Mv VT primary fuse burnt out

[azadani]

dear freinds.
There are some VT primary fuse burnt outs in the newly energized 24 Kv metal clad switchgear.there is no short circuit in vt secondary side.it mostly happens in the time of swiching of 132/20 KV transformers.Two 20 KV outgoing feeders supply 20KV overhead lines.
vt,s have 1/4 of their nominal outputs.
vt ratings: 25 VA ,2 core star connected .
i would like to know
1)if there is a risk of ferroresonance
and
2)if ferroresonance may cause primary fuse to burn out.
i would thank you if you could help me to find out more specially another probable causes .
Best regards.

 

[slavag]

What is a 20kV winding of trasnformer: grounding/compensated/ungrounded/resistance.

in case of compensated/ungrounded/probably resistance answers to both Q's-YES.

Best solution against ferroresonance is broken delta connection of VT with dumping resistor.

Best Regards.
Slava

 

[marks1080]

Not really enough information to determine if ferroresonance is an issue. How long does the switching operations take place? I would think it would be a short duration. Hard to say if coupling of a 20kV feeder would be enough to cause a VT fuse to blow in that situation. My best guess would be to say no...

 

[slavag]

Hi.
Good document on the issue:

http://www.schneider-electric.com/d...ntal-constraints/general-knowledge/ect190.pdf

 

[azadani]

Hi
The main trans. is YNd1 and there is a grounding trans. ZNyn5
connected in the vicinity of main trans.(2 20KV incoming feeders are in parallel mode)

there is something else ,i am not sure about it.
is there any inrush current for a VT during switching ,especialy when vt has a low burden?
and what is your idea for a 2 amp fuse for the VT(25 VA 20 KV prim 110v sec. 2cores both y connected.)
best regards

 

[dpc]

These low ampere rating fuses are somewhat fragile. It is not unknown for 1E and 2E fuses to open for no apparent reason.

Yes, there will be some inrush current on a VT. Normally not significant issue.

 

[marks1080]

Think he should try a 5A fuse?

azadani - does the fuse 'mostly' happen during switching or only during switching?

A couple simple resistance/insulation checks on the VT and associated circuit might be helpful as well.

 

[azadani]

Hi.
As far as subtation operators say it happens when CB,s trip
or close. no report during normal operation.
i would like to know what would happen if there were no fuse on primary side of VT.
best regards

 

[marks1080]

Would you not rather just size the fuse bigger than take it out all together?

A PT with no fuse can be a dangerous thing

 

[scottf]

Frankly, sizing the fuse higher than 2A (which is already very high for a primary VT fuse) is about the same as taking it away.

If a 20 kV VT's 2A primary fuse is tripping during switching, there's a good chance it's ferro-resonance.

As mentioned above, place a damping resistor (sized to the thermal burden of the VT) in a broken delta configuration.

 

[azadani]

Hi.
The VT manufactuerer also is,nt agree with sizing a bigger fuse as Scottf says.
We are going to reconnect second windings in open delta and insert damping resistors .
Although i am not still sure that the low value of cpacitance presents in switchgear busbars plus 250 m cable and 2 OHL feeder (20 KV) may cause such a problem.