POWER SYSTEMS- I

INSTRUCTIONS FOR EXPERIMENT NO. 4

1. Aim: To determine ABCD parameters of a medium transmission line.

2. Theory:

Medium transmission lines are modeled with lumped shunt admittance. There are

two different representations –nominal – π and nominal-T depending on the nature of the

network. Nominal – π representation is discussed as below.

In this representation the lumped series impedance is placed in the middle while

the shunt admittance is divided into two equal parts and placed at the two ends. The nominal-

representation is shown in Fig. a. This representation is used for load flow studies, as we

shall see later. Also a long transmission line can be modeled as an equivalent -network for

load flow studies.

Fig. a Nominal- representation.

Let us define three currents I1, I2 and I3 as indicated in Fig. a. Applying KCL at nodes M and

N we get

RRs

Rs

IVY

VY

IIIIII

22

3121

(2.1)

Again

RR

RRRRs

ZIVYZ

VIY

VZVZIV

12

22

(2.2)

Substituting (2.2) in (2.1) we get

RR

RRRRs

IYZ

VYZ

Y

IVY

ZIVYZY

I

12

14

21

22 (2.3)

Therefore from (2.2) and (2.3) we get the following ABCD parameters of the nominal-

representation

1

2

YZDA (2.4)

ZB (2.5)

mho 14

YZYC (2.6)

Obtain Vs and Is in terms of Vr and Ir. Write the ABCD constants in terms of line

impedance and shunt admittance.

3. Procedure:

i. The connections on the transmission line kit are made in such a way that all the

six indicators in each line are connected in series and at it’s both ends two shunt

capacitors are connected to form the π-model.

ii. For the open circuit test, the receiving end terminals are kept open and the

sending end voltage is raised to it’s rated value. Readings are taken for sending

end voltage, current and power.

iii. For the short circuit test, the receiving end terminals are shorted and current at the

sending end is raised to it’s rated value. Measurements are taken for the sending

end voltage, current and power.

iv.

4. Attach the following pages:

a. Single Line diagram of line showing connections for O.C. and S.C. test. (Draw only

for one phase. Represent total line with one pi-section only.)

5. Calculations:

a. Calculate Zoc, Zsc, cos ө, A, B, C, D as,

Zoc = Vs/Is; cos өoc = Ws/ (Vs*Is)

Zsc = Vs/Is; cos өsc = Ws/ (Vs*Is)

A = [Zoc/(Zoc – Zsc)]1/2

B = A*Zsc

C = A/Zoc

D = A

b. Verify that AD-BC = 1 < 0o

6. Observation Table:

Test Vs Is Ws

Open Circuit

Short Circuit

7. Conclusion: Write appropriate conclusion based on the values obtained.