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Electrical Power Transmission: Exam Questions and Answers

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Section A (10 marks in total)

A1. When the voltage of a transmission line is increased by 2 times, what will be the expected changes in the power transfer capability

A Increased by 2 times

B Decreased by 2 times

C Increased by 4 times

D Decreased by 4 times

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A2. The per unit impedance Z(pu) is given by

A Z?* (kVA)base/(KV)Â²base

B Z?* (MVA)base/((KV)Â²base *100)

C Z?* (MVA)base*100/(KV)Â²base

D Z?* (MVA)base/(KV)Â²base

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A3. The characteristic impedance Z0 of a transmission line is

A Directly proportional to its length

B Directly proportional to square root of its length

C Inversely proportional to its length

D Independent of its length

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A4. For a given transmission line, its voltage regulation is expressed as (Vs - Vr)/Vr *100%. Hence,

A the given line is a short transmission line

B the given line is a medium transmission line

C the given line has a shunt impedance

D the given line has a reactive compensation regulation

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A5. Please suggest the protection scheme(s) for a transmission line.

A Distance Protection

B Overcurrent Protection

C Earth Fault Protection

D All of the Above

Section B (30 marks in total)

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B1. A 345kV three phase transmission line delivers 500MVA, 0.866 of lagging power factor to Delta connected load:

a) Find load impedance per phase (3 marks)

b) Find load current and phase current (3 marks)

c) Real and reactive power absorbed per phase (2 marks)

d) Total real and reactive power by load (2 marks)

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B2. Explain the functions and basic requirements of a protective relay. What are the relays to be used in Transformer, Transmission Line, and Busbar (10 marks)

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B3. A three-phase 3-wire transmission line is feeding a load but the Y connected load has one of its phases (phase b) open-circuit. The load neutral is earthed, and the unbalanced line currents Calculate

a) positive sequence current (3 marks)

b) negative sequence current (3 marks)

c) zero sequence current, and (3 marks)

d) the neutral current (1 marks)

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Section C (60 marks in total)

C1. An industrial load consisting of a group of induction motors which aggregate 500 kW at 0.6 power factor lagging is supplied by a three phase distribution line having an equivalent impedance of (0.25 + j0.75) ohm. The voltage at the load end of the feeder is 2300V

a) Determine the line current (3 marks)

b) Find the voltage at the sending end of the feeder (4 marks)

c) Find the power, reactive power and voltampere supplied to the sending end of the feeder (8 marks)

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C2. A primary (33/11kV) two-transformer substation is supplied at 33kV through two 0.3 sq.in. 3- core solid type 33kV 0.84 ohm/km copper cables, each of 4.85 km route length, from a grid supply point. The fault level at the GSP (Grid Supply Point) 33kV busbars is 750MVA. The

impedance of each of the 33/11kV transformers at the remote end of each 33kV cable is 20% on 15 MVA. The impedance of the 11kV cables connecting the transformers to the 11kV switchboard is considered negligible.

a) Draw the Single Line diagram (3 marks)

b) The total equivalent impedance of the system (6 marks)

c) The fault level at the 11kV busbar (3 marks)

d) The fault current level at the 11kV busbar (3 marks)

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C3. A three phase transmission line has the sequence impedance given as follows:

Zo = 0.199?900 pu

Z1 = 0.175?900 pu

Z2 = 0.175?900 pu

(The impedances are in per-unit)

During operations, a single line to ground fault occurs,

a) Draw the sequence network (3 marks)

b) Determine the fault current for a Single line-to-ground fault (6 marks)

c) Determine the fault voltage for a Single line to ground fault (6 marks)

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C4. An overhead line with inductance and capacitance per km length of 1.3 mH and 0.09 ?F, respectively is connected in series with an ungrounded cable having inductance and capacitance of 0.2 mH/km and 0.3 ?F/km, respectively.

a) The definition of characteristic impedance (1 mark)

b) Find the characteristic impedance of the overhead line (2 mark)

c) Find the characteristic impedance of the Cable (2 mark)

d) Determine the values of reflected and transmitted waves of voltage and current at the junction due to a voltage surge of 100 kV travelling to the junction

i) Calculate along the line towards the cable (6 mark)

ii) along the cable towards the line (6 mark)

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