Question

Question 4 (20 marks) (a) State the main three objects for fault analysis. (6 marks) (b) Explain why it is necessary to remove faulted sections of a (4 marks) power system from service as soon as possible (c) A bolted short circuit occurs in the series R-L circuit of Figure 1.33 (see unit 1 page 96) with V=40KV. X=892, R=0.802 and with maximum DC offset The circuit breaker opens three cycles after fault inception. Determine 1. The RMS AC fault current (2 marks) il. The RMS "momentary" current at r=0.5 cycle, which (3 marks) passes through the breaker before it opens. (d) Name the three basic components in a protection system. (3 marks) (e) Name two specification factors where fuse is normally based (2 marks) on

Answer 1

- the system during the fault. of the required Circuit breakers. Selection of appropriate Schemes of protective relay unbalanced with the result that three-phase equip 4. Sometimes faults block the flow of power.. Question -4 : a) Objectives of fault analysis 1. To determine the values of vottages and Currents at different points of the nge 3. Determination of the ratings faults can damage to become. b) or distrupt power systems in several ways : 1. faults give rise to abnormal operating conditions, usua -lly excessive Vottages and Currents at certain points on the System Large Voltages stress insulation beyond their breakdown Value while large Currents result in over heating of power system components. 2. Sustained overheating may reduce useful life of the equipment. 3. faults can Cause the three-phase System -ment operates improperly. faults can System to become unstable. Cause 5. R = o.fr X = 852 V = 40 KV c) Series Given data R-L Circuit, circuit breaker has 3 Cycles after fault inception. AC fault Current : The 9) RMS

PAC - Nok 2 *3121= R + x 40 x1000 (0.8) + + 8 4,975.19 Amp ii) RMS "momentary" Current at T= 0.5 Cycles : K(C) (-41777(XIR) 14 Qe K(0.5) - At r=0.5 cycles, 64 TTX0.5 T20:5) 8 It al 1 -62.831100 +2 e -0.6283 it qe 2.0669 = 1,438 RMS momentary Current from k(o.5 Goycle ) * Lac 1.438 x 4975.19 1154,32 A d) Three basic components in a protection System :- Various components in a a protection system are 1. fuse २. Lighting arrestors 3. circuit breakers Relays 4.

1) Current rating (e) Specification factors of fuse :- of the fuse 2) Breaking capacity or interrupting Capacity 3) Ambient temperature Surrounding the fuse.