STUPIDSID
1 (a)
Give the merits and demerits of per unit system. Prove that the per-unit impedance of a transformer is the same regardless of the side from which it is viewed.
7 M
1 (b)
Consider the 50 Hz power system the single-line diagram of which is shown in Figure. The system ratings are:
Generator G1: 200 MVA, 20 kV, Xd=15%
Generator G2: 300 MVA, 18 kV, Xd=20%
Generator G3: 300 MVA, 20 kV, Xd=20%
Transformer T1: 300 MVA, 220Y/22 kV, X=10%
Transformer T2: Three single-phase units each rated 100 MVA, 130Y/25 kV, X=10%
Transformer T 3 : 300 MVA, 220/22 kV, X = 10%
The transmission line reactance are as indicated in the figure. Draw the reactance diagram choosing the Generator 3 circuit as the base.
Generator G1: 200 MVA, 20 kV, Xd=15%
Generator G2: 300 MVA, 18 kV, Xd=20%
Generator G3: 300 MVA, 20 kV, Xd=20%
Transformer T1: 300 MVA, 220Y/22 kV, X=10%
Transformer T2: Three single-phase units each rated 100 MVA, 130Y/25 kV, X=10%
Transformer T 3 : 300 MVA, 220/22 kV, X = 10%
The transmission line reactance are as indicated in the figure. Draw the reactance diagram choosing the Generator 3 circuit as the base.
7 M
2 (a)
With the simplifying assumptions explain the SC transient on a transmission line and hence prove that; I mm (max. possible) = 2[?2V/|z|], i.e. doubling effect. Also draw necessary waveforms to explain doubling effect.
7 M
Answer any two question from Q2 (b) or Q2 (c)
2 (b)
Explain different types of current limiting reactors. Show how they are connected at power stations. Write their uses also.
7 M
2 (c)
Explain how an unbalanced set of three phase voltages can be represented by systems of balanced voltages.
7 M
Answer any two question from Q3 (a), (b) or Q3 (c), (d)
3 (a)
Two synchronous motor are connected to the bus of a large system through a short transmission line as shown in figure. the rating of components are:
Motors (each): 1 MVA, 440 V, 0.1 p.u. transient reactance.
Line: 0.05 ohm reactance.
Large system: short circuit MVA at its bus at 440 V is 8.
when the motors are operating at 440 V, calculate the short circuit symmetrical current fed into a 3-phase fault at motor bus.
Motors (each): 1 MVA, 440 V, 0.1 p.u. transient reactance.
Line: 0.05 ohm reactance.
Large system: short circuit MVA at its bus at 440 V is 8.
when the motors are operating at 440 V, calculate the short circuit symmetrical current fed into a 3-phase fault at motor bus.
7 M
3 (b)
A delta connected balanced resistive load is connected across an unbalanced three phase supply as shown in figure. With currents line A and B specified. Find the symmetrical components of line currents.
7 M
3 (c)
What are the causes of unsymmetrical faults in power system? Derive an expression for the fault current for a single line-to-ground fault.
7 M
3 (d)
Justify the following statement:
"For a fault at alternator terminals, a single line to ground fault is generally more severe than a 3-ph fault whereas for faults on transmission lines, a 3-ph fault is more severe than other faults."
"For a fault at alternator terminals, a single line to ground fault is generally more severe than a 3-ph fault whereas for faults on transmission lines, a 3-ph fault is more severe than other faults."
7 M
Answer any two question from Q4 (a), (b) or Q4 (c), (d)
4 (a)
Derive the power angle equation: P=(EG EM/X) sinδ with usual notations. Also draw the power angle curve.
7 M
4 (b)
A synchronous generator of reactance 1.20p.u. is connected to an infinite bus (|v| = 1p.u.) through transformers and a line of total reactance of 0.60p.u. The generator no load voltage is 1.20p.u. and is inertia constant is H = 4 MW-s/MVA. The resistance and machine damping may be assumed negligible. The system frequency is 50 Hz. Calculate the frequency of natural oscillations if the generator is loaded to (i) 50% and (ii) 80% of its maximum power limit.
7 M
4 (c)
Derive the equation Ybus=A YA, Where Ybus=bus admittance matrix, A=bus incidence matrix, At=transpose of A and Y=primitive admittance matrix.
7 M
4 (d)
For the system of figure. Find the voltage at the receiving bus at the end of the first iteration. Load is 2+j0.8 p.u. Voltage at the sending end (slack) is 1+j0 p.u. Line admittance is 1.0-j4.0 p.u. Transformer reactance is j0.4 p.u. off-nominal turns ratio is l/l.04. Use the GS technique. Assume V R = 1∟0°
7 M
Answer any two question from Q5 (a), (b) or Q5 (c), (d)
5 (a)
Discuss the advantages and limitations of Gauss-Seidal and Newton Raphson methods. Of this two, which method is generally preferred for solving the load flow problem?
7 M
5 (b)
What are the conditions to be satisfied before a 3-phase alternator is synchronized to infinite bus bars?
7 M
5 (c)
Explain how the active and reactive power loading of an alternator working on infinite bus-bar is controlled.
7 M
5 (d)
Describe briefly the principle of operation of load dispatch organization coordinating different types of power plants in a power system.
7 M
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