Surge impedance of a line, Zo 'LC

.

SIL '(kVLL)2

Zo

, (MW)

I 2XL'V 2

Xc

, or, V 2

I 2'XLXc '

TLTC

, or, VI'

LC'Zo'Load impedance

SURGE IMPEDANCE LOADING (SIL)

A transmission line loaded to its surge impedance loading:

(i) has no net reactive power flow into or out of the line, and(ii) will have approximately a flat voltage profile along its length.

For (i) to hold:

This means that there will be no net reactive power flow at surge-impedanceloading.

Vs'AVr%BIr ; Is'CVr%DIr ,

And, Vs' (A%BZo

)Vr' (cos 2BR8

% j sin 2BR8

)Vr'Vrptan&1 2BR8

,

Is' (CZo%D)Ir' ( jsin 2BR8

%cos 2BR8

)Ir' Irptan&1 2BR8

At surge&impedance loading ,Vr

Ir

'Zo .

Then, A'D'cos 2BR8

; B' jZosin 2BR8

; C' jsin 2BR

8Zo

ZY' jTR LC'jTR<c

' j 2BfRf8

' j 2BR8

where, A'D'cosh ZY ; B'Zosinh ZY ; C'sinh ZY

Zo

; Z' jTLR ; Y' jTCR

For (ii) to hold:

This means that the line will have a flat voltage profile, i.e., no voltage drop.

LOADABILITY CURVES

Loadability of a line is limited by :

(i) thermal limitation (I R losses)2

(ii) voltage regulation(iii) stability limitation

(EPRI Publication EL-2500)