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CONFIDENTIAL EE/APR 2006/EPO341

UNIVERSITI TEKNOLOGI MARA

FINAL EXAMINATION

COURSE POWER ENGINEERING FOUNDATION

COURSE CODE EP0341

EXAMINATION APRIL 2006

TIME 3 HOURS

INSTRUCTIONS TO CANDIDATES

1. This question paper consists of six (6) questions.

2. Answer FIVE (5) questions only. Start each answer on a new page.

3. Do not bring any material into the examination room unless permission is given by theinvigilator

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CONFIDENTIAL 2 EE/APR 2006/EPO341

QUESTION 1

a) There are two tests required in determining the approximate values of reactances andresistances in the approximate transformer model namely the open circuit test and theshort circuit test. Describe by using related labeled diagrams and equations on how todetermine the approximate values of reactances (Xeq, Xm) and resistances (Req, Re)

(10 marks)

b) The results of the open circuit and short circuit tests for a 550/220 V, 60 Hz transformerare given in Table Q1b.

Table Q1bOpen Circuit Test Short Circuit Test

Poc = 500 W Psc = 300 Wl0c=15A lsc=12A

Voc = 220 V Vsc = 30 V

All data given were taken from the high-voltage side of the transformer.(i) Find the equivalent circuit of this transformer referred to the low-voltage side of the

transformer,(ii) Assume a load connected to the low-voltage side draws 10 kW and a current of 58 A

at the 0.85 lagging power factor. Find the efficiency of the transformer.(10 marks)

Q S O

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QUESTION 3

a) Assume that the voltage applied to a load is 230Z-300

V and current flowing through theload is 5Z15 A.

(i) Calculate the complex power S consumed by this load(ii) Is this load inductive or capacitive?(Hi) Calculate the power factor of this load.(iv) Calculate the reactive power consumed or supplied by this load. Does the load

consume reactive power from the source or supply it to the source.(6 marks)

b) Figure Q3b shows a one-line diagram of a small 1100-V distribution system in anindustrial plant. Assume that the lines in the system have zero impedance,(i) If the capacitor switch is open, find the total real, reactive and the apparent power

supplied to the loads. Find also the total current supplied to the system and thepower factor at the supply,

(ii) If the capacitor switch is now closed, find the total real, reactive and the apparentpower supplied to the loads. Find also the total current supplied to the system. Whatis the power factor of the system now?

Load 1Y-connected

Load 1 Z= 3 + j4 fi/phaseLoad 1

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QUESTION 4

a) A 750kW, 11kV, 3-phase, star-connected synchronous motor has a synchronousreactance of 35Q/phase and negligible resistance. Determine the excitation emf perphase when the motor is operating on full-load at 0.8 pf lagging with 93% efficiency.

(8 marks)

b) A 2300V, 1000kVA, 0.85 power factor lagging, 50-Hz, two-pole, Y-connectedsynchronous generator has a synchronous reactance of 1.1Q and an armatureresistance of 0.15Q. At 50 Hz, its friction and windage losses are 24 kW, and its corelosses are 18 kW. The field circuit has a dc voltage of 200 V, and the maximum l F is 10A. The resistance of the field circuit is adjustable over the range from 20 to 200 The OCCof this generator is shown in Figure Q4b in APPENDIX 1.

(i) How much field current is required to make VT equal to 2300 V when the generator isrunning at no load?

(ii) What is the internal generated voltage of this machine at rated conditions?(iii) How much field current is required to make VT equal to 2300 V when the generator isrunning at rated conditions?

(iv) How much power and torque must the generator's prime mover be capable ofsupplying?

(12 marks)

QUESTION 5

a) A 6 pole 50 Hz 3 phase induction motor running of full load develops a useful torque of

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QUESTION 6

Figure Q6 shows a simple electrical power system. Calculate the per unit values of allimpedances using 150 MVA as the base MVA for the entire system. Use 11 kV at Bus A asthe new base voltage. Thus, draw the equivalent single-phase impedance diagram for thesystem. If a 3-phase balanced fault occurs at Bus D, determine the actual fault current.

(20 marks)

Data:

Generator G1Generator G2Transformer T1Transformer T2Transformer T3

Transformer T4Transformer T5Line 1Line 2Line 3

rated MVA100 MVA120 MVA120 MVA100 MVA150 MVA100 MVA50 MVA

rated voltage10.4 kV20 kV11/110kV110/11kV20/110kV120/11 kV11/6.6kV

reactance15%12%

12%14%

15%

12%12%

J50QJ60QJ0.1Q

Bus A11kV

G1 T1

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