Switch Gear and Protection

Question 1
In a 132 kV system, the series inductance up to the point of circuit breaker locationis 50 mH. The shunt capacitanceat the circuit breaker terminal is 0.05 \mu F. The critical value of resistance in ohms required to be connected across the circuit breaker contacts which will give no transient oscillation is_____
A
100
B
250
C
500
D
1000
GATE EE 2019   Power Systems
Question 1 Explanation: 
\begin{aligned} L&=50mH\\ C&=0.05\mu F\\ R_{cr}&=\frac{1}{2}\sqrt{\frac{L}{C}}\\ &=\frac{1}{2}\sqrt{\frac{50 \times 10^{-3}}{0.05 \times 10^{-6}}}\\ &=500\Omega \end{aligned}
Question 2
The total impedance of the secondary winding, leads, and burden of a 5 A CT is 0.01 \Omega. If the fault current is 20 times the rated primary current of the CT, the VA output of the CT is ________
A
50
B
100
C
150
D
200
GATE EE 2019   Power Systems
Question 2 Explanation: 
\begin{aligned} I_{sec}&=5 \times 20=100A \\ V &=I_{sec}R =100 \times 0.01\\ &=1V \\ \text{VA } &\text{output of the CT} \\ &= VI_{sec}=100 \times 1\\ &= 100VA \end{aligned}
Question 3
A power system with two generators is shown in the figure below. The system (generators, buses and transmission lines) is protected by six overcurrent relays R_1 \; to \; R_6. Assuming a mix of directional and nondirectional relays at appropriate locations, the remote backup relays for R_4 are
A
R_{1},R_{2}
B
R_{2},R_{6}
C
R_{2},R_{5}
D
R_{1},R_{6}
GATE EE 2016-SET-2   Power Systems
Question 4
A 3-phase transformer rated for 33 kV/11 kV is connected in delta/star as shown in figure. The current transformers (CTs) on low and high voltage sides have a ratio of 500/5. Find the currents i_1 \; and \; i_2, if the fault current is 300 A as shown in figure.
A
i_{1}=1/\sqrt{3}A,i_{2}=0A
B
i_{1}=0A,i_{2}=0
C
i_{1}=0A,i_{2}=1/\sqrt{3}A
D
i_{1}=1/\sqrt{3}A,i_{2}=1/\sqrt{3}A
GATE EE 2015-SET-2   Power Systems
Question 4 Explanation: 
i_2=0, since entire current flows through fault
i_1=\frac{1}{\sqrt{3}}A
Question 5
The over current relays for the line protection and loads connected at the buses are shown in the figure.

The relays are IDMT in nature having the characteristic

t_{op}=\frac{0.14\times Time \; Multipller \; Setting}{(Plug \; Setting \; Multiplier)^{0.02}-1}

The maximum and minimum fault currents at bus B are 2000 A and 500 A respectively. Assuming the time multiplier setting and plug setting for relay R_B to be 0.1 and 5 A respectively, the operating time of R_B (in seconds) is_____.
A
0.12
B
0.23
C
0.42
D
0.58
GATE EE 2014-SET-1   Power Systems
Question 5 Explanation: 
Minimum and maximum fault currents are
I_{fmin}=500A, I_{fmax}=2000A
For relay R_B,\;TMS=0.1, plug setting =5A
\begin{aligned} \text{PSM}&=\frac{I_f}{I_{pk} \times \text{C.T. ratio}} \\ \text{C.T. ratio} &=\frac{\text{Full load current}}{I_{pk}} \\ &= \frac{100}{5}\\ \text{At realy, }&R_B\\ \text{with }I_{fmin}\Rightarrow & \text{PSM}_1=\frac{500}{\frac{100}{5}\times 5}=5\\ \text{with }I_{fmax}\Rightarrow & \text{PSM}_2=\frac{2000}{\frac{100}{5}\times 5}=20 \end{aligned}
Operating time with TMS=0.1,
and from IDMT characterisrics
t_{op_2}=\frac{0.14 \times 0.1}{(20)^{0.02}-1}=0.226sec
Question 6
A negative sequence relay is commonly used to protect
A
an alternator
B
an transformer
C
a transmission line
D
a bus bar
GATE EE 2011   Power Systems
Question 7
A nuclear power station of 500 MW capacity is located at 300 km away from a load center. Select the most suitable power evacuation transmission configuration among the following options
A
A
B
B
C
C
D
D
GATE EE 2011   Power Systems
Question 7 Explanation: 
For transmission of bulk power of very long distance high voltage (400 kV) is used.
To increase reliability, double circuit is used.
Question 8
Consider a stator winding of an alternator with an internal high-resistance ground fault. The currents under the fault condition are as shown in the figure The winding is protected using a differential current scheme with current transformers of ratio 400/5 A as shown. The current through the operating coils is
A
0.1875A
B
0.2A
C
0.375A
D
60kA
GATE EE 2010   Power Systems
Question 8 Explanation: 


I_1=200 \times \left ( \frac{5}{400} \right )=2.75A
I_2=250 \times \left ( \frac{5}{400} \right )=3.125A
Current through operating coil,
I_{OC}=I_2-I_1=3.125-2.75=0.375A
Question 9
A three-phase, 33 kV oil circuit breaker is rated 1200 A, 2000 MVA, 3s. The symmetrical breaking current is
A
1200A
B
3600A
C
35 ka
D
104.8 kA
GATE EE 2010   Power Systems
Question 9 Explanation: 
Rated MVA of circuit breaker = 200 MVA \;\sqrt{3} \times |V(line)|_{rated} \times Symmetrical breaking current =Rated MVA of CB
Symmetrical breaking current =\frac{200}{\sqrt{3} \times 33}=35kA
Question 10
Match the items in List-I (Type of transmission line) with the items in List-II (Type of distance relay preferred) and select the correct answer using the codes given below the lists.
A
a - 2 , b - 1, c - 3
B
a - 3 , b - 2, c - 1
C
a - 1 , b - 2, c - 3
D
a - 1 , b - 3, c - 2
GATE EE 2009   Power Systems
Question 10 Explanation: 
Impedance relay is a voltage restrained over current relay.
T=k_1I^2-k_2V^2
Reactance Relay is an over-current relay with directional restraint.
T=k_1I^2-k_3VI \sin \theta
mho relay is a voltage restrained direction relay.
T=k_3VI \cos (\theta -t)-k_2V^2
mho relay is inherently directional.
There are 10 questions to complete.
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