1: Choose the best answer of the following (20 marks)
1:The connection group at the low voltage side of a YNd11 power transformer is:
1. Zigzag
2. Delta
3. Star
4. None of the above
2: The synchronization between two power supply system is executed when
1. The phase sequence of the two system is the same.
2. The frequency of the two system is the same.
3. The magnitude of the voltage of the two system is the same.
4. All the above.
3: To protect a power transformer from surge over voltages, the surge arrestor is
preferred to be installed
1. as far as possible from the power transformer.
2. as close as possible to the power transformer.
3. in the middle span of the power line feeding the transformer.
4. None of the above.
4: The sensitivity of the ELCB (Earth Leakage Circuit Breaker) uses in houses is
1. 3000 mA.
2. 300 mA.
3. 30 mA.
4. 3 mA.
5: In a tow winding transformer 220/22kV, the On load tap changer (OLTC) is
preferred to be installed on:
1. The primary winding.
2. The secondary winding.
3. both windings.
4. All the above.
6: In parallel connection of similar transformers, the theoretical condition for the
connection is
1. Same rated power.
2. Same short circuit impedance on the principal tapping.
3. Same rated voltage ratio.
4. All the above.
7: In a step down distribution transformer 22/.04 KV, the secondary voltage is
increased by :
1. Decreasing the number of primary windings Np.
2. Increasing the number of primary windings Np.
3. Decreasing the number of secondary windings Ns.
4. Increasing the number of secondary windings Ns.
8: Transposition in long transmission lines is done to make :
1. Each conductor have the same average inductance over the whole length.
2. Each conductor have the same average capacitance over the whole length.
3. Each conductor have the same average conductance over the whole length.
4. All of the above.
9: What does each disk in insulator strings make with cross arms and steel tower
body?
1. Inductance.
2. Capacitance.
3. Conductance.
4. None of the above.
10: If a 220KV line 100Km in length is operated below its surge impedance loading
limit (economical dispatch), then the power line shall
1. supply reactive power from the system.
2. consume reactive power from the system.
3. the economical operation of the line has no impact on the system
4. None of the above.
11: The instrument transformers are used largely in the power system to:
1. Transformer the currents of the voltage from high to low values
2. Insulate the relays and metering equipment from the primary high voltage
system
3. Give feedback signals to the relays to detect abnormal conditions
4. All the above.
12: If the phase wire is short circuited with the neutral wire in a single phase protected
circuit, what breaker of the following shall operate?
1. The earth leakage circuit breaker ELCB.
2. The miniature circuit breaker MCB.
3. The system shall not respond to the fault.
4. None of the above.
13: The resistance of the conductor[R] increases due to what of the following
1. Increasing the length of the conductor .
2. Increasing the cross section of the conductor.
3. Increasing the conductor temperature
4. Both 1 and 3
14: The breakdown of the high voltage underground cable (UGC) insulation occurs
due to
1. Increasing the system voltage.
2. Increasing the temperature of the cable.
3. Aging of the insulation material
4. All the above.
15: If a conductor capacity rating is 100A dc, then the ac capacity shall be
1. Larger that 100A
2. Lower than 100A
3. Equals 100A
4. None of the above.
16: The breakdown strength of an overhead transmission line (OHTL) depends upon
1. The span of the conductor
2. The sag of the conductor
3. The weight of the conductor
4. All of the above.
17: The maximum value of tension of a transmission line is found at
1. The fixing point on the top of the pole
2. The middle span of the conductor
3. There are no tension forces on the line
4. None of the above.
18: which of the following equipment contribute to poor power factor?
1. Power and distribution transformers
2. Induction motors
3. Types of discharge fluorescent lamps
4. All the above.
19: The length of the insulator string is directly proportional to the
1. System voltage.
2. Line current
3. the length of the insulator is independent of the voltage and current.
4. None of the above.
20: Which of the following means are used to improve poor power factor?
1. Install series compensation on power lines.
2. Install shunt compensation on power loads
3. Increase the number of generation units feeding the grid
4. All the above.
2: Calculate and choose the best answer of the following (20 marks)
21: The complex power (S) for the following circuit is calculated to be
1. 5+j 4 kVA
2. 5 - j4 kVA
3. 2.8+j2.6 kVA
4. 2.8 - j 2.6 kVA
22: In a 22KV system, if a load is measured to be 97<45ْ MVA and the bus voltage at
the load is 19KV, then the amount of reactive power in (MVAr) required improving
the power factor to 0.85 is approximately:
1. 30 MVAr
2. 37 MVAr
3. 46 MVAr
4. 55 MVAr
23: If the monthly consumption for a factory is 135,200kWh and 180,020kVArh
while the maximum demand for that month was recorded to be 310kW, then the
maximum demand in Kva for that month(30days) for the same power factor is
1. 417 Kva
2. 505 Kva
3. 517 Kva
4. 720 Kva
24: A step down power transformer 66/22 KV having 16KW iron losses and 50KW
total losses at full load.The transformer is fully loaded for 12hours daily at 10 cents
per KWh, then the cost of losses in a year is approximately
1. 25000$
2. 27000$
3. 29000$
4. 31000$
25: The total losses for the above question at half shall be:
1. 24.5 KW
2. 33.5 KW
3. 45.5 KW
4. None of the above.
26: A balanced three phase Y connected source with V L-L = 200V is used to supply a
balanced Y connected load with 900W of power at a power factor of 0.9 lagging the
line current shall be:
1. 5.5 A
2. 3.5 A
3. 2.9 A
4. 6.4 A
27: The phase impedance (Z) for the above question shall be
1. 40 ‫ے‬25.8ْ ohm
2. 50 ‫ے‬26.9ْ ohm
3. 60 ‫ے‬27.2ْ ohm
4. 70 ‫ے‬21.4ْ ohm
28: In the following positive sequence star connected three phase system, if the angle
between the phases is 120ْ , then the current in the neutral wire (In) is calculated to
be
1. 10 ‫ے‬50.5ْ A
2. 10 ‫ ے‬-50.5ْ A
3. 36 ‫ے‬13.9ْ A
4. 36 ‫ے‬-13.9ْ A
29: if a three phase 220KV 50Km long transmission line loaded 10MVA. The
conductor used is ACSR with resistance equal to 0.0566 per Km. The active
transmission losses shall be:
1. TL = 8.5 KW
2. TL = 117 W
3. TL = 5.8 KW
4. None of the above.
30: For the circuit show below , if the symmetrical short circuit power on the LV side
of the transformer is 100MVA, then the short circuit current on the LV side of the
power transformer shall be
1.
2.
3.
4.
0.525 KA
5.25 KA
10.25 KA
None of the above