Unbalanced 120/240V 1 phase loads on 3 phase system

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dstampfli

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I am involved in designing a distribution system for a small pump house. My question relates to dealing with unbalance in a three phase system and keeping the system as balanced as possible.

The service entering into the building is 480V - 3 phase (unsure if delta or wye). This will be split to some balanced 480V 3 phase loads (motors) and to several single phase 120/240V loads in a 100A panel. Intuitively the best choice would be to use a 45kVA 480V to 120/208 delta-wye transformer to source these single phase loads however, the loads require 240V (cannot operate at 208). We were considering using a 480V delta-delta 120/240V center tapped transformer however it is my understanding that these transformers are severely de rated at for 120V 1 phase loads. Our loading on 120V circuits is relatively high- We'd have to oversize the delta-delta transformer to 75kVA to keep it under the recommended 5% kVA limit for 120V circuits. What about unbalance due to the 240V 1 phase circuits?

Would it be better to pull off of 1 leg of the 480V service, step this down to 120/240V with a single phase transformer at the entrance or should we stick with the oversized 75kVA delta-delta transformer further down the line? My concern is keeping the system as balanced as possible. Is it better to cause unbalance at the 480V level i.e. pulling off of one leg of the 480V service or better to keep it balanced down the line and cause unbalance at the lower voltages? I don?t have any experience with any buck boost type transformers? Are these an option? I?m open to suggestions. Thanks very much.
 
Re: Unbalanced 120/240V 1 phase loads on 3 phase system

I can only tell you I would probably pull off one phase with a 480-240/120V transformer.

But then, I don't know exactly what your loads are. I'm assuming the 240/120V loads are small compared to the 480V loads. If that is the case, why are you so worried about balanced? Let the power company worry about that. (Remember, even if you use a 3 phase transformer, the loads probably won't exactly ballance between the phases, and even if they did - if someone plugs a heater into a 120V outlet, there goes the balance.

Steve
 
Re: Unbalanced 120/240V 1 phase loads on 3 phase system

Thanks very much for your recommendation. I am concerned about causing negative sequence currents in the transformers which cause strain on the motors. I understand that we will have some unbalance, but at what point does unbalance not matter to the point where I can let the power company deal with it? Out of curiosity, why do you recommend that I pull off of one leg of the 480V rather than keeping it balanced and causing unbalance at the lower voltage?

We have one unit which will be drawing 14A @ 120V continuous and other small 120V loads. Other 1 phase loads include heater which will draw 10.4A @ 240V, another heater which will draw 16.7A @ 240V.

Do you have any recommendations or guidelines for dealing with unbalance? i.e. a percentage of variation between the phases perhaps? Thanks very much! I appreciate all your help as I learn about this subject.
 
Re: Unbalanced 120/240V 1 phase loads on 3 phase system

The loads you named add up to about 8.2 KVA. I presume that the remainder of the loads on your proposed 45 KVA transformer are three phase, and are probably motors. So without seeing your load calculation or having a complete list of your loads, I might guess that your total load is around 40 KVA. That makes the single phase loads around 20% of the total. That is a high enough percentage to cause a significant imbalance, if they were all placed on the same phase (e.g., with a 277 ? 120/240 transformer) or between just two phases (e.g., with a 480 ? 120/240 transformer).

Do you have space or budget for two transformers? You could use a 480 ? 120/208 transformer for the 120 volt loads, and use a 480 ? 120/240 transformer for the 240 volt (single phase and three phase) loads.
 
Re: Unbalanced 120/240V 1 phase loads on 3 phase system

The loads I mentioned are the bulk of the 1 phase loads. There are actually no 3 phase loads in the 120/240 range.... We are using a 45kVA since the panel is breakered at 100A and for any future expansion. All the 3 phase loads are ahead of the 120/240V transformer (480V) Thanks for your ideas.
 
Re: Unbalanced 120/240V 1 phase loads on 3 phase system

You said that you had a single phase panel with a 100A MB and that was the reason you chose 45kVA. At most this would only require a 25kVA single phase 480-120/240 transformer.

Are you worried about the unbalance on the secondary of the step down transformer or on the 480V service? How big is your service?
 
Re: Unbalanced 120/240V 1 phase loads on 3 phase system

The motors that I'm concerned about are 480V 3 phase. I don't have any information about the motors or the service size, as it's outside of my scope.. I just know that they exist....
 
Re: Unbalanced 120/240V 1 phase loads on 3 phase system

To answer your question, you need to know how big the system is. motors are affected primarily by voltage imbalance. You can not determine the voltage imbalance without knowing how much supply you have 9how "stiff" is it).

For example, if the utility is feeding you with a 50kVA transformer and you need 40kVA at 480V then your additional 9kVA of single phase load is a problem. However if the utility is feeding you with a 225kVA transformer for the same 40kVA loading, the additional 9kVA is not an issue.
 
Re: Unbalanced 120/240V 1 phase loads on 3 phase system

Thanks for your help ...

Is there a percentage that you would recommend perhaps as a percentage of the total power rating that would help protect motors and the transformer?

For example, if it's 33% - A 45kVA transformer can handle 14.85kVA 1 phase unbalance downstream.
I don't know if such a rating exists, I'm just curious.
 
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