POWER SYSTEMS-I

Answer any seven of the following questions :

A generator transformer unit is connected to a line through a circuit breaker. The unit ratings are : Generator : 15 MVA, 11 kV, $X''_d = 0.3$ p.u., $X'_d = 0.3$ p.u., $X_d = 0.6$ p.u. Transformer : 15 MVA, 12.7/66 kV, reactance $= 0.06$ p.u. The system is operating at no load at a line voltage of 66 kV, when a three-phase fault occurs on the line just beyond the circuit breaker. Find-

A single-phase 50 Hz generator supplies an inductive load of 8 MW at a power factor of 0.707 lagging by means of an overhead transmission line 15 km long. The line resistance and inductances are of 0.023 ohm and 0.56 mH per km. The voltage at the receiving end is required to be kept constant at 11 kV.

Discuss in detail about the over-current and directional protection scheme. Also, discuss in detail about the advantages and disadvantages of both the protection schemes and mention their applications.

Discuss in detail about the permanent magnet synchronous generator and induction generator for power generation through wind energy. Also discuss why there is a requirement of using two types of generators. What is the advantage of using one over the other generator?

Draw the sequence diagrams of all types of connections of a transformer.

A synchronous machine 1 generating 1 p.u. voltage is connected through a Y/Y transformer of reactance of 0.1 p.u. to two transmission lines in parallel. The other ends of the lines are connected through a Y/Y transformer of reactance of 0.15 p.u. to a machine 2 generating 1 p.u. voltage. For both transformers $X_1 = X_2 = X_0$. Calculate the current fed into a double line to ground fault on the line side terminals of the transformer fed from machine 2. The star points of machine 1 and of the two transformers are solidly grounded. The reactance (p.u.) of the machines and lines referred to a common base are as follows : Machine 1: $X_1=0.45, X_2=0.35, X_0=0.05$ Machine 2: $X_1=0.40, X_2=0.30, X_0=0.04$ Line each: $X_1=0.30, X_2=0.30, X_0=0.64$

Explain in detail about the sinusoidal steady-state representation of short, medium and transmission lines.

Describe the construction, principle of operation and application of (i) rod gap and (ii) expulsion gap.

Explain clearly how the rating of lightning arrester is selected. What is the best location of lightning arrester and why?

Explain about the typical waveform of synchronous generator under balanced terminal short-circuit conditions for steady-state, transient and sub-transient equivalent circuit. Derive the essential expressions also.

Answer any seven of the following questions :

A generator transformer unit is connected to a line through a circuit breaker. The unit ratings are : Generator : 10 MVA, 6.6 kV, $X''_d = 0.1$ p.u., $X'_d = 0.2$ p.u. and $X_d = 0.8$ p.u. Transformer : 10 MVA, 6.9/33 kV, reactance $= 0.08$ p.u. The system is operating at no load at a line voltage of 30 kV, when a three-phase fault occurs on the line just beyond the circuit breaker. Find-

A single-phase 50 Hz generator supplies an inductive load of 5 MW at a power factor of 0.866 lagging by means of an overhead transmission line 20 km long. The line resistance and inductance are 0.0195 ohm and 0.63 mH per km. The voltage at the receiving end is required to be kept constant at 10 kV.

Discuss in detail about the line commutated converters and voltage source converters for DC transmission systems. Also, discuss in detail about advantages and disadvantages of individual converters for DC transmission systems.

Explain the working of (a) vacuum circuit breaker and (b) $SF_6$ circuit breaker.

The line currents in amperes in phases a, b and c respectively are $500 + j150, 100 - j500$ and $-300 + j500$ referred to the same reference vector. Find the symmetrical components of the currents.

What is a neutral grounding? Explain the methods of neutral grounding.

Discuss in detail about the tap changing transformers. Explain how tap changing transformer is superior than other voltage control methods and also discuss the disadvantage of the tap changing transformer.

Explain about the following :

Discuss in detail about the steady state performance characteristics of a synchronous machine operation, when connected to infinite bus and also real and reactive power capability curve of the generators.