Delta-Wye vs Wye-Wye for Solar Systems

[jefftt]

Is there a steadfast rule for what type transformer is to be used to connect a 3-phase 208V Enphase M250,or S280 microinverter PV system to the 480Y/277V switchgear?
Wondering when and/or why one would use a 480V delta primary - 208Y/120V secondary transformer vs a 480Y/277V primary - 208Y/120V secondary.

thank you, jefftt

 

[GoldDigger]

Is there a steadfast rule for what type transformer is to be used to connect a 3-phase 208V Enphase M250,or S280 microinverter PV system to the 480Y/277V switchgear?
Wondering when and/or why one would use a 480V delta primary - 208Y/120V secondary transformer vs a 480Y/277V primary - 208Y/120V secondary.

thank you, jefftt

In general one should not connect the wye point of a wye primary delta secondary transformer.
But in this case the primary is the winding on the utility side and the secondary is the inverter side, independent of power flow.
A wye wye gives you the option of connecting the primary wye point or not.
Most inverters will want to see a neutral connection for voltage reference even if they source current only from line to line.

 

[CajunTech]

This is what I think is correct, at least from my experiences. I have only worked with utility grid tied systems so this is all I can speak too. The utility will dictate the primary side configuration for interconnection, the way I understood some sites interconnected with "Dominion" on a 34.5kv power line. On the inverter side we used a delta, with a wye on the primary side. The 4 wire system used by Dominion allowed for transmission line fault detection. This then required we use an NGR tied to the neutral tap of the primary. The downside however as I understand is neutral current returns to the point of generation which now was several 1.5 mw inverters. Not to mention inverters will create some current harmonics simply because they are fancy switch power supplies. I have placed an amp clamp on the ground conductor between the inverter and the 34.5 kv transformer and measured between 5 to 9 amps while pushing about 1.4 mw of kw power @ 690 volts ac three phase with a rated 1000 vdc input but under load on mppt measured 600 to 800 volts. These systems used a boost voltage circuit on the DC side.

 

[jefftt]

Thank you for offering these interesting perspectives. I reached out to our electrical engineer, and was told we used a Y-Y configuration when our design employed a grounding transformer (a practice Hawaiian Electric Co. required in the past). The Y-Y allowed zero sequencing currents to pass through. For our typical microinverter 208Y/120V systems tied to a 480V switchboard we will use the Delta PRI, Wye-SEC.

As far as the high voltage utility level response offered by CajunTech; I will have to pass. This is way above my pay grade, but find it fascinating and will save this for potentially future projects.

Aloha, jefftt

 

[pv_n00b]

This is a good reference about using WYE transformers: Whys of the Wyes Y-Y transformers have some characteristics that make them work well in some situations and not others. So you see them on utility transmission and distribution systems but not much in C&I settings. I see them very rarely used on C&I PV systems.

 

[CajunTech]

This is a good reference about using WYE transformers: Whys of the Wyes Y-Y transformers have some characteristics that make them work well in some situations and not others. So you see them on utility transmission and distribution systems but not much in C&I settings. I see them very rarely used on C&I PV systems.

Yes, the pad transformer was a 34.5 kv wye to the utility and delta back to the inverter. As stated in earlier email an NGR was added to the neutral of the high side (utility side) then we added a relay circuit that monitored the neutral current to force a signal to the inverter that neutral current was occurring. This signal would trip the ACB so that inverter would no longer continue supporting neutral current. However some sites the Distribution of the load was so poor that neutral current was allowed to flow between values of 5 and 7 amps without tripping otherwise the inverters would have never run during the summer.

 

[pv_n00b]

Yes, the pad transformer was a 34.5 kv wye to the utility and delta back to the inverter. As stated in earlier email an NGR was added to the neutral of the high side (utility side) then we added a relay circuit that monitored the neutral current to force a signal to the inverter that neutral current was occurring. This signal would trip the ACB so that inverter would no longer continue supporting neutral current. However some sites the Distribution of the load was so poor that neutral current was allowed to flow between values of 5 and 7 amps without tripping otherwise the inverters would have never run during the summer.

I’ve seen neutral currents up to 50A from the utility due to imbalances on the distribution system. The neutral current had nothing to do with the PV inverter. This is why when effective grounding is required often a neutral reactor is needed to limit the neutral current and set up the correct X/R ratio.