# 3phase double circuit trasmission line

Discussion in 'Homework Help' started by Jess_88, Mar 30, 2012.

1. ### Jess_88 Thread Starter Member

Apr 29, 2011
174
1
Hey guys

I'm trying to work out this question, using an equivalent Pi circuit, but that fact that it is a "double circuit" is confusing me.

Here is the background.
A 3-Phase, 50Hz, 330kV, 360MW at 0.82 PF is supplied by:
-330kV, 50Hz, Length (L) = 160km, 3-Phase double circuit.
Each circuit has per-phase
Resistance (R) = 120x10^-6 per-unit/km
Reactance (X) = 990x10^-6 per-unit/km
Shunt Susceptance to Neutral (jB) = 461.7x10^-3 per-unit/km
Neglecting Shunt Conductance.

All units have been given in Vbase = 330kV, Sb = 100MVA

So what I think so far:
Because its a balanced and double
-both transmission line circuits will have the same voltage and current
-Thus we can calculate perimeters of just one line
But -The receiving current will be multiplied by two

Can I do these things?
Dose this effect the values used for each line impedance and the Shunt Susceptance?

With the Pi network
Z(line) = L x(R + jB)
=160((120x10^-6)+j(990x10^-6))

Do I need to convert this from phase resistance

I understand I may also need to determine the load impedance and add this to the line impedance for the equivalent circuit.?

What do you guys think?

thanks heaps

Last edited: Mar 30, 2012
2. ### Jess_88 Thread Starter Member

Apr 29, 2011
174
1
ok, so I have a little bit more direction i think.
theta = cos^(-1)(PF)
Ir(line-toline) = Pr/[(sqr(3)*Vr(line-to-line)*PF]

Because it is a medium length
A=D=(1 + (YZ)/2)--- p.u
C = Y(1+(YZ)/4) ---S
Vs = AVr + BIr
Is = CVr + DIr

From my calculations, AD - BC = 1 seem to hold true. So i'm pretty sure that part is right.
But when I use
Is = CVr + DIr, I seem to get 85.86<89.702 (kA)
which seems way to big.

Am a suposed to do anything because it is a double connection?

3. ### Jess_88 Thread Starter Member

Apr 29, 2011
174
1
Can anyone confirm weather the equations I'm using for this circuit are correct?