Primary Fusible Cut Out Failure 4

[kevinopp]

Good Morning -

I have been faced with a problem for over a year and am seeking insight from professionals on this topic. Any help would be greatly appreciated.

Plant Basics
-4160/2400 v wye primary
-240/120 v delta secondary (Center Tap Ground)
-Peak Demand on primary is approx 97 amps per phase
-Approx 1800 kVA total transformer banks
-Fusible Cutouts on primary currently = 200 amps per phase
-Under normal secondary loading conditions, currents on each phase measure closely
-Not an abundance of startup/shutdown during the course of the day. Plant is starting in the morning, shutdown in the evening.

The problem:
Every two - three weeks, one of the three primary 200 amp cutouts will basically melt into. The cutout typically does not "blow". Of the (6) secondary banks, all connected in Delta, usually two of the cutouts on different banks will blow but sometimes neither of them. Always, one of the 200 amp primary cutouts will fail.

I have used a Fluke thermal imaging gun to look for hot spots on all of the primary and secondary connections. I did find one hot spot on one secondary bank. The lose connection however is on one of the two banks that sometimes has a cutout blown.

Very troubling to me. It appears to be perhaps an arrestor or failed insulator that is going to ground somewhere between incoming power and secondary. How could I find this?

Thanks in advance for your insight. Great site btw!

 

[magoo2]

Are the transformer banks connected floating wye or grounded wye on the primary?

 

[PHovnanian]

What kind of fuse links are you using? Type K, T, etc.?

Where are these cutouts mounted? Outdoors? Exposed to any unusual environmental conditions?

 

[kevinopp]

Magoo2 - I am not sure I can answer that question but I think they are connected grouned wye. The only reason I make that assumption is because we have a 4 wire primary.

PHovnanian - I would say that this situation is a bit unusual because we are processing explosives. There are mulitple small process buildings instead of one large plant, each having a three phase delta bank. Also the plant is divided into halves but both are getting power from one primary bank. When we began having problems with one side of the plant, I isolated the halves by adding additional protection or cutouts where the primary split in case this problem reoccured. Just gives us the ability to run the other half if there is a problem. Can't say what type of fuses today but I'll find out in the morning.

I can say that when we have a problem it is always isolated to one half now due to the cutouts blowing that we installed. This eliminates the thought that there is something wrong with the primary feed.

Really appreciate the insight. It's very puzzling to me.

 

[magoo2]

If your transformers are grounded wye on the primary, with a delta secondary you have a grounding bank. Any feeder unbalance plus your load will be handled by the grounded wye delta bank. This usually results in nusiance blowing of cutouts.

Need to make sure the neutral of the transformer bank in not connected to the feeder neutral.

 

[waross]

I have seen this enough times that I am sure that you will find the primary wye point connected to the system neutral.
If one primary phase to neutral voltage is not exactly equal to the other phase to neutral voltages or if there is a phase shift on the primary, there will be a circulating current in the delta. You are fortunate that you are only blowing fuses. I have seen countless instances where the repair crews increase the size of the fuse until the transformer melts instead of the fuse. In the event that one incoming phase is lost, the wye delta banks will try to supply the missing phase. Sometimes they can, sometimes they can't. Sometimes the fuse melts, some times the transformer melts.
Solutions;
Float the primary wye point. You may experience transient over voltages when the bank is energized.
In some instances I have been able to change the secondary to a wye connection. I have changed 240V delta and 480 V delta to 120:208V wye and 240:416 V or 252:436 V wye. (Taps on +5%)
If the transformers are over sized, consider running open delta. If you have more than one transformer bank, leave a different phase open on each bank.
Once or twice I was able to "break" the delta.


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

 

[PHovnanian]

Bill,
Perhape I misunderstood the original post. keninopp stated that the cutouts 'ment' but do not 'blow'.

Exactly what melts, he didn't say. If its the fuse links, knowing the exact type and rating might give a clue about how they behave at or near overload conditions. But usually once a fule link opens even a bit, it progresses to a full 'blow' with expulsion, etc. If its a contact melting, it could be hot spots due to chemical contamination from the plant running away as the resistance gets higher.

More details are needed.

 

[kevinopp]

waross - my concern is although our demand is relatively low for the size of primary lines we have ran, our contracted high voltage electricians continue to increase the size of the fuses.

As mentioned above I will gather more details in the morning on the fuse type, etc. I don't think I detailed the problem as I should have.

-12470 incoming from Power Company feeding transformers that drops it to 4160/2400. I call the 4160 my primary. We own the xfmr's.

-The 4160 feeds multiple delta secondary banks within our plant. The plant is divided into two halves. Going to the half of the plant where I am having problems, I have installed fusible cutouts to isolate both halves of the plant. These cutouts were placed on the downward side or the 12470/4160 xfmrs. I did this because instead of shutting down both halves, we just shut down one side for when this problem occurs.

