Imbalance Systems

[shnshn]

Hello,

I do have an very tricky question where I want to get a better understanding of.

I am an electrical engineer and want to install Solarinverters on my house. I do have a three phase 208V system and I have to feed in between the phases. How much of imbalance load is allowed?
Can I connect a let's say 12 kW inverter on Phase 1 and phase 2 and let Phase 3 open? This means I have a imbalance of 12kW between Phase 2 and Phase 3 as well as between Phase 3 and Phase 1?

Any ideas how the max. imbalance can be calculated or is there a general rule?
Thanks for your help!

 

[wareagle]

What you have is 58 amps on phase 1 and 2 and 0 on phase 3.
I don't think that will cause a problem but a good design would balance the load on all 3 phases. Why not use a 3 phase inverter?

 

[burnt2x]

shnshn,

12 kW on two lines of a 208-3 phase system will load the two lines at 100 amps (L-L volts = 120V) and not 58 amps!

If you have motor loads and your voltage is largely unbalanced, you get problems. Try using a Scott trafo to even out the load currents at the 3-phase system.

 

[davidbeach]

Gee, the last I checked, 12000/208=58, not 100. Probably need to check your math again.

 

[waross]

(L-L volts = 120V)

?????????????
I did come across a small generator with this configuration about 25 years ago. (120V delta). It was not in use.
120 volts line to line is not common in most parts of the world.

 

[itsmoked]

Find it in some passenger rail cars Bill.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[shnshn]

Hello Guys,

thanks a lot for your answers.
Unfortunately, there is no real chance to get such a 3phase 12 kW inverter. Nevertheless, I want to use hte single phase because it is much more easier to connect and handle the whole installation.

I am still unsure if I can do it?

Any Code issues with that?

Thanks,

 

[waross]

Well, and electric range may have a demand of 6 KW. A hot water heater is typically 4 KW. 12 KW is a little heavy but probably doable.
Can you get three single phase inverters at a reasonable price?
If not, you can probably go with all the input on one phase.
Not good design but I have seen a lot of poor designs that work.
respectfully

 

[ppaya]

I am not sure why do you need 12kW?
North America Utility usually calculate with demand 4KVA load for residential connection.
120/240V connection is typical for this. You say you have 120/280V What is your load you want to support on one leg ph-ph?
There are LDC requirements in place for connection of Utility type inverters, when you apply they will check your Net metering connection.

 

[shnshn]

Hi ppaya,

thanks for your answer. Well, 4KVA seems pretty low because a single heater has 3 kW. Nevertheless, when I do the calculation, I do have a 200 Amps Panel times 120V- I am at 24kVA.
So 4 KVA seems pretty low.

Any other ideas?

Thanks,

Norbert

 

[ppaya]

shnshn:
200A Panel is installed (connected ) KVA. Demand is something else.

 

[jghrist]

Before investing in PV cells and inverters, I suggest that you meter your total load for a while to find out what your total demand is. I assume that you have a fairly large house and have a 3Ø air conditioner which is why you have a 3Ø 120/208 volt service. 4 kV would be low, but I would also expect that the demand will be considerably less than the panel capacity of 200A per phase (72 kVA total).

Also check with the utility to find out what restrictions they have on net metering. With a 12 kVA single phase inverter, you will be presenting a large imbalance to the utility and probably exporting on one or two phases. Total utility load will also appear to have a poor power factor (leading or lagging) because of the large single phase Ø-Ø connected generation.

 

[shnshn]

Hello,

well first of all thanks for all your inputs.
It is really very helpful and give new ideas.

Does anyone know some rules eg. UL / NEC, in regards to this? Is there some kind of Code Standard or other restrictions?

Thanks,

Norbert

 

[LionelHutz]

I see 2 restrictions.

The first is that you can't exceed the 200A on each phase. Above this, Hydro is known to under size their transformer and feed to many building so you have to make sure you don't exceed their equipment. Remember, you also can't have 200A from hydro and 58A from the inverter at the same time because that would overload the 200A panel (258A in the panel buss).

The second is that you shouldn't cause an excessive voltage imbalance. So, if your inverter running won't cause more than 1% to 2% voltage imbalance then you should be OK.

Now, personally I would look at 3 x 4kW inverters that you can connect in a 3-phase arrangement. That way, everything in your house remains balanced. The other advantage of this is that if (when) an inverter fails you still get 2/3 energy production until you get it fixed.

 

[waross]

If you are drawing 150 amps from the panel and the inverter kicks in with 58 amps, it will reduce the loading on the service, not increase it.
The most important issues are the policys of the local utility to your proposed installation.
Consider the implications of the following scenario:
You are drawing about 6 KW and your inverter is supplying 12 KW. 6 KW is being supplied to the grid.
Question; What is the policy of the utility? Do they require special metering? Do they allow back feeding?
Next issue:
The utility crews are performing work on your transformer bank.
They remove the primary fuses and expect the transformer bank to be safe. However your inverter is back feeding your transformer bank with lethal voltages.
A lineman may die.
You, as an electrical engineer will probably be held to a more stringent standard of accountability than a non-technical homeowner.
Does your utility demand measures to prohibit back feeds? What is their policy?
respectfully

 

[LionelHutz]

Waross, he should be using a UL listed inverter for this. I can't remember the UL number but the inverter basically will not operate in island mode. You are correct in that the utility will have to know and approve of the equipment. That is a pretty big system to be going guerrilla with.

 

[shnshn]

Thank you very much guys.

Actually the inverter meets al the UL 1741 requirements and will shut down in case the grid is down. So nobody will die. We are safe on that side. I do know something about a 6kW imbalance rule for 240V systems. But it seems that this doesn't apply for 208V / 277V systems, unfortunately.
However, does anyone of you have any experience here?
Does anyone have some experience in regards to the approval of the utility company?

Thanks again guys,

Norbert

 

[ogbuehi]

I don't know if there is something in the NEC regarding the amount of unbalanced you are allowed but if that is wye system you're gonna get a whole bunch of current on your neutral.

On a side note, does anybody know how to calculate the total KW if the current on your phases isn't the same?