II. OPTIMAL RELAY COORDINATION PROBLEMS
Overcurrent relays are devices which have ability to interrupt electricity supply service due to some severe fault. In a modern electrical power system, network interconnection is very complicated. This affects the difficulty of key parameter setting of protective relaying devices [5]. When a total number of overcurrent relays to be coordinated is increased or even feeding in closed-loop configuration is required according to a complex transmission network, overcurrent relay coordination setting is more difficult.An overcurrent relay is a typical protective relay that allows a protected load operating within a preset value of the load current.
The overcurrent relay is placed at the secondary side of the current transformer. The operating time of the overcurrent relay can vary due to relay type, time-dial setting (TDS) and magnitude of fault currents. For the inverse time overcurrent relay which corresponds to the ANSI device number of 51, the operating time of the overcurrent relay can be expressed as shown in (1) according to the IEC standard 255-4 [6].