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United Internationaluniversity (Uiu)

This document contains a midterm exam for a power system protection course. It has 5 questions related to power system protection concepts and applications. The questions cover topics like: 1) Calculating fault MVA and current for parallel generator systems. 2) Principles of induction disk relays, using directional relays as overcurrent relays, and the importance of directional relays. 3) Definitions of ALF, uses of potential transformers and current transformers, and plotting CT saturation curves. 4) Calculating operating times for overcurrent relays on a radial feeder with two sections protected by different relays. 5) Explaining directional impedance relays,

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Masud Sarker
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78 views2 pages

United Internationaluniversity (Uiu)

This document contains a midterm exam for a power system protection course. It has 5 questions related to power system protection concepts and applications. The questions cover topics like: 1) Calculating fault MVA and current for parallel generator systems. 2) Principles of induction disk relays, using directional relays as overcurrent relays, and the importance of directional relays. 3) Definitions of ALF, uses of potential transformers and current transformers, and plotting CT saturation curves. 4) Calculating operating times for overcurrent relays on a radial feeder with two sections protected by different relays. 5) Explaining directional impedance relays,

Uploaded by

Masud Sarker
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOCX, PDF, TXT or read online on Scribd
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UNITED INTERNATIONALUNIVERSITY (UIU)

Dept. Of Electrical & Electronic Engineering (EEE)


Exam: Mid Term, Trimester: Fall, Year: 2015
Course: EEE 477 (Sec – A/B/C), Title: Power System Protection
Marks: 60, Time: 1 Hour45 minutes

There are five Questions. Answer any Four. Marks are indicated in the right column
1. Two 11kV generators are running parallel to feed a load through a j25Ω transmission line as [6+6+3]
shown in figure Q.1. The ratings of transformer and generators are shown in the figure. Find
fault MVA for faults at F1 (immediately after breaker B) and F2 respectively. Find the
breaking capacity of breaker B and the fault current flowing through breaker A.
G1: 11kV, 40MVA, 12%; G2: 11kV, 30MVA, 10%; TR: 100MVA, 11/66kV, 9%.

Figure: Q.1.

2. a) With the help of neat sketch, explain the principle of operation of an induction disc relay. [7+2]
Why is such relay free from vibration? Explain with the relevant mathematical formula.
b) How can you use a directional relay as a simple over current relay? [2]
c) Why is application of directional relay important in a power system? Explain with an example [4]

3. a) What is ALF? For which equipment the term ALF is used? Why is it important? [1+1+2]
b) Why is PT used in a power system? Explain with diagram. Why is output of broken delta PT [2+2]
zero under healthy condition of a power system?
c) Explain with diagram, how will you plot CT saturation curve? Why is knee point important [4+3]
for a CT? Explain with special reference to protection as well as measuring CTs.

4. a) A radial feeder has two sections as shown in figure Q.4. Each section is protected by over [12]
current relays R1 & R2 respectively. Plug setting for R1 is 150% and that of R2 is 100%.
TMS for R1 is 0.40 and the discrimination time between R1 and R2 is 0.50 seconds. For a
fault current of 8000A, find the operating times of R1 and R2 respectively. Also find TMS for
R1.

800/5 800/5
R1 R2
G

I F=8000A
Figure Q.4

b) What are the advantages of reactance relay over simple distance relay? [3]

5. a) Why is impedance relay non directional? How can you make it directional? Explain. [2.5+2.5]
b) A transmission line fed from both the ends has two sections as shown in figure Q.5. The [5]
sections are protected by four directional impedance relays as indicated in the figure. First a
fault occurs at F1(position of F1 is at a distance of 90% of the line 1 from G1). With the help
of impedance diagram and contact circuit diagram, explain which of the relays are going to
operate to clear the fault. Show the direction of operation of each relay.

Line1 Line2

G1 R1 R2 R3 R4 G2
F1
F2

Figure Q.5

c) After the fault at F1 has been cleared and the system operating normally, another fault occurs [5]
at F2. Explain the operation of the relays under the present fault condition.

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