2011POWER SYSTEM

Time Allotted: 3 Hours Full Marks: 70

GROUP A(Multiple Choice Question)

Choose the correct alternative answers of any ten questions from the following 10X1=10

1. Choose the correct alternative: 10x1

A. The pick up current of a relay is 7.5 A and the fault current in the relay coil is 30 A . Its plug setting multiplier is

a) 4 b) 2 c) 25 d) None of the aboveB. The operator “a” rotates the vector in the anticlockwise direction through an angle of : a) 90o b) 120o c) 180o d) 360o

C. The surge impedance of 50 miles long underground cable is 50 ohms. For a 25 miles length of the same cable, it will be : a) 25 ohms b) 50 ohms c) 100 ohms d) none of the above D. A three phase circuit breaker is rated at 2000 MVA, 33KV. Its making current will be: a) 35KA b) 49KA c) 70 KA d) 89 KAE. An acceleration factor is used for load flow studies by a) Gauss- Seidal Method b) Newton- Raphson Method c) Decoupled method d) None of the above F. The voltage of a particular bus can be controlled by controlling

a) Phase angle b) Reactive power of the Bus c) Active Power of the Bus d) Phase angle and reactive powerG. The order of lightening discharge current is : a) 10000 A b) 100A c) 1A d) 1µAH. The velocity of traveling wave in the transmission line depends on : a) L only b) C only c) both L and C d) R onlyI. A power system network with a capacity of 100MVA has a source

impedance of 10% at a point . The fault level at that point is : a) 10 MVA b) 30 MVA c) 3000 MVA d) 1000MVA

J. When a line to ground fault occurs, the current in a faulty phase is 100A. The zero sequence current is: a) Zero b) 33.3 A c) 66.6 A d) 100A

GROUP B(Short Answer Questions)

Answer any three questions

2. What is acceleration factor? Why it is used in load flow studies? 2+3 = 5 3. Why do we use reactors in power system? Discuss their advantages and disadvantages. 5

4. Derive the torque equation of an induction disc relay. 5

5. Write a short note on ‘Equal area criterion’. 5

6. (a) What is current chopping in a circuit breaker?. (b) What measures are taken to reduce it? 3 + 2 = 5

GROUP C(Long Answer Question)

Answer any three questions

7. a) Discuss various applications, locations and design features of current limiting reactor. b) The 33 KV bus-bars of a station are in two sections P and Q separated by a reactor. The section P is fed from four 10 MVA generators each having reactance of 20%. The section Q is fed from the grid through a 500 MVA transformer of 10% reactance. The CB has a rupturing capacity of 500 MVA. Find the reactance of the reactor to prevent the CB from being overloaded if a symmetrical short circuit occurs on an outgoing feeder connected to A. Take base MVA = 50 MVA

6 +9 = 15

8. a) Explain with sketch, the construction, use, advantages and disadvantages of SF6 circuit breaker.

b) What is stability? Derive the swing equation for a multi-machine system 8+7=15

9. a) Describe the operating principle of directional over current relay with neat diagram. b) A synchronous generator is operating at an infinite bus and supplying 0.45 p.u. of its maximum power capacity. A fault occurs and reactance between the generator and the line becomes four times its value before the fault. The maximum fault that can be delivered after the fault is cleared is 70% of the original maximum value. Determine the critical clearing angle. 7 +8 = 15

10. a) Develop load flow equations suitable for solution by Newton Raphson method b) What is the necessity of having slack bus in a power system? c) For the following single line diagram determine YBUS when shunt admittances are neglected 6 + 2+ 7=15

11. Write short notes on any three of the following: 3x5=15 a) Valve type lighting arrester b) Arc extinction in circuit breaker c) Basic impulse level d) Arcing Ground e) Mho relay