Protection coordination study, discrimination study, selectivity study or relay setting study helps to select and arrive at the protection setting of various protection devices in the Power system viz.Medium Voltage Relays, LV Circuit Breakers, Fuses etc. so that the fault at certain part of the power system is cleared by the protection device installed immediately upstream of the fault without affecting the healthy part of the electrical network.

The protection scheme design criteria are based on the following terminology :

• Reliability
  • Dependability - is a measure of the ability of the relay / scheme to operate when it is supposed to operate, i.e., an internal fault in the protection zone.
  • Security - is a measure of the relay/scheme ability to avoid operation when tripping is not desired, i.e., a fault external to the protection zone.
• Selectivity - this is the ability of different relays / schemes to co-ordinate with each other to minimize the area disconnected as a result of the fault.
• Operating Speed – the fault clearance time refers to the total of the relay operating time and circuit breaker opening time.
• Sensitivity – this refers to the minimum relay operating quantities required to detect an abnormal fault condition.

The protection setting calculations are carried out based on the arrived worst-case maximum and minimum short circuit KA values for the network. The Time current curves (TCC plots) are plotted showing the curves of MV relays up-to final LV MCBs in the network. Selective discrimination should be achieved between protection devices to allow for proper co-ordination and sequential operation during the event of a fault.

The settings applied to time delayed overcurrent and earth fault protection are selected to ensure that an optimum time discrimination margin exists between successive devices. The results from the Protection coordination study or Discrimination study ensures the coordination between the upstream and downstream devices ensuring continued operation. By conducting the relay setting study or discrimination study and properly arriving at the settings nuisance tripping of the breakers can be avoided.