VT/PT connection to generator bus duct 6

[slavag]

Dear All.
One customer ask connect VT to generator ( 120MVA, 15.5kV)bus duct via fuses and current limeter resistors.

First time, I see this kind of request, of course, in this type of application.
For what, what is a cons and pros?

Thanks in advance
best Regards.
Slava

 

[alehman]

Isolated phase bus?

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[ScottyUK]

I'm guessing a little, but:

1) Fuses to protect against short circuit on the tapping and primary winding.

2) Resistors to protect against secondary winding fault, which would never operate the HV side fuses.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

I'm guessing too, but I wonder about primary fuses. Won't they degrade the accuracy? Both voltage and phase angle?


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

 

[slavag]

Hi.
Yes, Alan, it's isolated phase bus 15.75kV, 7000A.

Thanksa lot, as I nderstood...client was used some very old tender documentation and don't remeber what is a reason .

OK, we'll supply it...

Best Regards.
Slava

 

[rcw retired EE]

Resistors limit the fault current to within the fuse capability. The current draw of the potential transformers is small. The voltage drop through fuses and contacts and resistors will be negligible. It is a relatively common application at 18 kV and below.

 

[alehman]

Seems very strange to me.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[scottf]

Primary fuses is common for indoor installed VTs. Normally a 0.5A fast acting type fuse.

Not familiar with the use of resistors. Doesn't sound like a good idea to me. Normally secondary fuses are standard on VT installed. Size the fuses to a bit higher than the thermal burden rating of the VT.

 

[slavag]

Hi.
scottf, you are right, in the MV indoor installation, used of primary fused is a common practic ( except few cases on the BB in the GIS SWG).
But never see it in the big generator applications.

Thanks a lot to all. We'll try explain it to customer again.

Best Regards.
Slava

 

[scottf]

slavag-

Not sure why it would make any difference.

 

[jghrist]

I'm not sure if it matters much whether or not there are promary fuses. They will be in a differential zone and a VT fault will trip the generator or transformer differential with or without fuses.

 

[slavag]

YES, jghrist, exactly!!!

we try explain to customer this, and he,in all cases, want fuses. We start think, maybe something wrong with us .

Never matter, we order with fuses..more simple for us .

Best Regards.
Slava

 

[scottf]

In certain cases, primary fuses could keep the VT from failing violently.

From my perspective, I don't understand the downside of using primary fuses.

 

[rcw retired EE]

"I don't understand the downside of using primary fuses."

Scott - Here's my experience.

We've tripped some important generators when the fuse clips gave intermittent connections. Units tripped on loss of voltage or voltage unbalance. A voltage comparison (function 60 ) relay or fuse failure detection scheme minimizes but does not eliminate this problem.

With the high short circuit levels on a generator, a fuse clears just about as fast as a piece of wire. A failed transformer ends up putting black smoke and gunk all over the equipment whether it is fused or not.

If the VT's are mounted in the generator circuit breaker, adding fuses reduces the clearances enough to drop the BIL rating of breaker assembly. To maintain high insulation levels, delete the fuses.

 

[scottf]

rcwilson-

Given the standard type fuse clips on modern indoor VTs, I've heard of, nor can I imagine intermittent connection problems. Did this happen on newer age VTs?

A common failure mode for VTs is a turn-to-turn short on the primary winding. In this case, a 0.5A fast acting fuse will generally blow well before enough heat is generated in the VT sufficient enough to cause a line-to-ground insulation fault.

I'm aware of the clearance issue in gen circuit breakers. Most folks buy the VTs without fuse clips and then use externally mounted primary fuses.

 

[rcw retired EE]

Scott - the VT fuse connection problem occurred on generator breakers built in 2001 by the Swiss division of a major international electrical manufacturer. Fuses were in tubes with contact made to the hot lead by screwing down a cover, just like the batteries in a flashlight. If the electrician screwed down the connection too tight, the wire bent and no longer made good contact. The VT's were mounted in the isolated phase compartments of the generator breaker and were subject to the shock and vibration of the breaker opening and closing. The shocks eventually broke the fuse contact.

One of the dual secondary windings was connected in broken delta for ground fault detection on the isophase bus. A missing phase voltage generated a ground fault trip. All other voltage sensitive protection functions were inhibited by fuse failure or voltage comparison relays.

 

[scottf]

rcwilson-

Most indoor VTs in the US don't use the tube type fuse holders, for the very issue you mention. Most use clips that hold the fuses tightly. However, the issue you mention is a design problem with that style VT. The tube-type fuses used by most don't have that problem.