Application

Without the ability to act on the direction of current flow, it is difficult to coordinate the settings of time

overcurrent relays on lines that interconnect a series of substations. Without this capability, either

undesired tripping of adjacent lines may occur or a fault may go undetected because of the high settings

required by non-directional relays.

With directional time overcurrent relays, the settings and time delays can be decreased and the undesired

tripping eliminated. Figure 1-5 illustrates the use of directional overcurrent relays on a group of

interconnected distribution substations fed from a common source. In this example, non-directional

overcurrent relays (51) are used to protect the lines leaving the supply bus because there is only one

source of fault current. However, the breakers at the load buses (C, D, E, and F) are protected by

directional time overcurrent relays (67) to prevent overtripping in the event of a fault. This will remove the

faulted line and retain service to the connected loads.

In the case where two sources of power can supply fault current, as shown in Figure 1-6. directional

overcurrent relays will need to be applied to each end of the protected lines to prevent undesired tripping.