/THE INSTITUTION OF ENGINEERS SRI LANKA

IESl ENGINEERING COURSE

PART" EXAMINATION -December 2013

209 POWER SYSTEMS I

Time allowed: 03 hours

This paper contains seven questions. Answer any five (5) questions. All questions carry equal marks.

1. i. Explain the advantages of using bundle conductors in high voltage overhead [3]transmission lines

ii. Why AC resistances of conductors differ from DC resistance? [2]iii. State three properties of overhead line conductors [3]iv. Conductor arrangement of three-phase overhead line is shown in figure Ql.

Conductor type used is ACSR. Diameter of each conductor is 16.52 mm. AC resistanceof the conductor at 20DCis 0.22 O/km. Temperature coefficient of ACSRconductorequals to 0.0043 Del. Operating frequency is 50 Hz.

12 m

-I-~I -Figure Ql

a) Calculate per-phase per unit length inductance and capacitance of the line [8]b) If the length of the line is 120 km and operating at the ambient temperature [4]

of 42DC, determine equivalent n:model of the line

2 A 132 kV, 50 Hz three-phase overhead transmission line is 100 km long. Per- phaseparameters ofthe line are:

R= 0.112 OjkmX=0.480/kmY=3.8 xlO·6 S/km

Determine ABCD parameters of the line [8]ii If the line delivers load of 150 MW at 0.9 power factor lagging determine the sending [8]

end voltage and current. Voltage at the load end is to be maintained at rated valueiii What is the transmission dficiency? [2]iv Determine voltage regulation [2]

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3 Explain briefly undesirable effects of the short circuit current to the electrical power [5]system

ii A three-phase synchronous generator is subjected to a three-phase short circuit at [4]the output terminal while operating at no-load. Sketch the variation of short circuitcurrent with the time and clearly indicate sub-transient, transient and steady stateperiods

iii A generator is connected to an infinite bus as shown in figure 03. A three-phase shortcircuit occurs at the midpoint (F) of the line. Per-phase reactance of the line is 15 0and resistance can be neglected. Parameters of the generator (G) and transformer (T)are as indicated in the figure. Parameters of G and T are given on equipment base

G T

F Infinite bus

50 MVA 75 MVA15.25 kV 15.5/138 kV

X'=0.28 p.u. X=12%

Figure Q3a) Calculate the fault current in kA [7]b) What is the current through generator during the fault [4]

4 Explain briefly how the electric arc is interrupted in following circuit breakers: [7]Air-Blast circuit BreakerOil circuit Breaker

ii List four properties of SF6 gas that makes it superior to the other types of circuit [4]breakers.

iii "In some respect, High Rapturing Capacity (HRC) fuses are better than circuit breakers [4]of same capacity". Justify this statement

iv List three nominal characteristics of a circuit breaker [3]v Explain why isolators cannot be used as lion load device" [2]

5 Explain the advantages of symmetrical component [4]ii Prove that positive and negative sequence currents do not flow through the neutral [4]iii A 75 MVA, 20 kV Wye connected synchronous generator is connected to a 100 MVA, [12]

20 V/66 kV, delta to Wye connected transformer. Both neutrals of the generator andhigh voltage side of the transformer are solidly grounded. Positive, negative and zerosequence reactance of the generator are 0.2 p.u.,O.2 p.u. and 0.05 p.u. Transformerleakage reactance is 10%. A single-line to ground fault occurs at the high voltage sideof the transformer. Determine currents in all the three-phases of the generatorduring fault

6 Explain why the distance relay is more suitable than the over current relay for the [5]protection of transmission lines

ii Certain transmission line is protected by a mho relay having maximum reach of 12 [8]ohm and maximum torque angle of 750

. At the 60% of the maximum reach, a faultoccurs and detected by the relay. Calculate maximum fault resistance seen by the

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relay assuming that the impedance angle similar to the maximum torque angle.

iiii A 40 MVA, 11 kV/33 kV Delta-Wye connected, three-phase transformer is protected (7)by a differential relay. CT ratios at LV and HV sides are 2000:5 and 500:5respectively. Calculate relay current setting for faults up-to 200% of the ratedcurrent.

7 i. Explain briefly the function of a pumped storage hydro-power plant. What is theadvantage of having this type of plants in power systems

ii. Sketch daily load curve of Sri Lankan power system and explain briefly the reason forthe shape of this pattern. What is the disadvantage of having a shape of this nature?What measures would you take to improve the shape of this pattern

iii. Discuss the use of various renewable sources of the country to meet the electricitydemand under the following headings:

a) Reliability of supplyb) Cost of generation per unitc) Generating capacity

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