Correct overcurrent relay application requires knowledge of the fault current that can flow in each part of the network. The data required for a relay setting study are:
- Single-line diagram of the power system involved, showing the type and rating of the protection devices and their associated current transformers.
- The impedances in ohms, percent or per unit, of all power transformers, rotating machine and feeder circuits.
- The maximum and minimum values of short circuit currents that are expected to flow through each protection device.
- The maximum load current through protection devices.
- The starting current requirements of motors and the starting of induction motors and the transformer inrush, thermal withstand and damage characteristics.
The relay settings are first determined to give the shortest operating times at maximum fault levels
and then checked to see if the operation will also be satisfactory at the minimum fault current
expected. Also, the basic rules for correct relay co-ordination can generally be
stated as follows:
- Whenever possible, use relays with the same operating characteristic in series with each other.
- 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 logic coordination. The common aim of all three methods is to give
correct coordination.
Time-Based Coordination (will update soon)
Current-Based Coordination (will update soon)
Logic Coordination (will update soon)
