EEE8082: Post-Exam Assignment on Electrical Power

02.10.2018 EEE8082: Post-Exam Assignment
ELECTRICAL POWER
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02.10.2018 EEE8082: Post-Exam Assignment
Post-Exam Assignment
Total marks = 30
Attempt all three parts.
PART 1
A busbar has four lines and its twelve 2000/5A CTs are connected in parallel with a
high impedance relay to provide earth fault protection.
The connection resistance from each group of three CTs to the relay is 0.3 Ω and the
winding resistance of each CT is 0.2 Ω. The relay has a setting voltage of 25 Volts and
the corresponding relay current is 25 mA. The excitation current of each CT at the
setting voltage is 80 mA.
i) Sketch the 3 phase connections of the CTs and the relay.
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02.10.2018 EEE8082: Post-Exam Assignment
ii) Calculate the value of a suitable stabilising resistor to ensure stability for a
through fault of 24000 A.
RS= Us
Is = 25
0.025 =1 K Ω
iii) On the sketch drawn, using the convention P1, P2 and S1, S2 indicate CT
polarities and show the secondary current flows for a single phase fault on the
busbar. Assume that all circuits contribute to the fault.
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02.10.2018 EEE8082: Post-Exam Assignment
iv) Calculate the minimum value of the fault current to cause the relay to operate
If =Us
Zf = 25
0.3+0.2+1000 =24.65 mA
v) v) How can the relay be made less sensitive?
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