# Switch Gear and Protection

 Question 1
A balanced delta connected load consisting of the series connection of one resistor $(R=15 \Omega)$ and a capacitor $(C=212.21 \mu \mathrm{F})$ in each phase is connected to three-phase, $50 \mathrm{~Hz}, 415 \mathrm{~V}$ supply terminals through a line having an inductance of $\mathrm{L}=31.83 \mathrm{mH}$ per phase, as shown in the figure. Considering the change in the supply terminal voltage with loading to be negligible, the magnitude of the voltage across the terminals $V_{AB}$ in Volts is ____ (Round off to the nearest integer). A 585 B 632 C 225 D 415
GATE EE 2023   Power Systems
Question 1 Explanation:
Given circuit : where,
\begin{aligned} X_{L} & =2 \pi \times 50 \times 31.83 \times 10^{-3}=10 \Omega \\ R & =15 \Omega \\ X_{C} & =\frac{1}{2 \pi \times 50 \times 212.21 \times 10^{-6}}=15 \Omega \end{aligned}

Convert $\Delta$ to star :
\begin{aligned} Z_{Y} & =\frac{Z_{\Delta}}{3} \\ & =\frac{15-j 15}{3}=5-j 5 \end{aligned}

Now, per phase equivalent circuit : Using voltage division,
\begin{aligned} \mathrm{V}_{\mathrm{AN}} & =\frac{5-\mathrm{j} 5}{5-\mathrm{j} 5+10 \mathrm{j}} \times \frac{415 \angle 0^{\circ}}{\sqrt{3}} \\ & =\frac{415}{\sqrt{3}} \angle-90^{\circ} \mathrm{V} \\ \therefore \quad \quad\left|\mathrm{V}_{\mathrm{AB}}\right| & =\sqrt{3} \times \frac{415}{\sqrt{3}}=415 \mathrm{~V} \end{aligned}
 Question 2
For the three-bus power system shown in the figure, the trip signals to the circuit breakers $B_1$ to $B_9$ are provided by overcurrent relays $R_1$ to $R_9$, respectively, some of which have directional properties also. The necessary condition for the system to be protected for short circuit fault at any part of the system between bus $1$ and the $R-L$ loads with isolation of minimum portion of the network using minimum number of directional relays is A $\mathrm{R}_{3}$ and $\mathrm{R}_{4}$ are directional overcurrent relays blocking faults towards bus 2 B $R_{3}$ and $R_{4}$ are directional overcurrent relays blocking faults towards bus 2 and $R_{7}$ is directional overcurrent relay blocking faults towards bus 3 C $R_{3}$ and $\mathrm{R}_{4}$ are directional overcurrent relays blocking faults towards Line 1 and Line 2, respectively, $R_{7}$ is directional overcurrent relay blocking faults towards Line 3 and R5 is directional overcurrent relay blocking faults towards bus 2 D $\mathrm{R}_{3}$ and $\mathrm{R}_{4}$ are directional overcurrent relays blocking faults towards Line 1 and Line 2, respectively.
GATE EE 2023   Power Systems
Question 2 Explanation: $B_3,B_4$ and $B_5,B_7$ are directional over current relays.
All remaining are $( B_1,B_2,B_6,B_8,B_9)$
Now directional over current relays.

 Question 3
The most commonly used relay, for the protection of an alternator against loss of excitation, is
 A offset Mho relay. B over current relay. C differential relay D Buchholz relay.
GATE EE 2022   Power Systems
 Question 4
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 4 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 5
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 5 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}

There are 5 questions to complete.