Assignment-1 1. Two generators rated at 10 MVA, 13.2 kV and 15 MVA, 13.2 kV are connected in parallel to a bus bar. They feed supply to two motors of inputs 8 MVA and 12 MVA respectively. The operating voltage of motors is 12.5 kV. Assuming base quantities as 50 MVA and 13.8 kV draw the reactance diagram. The per cent reactance reactance for generators generators is 15% and that for motors is 20%. 2. A 3-bus system is given in Fig. P1.3. The ratings of the various components are listed below: Generator 1 = 50 MVA, 13.8 kV, X″ = 0.15 p.u. Generator 2 = 40 MVA, 13.2 kV, X″ = 0.20 Generator 3 = 30 MVA, 11 kV, X″ = 0.25 Transformer 1 = 45 MVA, 11 kV, Δ/110 kV Y, X = 0.1 p.u. Transformer 2 = 25 MVA, 12.5 kV, Δ/115 kV Y, X = 0.15 p.u. Transformer 3 = 40 MVA, 12.5 kV, Δ/115 kV Y, X = 0.1 p.u. The line impedances are shown in Fig. P1.3. Determine the reactance diagram based on 50 MVA and 13.8 kV as base quantities in Generator 1.
3. For a system shown in fig. the ratings of Generator:25 MVA, 12.4 kV, 10% sub transient reactance Motor: 20 MVA, 3.8 kV, l5% sub transient reactance Transformer T,: 25 MVA, 11/33 ky, 8% reactance reactance Transfbrmer Tr: 20 MVA, 33/3.3 kV, 10% reactance Line: 20 ohms reactance. The system is loaded so that the motor is drawing 15 Mw at 0.9 loading power factor, the motor terminal voltage being 3.1 kv. Find the sub transient current in generator and motor for a fault at generator bus.
4. Asynchronous generator rated at 500kVA, 440v, 0.l pu sub transient reactance is supplying a passive load of 400 kW at 0.8 lagging power factor. Calculate the initial symmetrical rms current for a three phase fault at generator generator terminals. terminals. 5. For the three phase power network shown in fig. the rating of various component are:
Generators G1: 100 MVA, 0.30 pu reactance G2: 60 MVA, 0.18 pu reactance Transformers (each): 50 MVA, 0.10 pu reactance Inductive reactor X: 0.20 pu on a base of 100 MVA Lines (each): 80 ohms (reactive); neglect resistance. With the network initially unloaded and a line voltage of 110 kv, a symmetrical short circuit occurs at mid point F of line calculate the short circuit MvA to be interrupted by the circuit breakers A and B at the ends of the line. what would these values be, if the reactor X were eliminated? 6. Consider the system shown in fig. The generators are 100 MVA, with transient reactance of 10% each. Both the transformers are 100 MVA with a leakage reactance of 5%t. The reactance of each of the lines to a base of 100 MVA, 110 KV is 10%. Obtain the short circuit solution for a three-phase solid short circuit on bus 3. Assume pre fault voltages to be 1 pu and pre fault currents to be zero.
7. A 3-phase generator is rated for 60 MVA, 6.9 kV and sub transient reactance Xd ″ = j0.15 p.u. The generator feeds a motor through a line with impedance of j0.1 p.u. on generator rating. The motor is rated at 10 MVA and 6.9 kV with Xd ″ = j0.2 p.u. on the motor base. The voltage at the terminal of the motor is 1 p.u. and takes a load current of 1.0 p.u. at unity p.f. A symmetrical fault occurs at the motor terminals. Determine the sub transient r.m.s. current at the fault, in the generator and in the motor. 8. Two 50 MVA, 50 Hz, 11 kV alternators with sub-transient reactance X ″ = j0.1 p.u. and a transformer of 40 MVA 11 kV/66 kV and reactance of 0.08 p.u. are connected to a bus A. Another generator 60 MVA, 11 kV alternator with reactance of 0.12 p.u. is connected to bus B. Bus A and B are interconnected through a reactor of 80 MVA 20 per cent reactance. If a 3-phase fault occurs on the high voltage side of the transformer, calculate the current fed into the fault. 9. Three 6.6 kV, 12 MVA, 3-phase alternators are connected to a common set of busbars. The positive, negative and zero sequence impedances of each alternator are 15%, 12% and 4.5% respectively. If an earth fault occurs on one busbar, determine the fault current: (i) if all the alternator neutrals are solidly grounded; (ii) if one only of the alternator neutrals is solidly earthed and the others are isolated; (iii) if one of the alternator neutrals is earthed through a reactance of 0.5 ohm and the others are isolated. 10. Two generating stations having short circuit capacities of 1500 MVA and 1000 MVA respectively and operating at 11 kV are linked by an interconnected cable having a reactance of 0.6 ohm per phase, determine the short circuit capacity of each station.
