Exploring the Effects of Short Circuits in Parallel Generators: A Case Study

[ge.vasiliou]

Hi guys,
Kind of newbie in this forum, i would like to ask your opinion in the following issue.

I have an application using one Generator (50Hz-400Volts) connected in parallel to the grid (50Hz-20kV) thru a step up transformer (Ynd, 0,4 to 20kV).
I have performed a short circuit study for single phase to ground short circuit on the LV side of the system just after the generator.
You can have an overview of the SC results in the attached file.

Looking in the picture bellow, the SC result represents actually fault current at location F1.

My question is that in case short circuit (single line to ground) happens in F2, do you think that the fault current would be zero...?

Regards

 

[MASTERMAKESH]

Ya its 0 acording to ohms law in basics

 

[jim hardy]

Surely your generator has a resistance grounded neutral?

When i retired we were starting to look at third harmonics across that resistor to end of detecting your F2 faults for our generators. Surely there's lots more papers on the subject by now. Search generator ground protection harmonic

EDIT: to your question:

fault current will be low. Zero in theory but something finite in practice because it's in parallel with wire connecting neutral to earth. Hopefully that wire has some intentional impedance to limit fault current.

But your fault shorts out whatever impedance is between neutral and Earth point so we want to know about it. A second fault would do damage with nothing to limit current.

 

[ge.vasiliou]

Hi Jim,
Thank you for your time to reply.
Clarifications:
There is no resistance in the neutral. Neutral is solid grounded (thru a 4P circuit breaker but solid grounded).

When you say fault current would be low... how low...? Is there a way to determine that low fault current on F2 point...?

I hope that fault current at F2 point would not be as much as the full current of F1 point (40+46 = 86kA)... otherwise if 86kA pass all the way thru my winding it might destroy my alternator winding.

 

[jim hardy]

At the spot you have shown F2 probably a very few amps. Think about it - what is there to develop voltage? Just stray flux coupling the wiring, and whatever voltage is developed across the wire by neutral current from unbalance.

In large generators they worry about a fault partway up the winding giving a partial short. To that end we had a resistor bank in neutral that limits phase to ground fault current to a value that won't do much welding, ten amps or so.
For us at 22kv that's a lot of KW and the resistor was quite big. A relay across that resistor trips machine at a few % of phase-neutral voltage so protects the majority of the winding.

As i said, they were looking into better protection using third harmonic but i don't know how it works.

To your question - You'd probably not notice a fault at your F2. There's nothing to drive substantial current through it.

I once did some measurements on a 3 phase bus grounding system. Since third harmonic returns through neutral, total 150 hz component(180 for me) in neutral pole of breaker should be 3X any individual phase.. If not it's found a path around breaker.

 

[ge.vasiliou]

Ok... What really confuses me is the fact that in the same sketch, currents measured by differential protection device at two winding ends (F1 and F2) is exactly the same (when everything works ok).
So playing in reality with my 87G device in this application, i can read same load current on point F1 and F2.
So i was wondering by the fact that if i have same load current at point F2 (i.e 1000Amps, same as F1 = load consumption) maybe if a shortcircuit happens at point F2 that would give me the same fault current as of point F1

 

[jim hardy]

So i was wondering by the fact that if i have same load current at point F2 (i.e 1000Amps, same as F1 = load consumption) maybe if a shortcircuit happens at point F2 that would give me the same fault current as of point F1

Shortcircuit from where to where? Where your arrows show?

your F2 fault connects two places on the same neutral. So there's no voltage difference to drive current through that red arrow.
Your F1 fault connects opposite ends of a phase so there's full capacity of generator to drive current through that red arrow. Plus the contribution backward from grid through transformer.
Draw them both on paper and work it with laws of kirchhoff and ohm using a pencil instead of the computer program. Just do steady state to prove principle and it won't take so long.

Trouble with computers is they discourage thinking.
Get your thinking straight before approaching the keyboard. Otherwise you'll fall into "GIGO"* trap .

