United International University (Uiu)
United International University (Uiu)
United International University (Uiu)
There are five questions. Answer any four. Figures in the right-hand margin
indicate full marks.
1. a) How can you carry out “Overvoltage Inter-turn Test” and “Insulation Test” for [5]
a CT?
b) Explain the CT specification written as 25 15P 10 and define different terms [4]
specified.
c) Consider a current transformer of nominal rating 100/5A and having secondary [6]
winding resistance of 0.15 ohm. Burden connected to secondary is specified as
0.5 ohm. The magnetization curve of the CT is as shown in Figure 1(c). Draw
the impedance/ burden line and find the ratio error of the CT for a primary fault
current of 1000A.
Figure 1c
2. a) Draw the constructional detail of a directional relay with neatly shown CT and [6]
PT connections. Define the positive torque zone of the relay with the help of
relevant mathematical formulae.
b) A capacitor type voltage transformer is as shown in Figure Q2(b). The value of [6]
the three capacitors used are C1=15µF , C2=25µF , C3= 20µF . Find the value of
the inductor connected to the secondary circuit at power line frequency.
Figure Q2b
c) Explain the working of a “Broken Delta” PT connection in power system [3]
protection. What is the other name of this configuration?
3. a) Define Plug Setting Multiplier(PSM) and Time Setting Multiplier(TSM). How [2+1+
can we fix the plug/current setting and also the TSM for an Inverse Definite 2]
Minimum Time(IDMT) relay?
b) Time grading of inverse-time relays for a radial feeder is to be done for the [10]
power system shown in Figure Q3(b). Determine the time-setting and plug
setting values for each relay if the fault currents at the different buses are as
shown in the table below. Use the ‘Operating time vs. Multiple of plug-setting’
curve as attached with the question paper.
Figure Q3b
Table Q3b
Relay Position CT Ratio Fault Current
R1 300/5 3500A
R2 200/5 2000A
c) Realize a Reactance relay from universal torque equation. Is it directional or, [6]
non-directional? How can you add directional feature to it and how the
directional unit is named?
5. a) Two generators rated 15MVA, 11kV, having 15% subtransient reactance are [8]
interconnected through transformers and a 100km long line as shown in Figure
Q5(a). The reactance of the line is 0.12ohms/km. The transformers near the
generators are rated 30MVA, 11/66kV with leakage reactance of 10% each. A
symmetrical three phase fault occurs at a distance of 20km from one end of the
line when the system is on no-load but at rated voltage. Determine the fault
current and the fault MVA.
Figure Q5a
Figure Q5b