Previous Year Questions- Per Unit System | Power Systems - Electrical Engineering (EE) PDF Download

Q1: For the three-bus lossless power network shown in the figure, the voltage magnitudes at all the buses are equal to 1 per unit (pu), and the differences of the voltage phase angles are very small. The line reactances are marked in the figure, where α, β, γ and x are strictly positive. The bus injections P₁ and P₂ are in pu. If P₁ = mP₂, where m > 0, and the real power flow from bus 1 to bus 2 is 0 pu. then which one of the following options is correct?      (2024)
(a) γ = mβ
(b) β = mγ
(c) α = mγ
(d) α = mβ
Ans: (a)
Sol: 
Q2: A three phase star-connected load is drawing power at a voltage of 0.9 pu and 0.8 power factor lagging. The three phase base power and base current are 100 MVA and 437.38 A respectively. The line-to-line load voltage in kV is _____.      (SET-2(2014))
(a) 88.2
(b) 108.9
(c) 118.8
(d) 127.4
Ans: (c)
Sol: Given,
For a 3 − ϕ star connected systems,
∴ Actual line to line load voltage in kV = 0.9 x 132 kV = 118.8 kV

Q3: For the power system shown in the figure below, the specifications of the components are the following :
G1: 25 kV, 100 MVA, X = 9%
G2: 25 kV, 100 MVA, X = 9%
T1: 25 kV/220 kV, 90 MVA, X = 12%
T2: 220 kV/25 kV, 90 MVA, X = 12%
Line 1: 200 kV, X = 150 ohms
Choose 25 kV as the base voltage at the generator G1, and 200 MVA as the MVA base. The impedance diagram is      (2010)
(a) (b) (c) (d) Ans: (b)
Sol: 
Q4: A generator is connected through a 20 MVA, 13.8/138 kV step down transformer, to a transmission line. At the receiving end of the line a load is supplied through a step down transformer of 10 MVA, 138/69 kV rating. A 0.72 pu. load, evaluated on load side transformer ratings as base values , is supplied from the above system. For system base values of 10 MVA and 69 kV in load circuit, the value of the load (in per unit) in generator will be      (2006)
(a) 36
(b) 1.44
(c) 0.72
(d) 0.18
Ans: (a)
Sol: Base value on load circuit,
Value of load in ohm = Z_L 
= value of load in p.u. x Base impedance
Base impedance,

Q5: The p.u. parameter for a 500 MVA machine on its own base are:
inertia M = 20 p.u.; reactance X = 2 p.u.
The p.u. values of inertia and reactance on 100 MVA common base, respectively, are     (2005)
(a) 4, 0.4
(b) 100, 10
(c) 4, 10
(d) 100, 0.4
Ans: (d)
Sol: 
Q6: A 75 MVA, 10 kV synchronous generator has X_d = 0.4 p.u. The X_d value (in p.u.) is a base of 100 MVA, 11 kV is       (2001)
(a) 0.578
(b) 0.279
(c) 0.412
(d) 0.44
Ans: (d)
Sol: