[RE-wrenches] feed-in imbalance

[Bill Brooks]

Marco,

 

All 3-phase services, and single split-phase services has imbalances from
time to time. The question of whether the imbalance is problematic depends
on the size of the imbalance relative to the service transformer. Since
3-phase services are generally larger, a 10kW phase-to-phase connected
single-phase inverter in unlikely to cause an imbalance problem unless it is
indiscriminately placed on the service on the most lightly loaded phases.

 

If the above inverter is placed on the two most heavily loaded phases (since
it is a negative load), it will often reduce imbalance rather than increase
imbalance. A load recorder placed on the service before construction can
gather data so that when it comes time to choose the most heavily loaded
phases, there is plenty of data to make that decision.

 

The inverter in question has the same imbalance impact as a 10kW
single-phase water heater on a 3-phase service. If the service utility
requires a load analysis to add a water heater, then doing the same for PV
is justified. If they don't require it, they should not require the analysis
of the PV inverter.

 

Bill.

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Marco
Mangelsdorf
Sent: Friday, December 10, 2010 11:01 AM
To: 'RE-wrenches'
Subject: [RE-wrenches] feed-in imbalance

 

Now that SMA has come up with their new 8, 9 and 10 kW transformerless line
of single-phase 120/208V inverters, it brings up the question once again
about how practical and wise it is to have one or two of these inverters
feed into a three-phase service.  That is, is such an unbalanced feed much,
if any, of a big deal?

 

I spoke to my consulting E.E. about this and he said that the large majority
of utility-provided transformers are so large these days that an unbalanced
feed of, say, a 10 kW inverter that only has output power through L1 and L2
should not present a problem.

 

Any other thoughts out there on this matter?

 

Thanks,

marco 

 

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