*GIGO is a local term meaning "Garbage In Gospel Out", a sarcastitc phrase regarding people's tendency to believe anything that comes out of a computer.

 

[gerbi]

I support Jim on his answer. Plus as he said, some more thinking about what is what here is required. Don't rely on simulation programs only.

[...] As i said, they were looking into better protection using third harmonic but i don't know how it works. [...]

Third harmonic emerges when generator becames non symetric (in this case - non symetric short circuit). In normal operation symetric winding and magnetic circuit naturally eliminates third harmonic.

 

[jim hardy]

Thanks, gerbi ..

currents measured by differential protection device at two winding ends (F1 and F2) is exactly the same (when everything works ok).

Well sure, what goes in one end of a good winding will come out the other .

If a fault let's current bypass the CT at either end that's a dead giveaway the winding has grown another way in or out - it's faulted. And your differential relay trips the machine immediately.

If fault current is same at both ends of the winding, the fault is someplace else.

 

[ge.vasiliou]

Well, thank you guys for your valuable support.
I have to admit that was some kind of stupid (or "rush") thought of mine to worry for a short circuit fault current in F2 red arrow , since it is obvious there is not any kind of resistance , no voltage , no current.

I would like your opinion also in another issue that worries me about:
In the same configuration as shown in my initial sketch, taking into account the short circuit study performed (line to ground) i have the following results:
Faulty Point : F1
Fault Current by Generator : 46 kA (line to ground)
Fault Current by Grid : 40 kA (line to ground)
Total Fault Current : 86kA at F1

Do you think that is necessary the particular LV Generator to be grounded thru a resistance (or reactance) or it can be solidly grounded...?

For sure using a kind of resistance / reactance at alternator's neutral would result in much lower line-to-ground fault currents since grounding impedance will limit enough the line to ground fault current, but according to my understanding solid grounding is not mandatory as long as the alternator is not in danger and as long as all equipment is sized correctly to withstand 86 kA of total fault current (busbars, cables, circuit breakers, etc).

I would like your comments in this issue as well.

Thank you in advance.

 

[jim hardy]

For sure using a kind of resistance / reactance at alternator's neutral would result in much lower line-to-ground fault currents since grounding impedance will limit enough the line to ground fault current, but according to my understanding solid grounding is not mandatory as long as the alternator is not in danger and as long as all equipment is sized correctly to withstand 86 kA of total fault current (busbars, cables, circuit breakers, etc).

I would like your comments in this issue as well.

Thank you in advance.

Yes that's why high impedance grounding is used, it leaves the machine in need of a lot less extensive repair.
One of our huge generators developed a ground when a wrench that had been left inside fell down across a terminal. Of course it was much larger machine than yours, but imagine the explosion that would have resulted from unlimited current. Instead the guys retrieved the wrench and wiped away a little soot.

I don't know your installation so can't tell what makes best sense for you.
BUt the IEEE "Green Book" is the best educational reference on the subject that i know of. There should be a copy in your engineering lobrary. If not, spring for the $29 and get your personal copy. It conveys the concepts very well. You will use it often in your career

http://www.google.com/products/cata...=X&ei=ggd7T5ToOqX02QXQ4IChAw&ved=0CFYQ8wIwBA#

 

[ge.vasiliou]

Thank you for the book recommendation Jim. I'll manage to get a copy, seems interesting.

In any case, the configuration of my system is advised in the "attached" single line drawing found on my very first post in this thread.

For sure high impedance grounding would be much more "safer" and protect the generator better.
But in my case where my installation is completed , i want to avoid "modifications" meaning extra cost).
Finally i don't see any direct danger for my LV alternator being solidly grounded even if a phase to ground short circuit happens at LV side of the system (point F1).
(Internal short circuit in the stator windings like your case with the forgotten wrench it can be considered a different kind of fault).

 

[jim hardy]

Sounds perfectly logical and sensible.

I hope you enjoy the book.

Regards -- old jim