Delta:wye xfrmr cut out fuse blown 1

[Dyorsolutions]

I have multiple transformers (1000kv, 2300kv, 3200kv)

Each are 3phase 415/240v

Each transformer powers multiple units. When I does amperage readings on each phase of the 400amp breakers I get various results such as:

91a/175a/235a

Could uneven phases like this be the culprit of making its way back up the power pole to blow one of the cutout fuses?

Thank you.

 

[Dyorsolutions]

@facengrpe

Apparently we are contracted for 12470kva
But have to tap all our xfrmr’s to 10800kva position to get the correct secondary.

Not sure if we are getting HV line spikes, but none of our other xfrmr’s are giving us issues.

 

[wcaseyharman]

Just a couple thoughts:
In the US at least, liquid filled transformer often have fuses set much higher than the full load amps of the transformer. I am surprised the fuses are 150A, that seems really low. I would have expected at least 200A fuses if not higher with an FLA of 170A.
Unbalanced currents certainly can cause fuses to blow if they are really that severe (not the imbalance itself, but one phase will likely be higher than rated). With the lower primary tap 80% load is 136A, it wouldn’t take much imbalance to bring one phase over 150A.
I would be looking at attempting to measure the primary current using temporary CTs, an “amp clamp” or something while running “full load” and see what the current is.
The previous suggestions of changing the phases should be helpful, although I tend to prefer measuring the current directly if possible.

 

[bacon4life]

@waross-Thanks for explaining how an intermittent transformer fault is possible.

@Dyorsolutions-Many utilities now provide customers access to download energy interval data just by logging into the utility website. Depending exactly how the meter is configured, the meter may also store min/max/average interval readings for amps and/or volts on each phase. Although current and voltage readings are unlikely to be available for direct download on the website, the metering department may be able to email them to you.

 

[stevenal]

As others have said, the fusing seems low. Primary fuses are generally for fault conditions, and sized to handle significant overloads.

An unbalanced full three phase load will overload one or more windings of a three phase transformer.

Suggest using the Coordinaide tool at to model the system. What speed fuse links are in the cutouts?

 

[Dyorsolutions]

Utility wasn’t too thrilled to go with rolling the phases, so I didn’t want to push that too hard on them.

I will be getting Megger, TTR, & Doble test done on Monday.

I will be sure to update the results.

 

[jghrist]

What is the voltage on the secondary?
It can't be overload that blows the fuses. With a maximum secondary current of 235A, the primary will be about 235/26 = 9 A, much lower than the fuse rating. If the fuses blow when energizing the transformer, it could be inrush. There may be momentary outages on the primary, causing inrush when the circuit recloses after a temporary fault. Can you get the utility to give you a record of reclose operations on either the distribution circuit or the transmission line feeding the distribution substation?

 

[Dyorsolutions]

Okay so I don’t have emailed results yet however,
Our transformer passed all tests conducted.
-Doble
-TTR
-Megger


The tap dial is “marked” wrong as to indicating its place, but it in fact is set at “A” position for a 12470kva with current of 415/240.

We also tested the primary wire going from top of pole to the transformer and it tested good.

Right now we are going with the theory that our units are poorly balanced and the imbalance is causing a fuse to blow.

(Also this xfrmr did not seem to have any bayonet fuses on either primary or secondary side in the xfrmr cabinet).

 

[waross]

A question that should have been asked much sooner;
Does your transformer have a delta tertiary winding?
With a delta tertiary or a phantom delta, then unbalanced loads may easily cause primary fuse blowing on a wye primary.
Another symptom to look for is disproportionately high transformer temperatures for the amount of load.
For example, if a transformer is showing close to maximum temperature on half load or less, suspect a tertiary delta or phantom delta.
(Phantom delta: A three phase transformer built on a three legged core will exhibit similar effects as will a delta tertiary. These effects are called a "Phantom Delta".)

I apologize for not remembering this sooner.
In my defence, I tend to take specs at face value, and not consider a tertiary winding unless a tertiary is explicitly mentioned.

The possible presence of a delta winding or of a phantom delta changes the trouble shooting suggestions.

If you have a delta effect, there are three possible solutions:
1; Float the primary neutral. This may lead to overvoltage transients associated with switching transients. The utility may be able to mitigate some of these transients effects.
2. Open the delta at one corner. This is often not feasible with a buried tertiary winding and not at all possible with a phantom delta.
3. Balance the load.

Without going into detailed calculations, an percentage of unbalanced load in relation to the percent impedance of the tertiary may cause a circulating in the delta of over 100% of rated current.
I hope that you don't have a delta effect going.
Check the transformer temperature relative to the percent loading and check the nameplate to se if a tertiary winding is present.

Can you post a picture of the nameplate?



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Dyorsolutions]

 

[Dyorsolutions]

Heat has been an issue for us in summer with xfrmr’s and the 24/7 load demand we put on them.

 

[waross]

Ok delta primary. That rules out a delta tertiary issue.
The primary conductors will be easy to roll.
A to B
B to C
C to A
If your problem is a local problem, a different fuse will blow.
If the problem is a faulty cutout, the same fuse will blow.

My calculations show a rated secondary current of over 4000 Amps.
You are talking about 400 Amp breaker.
Do you know the actual current at the transformer secondaries?
The fuse may be blowing because you may be overloading the transformer.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!