Delta Transformer Install Issue for PV

[Vince P]

Any insight on this would be most helpful.

We installed 240 Delta with a 208 high leg to 480/277 WYE transformer. The 240 Delta is the line side and the 480 is the load it feeds, or is fed depending how you look at it the PV Inverter. The 240 Delta is connected on the line side of the system between the meter and the MDP of the facility. When we went to test the system the 208 leg immediately blew the fuse. There was no load on the 480 side as the disconnect was off. I double checked to make sure we had connected the 208 high leg to right location on the transformer. This transformer has the neutral on both the delta and the WYE sides. Our construction drawings indicate that we are to land on the neutral on both the Delta and the WYE sides. I contacted the Inverter manufacture and the inverter does need a neutral to work properly. My question after doing some digging is the neutral on the line side causing the problem? I did a conductivity check on the 208 conductor to ground ( I disconnected the conductor and checked to make sure there was no conductivity ie. we skinned the wire in the pull or something). When the high leg conductor was disconnected from the transformer there was no conductivity, however when I reconnected it to the transformer it was showing conductivity to the ground. Since in this case the 240 delta is the line side I am not sure the neutral is even needed.

Thanks for any help
Vince

 

[ActionDave]

Welcome to the forum.

I have never seen a delta/wye transformer with a "neutral" on both the primary and secondary. The general rule is you don't run a neutral to the primary. Can you give any more details about your transformer.

 

[electrofelon]

Welcome to the forum.

I have never seen a delta/wye transformer with a "neutral" on both the primary and secondary. The general rule is you don't run a neutral to the primary. Can you give any more details about your transformer.

Indeed. Is the lower voltage side a wye? :? if so, yes bad things would happen connecting that 240 system grounded conductor to the wye point

 

[Vince P]

My best guess as to why it has the neutral is because it was special made for the existing 240 service, and to get the 208 high leg you need to pull the neutral from the center of two of the phases. This is what I don't understand does it do something weird to the system if it is connected as the primary instead of the secondary? When there PV system is turned on and starts to feed the system will it matter? As line to line is all 240 anyway. I am really confused at this point as to the need for the neutral if the Delta is on the line side and is acting as the primary.

 

[Vince P]

Indeed. Is the lower voltage side a wye? :? if so, yes bad things would happen connecting that 240 system grounded conductor to the wye point

The Delta is 240 the WYE is 480. This is done because it reduces the number of PV inverters required. In this case only needed 1 inverter insead of 4. As you can get the inverters in a larger capacity in 480 volt (I am not convinced this is really a cost savings).

 

[synchro]

Was the transformer designed for step-down usage? If so then the inrush current may be higher than normal because of the lower resistance typical in the secondary windings, and that's causing the fuse to blow. If that's the case you'll need to find an appropriate fuse that can handle the inrush and still meet code requirements.

 

[GoldDigger]

If the POCO side windings of your transformer are, for some inexplicable reason, a high leg delta configuration, then, just as for a wye primary with a delta secondary, you must NOT connect the neutral (center tap) of that winding set to the POCO neutral. To do so would allow for high circulating currents in the "primary" windings if the POCO voltages are not perfectly balanced.

 

[ActionDave]

..... I am really confused at this point as to the need for the neutral if the Delta is on the line side and is acting as the primary.

My opinion is don't connect it.

 

[electrofelon]

The Delta is 240 the WYE is 480. This is done because it reduces the number of PV inverters required. In this case only needed 1 inverter insead of 4. As you can get the inverters in a larger capacity in 480 volt (I am not convinced this is really a cost savings).

That is indeed an odd duck if it has 240 delta with center tap winding on the utility side and a wye on the PV side. If someone had that specially made for this system, they should not be designing electrical systems :dunce:. OTOH it is a common thing where so many people are just determined to bring the primary neutral along to the party :hug:
Just disconnect the neutral and let the center tap float (no snuggling with anything) and let us know how it goes.

 

[Vince P]

Update: after talking to the manufacture tech support and the great input from my new friends here on the forum we have solved the problem. The neutral should not be installed :happysad: on the line side in this configuration. We got the system up and running no further issues.

Thanks to everyone that gave input.