Unbalanced three phase loads

[brentp]

Putting efficiency to the side, what "harm" is done with unbalanced loads? Example... A=9A, B=50A, C=15A on a 100A 208 feeder. I've done some research but cannot find anything that actually deals with any detrimental effects from phase imbalance.

If anyone has the time, please point me in the right direction on this 'balanced load' topic.

Brent

 

[Dennis Alwon]

Putting efficiency to the side, what "harm" is done with unbalanced loads? Example... A=9A, B=50A, C=15A on a 100A 208 feeder. I've done some research but cannot find anything that actually deals with any detrimental effects from phase imbalance.

If anyone has the time, please point me in the right direction on this 'balanced load' topic.

Brent

The only problem I can see is if there were a 3 phase load added later that caused on phase to be over. I don't see the harm in unbalanced loads. Almost all loads are unbalanced.

 

[LarryFine]

Almost all loads are unbalanced.

Well, that makes sense; look at us!

 

[jim dungar]

Putting efficiency to the side, what "harm" is done with unbalanced loads? Example... A=9A, B=50A, C=15A on a 100A 208 feeder. I've done some research but cannot find anything that actually deals with any detrimental effects from phase imbalance.

If anyone has the time, please point me in the right direction on this 'balanced load' topic.

Brent

Unbalanced currents can lead to unbalanced voltages and poor voltage regulation.

 

[charlie]

You can also be in violation of the serving electric utility's rules. We require the load to be balanced to within 10% in order to utilize the transformer more efficiently and to keep our system balanced. :smile:

 

[drbond24]

Putting efficiency to the side, I believe the answer to your question would be 'none.'

However, putting efficiency to the side can be expensive.

 

[zog]

Depending on the methods used for ground fault protection you can get false ground fault trips.

 

[brentp]

Perfect!

Perfect!

Thanks for your responses.

Brent

 

[Cold Fusion]

Depending on the methods used for ground fault protection you can get false ground fault trips.

Zog -
Give me some science on this one - I'm not seeing it.

cf

 

[mivey]

Zog -
Give me some science on this one - I'm not seeing it.

cf

This is one example:

http://ecmweb.com/power_quality/electric_groundfault_current_problems/

 

[Cold Fusion]

This is one example:

http://ecmweb.com/power_quality/electric_groundfault_current_problems/

Interesting read. My translation is that if the neutrals are incorrectly connected, unbalanced load can cause the GFP to trip. Okay, I guess that could happen on a lot of mis connected equipment.

I'll rephrase my question:
zog -
For a correctly connected and installed system, I don't see how unbalanced loads can cause a GFP to trip. Can you give me some science on this one.

cf

 

[zog]

Interesting read. My translation is that if the neutrals are incorrectly connected, unbalanced load can cause the GFP to trip. Okay, I guess that could happen on a lot of mis connected equipment.

I'll rephrase my question:
zog -
For a correctly connected and installed system, I don't see how unbalanced loads can cause a GFP to trip. Can you give me some science on this one.

cf

Most electronic trip units on power circuit breakers that have a ground fault trip function detect ground faults by comparing the 3 phase currents. When 1 phase has a ground fault that causes an imbalance between the 3 phases and the trip unit trips the breaker. This is the most common type of ground sensing used in modern trip systems and is called residual ground sensing.

For MV breakers, there are 2 basic types of GF relays, 51G and 51N. 51N is more common becasue most MV systems are ungrounded, the 51N relay get its inpout from the common nuetral connection from the 3 51 phase relays, so again, it is sensing an imbalance between the 3 phases. See attachment.

 

[Cold Fusion]

---For MV breakers, there are 2 basic types of GF relays, 51G and 51N. 51N is more common becasue most MV systems are ungrounded, the 51N relay get its inpout from the common nuetral connection from the 3 51 phase relays, so again, it is sensing an imbalance between the 3 phases. See attachment.

I'm not familiar with ungrounded MV systems. The ones I deal with are solidly grounded 4160v and impedance grounded 13.8kV - that's okay, the math still works.

Looking at the picture you posted, and assuming unbalanced loading:
If there is no ground on the system, then ∑(I)=0. doesn't matter if the loads are unbalanced - there is no "neutral" CT current.

How about if there is a ground fault on the system? Still doesn't matter - it's an ungrounded system, no current flows over the ground. The CTs still show ∑(I)=0. Still no ground fault trip.

I'm not seeing it

gf

 

[zog]

I dont how to explain it any clearer, maybe someone else can describe it better.

 

[mivey]

Interesting read. My translation is that if the neutrals are incorrectly connected, unbalanced load can cause the GFP to trip. Okay, I guess that could happen on a lot of mis connected equipment.

I'll rephrase my question:
zog -
For a correctly connected and installed system, I don't see how unbalanced loads can cause a GFP to trip. Can you give me some science on this one.

cf

Because they can cause zero sequence currents.

 

[Cold Fusion]

Because they can cause zero sequence currents.

I'll have to think about that a bit. I must have slept through that day of symetrical components.

The members of the zero-sequence set of rotating phasors are always equal in magnitude and always in phase. Once again, if zero sequence currents or zero sequence voltages exist, they must exist in all three phases, never alone or in one phase.

cf

 

[mivey]

It's a leakage thang

It's a leakage thang

I must have slept through that day of symetrical components.

I doubt it. You probably just came in late, right after they said something like: "For transmission lines without earth capacitance unbalances caused by something like non-uniform phase transpositions or voltage imbalances (like from unbalanced single phase loads)..."

A common misconception is that zero-sequence current only exists under fault conditions. However, zero-sequence current can and does exist under nonfault, normal load conditions. Unbalanced system conditions, such as those caused by nontransposed transmission lines or unbalanced loading, can cause zero-sequence current to flow.

 

[iwire]

I am still at a loss to understand this, I see it like Cold F sees it.

A common example in my world is the use of a two pole GFCI circuit breaker and no matter how unbalanced the loading is the breaker will not trip as all circuit conductors are being measured.

 

[Cold Fusion]

---"For transmission lines without earth capacitance unbalances caused by something like non-uniform phase transpositions or voltage imbalances (like from unbalanced single phase loads)..."

Good thing I came in late that day -If I had heard that, I'd have dropped to the floor and crawled back out - probably whimpering in defeat.

I didn't think the OP or zog was talking about transmission lines. Even your post was about mis-connected SDS.

cf

 

[Cold Fusion]

---A common example in my world is the use of a two pole GFCI circuit breaker and no matter how unbalanced the loading is the breaker will not trip as all circuit conductors are being measured.

That would be right in line with my understanding. The only difference with the three phase is the currents get twisted to snot (precise engineering term), but they still sum to zero.

cf