Semester Two Final Examinations, 2021 ELEC4302 Power System Protection
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School of Information Technology and Electrical Engineering
EXAMINATION
Semester Two Final Examinations, 2021
ELEC4302 Power System Protection
This paper is for External students.


Answer ANY four (4)
questions.
All parts of each question
should be together.
Total Number of Questions - 5.
Total Number of marks -100
(weightage 50%).
















Semester Two Final Examinations, 2021 ELEC4302 Power System Protection
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Answer ANY four (4) questions.
Question 1. (25 Marks)
Fig Q1. shows a ring circuit with two generators operating at 11 kV. Sub transient reactance
of both generators is 5 Ω. Answer the following questions.
a) Calculate the maximum and minimum 3 phase fault current of the ring circuit.
Assume that minimum fault current occurs when only one generator is in operation.
[7 Marks]
b) Propose suitable CT ratios for relays at E, F and K. [6 Marks]
c) Relays B, C, E, F, H, I, K, and L are directional over current relays and A, D, G and J
are non-directional relays. Explain how one can coordinate the operation of relays F,
I and L. [4 Marks]
d) Identify the backup relays for bus 4. [4 Marks]
e) Which relay/relays will have the fastest time dial setting and which relay/relays will
have the slowest time dial settings? Explain your answer. [4 Marks]


Fig Q1.

Question 2. (25 Marks)
A section of a typical power system is shown in Fig Q2.
a) Three zone MHO type distance protection relays are located at busbar 1 of
transmission line A as shown in the Fig Q2. The VT ratio is 400 kV/110 V and CT ratio
is 2000/1. The line between (1 – 2) is 100 km and between (2 – 3) is 50 km. All the
Semester Two Final Examinations, 2021 ELEC4302 Power System Protection
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lines have impedance of 0.025 + j 0.25 Ω/km. Find the impedance setting for zone 1
(80% of line impedance) and zone 2 (120% of line impedance) protection of the relay.
[10 Marks]
b) A ground fault occurred at bus 2. What is the maximum allowable fault resistance if
this fault should be detected by the MHO relay in zone 2? [10 Marks]
c) What will be the Zone 3 settings if the 500 MVA transformer is included within the
Zone 3 of the MHO relay when both transmission lines (A and B) are in operation.
[5 Marks]

Fig Q2.

Questions 3. (25 Marks)
A transformer ground differential protection scheme is given in Fig Q3. High voltage side of
the transformer is delta connected and low voltage side is Y connected. Answer the
questions given below.

Fig Q3.
a) Identify suitable winding connection types for two line CT secondaries. Explain your
selections. [5 Marks]
b) Calculate the CT ratio at high voltage side. Available CT ratios are:
Semester Two Final Examinations, 2021 ELEC4302 Power System Protection
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50:5 100:5 150:5 200:5 250:5 300:5
(Hint: Use the simple differential scheme principal to find the CT ratio) [3 Marks]
c) As shown in Fig Q3, transformer neutral is grounded through a 15  resistor. Calculate
what percentage of the winding can be protected by a simple differential protection
scheme (without earth relay scheme) if the minimum relay operating current is 0.5 A.
[5 Marks]
d) The CT at neutral is 300:5. Calculate what percentage of the winding can be
protected through the neutral CT if the minimum relay operating current is 0.5 A. [5
Marks]
e) Explain the purpose of ACT winding. [2 Marks]
f) Calculate the turns ratio of ACT to achieve the purpose you proposed in section e).
[5 Marks]
Questions 4. (25 Marks)
a) Identify three important parameters needs to be considered when selecting a CT for
protection measurements. [5 Marks]
b) Discuss the operation of capacitive VT and mention the advantages of capacitive
VTs over inductive VTs. [5 Marks]
c) Explain why a fuse in primary circuit cannot protect a VT during a secondary short
circuit incident. [5 Marks]
d) The open circuit voltage requirement of a CVT is 110 V. The system voltage is 132 kV
at 50 Hz. The leakage inductance of the transformer is 2 mH and turns ratio is 110:1.
Find the values of capacitances used in CVT such that there is no phase displacement
between the line voltage and the output of the CVT. [10 Marks]

Question 5 (25 Marks)
a) Describe 3 different bus configuration schemes with appropriate diagrams.
[5 Marks]
b) Explain the purpose of adopting IEC 61850 in substation automation. [5 Marks]
c) What is a GOOSE messaging and describe its typical applications in protection
systems? [5 Marks]
d) Explain the advantage of pilot relays over impedance relays and describe the
operation of one type of pilot relay scheme. [5 Marks]
e) Explain the difference between Class C and Class T type CTs. [5 Marks]

END OF EXAMINATION

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