POWER SYSTEMS 2 – EE521
ASSIGNMENT 1
DUE DATE: 5th APRIL, 2023.
QUESTION ONE
In the three-phase system shown in Figure 1, phase a is on no load and phases b and c are short
circuited to ground. The following currents are given: 𝐼𝑏 = 91.65∠160.9°, 𝐼𝑛 = 60.00∠90°.
Find the symmetrical components of current 𝐼𝑎0 , 𝐼𝑎1 , and 𝐼𝑎2 .
Figure 1
QUESTION TWO
A generator is connected through a transformer to a synchronous motor. The subtransient
reactances of generator and motor are 0.15 and 0.35 pu respectively. The leakage reactance of
the transformer is 0.1 pu. All the reactances are calculated on a common base. A three-phase
fault occurs at the terminals of the motor when the terminal voltage of the generator is 0.9 pu.
The output current of generator is 1 pu and 0.8 pf leading. Find the subtransient current in pu in
the fault, generator and motor. Use the terminal voltage of generator as a reference vector.
QUESTION THREE
The one-line diagram of a three-bus power system is shown in Figure 2. Each generator is
represented by an emf behind the transient reactance. All reactances are expressed in per unit on
a common 100 MVA base. The generators are running on no load at their rated voltage.
Determine the fault current, bus voltages and line currents during fault when a balanced threephase fault with fault impedance j0.16 per unit occurs on bus 2.
Figure 2.
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QUESTION FOUR
The system shown in Figure 3 shows an existing plant consisting of a generator 100 MVA, 30
kV with 20% subtransient reactance and a generator of 50 MVA, 30 kV with 10% subtransient
reactance, connected in parallel to a 30 kV bus bar. The 30 kV bus bar feeds a transmission line
via the circuit breaker C which is rated at 1250 MVA. A grid supply is connected to the station
bus bar through a 500 MVA, 400/30 kV transformer with 20% reactance. Determine the
reactance of a current limiting reactor X in ohm to be connected between the grid system and the
existing bus bar such that the short-circuit MVA of the breaker C does not exceed the breaker
capacity.
Figure 3.
QUESTION FIVE
The reactance data for the power system shown in Figure 4 in per unit on a common base is as
follows:
Figure 4.
Obtain the Thevenin sequence impedances for the fault at bus 1 and calculate the fault current in
per unit for the following faults:
(a) A three three-phase fault at buss 1.
(b) A single line-to-ground fault at bus 1.
(c) A line-to-line fault at bus 1.
(d) A double line-to-ground fault at bus 1.
QUESTION SIX
A generator having a solidly grounded neutral and rated 50 MVA, 30 kV has 𝑋1 = 0.25 𝑝𝑢,
𝑋2 = 0.15 𝑝𝑢, and 𝑋0 = 0.05 𝑝𝑢. What reactance must be placed in the generator neutral to
limit the fault current for a double line-to-ground fault to that of a three-phase fault?
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QUESTION SEVEN
Figure 5 shows three 15 MVA, 30 kV synchronous generators A, B and C connected through
three reactors to a common bus bar. The neutral of generator C is grounded via reactor of 2.0 Ω
and the neutrals of generators A and B are solidly grounded. The generator data and reactance of
the reactors are tabulated below. A line-to-ground fault occurs on phase a of the common bus bar.
Determine the fault current in phase a. Neglect prefault currents and assume generators are
operating at their rated voltage.
Figure 5.
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