Relay coordination is necessary to ensure that while protection relays operate as fast as possible, they are also able to isolate only the faulted parts of the system from the network, ensuring that a power system disturbance does not result in interruption of the power supply to a larger part of the power system network. Optimal relay coordination for overcurrent relays depends on two parameters, namely, Time Multiplier and Pickup Current Setting. The conventional method of setting these two parameters for overcurrent relays applied on the power system network is to first determine the main and backup relay pairs which form part of the clockwise and anti-clockwise loops around the power system network. The relays are then set through an iterative process to ensure coordination. Initially, a general rule of setting relays to operate in 0.2 seconds for faults in the primary zone, to ensure fast operation, and in 0.2 seconds plus additional grading time, to ensure coordination, for faults in the backup zone is applied. The next relay in the loop is tested to check if it fulfils the requirements of the initial general rule. If the conditions of the general rule are not met, the previous relay’s setting is adjusted to meet the requirements. This process is repeated until all the relays around the loop are set. Conventional relay coordination process has a limitation in the sense that it is deterministic and the settings of subsequent relays depend on the initial guess of the settings of the initial relay. Therefore, this method does not necessarily provide solutions which guarantee optimal relay coordination but the best of the solutions tried.

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