Transformer fire

[tzclark]

I got a call out for a transformer fire at a place i do maintenance at, its a 208y to 480 delta 15kva tranny. The reason i am asking a question is, there is no load on it. It is used to power highway reefers while the trucks are switching. I did call the utility company and they said they had an outage in the area last night. Can a surge with no load create enough inbalance to melt the insulation and cause a fire. At the time this happened there was not a truck connected. It is protected on the primary side by 80a 250v fuses. Any insight would be appreciated.
One of my buddies said that he was in that area last night fixing a lighting kiosk because of a surge but i am trying to wrap my mind around the transformer issue

thanks

 

[texie]

I got a call out for a transformer fire at a place i do maintenance at, its a 208y to 480 delta 15kva tranny. The reason i am asking a question is, there is no load on it. It is used to power highway reefers while the trucks are switching. I did call the utility company and they said they had an outage in the area last night. Can a surge with no load create enough inbalance to melt the insulation and cause a fire. At the time this happened there was not a truck connected. It is protected on the primary side by 80a 250v fuses. Any insight would be appreciated.
One of my buddies said that he was in that area last night fixing a lighting kiosk because of a surge but i am trying to wrap my mind around the transformer issue

thanks

You are saying 208Y primary and 480 delta secondary? Is the Y grounded on the primary side? Did the POCO just lose a phase? If yes, yes and yes that may be the cause. I had this happen but on a much bigger transformer and it was ugly. As a side question, how is the 480 secondary grounded?

 

[jim dungar]

When a transformer is exposed to overvoltages, such as surges, the loading on the secondary is immaterial.

A small 15kVA 3-phase transfromer is probably not a 3-winding device, they are most commonly 2-windings connected in a T arrangement which approximates a delta-wye connection.

Is your transformer a true step up design?

 

[tzclark]

Yes it is a primary 208y to 480 delta secondary, if my memory serves me well the XO is grounded on the primary side, I think that the issue was signal phasing. The 480 side is grounded through the lugs on the tranformer chassis. It is not a true step up transformer.

 

[jim dungar]

Yes it is a primary 208y to 480 delta secondary, if my memory serves me well the XO is grounded on the primary side, I think that the issue was signal phasing. The 480 side is grounded through the lugs on the tranformer chassis. It is not a true step up transformer.

If this is a normal 15kVA 480V-208Y/120V step down transformer, it is most likely likely a T connection of two windings, especially if it is one of those 'sealed' epoxy encapsulated ones where all you can see is the leads sticking out. Most manufacturers do not recommend backfeeding small transformers like this.

Regardless: Do not ever bond the X0 bushing on a reverse fed transfromer, the result can be overheating and fire during periods of severe unbalanced loading.

 

[kwired]

Regardless: Do not ever bond the X0 bushing on a reverse fed transfromer, the result can be overheating and fire during periods of severe unbalanced loading.

Kind of sounds exactly like what the OP may have doesn't it. From other posts on this area of discussion, I don't think the unbalanced loading needs to be all that severe.

 

[jim dungar]

Kind of sounds exactly like what the OP may have doesn't it. From other posts on this area of discussion, I don't think the unbalanced loading needs to be all that severe.

The OP saying the secondary was unloaded, so in this case it may have been circulating currents that were the problem. But regardless single phasing of the primary 208V feed was definitely a severe unbalance.

 

[tzclark]

Why would grounding the xO create a severe imbalance? I am asking not out of doubt but just so I have the right answer next time. I remember very vividly when I was an apprentice a journeyman giving my sh!t for not connecting the XO in the exact same setup.

 

[kwired]

Why would grounding the xO create a severe imbalance? I am asking not out of doubt but just so I have the right answer next time. I remember very vividly when I was an apprentice a journeyman giving my sh!t for not connecting the XO in the exact same setup.

Only when the XO is on the input side of the transformer is when it is important to leave the XO unbonded. The primary of the transformer (which is what this is in this case) does not need the neutral of the primary supply to operate. If it is connected it makes a path of less impedance than what is available through another coil to get back to one of the other two phases. Less impedance means more current. If the transformer is perfectly balanced it theoretically should not carry any current. How often is input voltage and impedance of each coil matched perfectly? It is unlikely to be perfectly balanced.

 

[jim dungar]

Why would grounding the xO create a severe imbalance? I am asking not out of doubt but just so I have the right answer next time. I remember very vividly when I was an apprentice a journeyman giving my sh!t for not connecting the XO in the exact same setup.

Your journeyman was wrong.

The bonding of the X0 does not cause the unbalance, rather it prevents the transfromer from 'going with the flow' and tolerating an imbalance.