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Transmission Lines in Parallel

09/09/2014 11:57 AM

There are two transformers operating in parallel operation. The bus fed by these transformers supplies power to another bus through two transmission lines with equal impedances and lengths, but magnitudes of the currents flowing through the two transmission lines are not the same. What could be the reason for that?

Thanks in advance.

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#1

Re: Transmission Lines in Parallel

09/09/2014 12:15 PM

Your previous posts suggest that this is work related. You may wish to add a few more details to avoid having this look like homework. On CR4 responses tend to be friendly and helpful for work related questions and questions where the poster has put effort into looking for the answer.

If someone thinks this is a homework question the responses might be less helpful.

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#2

Re: Transmission Lines in Parallel

09/09/2014 12:16 PM

Even if it isn't homework, it is hypothetical.

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#3

Re: Transmission Lines in Parallel

09/09/2014 12:20 PM

Nothing is known about the feed to these transformers. I bet the feeds are not symmetrical. If they were, then the question would not arise.

You are also assuming the transformers are absolutely identical, which may not be the case.

All clear now?

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#4
In reply to #3

Re: Transmission Lines in Parallel

09/09/2014 12:47 PM

The transformers feed the same first bus, and the loads are fed by the same second bus. They are not split buses. If they were, I would have examined those options before asking this question as well.

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#5
In reply to #4

Re: Transmission Lines in Parallel

09/09/2014 1:03 PM

Well you could have said. How am I supposed to know that?

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#6
In reply to #5

Re: Transmission Lines in Parallel

09/09/2014 1:06 PM

I added a drawing of the system while asking the question.

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#7

Re: Transmission Lines in Parallel

09/09/2014 1:22 PM

Look at the lines with an IR camera.

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#9
In reply to #7

Re: Transmission Lines in Parallel

09/09/2014 4:53 PM

The OP has verified the current flow is higher is one than the other with through direct measurement. IR may bolster that information.

What type of fault in the lines would you expect to be revealed using IR? I could see if there were terminal/lug issues somewhere, though.

That would be some pretty healthy heating!

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#13
In reply to #7

Re: Transmission Lines in Parallel

09/09/2014 9:17 PM

Probably not the lines, but there's a "hot" connection at one or more of the terminations. Nice interview question, perhaps the OP will share with us how he answered it.

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#8

Re: Transmission Lines in Parallel

09/09/2014 1:57 PM

Why is this bus being fed from 2 transformers simultaneously?

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#10

Re: Transmission Lines in Parallel

09/09/2014 5:06 PM

Have you taken current readings before and after every device on the line? I see several OCPD's there which provide opportunity.

What is the frequency of the system? What are the lengths of your transmission lines? How are they routed? Are they distribution or transmissions lines?

How did you test the characteristic impedance of the transmission lines?

Are you sure nobody has gone in behind you and split/tapped one of your lines?

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#11
In reply to #10

Re: Transmission Lines in Parallel

09/09/2014 5:18 PM

Thanks for the answers, I will re-examine the system tomorrow.

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#12

Re: Transmission Lines in Parallel

09/09/2014 8:22 PM

Why would you have LV transmission lines of any length carrying current of that magnitude, why run them paralleled?

This looks, feels and smells like homework.

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#14
In reply to #12

Re: Transmission Lines in Parallel

09/09/2014 9:40 PM

It is not a homework, they are actually distruition lines though.

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#20
In reply to #14

Re: Transmission Lines in Parallel

09/10/2014 12:12 PM

Ah, distribution lines.

Does each transformer provide a neutral? Is the neutral common?

Harmonics for voltage and current of one xfmr vs. the other?

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#15

Re: Transmission Lines in Parallel

09/10/2014 1:10 AM

Check and retighten terminations/switches/contact points between bus along 1200A. This might be a case of lose connection technically and theoretically.

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#16

Re: Transmission Lines in Parallel

09/10/2014 2:07 AM

To be sure, 600A on busbar to the left and 1200A on the right. Is that what you are stating in your drawing? And it is between the Tfx and the 3rd, 4000A breaker or isolator on the drawing.

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#17

Re: Transmission Lines in Parallel

09/10/2014 7:57 AM

Was it from the day of commissioning or started later?. What are the vector groups?. Are the output voltages same?. Are tap changers manual or OLTC?.

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#19
In reply to #17

Re: Transmission Lines in Parallel

09/10/2014 11:03 AM

What are the pf of 2 loads?.

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#21
In reply to #17

Re: Transmission Lines in Parallel

09/10/2014 7:42 PM

While I agree with your question about commissioning, would you please explain how vector groups, output voltages, and manual or on-load tap changers would have any effect on this problem given the fact the two transmission lines are shown as between two solid busses; therefore any circulating current problems would occur within the paralleled transformer loop which is before the solid busses.

Besides the already stated "poor connection" hypothesis there are other possible reasons, but since OP has not answered whether this is an interview question or not, they will have to wait until it is answered.

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#18

Re: Transmission Lines in Parallel

09/10/2014 10:43 AM

It seems obvious that the 2 loads are not the same. Can you switch them and remeasure?

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#22

Re: Transmission Lines in Parallel

10/12/2014 12:37 PM

When two transmission lines operate in parallel, the currents flowing in each line are always the same. They are only different from each other if the lower bus-bar is split into two parts and the loads are not the same.

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