Question

A distribution feeder has three relaying points A,B and C, with A closest to the source. The relays used are all standard inverse-current (IDMT) types nominally rated at 1 A. The PSs for the relays at A,B and C are 95%,85% and 85%, respectively. The corresponding TMSs are 0.15,0.13 and 0.1 , respectively. The relays are supplied via 600/1 A, 400/1 A and 200/1 A CTs, respectively, at the three relaying points. The CBs at the three relaying points have operating times of 0.30 seconds. Determine whether the system can selectively isolate an affected section of the feeder given that the fault currents at A,B and C are 6.1kA,3.7kA and 1.26kA, respectively.

Answer 1

The evaluation of a protection system's selectivity in a distribution feeder. This involves calculating the relay operating times using inverse definite minimum time (IDMT) characteristics and assessing if the affected section can be isolated without impacting the rest of the system.

Step-by-step explanation:

To determining the selectivity of a protection system in a distribution feeder using standard inverse-current (IDMT) relays at different relaying points A, B, and C. The question provides plug setting (PS) and time multiplier setting (TMS) values for each relay, the current transformer (CT) ratios, and the fault currents at each point. The goal is to check if the system is capable of isolating only the affected section when a fault occurs.

Selectivity requires that only the relay closest to the fault operates. To verify selectivity, we would typically calculate the operating time for each relay using the provided information, which includes relay characteristics, CT ratios, fault current, PS, and TMS. These calculations involve the use of standard IDMT relay equations, which relate the operating time to the current multiple of the relay set value.

After calculating the operating times for each relay during the faults at different points, we add the circuit breaker (CB) operating time to determine the total clearing times. Finally, we compare the clearing times to ensure that the relay at the fault point operates first and that there's enough time margin between the operation of downstream and upstream relays.