MECH0071 Main Exam period - Alternative Assessment (Coursework) 2019/20
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MECH0071 Main Exam period - Alternative Assessment (Coursework) 2019/20
Answer All FOUR Questions
Marks for each question are provided.
Each question is equally weighted with 25 marks total .
There are 100 marks available on this paper.
The assessment should take between 4 to 6 hours to complete.
1.
a) A double-phase-to-ground bolted fault develops in a power system. Use sequence components to describe the amplitudes and phase-angles of the voltages and currents, stating all assumptions made. Hence sketch a circuit that represents the fault. [8 marks]
b) An electrical machines manufacturer produces a 3,000 kVA, star connected generator having sequence reactances X1 = 60%, X2 = 25% and Xo = 15%. Using an appropriate base rating, suggest values for a reactance (Xn) and a resistance (R) that could be added to the generator neutral such that the current for a single-line-to-ground fault will not exceed rated current. Give your two answers in per unit form. [7 marks]
c) The power circuit shown in Figure 1 consists of two three-phase 11 kVA generators each rated at 3,000 kVA, each having reactance 10%, and both connected to busbar B. A grid supply feeding Bus A is to be connected in parallel via a transformer rated at 9,000 kVA, having a leakage reactance of 5% and with the source impedance of the grid being effectively zero. Calculate the reactance(X) to be placed between bus A and bus B that will limit the three- phase short circuit fault level to 150 MVA at the fault position indicated. Give your answer in per unit form. [10 marks]
2.
a) Briefly discuss four principal effects that an automatic voltage regulator has on an isolated power supply system [6 marks]
b)A 25,000 kVA, 11 kV three-phase generator with 15% subtransient reactance and 15% transient reactance is connected through a transformer having a reactance of 10% to a bus that supplies 4 identical motors, as shown in the electrical line diagram in Figure 2. Each motor has Xd”=30%,Xd'=30% based on 5,000 kVA and 6.6 kV. The three-phase rating of the transformer is 25,000 kVA, 11/6.6 kV and leakage reactance X is 10%. Bus voltage is 6.6 kV, when a balanced three-phase fault occurs at the terminals of one of the motors. Determine:
i) The maximum symmetrical subtransient fault current in pu. [3 marks]
ii) Generator contribution to the subtransient fault current as a percentage. [4 marks]
iii) Momentary or asymmetrical current in breaker A in pu. [3 marks]
iv) The transient symmetrical fault current of the generator and each motor. [4 marks]
v) Current to be interrupted by breaker A in this fault scenario. [5 marks]
3.
a) The reactance of a short transmission line is j0.06 pu. The load current at the receiving end is (1+ j0.6) pu and the receiving end voltage is (1+j0) pu. Calculate the sending end voltage and average reactive power flow in the line. [5 marks]
b) Describe an algorithm which may be used by a computer load flow analysis program to obtain the bus admittance matrix of the impedance network of transmission lines shown in Figure 3, hence write a bus admittance matrix for the network. [8 marks]
c) Write a short essay entitled "The importance and objectives of load flow studies". Ensure you address the following aspects:
i) The need for load flow studies;
ii) the data that is obtained from load flow studies; and
ii) how the data obtained would be used?
The essay should not include diagrams nor graphs and has a 400-word limit Your answer should use double line spacing
[12 marks total: 4 marks are attributed to each of the 3 subtopics]
4. You are requested by the engineering manager to produce a report describing "power quality issues in modern power systems". In your report, you are
asked to describe:
a) The likely sources of distortion and their characteristic harmonics;
b) how power quality can be measured, qualified and quantified;
c) explain how distortion can impact a power system;
d) by what means distortion can be mitigated using filtering methods, and
e) identify any important regulations that should be complied with highlighting permitted levels for either a land based or a marine application.
The report can include diagrams and graphs as appropriate and has a 1,000-word limit. Your answer should use double line spacing. Additional resources may be used and should be referenced.
[25 marks total: 5 marks are attributed to each of the 5 subtopics]
2023-08-12