20-07-2012, 04:16 PM
Overcurrent Protection for Phase and Earth Faults
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INTRODUCTION
Protection against excess current was naturally the
earliest protection system to evolve. From this basic
principle, the graded overcurrent system, a discriminative
fault protection, has been developed. This should not be
confused with ‘overload’ protection, which normally
makes use of relays that operate in a time related in
some degree to the thermal capability of the plant to be
protected. Overcurrent protection, on the other hand, is
directed entirely to the clearance of faults, although with
the settings usually adopted some measure of overload
protection may be obtained.
CO-ORDINATION PROCEDURE
Correct overcurrent relay application requires knowledge
of the fault current that can flow in each part of the
network. Since large-scale tests are normally
impracticable, system analysis must be used – see
Chapter 4 for details. The data required for a relay
setting study are:
i. a one-line diagram of the power system involved,
showing the type and rating of the protection
devices and their associated current transformers
ii. the impedances in ohms, per cent or per unit, of
all power transformers, rotating machine and
feeder circuits
iii. the maximum and minimum values of short circuit
currents that are expected to flow through each
protection device
iv. the maximum load current through protection
devices
v. the starting current requirements of motors and
the starting and locked rotor/stalling times of
induction motors
vi. the transformer inrush, thermal withstand and
damage characteristics
vii. decrement curves showing the rate of decay of
the fault current supplied by the generators
viii. performance curves of the current transformers
The relay settings are first determined to give the
shortest operating times at maximum fault levels and
then checked to see if operation will also be satisfactory
at the minimum fault current expected. It is always
advisable to plot the curves of relays and other
protection devices, such as fuses, that are to operate in
series, on a common scale. It is usually more convenient
to use a scale corresponding to the current expected at
the lowest voltage base, or to use the predominant
voltage base. The alternatives are a common MVA base
or a separate current scale for each system voltage.
PRINCIPLES OF TIME/CURRENT GRADING
Among the various possible methods used to achieve
correct relay co-ordination are those using either time or
overcurrent, or a combination of both. The common aim
of all three methods is to give correct discrimination.
That is to say, each one must isolate only the faulty
section of the power system network, leaving the rest of
the system undisturbed.
Discrimination by Time
In this method, an appropriate time setting is given to
each of the relays controlling the circuit breakers in a
power system to ensure that the breaker nearest to the
fault opens first. A simple radial distribution system is
shown in Figure 9.1, to illustrate the principle.