How Much Unbalance?

Status
Not open for further replies.
yomatt

yomatt

Member
Location
Arlington, VA, USA
Occupation
RF Engineer
My customer has two banks of DC rectifiers (minimum input 185V) with 12 kVA and 24 kVA maximum loads to be connected line-to-line on a 120/208 3ɸ service.
There are de minimis single phase loads. 120/240 1ɸ isn't available.

I calculate 150, 50, 100A on each phase, using panel VA addition, or 153, 58, 115A using phase current math.

This is pretty unbalanced, but is it enough to bother the utility servicing the installation?
Is 200A service adequate? (it seems to me it is)

The reach alternate is to step the voltage up to 277/480 and re-wire the rectifiers balanced on 3ɸ.
Of course this is costly, so would rather avoid it unless there is no choice.

Bonus question: does the answer change if taking house power from a facility that has 3ɸ motors on the same branch? (I think yes)

First time posting, so take it easy on me :)
 
Welcome to the forum!
Would you please go into your profile and list your occupation?
If you don't know how, please post it here and we can add it for you.
 
yomatt said:
This is pretty unbalanced, but is it enough to bother the utility servicing the installation?


Bonus question: does the answer change if taking house power from a facility that has 3ɸ motors on the same branch? (I think yes)
Click to expand...

You would have to ask the utility but it's a good question. I would see three variables but definitely the utility will see it differently. You could be solidly on a big trunk line so 200 kw is like a flea biting an elephant, or you could be way out at the end of the line, so voltage drop imbalance could be a concern.

They may want to know, and, they may already have an imbalance, and can preferentially connect your phases to mitigate an existing imbalance. It's easier for them to swap phases at one service than to go up and down the trunk line, looking for phases to swap to get an existing imbalance to be less. They can mitigate the issue maybe, talk to them.

Not sure what you're asking in the bonus question, and I'm sure I'm the only guy who would hold this view, but I have regarded high aggregate 3 phase line connected motors as a sort of rotating line conditioner. Specifically to your load imbalance, no, I would not guess it will generate the missing component imbalance. But if you happen to have a lot of nasty line distortion, my guess would be an extra 1000 hp of line connected induction motors could help you with that.
 
Thanks Dan. I guess the "talk to them" is the important take away.

My cryptic bonus question is how best to understand such imbalance causing harm to motors or other equipment on the branch and I'm understanding from your answer that large motors will help smooth things out, which in some ways refers back to your first answer relating to relative sizes of the flea and the elephant.

Is there a back of the envelope ratio to determine concern or prompt further investigation?
 
yomatt said:
My customer has two banks of DC rectifiers (minimum input 185V) with 12 kVA and 24 kVA maximum loads to be connected line-to-line on a 120/208 3ɸ service.
...
I calculate 150, 50, 100A on each phase, using panel VA addition, or 153, 58, 115A using phase current math.
...
The reach alternate is to step the voltage up to 277/480 and re-wire the rectifiers balanced on 3ɸ.
Of course this is costly, so would rather avoid it unless there is no
Click to expand...
So it sounds like each of the two rectifier banks has a single-phase two wire 208V input?
If the rectifiers can be reconfigured as a 3-phase bridge across all three 208V phases, not only does that balance the AC input current across the phases but the output DC ripple will be significantly less for a given amount of filtering. (Or conversely require significantly less filtering for the same amount of ripple).

The minimum value of the output voltage waveform from a 3-phase 6-diode bridge rectifier is ideally 86.6% of its peak value (i.e., 13.4% ripple), while the output of a single-phase full-wave rectifier goes all the way down to zero every half cycle.
 
That is correct, each bank is single-phase two wire. Rewiring the rectifiers as a bridge would be nice but not feasible. The only option is to configure them 277/480 phase to neutral.
 
yomatt said:
My cryptic bonus question is how best to understand such imbalance causing harm to motors or other equipment on the branch ...
Click to expand...

For 3 phase across the line connected induction motors, if the line to line supply voltages are matching, system voltage may vary 10% affecting only motor torque and efficiency marginally. If an unbalanced loading on the system upstream causes a line to line supply voltage imbalance exceeding 1%, motor heating may increase by 25%.

This would be how a 3 phase motor may be exposed to harm, if the line to line suply voltage imbalance exceeds 1%, caused by voltage drop over an unbalanced current loading elsewhere.
 
yomatt said:
That is correct, each bank is single-phase two wire. Rewiring the rectifiers as a bridge would be nice but not feasible. The only option is to configure them 277/480 phase to neutral.
Click to expand...
Assuming these rectifiers use diodes and not actively controlled devices like SCRs, then 2 diodes would need to be added to the 4 diodes already there in the single-phase bridge to make a 3-phase bridge rectifier. That and of course bringing in the remaining phase off the 120/208 supply to drive these added diodes. Then the load from the rectifiers would be balanced across all three phases.
 
I'm with synchro. Simple rectifiers are cheap and for a given power and ripple 3 phase is cheaper and easier than single phase. An $80 part would easily handle 24kW at 208V.

However the actual rectifiers are the cheapest part of the system. Things such as the control contactor, precharge circuitry, and other such essentials dominate the price. These other components may not be so easy to reconfigure for 3 phase use.

If these are controlled rectifiers that electronically adjust output voltage then the controller could also introduce a significant barrier to 3 phase operation.

Jon
 
Ahh, that does make a difference.

I think you already have the answer you need: ask the POCO, however I am curious: you state that these units could be wired to be a balanced 480/277 load...this would seem to imply that they could be wired as a balanced 208V delta load. Am I missing something?

Thanks
Jon
 
The implication is there, I thought that at first, but alas the multiple rectifiers are all tied to a common bus on one side. I understand it would be exceedingly difficult to rewire them delta, even if I could get schematics.

If wired 3ɸ this common bus is neutral but the rectifier design requires the input be at least ~180V...

The trouble with manufacturing something for a global market is it isn't quite right anywhere.
 
I feel your pain. Hopefully this load is small enough relative to the rest of the connected load that the unbalance will not be a problem.

Given that hard common bus, I suppose you could re-wire 'open jaw', where the common bus is connected to one phase and the rectifier shelves modules are split between the other two phases. If you do this to the 24 kVA unit and put the 12 kVA unit across the open jaw then you get pretty close to balanced.

-Jon
 
Status
Not open for further replies.
Top