Parallel single circuit vs double circuit transmission

[zeyazzy]

Hi all

Please explain the difference between parallel single circuit and double circuit transmission lines?

I wish to understand them in terms of impedance calculation. Also, am i right to assume that the equivalent impedance of a double circuit transmission line is half of a single line transmission (assuming the line parameters are the same)?

Many thanks.

 

[davidbeach]

You're going to need to explain what you mean by both terms. As I'd use the terms, you have twice as many structures and somewhat less mutual impedance for the parallel, but your questions suggest you might have something else in mind.

 

[zeyazzy]

Hi all

I'm undertaking an optimization of the transmission network with capacitor bank installation. 2 common objectives are - 1) reduce active power loss, 2) reduce voltage deviation from 1pu (i.e. make buses V close to 1pu).

I want to know the economic benefits for reducing the voltage deviation from 1pu as I need to justify the investment cost.

Thanks.

 

[zeyazzy]

Sorry please ignore my reply above.

Thanks David for your reply. I wish to know that if 2 transmission lines with the same parameters connect two buses, can i take the equivalent impedance to be half of a line?

 

[davidbeach]

No.

On a very basic positive sequence only power flow model you might get away with that. But if your lines are in close proximity, whether on the same or different structures, there will be mutual coupling between the lines, particularly in the zero sequence.

It is completely impossible to consider fault cases for those lines if they are not each included in the model.

 

[zeyazzy]

Thanks David.

I'm undertaking a reactive power optimization project - where i have to optimize the capacitor allocation problem.

Can i use the positive sequence assumption as i will not consider fault conditions, and only the power flow in max load (steady-state)? If i can assume positive sequence, then equivalent impedance of parallel connected buses is half of one line right?

Thanks!

 

[davidbeach]

Well, you wouldn't be getting an accurate picture of the line charging currents if you do that.

 

[zeyazzy]

Thanks David.

So what should i do? Please have a look at my attached picture. How can i represent the double circuit connection in the bus matrix?

I need to form the bus matrix (which requires equivalent impedance between two buses). I assume single line representation of the grid is used to do power flow studies?

 

[zeyazzy]

The picture uploaded before is not that clear. This one is better.

 

[davidbeach]

I really don't understand the question, just put both lines in the model.