Isolated/solidly grounded neutral ground foult phasor diagrams

[angryB]

Hello,

Let's say we have transformer. Primary(delta or star) is connected to medium voltage, neutral is isolated. Secondary(star) connected to low voltage, neutral is solidly grounded.
What primary and secondary phasor diagrams would look like in case of 1 phase ground fault in medium voltage network?

 

[Skogsgurra]

Secondary: no difference.
Primary: if capacitance balanced before, it will change from rotating around center of phasor triangle to rotating around one of the apexes/corners of the phasor triangle.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Look for ampere-turn balance in the transformer windings. If you know your fault current path on the secondary you can deduce where it will flow on the primary in order to maintain A-T balance through the transformer.

 

[Skogsgurra]

Under the idealized (and not specified) conditions - would there be any need to look at the transformer's AT-balance if one primary phase touches ground?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Don't do their homework for them Gunnar! And yes, I might have mis-read the question.

 

[Skogsgurra]

Didn't do much work on that one. So, I don't feel too sorry. Don't know what that picture is meant to say. So use it to confuse.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[angryB]

It's not homework

In case of fault one phase voltage remains unchanged and other two phase voltage becomes equal to line voltages.
What I don't get it why no change in secondary if primary voltages have chaneged?

 

[davidbeach]

What primary voltages have changed? What primary voltage changes affect the secondary voltage?

 

[angryB]

What primary voltages have changed?

Let's say we have 3 phases A, B and C
Normal operation phase voltages:
VAB=V
VAC=V
VBC=V

A phase grounded:
VAB=V/sqrt3
VAC=V/sqrt3
VBC=V
Correct me if i'm wrong.

 

[Skogsgurra]

Yes, you are wrong.
The voltage that the transformer primary sees doesn't change.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[angryB]

Could you explain why transformer sees normal voltages?

 

[jghrist]

The voltages in the medium voltage system depend on the medium voltage source which was not specified. If it is solidly grounded, with zero ground impedance, then the voltages will be as indicated by angryB. This will affect the secondary voltage. If the medium voltage source is ungrouded, there will be no change in the line-line voltages or the transformer secondary voltage.

 

[Skogsgurra]

Question: "Let's say we have transformer. Primary(delta or star) is connected to medium voltage, neutral is isolated. Secondary(star) connected to low voltage, neutral is solidly grounded.
What primary and secondary phasor diagrams would look like in case of 1 phase ground fault in medium voltage network?"

From this, we get:
1. The MV side is isolated (neutral is isolated).
2. Transformer primary is connected to medium voltage.
3. 1 phase ground fault in medium voltage network.

Since there will not be any extra currents flowing in the MV network (isolated neutral and there was nothing said about capacitance, so we do not know what influence it may have), there will not be any changes in the medium voltage that the transformer primary is connected to.

There will be a change in L-GND voltages, yes. But that does not influence L-L or L-N voltages.

Sorry, I cannot be any clearer than that.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jghrist]

I read "neutral is isolated" to mean that the transformer primary was ungrounded, not that the medium voltage system was ungrounded.

 

[Skogsgurra]

OK, I do not read that.

If that were the case, the line would be shorted and the breaker would trip when there is "1 phase ground fault in medium voltage".

Don't you agree?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

I assume that "wye point" applies to the transformer primary and "isolated neutral" applies to the supply system.
An assumption based on my perception of the usage of the terms.
For some more information on the effect or lack of effect of one line to ground connection on an otherwise ungrounded system, search this site for "Corner Grounded Delta".

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

You were right ScottyUK on 2 Mar 14 11:31

Some questions are best left unanswered. Like this one. There is nothing to question - and still, there is a discussion.

Good tip on "effect or lack of effect of one line to ground connection on an otherwise ungrounded system", Bill. Especially that "lack of effect" phrasing. That is what I said from the beginning.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[wroggent]

I'm probably way off but I believe the only two scenarios where you'd get a change in the secondary voltage is if the primary of the transformer was solidly grounded and there was significant fault impedance (so the breaker won't trip) or for a resistance grounded primary as this would cause a shift in the neutral point which would raise the line to neutral voltage for the two unfaulted phases. On the secondary I imagine the phasors for the two voltages derived from the unfaulted phases would approach 180 degree angle between and the remaining phase would be at 0V for a solidly grounded secondary. For a delta or ungrounded wye secondary I'm not really sure.

This wouldn't happen for an ungrounded wye primary or delta as stated above.

 

[jghrist]

Only the OP can say what he meant by isolated neutral. He could want to know what the secondary voltages are during a primary system fault while the MV system overcurrent protection is operating.