TRANSMISSION LINE 1. Instructed by: Miss. R T Gunasekara

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1 TRANSMISSION LINE 1 Instructed by: Miss. R T Gunasekara Name :- D.K.Pathirana Index No : P Group :- EE9 Date of Per. :- 24/01/2011 Instructed by :- R.T.Gunasekara

2 OBSEVATION SHEET Name :- D.K.Pathirana Index No : P Group :- EE9 Date of Per. :- 24/01/2011 Instructed by :- R.T.Gunasekara Observations Open circuit test, V s =74V W s = 10W I s = 0.9A Short circuit test, V s = 5.2V W s = 5W I s = 1.1A

3 EXPREIMENT: Transmission Line 1 OBJECTIVES: To determine the general parameters of a transmission line from 1. Open circuit test 2. Short circuit test APPARATUS: Transmission Line model Variac (0-230V, 8A) Wattmeter Rheostat (100Ω, 8A) Capacitor (4μF) 1 No 1 No 1 No 1 No A. C. Voltmeter (0-150V) 2 Nos A. C. Ammeterers (0-5A) 2 Nos TRANSMISSION LINE DATA Three phase, 50Hz, 25km long transmission line. Constants per phase r = 0.3Ω/km l = 2mH/km c = 0.256μF/km and g is negligible Scale factors: Voltage scale: 1000:1 Current scale: 200:1 THEORY Think that this is a part of a transmission line A δx Vx Ix X Vr Ir B

4 then, A- Sending end B- Receving end Vx- Voltage at X distance from the receiving end Ix- Current at X distance from the receiving end Δv- Voltage drop across δx distance δi- Current though δx section Impedance per unit length(z) = r+ jωl Admittance per unit length(y) = g + jωc δv = Iz δx δi = yv δx Differentiating (1) and (2) w.r.t x we get, Substituting from (1) and (2) Therefore, Differentiating (3) w.r.t x

5 From (1) and (4), Equating coefficients, Characteristic impedance Then, When x= 0, from (3) and (4) Substituting above result From (9) and (10), Substituting in (3) and (4)

6 So, In matrix form

7 CALCULATIONS: Finding ABCD parameters theoretically R = (0.3x25)/5 = 1.5Ω L = (2x25)/5 = 10mH C = (0.256x25)/0.2 = 32μF Characteristic impedance Propagation constant Using equation (11) and (12), A =cosh(γx) =cosh( = B = C = D = A =1.0185

8 Finding ABCD parameters practically, For open circuit test, I r =0; Then So, From short circuit test; V r = 0; So,

9 Now we know that; A = D and also AD-BC = 1 from above we can proof that A = D = so; A= D= A = D = So,

10 DISCUSSION Capacitors that we used, haven t have the represent value. So it may cause some errors. Reading errors also can be occurred. The instruments that we used are not 100% accurate. Used transmission line model was a very old one which would not work very accurately, Since we are scaling the transmission line there will be some errors due to scaling factor, Per unit values would not constant along the line. Due to these errors practical and theoretical values can be differ. Some of things must be considered in the design and operation of a transmission line is the determination of voltage drop, line losses and efficiency of transmission. These values are greatly influenced by the line constants, resistance, impedance and capacitance of the transmission. For instance, the voltage drop in the line depends upon the values of above three line constants. In any four terminal network, where the net work should be passive i.e. containing no source of e.m.f., and linear i.e. impedances independent of current flowing, and bilateral i.e. impedances independent of direction of current flowing, the input voltage and input current can be expressed in terms of output voltage and output current. Incidentally, a transmission line is a four-terminal network; two input terminals where enters the network and two output terminals where power leaves the network, and the conditions for a network are fully met in transmission lines. Due to smaller length and lower voltage in these lines, the effect of line capacitance is small and hence can be neglected. Therefore the line is considered to have series impedance i.e. only resistance and inductance of the line are taken into account. There are two methods to represent medium lines. they are Nominal T-model and Nominal model. In this transmission line the whole line to neutral capacitance is assumed to be concentrated at the middle point of line and the half the line resistance and reactance are lumped on either side. Here the lengths of lines are more than 150km and the voltage used is higher than 100kV. In this case, the line constants i.e. resistance, inductance and capacitance are considered uniformly distributed and line to neutral capacitance is divided into two halves; one half being concentrated at the load end and the other half at the sending end. By doing this experiment we identified the four parameters governing the characteristics of a transmission line. They are Line resistance, Line inductance, Line capacitance, Line conductance. And also we knew how to find those parameters theoretically and practically. In designing a transmission line it s very important to keep the voltage drop at a minimum and to achieve maximum possible efficiency.

Exercises on overhead power lines (and underground cables)

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