# 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.