TABLE OF CONTENTS Directional overcurrent protection is used when it is necessary to protect the system against fault currents that could circulate in both directions through a system element, and when bi-directional overcurrent protection could produce unnecessary disconnection of circuits. This can happen in ring or mesh-type systems and in systems with a number of infeed points. The use of directional overcurrent relays in the two situations is shown in Figure 7.1.
Directional overcurrent relays are constructed using a normal overcurrent unit plus a unit that can determine the direction of the power flow in the associated distribution system element. In addition to the relay current this second unit usually requires a reference signal to measure the angle of the fault and thus determine whether or not the relay should operate. Generally, the reference or polarisation signal is a voltage but this can also be a current input.
Basically, there are two methods for obtaining the direction of the power flow-supervision and control; both cases are illustrated in Figure 7.2 where D indicates the directional unit and OC the overcurrent unit. It is better to use the control system to determine the direction of the power flow, since the overcurrent unit only picks up when the flow is in the correct direction. With the supervision method, the overcurrent unit can pick up for the wrong power flow direction. In addition, when a breaker is opened in a...
