ELX303 Jan 2019 Exam PDF

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University of SunderlandFACULTY OF TECHNOLOGYELXElectrical PowerEXAMINATIONDate: 16 th January 2019 Time: 1.00pm-4Instructions to Candidates:Time allowed - 3 hoursThere are SIX questions set. You must attemptFOUR questions. All questions carry equal marks.This is a closed book examination – this mea...

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University of Sunderland FACULTY OF TECHNOLOGY ELX303 Electrical Power

EXAMINATION

Date:

16th January 2019

Time:

1.00pm-4.00pm

Instructions to Candidates: Time allowed - 3 hours There are SIX questions set. You must attempt FOUR questions. All questions carry equal marks. This is a closed book examination – this means you are not permitted to use any text books or study aids in the examination. Non-programmable calculators are allowed. You must answer the required number of questions only. You should put a cross through any work you do not wish to be marked.

Question 1 a) A single-phase input voltage Eo is feeding a thyristor that has a resistive load in series, as shown in Figure 1. Derive the mathematical expression for the voltage across the load based on the graph also shown in Figure 1.

Figure 1 – Resistive Load (4 marks) b) A single-phase input voltage Eo is feeding a thyristor that has a complex load in series, as shown in Figure 2. Derive the mathematical expression for the voltage across the load based on the graph also shown in Figure 2.

Figure 2 – Complex Load (5 marks) c) Draw the schematic circuits for the following converts and include a load across the output of each converter: 

Single-Phase, Single-Way, Uncontrolled



Three-Phase, Single-Way, Uncontrolled



Single-Phase, Double-Way, Uncontrolled



Three-Phase, Double-Way, Uncontrolled



Single-Phase, Half-Controlled



Single-Phase, Fully-Controlled



Three-Phase, Half-Controlled



Three-Phase, Fully-Controlled

(2 marks) (2 marks) (2 marks) (2 marks) (2 marks) (2 marks) (2 marks) (2 marks)

Question 2 a) Determine the average and effective values of the full-wave rectified sinewave as shown in Figure 3:

Figure 3 – Full-Wave Rectified Sinewave (8 marks) b) Determine the firing angle α of a delayed full-wave rectified sinewave which has an average value of half the maximum value Ym, as shown in Figure 4:

Figure 4 – Delayed Full-Wave Rectified Sinewave (8 marks) c) Classify the flowing converters as shown in Figures 5, 6 & 7 as either A.C./D.C., A.C./A.C. direct or A.C./A.C. indirect, and state whether they have unidirectional or bidirectional power flow.

Figure 5 – Converter

Figure 6 – Converter

Figure 7 – Converter (9 marks)

Question 3 A 30 MW, 38MVA, 33 kV star connected alternator is protected by a differential protection scheme using 600/1A current transformers and unbiased relays set to operate at 11% of their rated current of 1 A. a) If the earthing resistor is 89% based upon the machine’s rating, estimate the percentage of the stator winding that is not protected against an earth fault. (8 marks)

b) If it is required to protect 89% of the winding, estimate the value of the earthing resistor in ohms and percentage based on the machine rating. (6 marks)

Identical shunt reactors are connected from each phase conductor to neutral at both ends of a 300 km line during light load conditions, providing 65% compensation. The reactors are removed during heavy load conditions. Full load is 1 kA at unity power factor and at 400 kV. Assuming that the sending-end voltage is constant, determine the following: c) Percent voltage regulation of the uncompensated line. (5 marks)

d) The equivalent shunt admittance and series impedance of the compensated line. (3 marks)

e) Percent voltage regulation of the compensated line. (3 marks)

The A and B parameters for the line are A = 0.9313∠0.209˚ and B = 97∠87.2 ˚Ω and the admittance of the equivalent π circuit is Y’/2 = 3.7x10-7 + j7.094x10-4 S.

Question 4 a) It is known that the UK typically uses larger number of smaller machines rather than fewer large machines. Explain the advantages and disadvantages of using smaller generators. (4 marks) b) To reduce fault power, reactors maybe introduced into the system. The diagram below shows parallel 11kV generators in series with 11/33kV step up transformer feeding a line through a reactor:

Using a base of 30MVA, calculate the following: (i)

The fault current with the reactor in place.

(ii)

The fault current with the reactor removed.

(12 marks) (3 marks) c) Explain with the aid of a diagram how two generators will assist each other when the voltage falls on one machine if they are connected in parallel. (6 marks)

Question 5 Certain countries including the UK have strict legal requirements which stipulate that certain transmission and distribution voltages are kept within given parameters. a) Explain how poor power factor affects lines losses. (3 Marks) b) List and explain two methods that can be employed to provide compensation. (6 Marks) c) Describe how the theoretical “infinite busbar” works, your explanation you must discuss in the context of a large interconnect grid. Your answer should include a suitable diagram. (8 Marks) The sensitivity of a node to real power changes is 200 MW/kV of line voltage and that to reactive power changes is 12 MVAr/kV of line voltage. d) much reactive compensation would have to be introduced at a point to maintain the voltage constant for a real power change of 55MW?

(8 Marks)

Question 6 a) The load at the receiving end of a three-phase overhead line is 26 MW, power factor 0.85 lagging, at a line voltage of 33 kV. A synchronous compensator is situated at the receiving end and the voltage at both ends of the line is maintained at 33 kV. The line has resistance of 7 Ω per phase and inductive reactance of 23 Ω per phase. Calculate the MVAr of the compensator. (6 Marks)

b) Determine the power factor the generator must operate at in the figure below to deliver power at the power factors stated. All p.u. values are referred to the voltage bases shown and 100 MVA. You may ignore the reactive loss in the generatortransformer.

(19 Marks)

END OF PAPER...