power system HNC 4th assignment (total 6 questions..plz check the questions before buying the answer)
julian kay
questions_2_2.pdf
1. A CT has a current ratio of 100/1. If the primary load current is
100 A, determine the excitation current, the maximum ratio error and
the maximum phase angle error if the burden is 5 ohms. The CT
characteristic gives an excitation current of 0.014 A for a secondary
voltage of 5 V.
2
a) When should a CT with a 5 A secondary rating not be used?
b) Determine the required knee point voltage of a 3 VA CT with a setting of 0.2 A and an Accuracy Limit Factor of 20 (neglect
saturation).
3.
(a) A 400 A circuit is to be protected by a CT and relays. The relays present a 2.5 VA burden. If the setting is 20% and there is saturation
of 50% at 20 × current setting, specify a suitable CT.
4.
(a) In FIGURE 1, time graded relays control the circuit breakers. Which will have the shortest delay in operating for a fault at the location
shown?
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FIG. 1
(c) A voltage transformer for use on an 11 kV system has a star
connected primary. If the turns ratio is 100/1 what will be the
residual voltage of the open delta winding?
(d) A 1 A electromagnetic relay has 80 turns on minimum setting. If the
settings are from 50% to 200% in 25% steps, calculate the plug
bridge turns for each setting. If the relay presents a burden of 3 VA
determine the number of turns required if the relay was to operate at
12 V.
5.
(a) A motor with differential protection has a full load current of 200 A. When starting, the motor takes 5 × FL current. The motor is remote from the switchgear which houses one set of CTs and the relay. If the
CT ratios are 200/1, the secondary winding resistance is 1.0 ohm and
the protection loop lead resistance to the motor is 6 ohms, calculate
the minimum relay setting voltage. State what the CT knee point
voltage should be.
(b) What value of resistance must be added to the relay circuit if the relay current has not to exceed 0.02A? See FIGURE 2. Neglect
relay resistance.
A B C D E
3
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© Teesside University 2011
FIG. 2
6. Determine the voltage setting of the high impedance residually connected
relay shown in FIGURE 3 to protect the transformer secondary winding
against earth faults. The 11 kV/415 V Delta/Star transformer rating is
2 MVA and X% = 8%. The source fault MVA is 25 MVA, the CT ratio
is 1000/1 and its secondary resistance is 2.5 ohms. The lead loop
resistance to the neutral CT is 2 ohms. Use a 1 MVA base.
FIG. 3
E/F
Transformer secondary winding
Relay
CT1 CT2
RL2 RS2
RR
4
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© Teesside University 2011
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