Photovoltaic; Single-Phase Inverter on Three-Phase System

[AFerguson]

Multi-tenant (nine meters) warehouse, fed 240V-delta. Three customers use the 240V three-phase service, remaining customers use the 120/240V single-phase service. Three single-phase customers plan to install PV using 240V single-phase inverters.

Installation is allowed per NEC 690.63(A); however, the wording "shall not be connected to 3-phase power systems unless the interconnected system is designed so that significant unbalanced voltages cannot result" is a tough one as this is a 1980s-era building that has experienced multiple interior alterations (new install could at least have a good shot at balancing the loads, including tenants not benefiting from the PV).

Will connect the inverter outputs across the center-tapped phases, but would appreciate any input on the best way to approach the (possible) unbalanced issue.

 

[davidbeach]

Ideally your load absent the PV and with maximum PV would have the same level of unbalance. The no PV would be with the single phase exceeding the 3-phase and the maximum PV with the 3-phase exceeding the single phase.

 

[alehman]

Yes, if your PV contribution significantly alters the balance you may see voltage variations as the PV output comes and goes. It would be better to connect the PV's on different phases.

Alan
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"It’s always fun to do the impossible." - Walt Disney

 

[waross]

I would suggest that voltage unbalances are inherent in a mixed three phase/ single phase, four wire delta installation.
Three single phase customers on a four wire delta system will probably cause slight voltage unbalances as will 120V loads and some 240 volt single phase loads on the three phase services.
I don't see a serious voltage balance issue with a reasonable amount of PV input.Most of the time it will act to partially correct the voltage unbalances inherent in the basic connection.
The Authority Having Jurisdiction and the power utility will have to approve the installation.

Bill
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"Why not the best?"
Jimmy Carter

 

[dpc]

With nine customers on a wild-leg delta system, voltage balance may not be too good as it is. This does not seem like a good application of a 240 V delta system.

Agree with Ahleman - it would be best to connect the inverter outputs to the non-wild legs. But that probably complicates the installation quite a bit.

Might be a good time to think about cleaning this service up. Separate out the few needing three-phase and put in new transformers to serve the single-phase customers.



"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[EEJaime]

It may be a more costly installation but depending on the level of unbalance already in the system, you may be able to distribute the load amongst the phases. Normally these systems are designed with a certain level of unbalance expected. Often the transformer with the 120V tap is larger than the other two. If it isn't sufficiently large to absorb the added unbalance of the varying PV input it may be possible to connect the PV system to the other phases. You will need to meter power at 240V, single phase and then install a transformer to establish the 120 forth customer. More equipment, but an alternative.

 

[alehman]

What's the capacity of the PV inverters relative to the service?

Alan
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"It’s always fun to do the impossible." - Walt Disney