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?"

 

Responding to Kevin's 19-Jan-07 12:27am, query...

Please answer the following:
o Fuse mfg, type and rating?

o Primary supply: overhead transmission; underground cable?

o Does the same fuse "melt" all the time? Or does it randomly occur on all three phases?

o Can you correlate the time of fuse operation with a change of plant or utility operation? For example: upon energization of a large load; switching of secondary pf capacitors; change in utility transmission voltage regulator; shift change; stormy weather; etc?

o Have you done temperature monitoring of the three fuse holders?

o Trafo winding connections: Y-Y; Y-D, D-Y, or D-D?

o Trafo tap-changing: fixed; on-line; auto (TCUL equipped?)

o Are these incidents recent? Or have they been occuring over a long period of time?

o Proximity to utility transmission pf correction caps?

o Proximity to utility metering PTs?

Regards, Phil Corso, PE ([email protected])

 

You need to get a scope on this to figure out hwat is happening. Simply put, the fuse clears due to too much current - you just need to find out what kind of current. Look for phase delay between voltage and current. Look for upper harmonics. Look for net DC. The transformer could be saturating if you have downstream loads that are significantly unbalanced.

I am assuming a QA200 fuse is a conventional high voltage fuse and not and I-squared T fuse.

Mitch

 

Thanks Phil for your response.

In answer to your questions:
1. The fuses are from Kearny type QA 200 amp fuses

2. The supply is underground- approx. 100 meters to the riser.

3. The last several instances have been the “B” phase, although other phases have been problematic in the past.

4. Up until the last incident, the fuses have blown while the plant was fully loaded, however, the last event occurred while the facility was lightly loaded.

5. The utility have performed IR scanning of the risers and say that all is normal.

6. I believe that the xformer is Y-Y. Certainly the load side is Y.

7. There is the capability to change the taps manually.

8. The events have occurred steadily--almost monthly for the past two years.

9. The closest utility caps are several spans away. The utility says they should not be posing a problem.

10. I'm not sure about PTs. I have not seen any in close proximity.

Thanks for your consideration,
Kevin

 

Kevin, your answers thus far have been quite helpful. What you are experiencing is not unusual nor rare. Furthermore, it is often impossible to determine the triggering mechanism. Instead, it is more helpful to determine if system parameters are such as to cause overvoltages. So, in keeping with this approach following is a second round of questions. Note that one or two may seem silly or trivial, but please bear with me:

Q10) You mentioned that the transformer's primary voltage is 12.47 kV. Or is the trafo rated 13.8 kV, but set on the minus 10% tap?

Q11) What is relative size, in km, of utility overhead system?

Q12) Is OH system grounded? Solidly? Limited-current? Other method?

Q13) Is your plant the only one connected to the OH utility? If there are others, do they have similar sized step-down trafos? Also, if there are others, are they experiencing like (or even un-like problems?)

Q14) Is underground portion 1x3c cable or 3x1/c cable? Direct buried? In magnetic or non-magnetic duct or conduit? If cable is metal-sheathed, then are both ends grounded? What is size in kcmil? Or mmq?

Q15) Where are fuses located? On pole? Near trafo?

Q16) Although a utility will always tell you what you want to hear, they will never tell you what they don't want you to hear! I suggest that you have fuses x-rayed to determine interruption-mechanism, i.e., low or high-current. For such non-destructive tests I have used in-plant NDT X-ray equipment used on welds. In one case, I used a dentist's X-ray machine?
Q15) Is OH system provided with coordinated air-gap insulators? Expulsion tubes? At pole? At trafo? Other types?

Q16) If trafo is Y-Y, it is imperative to know if primary neutral point is grounded (earthed?)

Q17) Where are you located? I'm suprised that the utility doesnt have high-speed event recorders!

P.S.: If you would rather not present the info in this forum, contact me at my email address!

Regards, Phil Corso, PE ([email protected])

 

Further to my 23-Jan-07 - 11:14 pm comments:
Kevin, can you describe how the fuse clearing manifests itself within your plant? And/or to the Utility Co?

Regards, Phil Corso PE ([email protected])

 

Phil, The entire plant goes single phase resulting in- after about 5-10 seconds- The main CB (SquareD SE 4000 amp) opening.

 

A new development:
The Utility changed the transformer and we haven't had an outage since it was changed. We have, however lost two banks of capacitors in our fixed bank unit (100 kvar each).

 

Responding to Kevin’s 25-Jul-07 (23:54):

I suggest you post this as a new thread, and provide the information requested in earlier ones covering ike problems! They can be found in the List Archive as “… Capacitor Bank Problem(s)… ”, and can be found as threads:

http://www1.control.com/1026165423/index_html and http://www1.control.com/1026202855/index_html

Regards, Phil Corso ([email protected])