1. Introduction
2.Co-ordination procedure
3.Principles of time/current grading
4.Standard I.D.M.T. overcurrent relays
5.Combined I.D.M.T. and high set instantaneous overcurrent relays
6.Very inverse (VI) overcurrent relays
7.Extremely inverse (EI) overcurrent relays
8.Other relay characteristics
9.Independent (definite) time overcurrent relays
10. Relay current setting
11. Relay time grading margin
12. Recommended grading margins
13. Calculation of phase fault overcurrent relay settings
14. Directional phase fault overcurrent relays
15. Ring mains
16. Earthfault protection
17. Directional earthfault overcurrent protection
18. Earthfault protection on insulated networks
19. Earthfault protection on Petersen Coil earthed networks
. Examples of time and current grading
. References
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 [A3: Fault Calculations]
for details. The data required for a relay setting study are:
a. a one-line diagram of the power system involved, showing
the type and rating of the protection devices and their associated current transformers
b. the impedances in ohms, per cent or per unit, of all power transformers, rotating machine and feeder circuits
c. the maximum and minimum values of short circuit currents that are expected to flow through each protection device
d. the maximum load current through protection devices
e. the starting current requirements of motors and the starting and locked rotor/stalling times of induction motors
f. the transformer inrush, thermal withstand and damage characteristics
g. decrement curves showing the rate of decay of the fault current supplied by the generators
h. 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.
The basic rules for correct relay co-ordination can generally be stated as follows:
a. whenever possible, use relays with the same operating characteristic in series with each other
b. make sure that the relay farthest from the source has current settings equal to or less than the relays behind it, that is, that the primary current required to operate the
relay in front is always equal to or less than the primary current required to operate the relay behind it
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.
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