furthermore, during an emergency,
an operator has other important things to do for which he is better fitted.
An unnecessary generator outage is undesirable, but one should not try to avoid it by the
omission of otherwise desirable automatic protection. It is generally agreed that any well-
designed and well-operated system should be able to withstand a short unscheduled
outage of the largest generating unit.
9
It is realized that sometimes it may take several
hours to make sure that there is nothing wrong with the unit and to return it to service.
Nevertheless, if this is the price one has to pay to avoid the possibility of a units being out
of service several months for repair, it is worth it. The protection of certain generators
against the possibility of extensive damage may be more important than the protection of
the service of the system.
9
The practice is increasing of using centralized control, which requires more automatic
equipment and less manual on the spot supervision, in order to provide higher standards
of service with still greater efficiency.
10
Such practice requires more automatic protective-
relaying equipment to provide the protection that was formerly the responsibility of
attendants.
8
SHORT-CIRCUIT PROTECTION OF STATOR WINDINGS
BY PERCENTAGE-DIFFERENTIAL RELAYS
It is the standardized practice of manufacturers to recommend differential protection for
generators rated l000 kva or higher,
2
and most of such generators are protected by
differential relays.
11
Above 10,000 kva, it is almost universally the practice to use differential
relays.
9
Percentage-differential relaying is the best for the purpose, and it should be used
wherever it can be justified economically. It is not necessarily the size of a generator that
determines how good the protection should be; the important thing is the effect on the rest
of the system of a prolonged fault in the generator, and how great the hardship would be
if the generator was badly damaged and was out of service for a long time.
The arrangement of CTs and percentage-differential relays is shown in Fig. 1 for a wye-
connected machine, and in Fig. 2 for a delta machine. If the neutral connection is made
inside the generator and only the neutral lead is brought out and grounded through low A-C GENERATOR AND MOTOR PROTECTION
173
impedance, percentage-differential relaying for ground faults only can be provided, as in
Fig. 3. The connections for a so-called unit generator-transformer arrangement are
shown in Fig. 4; notice that the CTs on the neutral side may be used in common by the
differential-relaying equipments of the generator and the transformer.
For greatest sensitivity of differential relaying, the CT primary-current rating would have to
be equal to the generators rated full-load current. However, in practice the CT primary-
current rating is as much as about 25% higher than full load, so that if ammeters are
connected to the CTs their deflections will be less than full scale at rated load. It may be
impossible to abide by this rule in Fig. 5; here, the primary-current rating of the CTs may
have to be considerably higher than the generators rated current, because of the higher
system current that may flow through the CTs at the breakers.
The way in which the generator neutral is grounded does not influence the choice of
percentage-differential relaying equipment when both ends of all windings are brought
out. But, if the neutral is not grounded, or if it is grounded through high enough
impedance, the differential relays should be supplemented by sensitive ground-fault
relaying, which will be described later. Such supplementary equipment is generally
provided when the ground-fault current that the generator can supply to a single-phase-to-
ground fault at its terminals is limited to less than about rated full-load current. Otherwise,
the differential relays are sensitive enough to operate for ground faults anywhere from the
Fig. 1. Percentage-differential relaying for a wye-connected generator. 174
A-C GENERATOR AND MOTOR PROTECTION
terminals down to somewhat less than about 20% of the winding away from the neutral,
depending on the magnitude of fault current and load current, as shown in Fig. 6, which
was obtained from calculations for certain assumed equipment. This is generally
considered sensitive enough because, with less than 20% of rated voltage stressing the
insulation, a ground fault is most unlikely; in the rare event that a fault did occur, it would
simply have to spread until it involved enough of the winding to operate a relay. To make
the percentage-differential relays much more sensitive than they are would make them
likely to operate undesirably on transient CT errors during external disturbances.
The foregoing raises the question of CT accuracy and loading. It is generally felt that CTs
having an ASA accuracy classification of 10H200 or 10L200 are satisfactory if the burdens
impo