Distribution side fuse getting blown

[Nadimuthu]

We are being fed from a 22Kv feeder and the sanctioned demand is 1750kVA. The fuse from the distribution side is 60A and on our side is 30 A. These are horn gap fuses.
When we raise our furnace load the fuse on both sides gets blown. This is in spite of the fact that all the OCBs in our side and also the Underground cables have been replaced with XLPE cables when this defect first occured. Also the phenomenon of fuse getting blown is not following any pattern.
Can anyone help us trouble shoot this problem?
Changing fuses, checking the protection relays , checking and replacing transformer oil, checking for gas in transformer oil and clearing this problem has not given any result.

 

[Humble2000]

Have some monitoring done?
Is the furnace acting up? may be there is a short on your furnace side?

 

[rmiell]

You state 22Kv for your primary voltage. What is your secondary voltage? What is KVA rating of transformer?

Rick

 

[jghrist]

1750 kVA is 46A. Seems like you fuses are undersized.

 

[Nadimuthu]

Dear All,
THanks for the quick replies. The secondary voltage is 440V.
The furnace transformers have been checked and we also changed to a different furnace to operate but the problem persists.
The first suspicion was on the Group Control transformer whose rating is 1000kVA to which the supply goes from the distribution point and this transformer was checked and oil replaced and also Insulation resistance checked.But when the first fault occured this transformer showed a Insulation resistance of 6Mohms.I would like to mention that the whole unit of which i am talking about is 30 years in operation.
Our engineers calculate the amperage for fuse as 22A.
May I know how you could arrive at 46A so that I can guide them and also implement this fuse rating immediately.

Thanks a lot and look forward to more help and tips.
Nadi.

 

[edison123]

46 Amps is based on your sanctioned demand of 1750 KVA. Your 1000 KVA trafo primary current os 26 A.

May be your spike current is more than your fuse size causing them to blow.

If you have ruled out other possibilties (like earth faults of bus and cables, shorts between phases, furnace faults etc), try changing to higher size fuse.

 

[waross]

Can you tell us a little more about your system please?
I understand that you have a 1750KVA transformer feeding a 1000KVa transformer and furnaces.
What are the voltages of the 1000 KVA transformer?
Do the fuses blow as soon as you energise the 1750 KVA transformer?
Are you able to energise the 1750 KVA transformer?
Do the fuses blow when you energise the 1000 KVA transformer?
Are the furnaces fed from the 1750 KVA transformer or the 1000 KVA transformer?

46 amps is the transformer primary current. Type K fuses will be 250% to 300% of this value.
Your original fuses may have been time delay fuses. If properly sized time delay fuses are replaced with type K fuses of the same rating, the new fuses will blow.
If you are installing non time delay fuses, you may need 120 amp up to 150 amp fuses.
It sounds as if your engineers are calculating the primary fuse sizes based on the connected load.
Primary fuses for a transformer primary must be based on the inrush current of the transformer. For simple non time delay fuses the primary fuses must be 250% or more of the rated full load primary current.
respectfully

 

[Nadimuthu]

Dear Waross,
The unit is fed from the nearest distribution substation with a 22kV supplyAt the outdoor - both at the distributor point and at our receiving point we have horn gap fuses of 60 and 30 amps respectively.This is received by us at the outdoor and though underground cable taken to Group control OCB.
.
From here through HT bus tappings are taken for Dsitribution OCB, two 950kv/560 secondary voltage and one 900kv /560 transformers MOCBs..From the distribution OCB there is one UG cable to HV SSB which has HRC bottle fuse .There is one u/g cable from SSB(Sub swithcing board) to the 1000kVA /433Vdistribution transformer.
All auxiliary supplies like blowers , cleaning equipment, sand plant are fed from this distribution transformer.Also the control supply to the panels for operating the furnaces through the other transformers is from this 1000kVa transformer.

The unit was closed for 4 years and after checking healthiness of transfomers, cables and transformer oil checks, the unit was restarted.
After one month of operation, on a night of heavy rain, the first fuse blow out occured on the B phase horn gap fuse at our end outdoor and also at distributor end and their feeder tripped with earth fault indication in their settin of 120 milliseconds.
Suspicion was placed on the 1000kVa transformer as its Insulation resistance was found to be 6Mohms.
The next step was to check and replace the oil, take the core out for inspection and all these were done while a spare 350kVA transformer was brought in and put into service. Again the fuse got blown in B fuse.
The suspicion was on cable between ssb and 1000 kvA transformer and this was changed from the old paer insulated cable to XLPE cable.
Now when the power was resumed and after 10-12 hours of operation, the Y phase both outdoor fuses and the feeder ACSR conductor got cut.
By this time the two other UG cables were relaid with new XLPE cables, the 100kVa transformer sent outside was found to have a Body to LV side arc and therefore repaired, oil checked, IR value measured and brought back into service.
After this on restoring operations, B phase both outdoor fuses and ACSR conductor got cut and feeder tripped at the distributor end.
They were replaced and now we changed to a different furnace. The previous furnace was Medium frequency induction melting and a Main frequency furnace was brought into service.
After 18 hours of warm up and when increasing power to 300 Kw the fuses got blown again.
Now we are analysing the problem and consulted some experts also.
Their views.
1.Our indoor relays are analog and response time is slower than distribution end and needs to be replaced.
2.Horn gap fuses ends are nearer to other pole lements and even after fuse blown arcing is causing distribution side feeder trip through earthfault.
3.Furnace transformers tap chaningers and core have to be checked.
4.Grading of fuses has to be done.

Please see if all these inputs can help you give us your suggestions.

 

[waross]

Dear Nadimuthu;
Thank you for your explanation. I will refrain from commenting for a day or so. There are some experts on this forum with more direct knowledge of your type of installation than myself.
With the description of the course of events that you have now provided, they should be able to give you more accurate information than myself. Let's give them a chance to get involved.
respectfully