Assignment

1. FIGURE 1 shows four generators feeding into a sectionalised busbar with a fault limiting reactor.

FIG. 1

Each generator is rated at 11 kV and 30 MVA and has a reactance of 0.2 p.u.

Calculate the value of the reactance to limit the fault level on a feeder to 500 MVA.

2. FIGURE 2 shows the sectionalised busbar of a two generator power station connected to the national grid.

Calculate, using a 40 MVA base, the fault level for a 3-phase symmetrical short circuit on the 11 kV feeder.

FIG. 2

3. State:

(i) The function of a plug bridge on an IDMT electromagnetic protection relay.

(ii) Two ways in which the setting of an IDMT electromagnetic relay may be altered.

4. (a) An 11 kV, single phase system is protected by a scheme using 1000/5 CTs and an IDMT relay having a plug setting of 150% and time multiplier setting of 0.68 (FIGURE 3).

Using the standard PSM/time characteristic, find the relay operating time (TMS = 1) for a 236 MVA fault.

FIG. 3

(b) The circuit breakers CB1, CB2, CB3 and CB4 are part of a scheme which uses 1000/5 CTs and IDMT relays.

If the maximum fault current capability is 6700 amps, clearance of which must be initiated in 2.25 secs, select, using the standard characteristic:

(i) plug setting multipliers for CB1 at plug settings of 100% and 50%

(ii) the time multiplier setting of each IDMT relay to afford effective discrimination of 0.4 sec at a plug setting of 50%