Question 1 |

The most commonly used relay, for the protection of an alternator against loss of
excitation, is

offset Mho relay. | |

over current relay. | |

differential relay | |

Buchholz relay. |

Question 2 |

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_____

100 | |

250 | |

500 | |

1000 |

Question 2 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 3 |

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 ________

50 | |

100 | |

150 | |

200 |

Question 3 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 4 |

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

R_{1},R_{2} | |

R_{2},R_{6} | |

R_{2},R_{5} | |

R_{1},R_{6} |

Question 5 |

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.

i_{1}=1/\sqrt{3}A,i_{2}=0A | |

i_{1}=0A,i_{2}=0 | |

i_{1}=0A,i_{2}=1/\sqrt{3}A | |

i_{1}=1/\sqrt{3}A,i_{2}=1/\sqrt{3}A |

Question 5 Explanation:

i_2=0, since entire current flows through fault

i_1=\frac{1}{\sqrt{3}}A

i_1=\frac{1}{\sqrt{3}}A

Question 6 |

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_____.

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_____.

0.12 | |

0.23 | |

0.42 | |

0.58 |

Question 6 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

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 7 |

A negative sequence relay is commonly used to protect

an alternator | |

an transformer | |

a transmission line | |

a bus bar |

Question 8 |

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 | |

B | |

C | |

D |

Question 8 Explanation:

For transmission of bulk power of very long distance high voltage (400 kV) is used.

To increase reliability, double circuit is used.

To increase reliability, double circuit is used.

Question 9 |

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

0.1875A | |

0.2A | |

0.375A | |

60kA |

Question 9 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 10 |

A three-phase, 33 kV oil circuit breaker is rated 1200 A, 2000 MVA, 3s. The
symmetrical breaking current is

1200A | |

3600A | |

35 ka | |

104.8 kA |

Question 10 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

Symmetrical breaking current =\frac{200}{\sqrt{3} \times 33}=35kA

There are 10 questions to complete.