12,470 fuse (QA200) melting- Could capacitors cause problems?

[zaphod1]

About once a month a QA200 fuse clears on the utility riser at our plant. There has been no indication as to why it blows. We have a 2000 KVA 12470-> 480volt transformer. On the load (480v)side beyond the main CB there is a fixed bank of 400 KVar caps installed. We have had no indication of faults in the load side, and in fact, as soon as the fuse is replaced we can start up and run everything as normal. The Utility is at a loss to explain the problem. My question is "can the fixed caps be creating problems on the line side of the transformer causing the fuse to heat?"

 

[jghrist]

The capacitors could cause resonance with the transformer at certain frequencies. If there are harmonic sources in the plant at these frequencies, you could have a high current in the transformer secondary and in the capacitors, but not on the transformer primary. If there is resonance, it would result in high harmonic voltage distortion.

 

[Skogsgurra]

I doubt that a 400 kvar bank can cause the fuses to blow. At least not through a resonance phenomenon. In such a case, the capacitors or their overcurrent protection usually are the first to go.

On the other hand, if your transformer is loaded close to rated and you have an additional harmonic load added to that, your fuse will perhaps overheat. But that harmonic has to come from the transformer primary and propagate through transformer to capacitors. And that is normally not possible - the 12,5 kV side is usually relatively free from harmonics.

Switching the secondary directly to the capacitor bank *could* be the reason for large peaks. But again, the capacitor bank is only 20 % of transformer - so that should not happen either.

I would put a transient recorder there (three voltages, three currents - and fast enough to sample at least every millisecond, faster is better. Then just sit and wait to see what the reason is.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[wbd]

Is it always the same fuse (the same phase)?

 

[zaphod1]

In the last several instances it has been the "B" phase fuse.

 

[DanDel]

FLA of a 2000kVA transformer is about 93A; it's unlikely that the 200A fuse is opening under overload.
Putting on a recorder would be my first suggestion also, but you may want to look at the fuse and fuseholder with an IR camera to make sure there is no bad connection causing heat in the fuse.

 

[zaphod1]

The fuse and fuseholder have been scanned by the utility and they inform me that everything looks normal. We had an occurence this morning, while the utility had a monitoring instrument on the secondary of the xformer as well as line monitors on the primary. I am waiting to hear the results from them.
Thanks all,

 

[zaphod1]

Please feel free to continue to contribute to this thread. the preliminary information from both sets of monitoring mentioned above shows nothing that might cause this problem. Quite an enigma.

 

[dpc]

A bad batch of fuses?

 

[itsmoked]

Be very interesting to video the thermal activity of all three at once, for a day or so. I would think that the B phase would be in the physical middle and hence actually run warmer.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[apowerengr]

200QA is relatively fast and rated 100% continuous as compared to other standard fuses with 150% continuous ratings. As for heating, if the fuse links have removable button-heads for use with link extenders (increases the fault current interrupting ratings for cutouts) then sometimes the button heads get loose and heat up. You could have a bad arrester, failing cable, etc. The utility should be able to tell if the fuse has a slow burn (overload) vs. a fast burn (fault clearing operation) and start troubleshooting from there.

 

[waross]

There may be a hot connection in side the cutout that is, for some reason, not apparent to the thermal scan.
If this is a wye/delta transformer with the primary neutral connected, a fault elswhere on the system can blow a primary fuse. In this case here will be increased primary current and increased current in the delta, but the secondary monmitoring will not be "in the delta" and so will not see the secondary current.
My primary suspicion would be a hot connection on the energised side of the cutout. It may not be readily apparent visually. If not the first time, by the second or third time they replaced the fuse the linemen would have cleaned and inspected the removable fuse holder. The crossarm mounted hardware is more difficult to examine because it can not be approached closely, safely.
respectfully

 

[gfaith]

I assume the 2500 KVA transformer is 3 phase and has bayonet fuses for it's protection.
I would be looking for primary underground cable problems.