Cork Institute of Technology Bachelor of Science (Honours) in Electrical Power Systems - Award Instructions Answer FIVE questions. (EELPS_8_Y4) Autumn 2008 Electrical Energy Systems (Time: 3 Hours) Examiners: Mr. S. Boljevic Mr. M. Hennessy Prof. E. McQuade Q1. (a) A 3 phase 4 wire 400V system supplies three star connected loads. 0 Z = 10 0 Ω, 15 30 0 0 Z = Ω, Z = 10 30 Ω A B The phase sequence is ABC. Neatly sketch the system described above clearly showing each label and calculate (i) line currents (ii) neutral currents (iii) total active power consumed by the system. C A 3 phase balance delta connected load of (15 + j5) Ω in each phase is fed from a 400V 3 phase system. The phase sequence is ABC. Neatly sketch the system described above, clearly showing each label and calculate: (i) magnitude of phase currents (ii) magnitude of line currents (iii) total active power consumed by the system (v) total reactive power consumed by the system (vi) total apparent power consumed by the system (vii) power factor of the system.
Q2. (a) A generator supplies a load through a feeder whose impedance Z foh = 1 + j2ω. The voltage across the load is 230V. Sketch the circuit and calculate: (i) the real power supplied by generator (ii) the reactive power supplied by generator (iii) draw the phasor diagram of the circuit [Note: Take the load voltage as the reference phasor] Two voltage sources V 1 = 240 5V and V 2 = 220 0 V are connected by a short line of impedance. Z = 2 + j7 Ω as shown in Fig 1. Determine the real and reactive power supplied or received by each source and the power loss in the line. R jx V 1 V 2 2
Q3. (a) Outline and explain the factors that govern the terminal voltage of an alternator when the load changes. Why is the knowledge of voltage regulation of an alternator important? A 3-phase, 50 H Z star-connected 1000 kva, 2300V alternator gives a short-circuit current of 400A for a certain field excitation. With the same excitation, the open-circuit voltage was 1328V (phase value). The d.c. resistance between two lines is 0.412 Ω. Calculate: (i) effective armature resistance (ii) synchronous reactance (iii) the full-load voltage regulation at 0.8 p.f. lagging. Q4. (a) A 3300/400V, 10 kva transformer has primary and secondary winding resistance of 13Ω and 0.15 Ω respectively. The leakage reactance referred to primary is 45Ω. The magnetising impedance referred to primary is 6kΩ and the resistance corresponding to the core loss is 12kΩ. Determine the input current: (i) when the secondary terminals are open-circuited (ii) when the secondary load current is 25A at a power factor of 0.8 lagging. Use simplified equivalent circuit. (i) (ii) The primary and secondary windings of a 40 kva, 6600/250 V single-phase transformer have resistance of 10Ω and 0.2Ω respectively. The leakage reactance of the transformer referred to the primary is 35Ω. Calculate: the primary voltage required to circulate full-load current when the secondary is shortcircuited. the full-load regulation at 0.8 p.f. lagging. NEGLECT the no-load current 3
Q5. (a) Explain how transmission lines are classified into short, medium and long lines and explain their characteristics. A 25km 3 phase overhead line delivers 8 MW at 11kV at 0.85 lagging p.f. line loss is 12% of power delivered. Line inductance is 1.1mH per km per phase. Calculate: (i) sending-end voltage (ii) voltage regulation A 3-phase transmission line delivers a load of 5MW at 0.8p.f. lagging. Resistance of each conductor is 0.5 km Ω. Receiving-end voltage is 33kV. If the line loss is not to exceed 10Y, determine the length of the line. Q6. (a) Describe with aid of a neat sketch off-load and on-load tap changing transformer method of voltage control. What are the limitations of each method? A 3-phase overhead line has resistance and reactance per phase 6 Ω and 22Ω respectively. The load at the receiving end is 20MW at 33kV and a power factor of 0.85 lagging. Find the capacity of the synchronous condenser required for this load condition if it is connected at the receiving end and the line voltages at both ends are maintained at 33kV. 4
Q7. (a) Outline and explain steps for symmetrical fault calculations. What do you understand by percentage reactance? Why do we prefer to express the reactance of various elements in percentage values for short-circuit calculations? A 3-phase transmission line operating at 10kV and having a resistance of 1Ω and reactance of 5Ω is connected to the generating station bus-bars through 10MVA step-up transformer having a reactance of 5%. The bus-bars are supplied by a 10MVA alternator having 10% reactance. Calculate the short-circuit kva fed to symmetrical fault between phases if it occurs (i) at the load end of transmission line (ii) at the high voltage terminals of the transformer 10MVA 10% 10MVA 5% F 1 1Ω 5Ω F 2 10kv LOAD 5