Neutral Bonding

[tkb]

I have a 2mw PV system that has 46 inverters feeding 9 power panelboards to a switchgear to a padmount transformer to 13.8kv into a powerhouse of a manufacturing plant in MA.

The inverters generate 480/277v 4w. The 4w is brought through the whole system to the main switchgear.
The engineer has designed use of 277v to feed power supplies for the monitoring off of the inverters at the panelboards.

Do the neutrals get bonded at the panelboards that are fed from the inverters (9 connections), or at the main switchgear at the transformer?
We have a 3/0 ground loop throughout the system.

Also the engineer has designed a transformer the has a 480/277v 4w wye primary and a 13.8kv delta secondary.
He is showing the neutral from the PV system connected at the transformer.

Should the neutral be connected at the transformer?
Whenever we used a transformer in this configuration, it has always been understood to not connect the neutral to a wye primary.

 

[GoldDigger]

The neutral at the PV system should be connected to the wye side since the voltage from the inverter will automatically match the utility voltage, as set by the POCO voltage on the delta winding.
In this configuration the POCO side should be considered the primary regardless of the direction of energy flow.

If you had a freestanding inverter or generator driving the wye side and a load on the delta side the wye would indeed be the primary and the wye point should be left unconnected.

 

[jaggedben]

You do not bond the neutral at more than one place. The appropriate place would be at or near the transformer. Sounds like your designer generally knows how this is supposed to work. The only question I might raise is whether the transformer manufacturer approves of their equipment being used 'backwards'.

 

[pv_n00b]

You don't say what inverters you are using but most likely they require a neutral connected to operate so you have to connect the neutral of the transformer. There are reasons to leave the neutral unconnected in other systems but with PV it's rarely the case.

The LV side of the transformer is an isolated system that needs to be grounded at the source. Since the utility is the larger source of fault current than the PV system it is appropriate to make the system bond as close to the transformer as possible, the transformer switchgear in this case.