Question

Figure 1 shows the one-line diagram of a radial system. There are four (4) relaying units:three (3) phase-fault units and one (1) ground-fault unit as explained in class. It is desired to determine the operation time of the relays for faults (short-circuits) over the system, as indicated in Figure 1. Table I shows the primary currents for four types of faults. The relays are of the SEL-501 type (a simple box contains all units for this relay), with the following settings: Assuming that the current transformers do not saturate, find the operation times of each unit (phases a, b, c and ground) for the faults indicated on Table I.

Fig: 1

Fig: 2

Fig: 3

Fig: 4

Fig: 5

Fig: 6