# Sathyabama University Question Papers

# BE Power System Analysis Fifth sem

Answer All the Questions

1. What is meant by per unit quantity? Why is the per unit method

considered superior to percent method for short circuit calculations?

2. Draw a general circuit which can be used to determine the zero sequence

network of a two winding transformer. Using this circuit draw the zero

sequence network of a star – star transformer with star point grounded.

3. What is off nominal transformer ratio?

4. Write the importance of load flow, studies in power system analysis.

5. What is short circuit KVA?

6. What is the use of reactor in power system? Discuss different types of

reactors.

7. Distinguish between a balanced and unbalanced system by means of

symmetrical components of vectors.

8. Draw a diagram showing inter connection of sequence networks for a

double line fault.

9. Distinguish between transient and steady state stability of a power

system.

10. Discuss why an early fault clearing means better chances of maintaining

system stability.

PART – B (5 x 12 = 60)

Answer All the Questions

11. (a) Figure shows a generator feeding two motors through transformer and

line. The rating and reactance are as follows.

Generator : 100 MVA, 11 KV, 3 PHASE, X = 20%

Transformer T1 : 3 Phase, 100 MVA, 11/132 KV, X = 5%

Transformer T2: Bank of three single phase transformers, each rated at 35

MVA,, 66/11KV, X = 4%

Motor M1 : 40MVA, 3 PHASE, 10 KV, X = 20%

(or)

14. Give reasons why a) NR method is preferred to GS. b) A majority of

buses in PS are load buses. c) One of the buses is taken as slack bus in

load flow studies. D) Order of Jacobias in NR method using polar

coordinates is smaller then that in NR method using rectangular

coordinates) A direct solution of load flow problem ins not possible) An

acceleration factor is commonly used in load flow studies using GS

method.

15. (a) Draw the diagram showing the short circuit current as a function of

time for a synchronous generator and explain the terms synchronous

reactance transient reactance and sub transient reactance.

(5)

(b) A 30 MVA, 11 KV generator has Z1 = Z2 = j0.2pu, Z0 = j0.05 pu.

A 3 PHASE fault occurs on the generator terminals. Find fault

current. Assume that the generator neutral is study grounded and that the

generator is operating at no load and rated voltage at the occurrence of

fault. (7)

(or)

16. (a) A synchronous generator feeds bus 1 of a system. A power network

feeds bus 2 of the system. Buses l and 2 are connected through a

transformer and a transmission line. Per unit reactances of various

components are,

Generator (connected to bus bar 1) 0.25

Transformer 0.12

Transmission line 0.28

The power network can be represented by a generator with a reactance in

series. With the generator on no load and with 1 pu voltage at each bus

under operating condition, a 3 PHASE short circuit occurring on bu 1

causes a current of 5 pu tp flow into the fault. Determine the equivalent

reactance of the power network.

(b) For the 3 bus network shown build Z bus (Fig 2)

17. Draw the sequence, – ve sequence and zero sequence network for the

system data is as follows:

Equipment MVA rating Voltage rating XI X2 X0

G1 100 11KV 0.25 0.25 0.05

G2 100 11KV 0.2 0.2 0.05

T1 100 11/220KV 0.06 0.06 0.06

T2 100 11/220KV 0.07 0.07 0.07

Line 1 100 220KV 0.1 0.1 0.3

Line 2 100 220KV 0.1 0.1 0.3

(or)

18. A 25 MVA, 11 KV, 3 phase synchronous generator with solidly grounded

neutral has sub transient reactance of 0.25pu. The –ve and zero sequence

reactances are 0.35pu and 0.1 pu respectively. Determine the generator

and line to line voltage when a single line to ground fault occurs at the

terminals of an unloaded generator operating at rated voltage. Resistance

may be neglected.

19. (a) Explain clearly the application of equal area criterion for studying the

transient stability of a power system. (5)

(b) The steady state limit of a power system is 100MW. A generator with

constant excitation is supplying 50MW to the system. Estimate the

maximum permissible sudden increase in generator output without

causing instability. (7)

(or)

20. (a) What is meant by “Power – angle” diagram? Show how this may be

used to determine the stability of a simple system under a transient fault

condition. (5)

(b) A 50 Hz turbo generator in delivering 50% of power that is capable of

delivering through a transmission line to an infinite bus. A fault occurs

that increases the reactance between generator and infinite bus to 400% of

the value before the fault. When the fault is isolate, the maximum power

that can be delivered is 75% of the original maximum value. Determine

critical clearing angle for the conditions given above.