TABLE OF CONTENTS The procedure for determining the settings of overcurrent relays, as illustrated in the past sections, is relatively simple for radial or medium-sized interconnected systems. However, for large systems, the procedure becomes cumbersome if performed manually and therefore software techniques are required, especially if different topologies have to be analysed. This section introduces a very simple procedure to set overcurrent relays using different algorithms. The entry data required are the short-circuit currents for faults at all busbars, the margins and constraints of the system and the available settings of the relays being co-ordinated. In addition, the settings of those relays closest to the loads and at the boundaries of other networks have to be considered.
The overall process consists basically of three steps:
Locate the fault and obtain the current for setting the relays.
Identify the pairs of relays to be set, determining first which one is furthest away from the source, and which is acting as the back-up. The program should define the settings in accordance with the criteria given in Section 5.3.
Verify that the requirements given in Section 5.4 are fulfilled; otherwise the process should be repeated with lower discrimination margins, or new relays should be tried.
The single line diagram given in Figure 5.20 can be used to illustrate simply how a typical computer program can tackle a co-ordination problem.
The algorithm files the discrimination margins required, the setting of the relays closest to the loads, (1,2 and 3 in...
