Phase-GND(N) Voltage fault (Urgent) 1

[yk052052]

We've got connection as following in our substation.

6/0.4kV Transformer (N is grounded) -----------BUS BAR(U/V/W/N)-------------LV Switchgear

And our system's Neutral is grounded.

In SWGR A side, all voltage measurement is OK.
But we have some problem on voltage measured from SWGR B Side.

L1 - L2 : 400V
L2 - L3 : 400V
L3 - L1 : 400V

N - L1 : 230V
N - L2 : 230V
N - L3 : 230V

GND - N : 230V
GND - L1 : 400V
GND - L2 : 0V
GND - L3 : 400V

GND(SWGR A) - GND(SWGR B) : 0V

We're in very urgent situation since P.T (that is supposed to be supply with 230V) is now supplied by 400V. (GND - L1&L3 : 400V)
If we have any damage on P.T, then our system will shutdown.

Can you give any possible reasons and what we have to try to solve this?

For now I can't understand this voltages measured.

 

[davidbeach]

Obviously L2 is grounded and N isn't. If N was grounded then the L2 ground would have caused a fault and tripped something.

 

[yk052052]

If L2 is grounded, shouldn't the system have already been tripped by Overcurrent or GDN fault relay protection?

 

[HamburgerHelper]

I think you have created a fault on an ungrounded system but there is nothing but ground capacitance to supply ground current, which will be very little.

 

[davidbeach]

If N isn't grounded then an L2 ground won't cause any protection to do anything.

 

[unclebob]

Remove primary fuse on the PT.

 

[yk052052]

<HamburgerHelper>
You also think that N is not grounded, and L2 is grounded. So there is current flowing to ground from L2. Am I understanding right?

<davidbeach>
I think I need to look in to GND fault protection. But simply followings came to my mind.

1. If L2 is grounded, L2 and GND are connected and there should be big current flowing so overcurrent protection relay should operate.
Can you explain me why this doesn't happen?


We just checked Netural in transformer and it has 230V to GND(TR enclosure that is connected to isolated grounding rod main GND grid which is different from same as above GND(GND of SWGR enclosure))- (Sorry for my confuse. Only Neutral in transformer is connected to isolated GND Rod.)
And now we're considering to make Neutral connected to GND again(if N is not grounded for sure, We'll check this first).

 

[unclebob]

There is more than one issue here.

 

[yk052052]

<unclebob>
Can you please tell me your idea?

 

[waross]

Be very careful. You may have lethal step voltages present, if I understand your post.

(TR enclosure that is connected to isolated grounding rod which is different from above GND)

This is a very bad situation. It is dangerous. It is contrary under any code that I have worked under.

I suspect that you had a ground fault which blew off your system ground jumper.
Alternately
The system grounding jumper is damaged, corroded or disconnected.
Be very careful.
Someone could DIE from touching some metal part that is normally at ground potential.
If I am wrong, then nobody dies. That's good.

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

 

[jghrist]

Is GND connected to a grounding electrode? If so, there is 230 V between the GND electrode and the transformer neutral isolated ground rod. This would cause current to flow from GND to N. It would also cause step- and touch-voltages, which as waross points out could be lethal. If GND is not connected to a grounding electrode, then there could be lethal voltages between anything connected to GND and points on earth. Connecting N to an isolated ground rod is a very bad idea.

As things stand, if you connect N to GND, you will cause a short circuit because GND is connected to L2.

 

[pwrtran]

Measure voltage between L2 phase on swgr A and swgr B you will confirm whether L2 on swgr B is grounded or not!

 

[yk052052]

I found the reason of those.

At one of the load connected to the SWGR A, L2 was connected to enclosure.

After removing this, all came back to normal. And unlike my assuming, we found Neutral line is grounded.

But still unclear since GND(SWGR A) was connected to L2, but GND(SWGR A) - GND (SWGR B) was measured 0V even though all enclosure GND are connected to main GND grid.

Anyway I'm so glad that the problem is solved.

 

[waross]

Thank you very much for the update.

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

 

[jghrist]

You appear to have a major problem if the switchgear enclosure was connected to L2 and nothing tripped. You need to connect the neutral to GND and check the GND connections to the ground grid.