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# Important Short Questions and Answers: Symmetrical Components And Unbalanced Fault Analysis

Power System Analysis - Symmetrical Components And Unbalanced Fault Analysis - Important Short Questions and Answers: Symmetrical Components And Unbalanced Fault Analysis

1. What is meant by a fault?

A fault in a circuit is any failure which interferes with the normal flow of current. The faults are associated with abnormal change in current, voltage and frequency of the power system.

2. Why fault occur in a power system?

The faults occur in a power system due to insulation failure of equipments,flashover of lines initiated by a lightning stroke, due to permanent damage to conductors and towers or due to accidental faulty operations.

3. List the various types of shunt and series faults. The various types of shunt faults are:

(i)Line to Ground fault

(ii) Line to Line fault

(iii)Double line to Ground fault

(iv) Three phase fault

The various types of series faults are:

(i)One open conductor fault

(ii) Two open conductor fault

4. What is symmetrical unsymmetrical fault?

The fault is called symmetrical fault if the fault current is equal in all the phases.The fault is called unsymmetrical fault if the fault current is not equal in all the phases.

5.                 Name any two methods of reducing short â€“circuit current.

(i)By providing neutral reactance.

(ii)By introducing a large value of shunt reactance between buses.

6.                 What is meant by fault calculations?

The fault condition of a power system can be divided into subtransient, transient and steady state periods. The currents in the various parts of the system and in the fault are different in these periods. The estimation of these currents for various types of faults at various locations in the system are commonly referred to as fault calculations.

7. What is the need for short circuit studies or fault analysis?

The short circuit studies are essential in order to design or develop the protective schemes for various parts of the system. The protective schemes consists of current and voltage sensing devices , protective relays and circuit breakers. The selection of these devices mainly depends on various currents that may flow in the fault conditions.

8. What is synchronous reactance?

The synchronous reactance is the ratio of induced emf and the steady rms current. It is the sum of leakage reactance and the reactance representing armature reaction.

9. Define subtransient reactance.

The subtransient reactance is the ratio of induced emf on no load and the subtransient

symmetrical rms current.

10. Define transient reactance.

The subtransient reactance is the ratio of induced emf on no load and the transient symmetrical rms current.

11. Name the fault in which positive, negative and zero sequence component currents are equal.

In Single line to ground fault positive, negative and zero sequence component currents are equal.

12. Name the fault in which positive and negative sequence component currents together is equal to zero sequence current in magnitude.

Double line to ground fault.

13. Define positive sequence impedance.

The positive sequence impedance of an equipment is the impedance offered by the equipment to the flow of positive sequence currents.

14. Define negative sequence impedance.

The negative sequence impedance of an equipment is the impedance offered by the equipment to the flow of negative sequence currents.

15. Write the boundary condition in single line to ground fault.

Va = 0 ; Ib = Ic = 0

16. What are the boundary conditions in line to line fault?

Ia = 0 ; Ib + Ic = 0 ; Vb = Vc

17. Write down the boundary condition in double line to ground fault. Ia = 0 ; Vb = 0 ; Vc= 0

18. Give the boundary condition for 3-phase fault.

Ia + Ib = Ic = 0 ; Va = Vb = Vc= 0

Part-B

1.A balanced delta connected load is connected to a three phase system and supplied to it is a current of 15 amps. If the fuse is one of the lines melts, compute the symmetrical components of line currents

2. 2.Draw zero sequence network of the power system as shown in fig.

3.A 50MVA, 11KV, synchronous generator has a sub transient reactance of 20%.The generator supplies two motors over a transmission line with transformers at both ends as shown in fig. The motors have rated inputs of 30 and 15 MVA, both 10KV, with 25% sub transient reactance. The three phase transformers are both rated 60MVA, 10.8/121KV, with leakage reactance of 10% each. Assume zero sequence reactance for the generator and motors of 6% each. Current limiting reactors of 2.5 ohms each are connected in the neutral of the generator and motor number 2. The zero sequence reactance of the transmission line is 300 ohms. The series reactance of the line is 100 ohms. Draw the positive, negative and zero sequence networks.

4.A 30 MVA, 13.2KV synchronous generator has a solidly grounded neutral. Its positive, negative and zero sequence impedances are 0.30, 0.40 and 0.05 p.u respectively. Determine the following:

i.  What value of reactance must be placed in the generator neutral so that the fault current for a line to ground fault of zero fault impedance shall not exceed the rated line current?

ii. What value of resistance in the neutral will serve the same purpose?

iii.              What value of reactance must be placed in the neutral of the generator to restrict the fault current to ground to rated line current for a double line to ground fault?

iv.              What will be the magnitudes of the line currents when the ground current is restricted as above?

v.                 j) As the reactance in the neutral is indefinitely increased, what are the limiting values of the line currents?

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