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THE INSTITUTION OF ENGINEERS SRI LANKA

IESL ENGINEERING COURSEPART II EXAMINATION -June 2013

209 POWER SYSTEMS I

Time allowed: 03 hours

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

Q1.

A 400 V, 10 kVA generator G connected to a load via transformers Tl, T2 and transmission line l as

shown in figure Q1. Both transformers are considered as ideal ones. Load impedance equal to 14+jS

n. Transformation ratio of Tl and T2 are 1:10 and 30:1 respectively. Series impedance of the line is

2S+jSOo

G T1L T2

2S+j50 o30:11:10

14+j50

Figure 01

a) Determine p.u. values on common base of 10 kVA and 400 V at generator G

b) Draw the impedance diagram of the system and indicate p.u. values

c) Calculate:I. Voltage at load in volts

II. Power developed by the generator Gin kW

III. Power loss in transmission line in kW

[S][3]

[6]

[4][2]

Q2

a) Describe the advantages of using of bundle conductors in Extra High Voltage transmissionlines [2]

b) State why the "transposition" is important in high voltage transmission lines? Sketch the

conductor arrangement of three-phase transmission line for which transposition is not

applicable [3]

1

r.) certain 50 Hz single phase transmission line consists of three solid conductors with 0.22 cm

radius each in one circuit (conductor group X ) and two solid conductors with radius 0.6 cm

each in return circuit (conductor group V). Conductor arrangement is shown in figure Q2.

oI~

oo o o~14"" ~I( ~16 ~I

6m 9m 6m

Conductor group X Conductor group Y

Figure Q2

I. Calculate GMD and GMRs [7]

II. Determine inductance and capacitance in each circuit [4]

III. Calculate per unit length inductive reactance and capacitive reactance of the complete line[4]

Q3

a) What do you understand by II a fault II in electrical power system [2]

b) Explain the types of fault that occur in electrical power system [2]

c) Why the fault current calculation is is important in power system analysis? [2]

d) 30 MVA, 12.5 KV three-phase, star connected generator operating at no load condition is

subjected to a single line to ground fault at an output terminal when the generator. Positive,

negative and zero sequence reactance ofthe generator 0.22 p.u., 0.35 p.u and 0.1 p.u

respectively. Neutral point is grounded through 0.15 p.u. reactance.

Calculate:I.

II.Short circuit current in kA

Output terminal voltage at each phase

[6][8]

Q4.

A 132 kV, 50 Hz, 200 km three-phase transmission line has following per-phase parameters:

R=0.12 O/kmX=j 0.45 O/km

Y = j3.33 ~S/kma) Calculate ABCD parameters of the line [8]

2

b) If the line delivers 85 MW at 0.85 power factor laggingdetermine sending end voltage, powerand power factor. Voltage at load equalsto 132 kV [8]

c) If the load is disconnected at the receivingend what would be the voltage at the load end ofthe transmission line? Explainyour answer? . [4]

Q5.

a) Explain briefly the arc interruption methods in acand dc circuits [6]b) Explainhow grading of insulators improve the string efficiency [4]c) List the main types of insulators that are used in over-head transmission lines [2]d) Certain string insulator is made out of 9 similar units. The capacitance between each interlink

and earth is one tenth of the capacitancebetween the metal interlinks. The flashover voltageof one unit is 75 kV.Calculate the voltage at which this assembleswill flashover. [8]

Q6.

a) Calculate the positive, negative and zero sequence currents if the unbalanced currentsare as follows:, , [5]

Ia = 9LlO° A Ib = 4LI0o A I, = 3.3L140o Ab) Calculate the voltages in three phases if the positive, negative and zero sequence

voltages areVao= 26.5L140o V, Vb2= 12L385° V Vel = 38L190o V [5]

c) A star connected impedances (Zy) with neutral is connected to the ground through ZN isshown in figure Q6. Show that

I. No positive or negative sequence currents flow from neutral to ground. [4]II. Impedance to the zero -sequence current is [6]

z, = z, +3ZN

ZN

FigureQ6

3

Q7

a) List the types of turbines that are used in hydro power plants. Explain briefly their operatingprinciple and the suitability . • [5]

b) What is the function of cooling tower in thermal power plant and how does it operate [4]

c) Sketch block-schematic diagram of combined cycle power plant and explain briefly theoperating principle [5]

d) Explain the functions of the following components in a nuclear reactorI. Control rod

II. ModeratorIII. Reflector [6]

4