11-10-2012, 12:28 PM
LINE PROTECTION WITH OVERCURRENT RELAYS
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Introduction
Lines are protected by overcurrent-, distance-, or pilot-relaying equipment, depending on
the requirements. Overcurrent relaying is the simplest and cheapest, the most difficult to
apply, and the quickest to need readjustment or even replacement as a system changes. It
is generally used for phase- and ground-fault protection on station-service and distribution
circuits in electric utility and in industrial systems, and on some subtransmission lines
where the cost of distance relaying cannot be justified. It is used for primary ground-fault
protection on most transmission lines where distance relays are used for phase faults, and
for ground back-up protection on most lines having pilot relaying for primary protection.
However, distance relaying for ground-fault primary and back-up protection of
transmission lines is slowly replacing overcurrent relaying. Overcurrent relaying is used
extensively also at power-transformer locations for externa-fault back-up protection, but
here, also, there is a trend toward replacing overcurrent with distance relays.
It is generally the practice to use a set of two or three overcurrent relays for protection
against interphase faults and a separate overcurrent relay for single-phase-to-ground faults.
Separate ground relays are generally favored because they can be adjusted to provide faster
and more sensitive protection for single-phase-to-ground faults than the phase relays can
provide. However, the phase relays alone are sometimes relied on for protection against all
types of faults. On the other hand, the phase relays must sometimes be made to be
inoperative on the zero-phase-sequence component of ground-fault current. These subjects
will be treated in more detail later.
Overcurrent relaying is well suited to distribution-system protection for several reasons. Not
only is overcurrent relaying basically simple and inexpensive but also these advantages are
realized in the greatest degree in many distribution circuits. Very often, the relays do not
need to be directional, and then no a-c voltage source is required. Also, two phase relays
and one ground relay are permissible. And finally, tripping reactor or capacitor tripping
(described elsewhere) may be used.
HOW TO SET INVERSE-TIME-OVERCURRENT RELAYS FOR COORDINATION
The first step is to choose the pickup of the relay so that it will (1) operate for all short
circuits in its own line, and (2) provide back-up protection for short circuits in immediately
adjoining system elements under certain circumstances. For example, if the adjoining
element is a line section, the relay is set to pick up at a current somewhat less than it
receives for a short circuit at the far end of this adjoining line section under minimum
generatingÐor otherÐconditions that would cause the least current flow at the relay
location. This is illustrated in Fig. 1.
For a phase relay, a phase-to-phase fault would be assumed since it causes less current to
flow than does any other fault not involving ground. However, a phase relay must not be
so sensitive that it will pick up under emergency conditions of maximum load over the line
from which it receives its current. For a ground relay, a single-phase-to-ground fault would
be assumed; load current is not a factor in the choice of a ground-relayÕs pickup except in
a distribution system where there is ground current normally because of unbalanced
loading. If there are two or more adjoining line sections, the fault should be assumed at
the end of the section that causes the least current to flow at the location of the relay being
adjusted.
Because of the effect of parallel circuits not shown, less current will flow at the relay
location of Fig. 1 if breaker A is closed than if A is open. If satisfactory adjustment can be
obtained with A closed, so much the better. However, the relay under consideration is
being adjusted to operate if breaker B fails to open; it is not generally assumed that breaker
A will also fail to open. There may be some occasions when one will wish to assume
simultaneous equipment failures at different locations, but it is not the usual practice.
Hence, it is permissible to assume that breaker A has opened, which is usually very helpful
and may even be necessary.
ARC AND GROUND RESISTANCE
Although there is much difference of opinion on the interpretation of test data, the
maximum value of rms volts per foot of arc length given by any of the data1,2,3 for all arc
currents greater than l000 rms amperes is about 550. For currents below l000 amperes, the
Fig. 3. Operating time of overcurrent relays with inverse-time characteristics.
LINE PROTECTION WITH OVERCURRENT RELAYS
formula V = 8750/I 0.4 gives the maximum reported value of rms volts per foot (V) for any
rms value of arc current (I); from this formula, values considerably higher than 550 will be
obtained at low currents. Actually, this formula gives a fairly good average of all the
available data for any value of arc current, as will be seen by plotting superimposed the
data of Fig. l of Reference 1, Fig. 5 of Reference 2, and the foregoing formula which is
obtained from the formula given in Reference 3.
EFFECT OF LOOP CIRCUITS ON OVERCURRENT-RELAY ADJUSTMENTS
Figure 3 best serves the purpose of illustrating how selectivity is provided with inverse-timeovercurrent
relays. But, lest it mislead one by oversimplifying the problem, it is well to
realize that, except for some parts of distribution systems, Fig. 3 does not truly represent
most actual systems where loops are the rule and radial circuits are the exception. The
principles involved and the general results obtained in the application and adjustment of
overcurrent relays are correctly shown by reference to Fig. 3, but the difficulties in arriving
at suitable adjustment in an actual system are minimized. This consideration is important
because it is often the deciding factor that leads one to choose distance or pilot relaying in
preference to overcurrent relaying.