HNC power system 3rd assignment (plz check the questions first --total 7 questions)

julian kay

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Home >Science homework help >HNC power system 3rd assignment (plz check the questions first --total 7 questions)

1. Determine the voltage drop at the motor frame for an earth fault at the

motor in the circuit shown in FIGURE 1. The phase voltage is 240 V.

FIG. 1

240 V

Motor

Fault

Motor control centre

Switchgear

Transformer

Earth continuity conductor

0.01Ω

0.0002Ω 0.0002Ω

TXr winding

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2. An 11 kV motor is fed by cables from a transformer via switchgear,

having a phase impedance of 0.3 + j0.3 ohm. The earth return path to

the transformer neutral has a resistance of 0.42 ohm. Determine a

suitable value of neutral resistance if the voltage rise at the motor in the

event of an earth fault at the motor is not to exceed 430 V. Neglect

transformer winding resistance.

3. Using TABLE 1, determine the resistance of a conductor and the volt

drop per ampere metre at 70°C for a 95 mm2 pvc/swa/pvc copper cable.

The temperature coefficient of resistance is 0.004 per °C.

TABLE 1 Resistances of Copper Conductor and Steel Wire Armour for

3-core copper PVC Insulated Cables at 20°C.

Copper Conductor

Size

Resistance/metre of Conductor

(microhms per metre)

Resistance/metre of Steel Armour

(microhms per metre)

6 mm2

10 mm2

16 mm2

25 mm2

35 mm2

50 mm2

70 mm2

95 mm2

120 mm2

150 mm2

185 mm2

240 mm2

300 mm2

3080

1830

1150

727

524

387

268

193

153

124

4600

3700

3200

2400

2100

1990

1400

1200

1100

740

680

600

540

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4. A load, 100 m from a 415 V Supply Authority point is connected by a

35 mm2 pvc/swa/pvc copper cable. The current rating is 119 A and the

volt drop per ampere metre is 1.1 mV. If the load current is 90 A,

determine whether the volt drop at the load is within the 17th Edition of

the IEE Wiring Regulations 5% limit. Using TABLE 1, calculate the

earth loop impedance, minimum fault current (neglect the supply/earth

impedance) and select an appropriate fuse for the circuit with reference to

B.S. 88 part 2 from Figure 3 Below.

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FIG. 3

6 A

20 A

32 A

50 A

80 A

125 A

200 A

315 A

500 A

800 A

100

101

102

103

104

105

Time, s

P ro

sp ec

ti ve

c u rr

en t,

r m

s a m

p er

10–2 10–1 100 101 102 103 104

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1. (a) State two disadvantages of Time Graded Protection Systems.

(b) State two advantages and a disadvantage of Differential Protection

Systems.

Differential Protection System? Why is this rating preferred?

(d) State the minimum time grading interval between two relays and give

reasons for this interval.

(e) Complete the following:

1. Desired relay operating time = TMS × ....................

2. Saturation can be a feature of CTs and ....................

(f) State the two rules of successful time grading of relays.55

5 Calculate the primary setting current and TMS for an inverse-time overcurrent relay in a system where the maximum load is 400 A,

the maximum fault current is 7000 A and CT ratio is 600/1, if the relay

has to operate in 0.7 s.

Assume settings are 50% to 200% in 25% steps. (110% of max. load has

to be catered for.)

Use the BS curve of an inverse - time overcurrent relay, FIGURE 1 for

reference.

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FIG. 1 B.S. Characteristic Curve of an Inverse - Time Overcurrent Relay

100

1 2 3 5 10 20 100

Multiple of setting

T im

e (s

ec s)

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6. For the radial power system shown in FIGURE 2, the outgoing circuit at C is fused and will operate in 0.1 s for the fault level at C. The fault level

at B is 6000 A. If the required discrimination time between the relay at B

and the fuse at C is 0.3 s, calculate a suitable TMS setting for the relay at

Assume CT settings are from 50% to 200% in 25% steps. 110% of

maximum load is to be catered for.

FIG. 2

7 FIGURE 3 shows the use of unit protection where differential relays are protecting transformers. For each of the circuit breaker configurations:

(i) CB A open and CB B closed

(ii) CB A closed and CB B open

(all other CBs closed)

(a) To a base of 10 MVA determine initially the fault levels at the 11 kV

and 3.3 kV busbars for a fault at the transformer, T1, output as

shown.

(b) Determine the current settings and TMSs for the relays shown if the

operating time of the directional overcurrent relays on the 3.3 kV side

of the transformers is 0.5 s. (Use a 0.4 s discrimination interval.)

State with which of the CB configurations the directional overcurrent

relays (if any) will not operate.

800/1 400/1

300 A max load

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FIG. 3

G G

A 11 kV

3.3 kV

5 MVA 10%

T2T1

CT 250/1 CT 250/1 Non-directional

overcurrent relays

CT 1000/1

Directional overcurrent relays

CT 1000/1 t = 0.5s

Fault current

G2G1

8MW, 0.8pf 20%

t = 0.5s

8MW, 0.8pf 20%

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