-The contracted electrical company has increased the size of the primary isolation cutouts to over twice (200amp each) what our peak demand (90 amps per phase) is. One cutout continues to blow or slow burn no matter the size.

-When the "meltdown" in the main primary (200 amp)occurs, we often have two secondary banks where fusible cutouts sometimes blow. One bank I used a thermal imaging gun to find a hot spot on one of the transformer connections. This bank has probably our highest plant load, large compressor, etc. I am not sure if the cutouts protecting the secondary bank or the main (200 amp) cutouts blow first. The secondary bank at this location consists of two 37.5 kVA and one 50 kVA transformers.

-My power company and I connected an ammeter to each primary phase and noted that:
Primary Line 1 - 67 amps
Primary Line 2 - 73 amps
Primary Line 3 - 67 amps

We do not have alot of starups/shutdowns throughout the day so I think the amp draw readings are consistent day to day. Will these readings tell me if I have a balanced load?

Also, if I have an unbalanced load on a delta secondary due to incosistent single phase distribution, will the load be equally spread through the primary cutouts on all three phases or just one phase?

-Just wondering if incosistent distribution of single phase loads on all secondary banks could "add together" and cause the main primary fuses to blow and not the fuses that are directly above the delta banks?

Thanks again for the help...I am learning as I go here!

 

[kevinopp]

BTW waross when one phase is lost due to a main blowing, two of transformers try to do the work of three causing low supply voltages. Usually something like:

Line to Neutral - 105
Line to Neutral - 204
Line to Neutral - 105

Line to Line - 215
Line to Line - 225
Line to Line - 230

 

[jtester]

kevinopp

It's not just the load on the secondary, delta, side of your transformers that cause the fuses to blow. If there is an unbalance on the 2400/4160 side, a grounded high side will allow the transformer to attempt to balance the high side imbalance.

If you have a grounded high side, the solution is simple as previously pointed out. Lift the HO connection to ground.

Jim T

 

[waross]

There is either a VOLTAGE imbalance on the primary or a PHASE ANGLE ERROR on the primary, or both. You have noticed that the transformer bank tries to supply a missing phase. It will also try to correct an unbalanced voltage or a phase angle error.
The linemen increasing the size of the fuses is also part of the expected result of this condition.
Float the primary wye point.
The Rural Utilities Authority, previously the Rural Electrification Authority recommends that wye:delta transformer banks be connected with the primary wye point floating.
Several years ago, in the space of about 6 weeks I saw three transformers destroyed at two locations due to this issue.

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

 

[kevinopp]

Couple of things I found today from my electrical contractor and by me making a general observation on the system.

1.) The type of fuse cutouts are QA

2.) The secondary ground and primary neutral are tied together at the pole where the transformers are mounted on several but not all of the three phase banks. The place where they are tied together are the banks where we often blow secondary fuses when the primary blows.

3.) We are going to install metering devices in several locations throughout the plant to help isolate the issue. We will also remove the primary neutral and the secondary ground.

 

[waross]

Why bother to ask the question if you don't comprehend the answer?? Float your primary wye points. The problem will go away.

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

 

[davidbeach]

What Bill said.

 

[magoo2]

Amen.

 

[kevinopp]

I was simply stating what the plan was going forward. I appreciate all of the insight and don't understand the criticism. Thanks for the help in getting this resolved.

 

[jghrist]

I agree with others that if your primary neutral is grounded, that this is the problem. Does

The secondary ground and primary neutral are tied together at the pole where the transformers are mounted

mean that the transformer primary neutral bushing is grounded and connected to the primary neutral conductor?

I also ran into a situation with fuses "melting but not blowing". The problem was that 200A fuse tubes were used with 100A fuse links. According to Cooper Power, using 100A fuse links or smaller in 200A fuse tubes may result in the current not interrupting properly. The diameter of the 200A fuse tube is larger and the gases generated may not provide the force necessary to extinguish the arc.

 

[waross]

Many many many years ago, distribution systems in many areas tended to be delta:delta.
Many many years ago, as loads increased, many systems got a quick and economical boost to their system capacity by running one more conductor and reconnecting their transformers from line to neutral.
Many went from 2400V delta to 4160V wye. (Still 2400V line to neutral.) This worked well for single phase loads and this connection is still in use today at various voltages for single phase service. Unfortunately this gave the three phase transformers and banks a wye:delta connection with the wye point connected to the primary neutral.
Many years ago most utilities found that a "wye:delta connection with the wye point connected to the primary neutral" was a very bad idea.
For all the reasons that you are discovering and several other reasons as well. There is a long list of issues and effects with this connection and while one or two may not be bad, none are really good.


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