Unbalanced current in 3 phase

[Jim90]

Hi guys,

Just after a bit of clarity. After installing an energy logging device as per a client’s request we have highlighted that we have over 200A difference between L1 to L2 and L3. The supply is a big commercial science building with 400A per phase.
This isn’t due to inrush and is constant. There hasn’t been any issues yet but in the past I have had 3 phase MCCBs trip out due to this issue. There is an awful large amount of business critical equipment and sensitive scientific research kept at very low temperatures.

Is anybody aware of any regulation stating any phase imbalance must be within a certain %?
Or, do you think this will hold? Doesn’t give much room for expansion on L1 so any new high load circuits or sub mains will take it out eventually. That’s all I got to put to them at the minute!

I need to put together a case to prove to the landlord!

Cheers
Jim

 

[westward10]

Not sure you will find a Regulation but I work to a 10% rule of thumb out of balance but where this came from I don't know. High out of balance percentages may give neutral vd and unbalanced voltages across three phase distribution which could have an impact on motor performance. Another issue may be EMI on sensitive equipment and inaccurate power consumption through the metering.

 

[marconi]

You ought to measure and report the line and phase voltages also to see what imbalance there is due to volt drop.

See these links for the effect of voltage imbalance on 3 phase motors - it causes them to run hotter and less smoothly thus they wear out faster:

Voltage Imbalance in 3-Phase Systems - https://yorkcentraltechtalk.wordpress.com/2012/07/03/voltage-imbalance-in-3-phase-systems/

and

https://library.e.abb.com/public/0c4186f3e7354f6982124decf92742d8/AVC-20 white paper (low res).pdf

 

[Lucien Nunes]

to see what imbalance there is due to volt drop.

It's important to distinguish the cause and the effect here. The OP has discovered significantly unbalanced supply currents. These won't be due to voltage drop, since the VD is only ever a few percent of the supply voltage. Rather, the unbalanced currents will probably be causing an assymetric voltage drop that is undesirable if it affects 3-phase loads (it's of less consequence if it affects only single-phase loads).

For a resistive load, VD fractionally compensates imbalance by reducing the load power and current. For a constant-power load such as an SMPSU, it exacerbates it by increasing the load current. For a mixed load the likely effect is trivial, and in any case not more than a few percent.

The disadvantages of having one phase heavily loaded are as mentioned though; lack of expansion capacity on the affected phase, excessive loss through elevated VD (power loss is highest when the current is concentrated in the smallest fraction of the available copper in a cable as it is I²R loss), and excessive heating and slight loss of performance in 3-phase motors.

 

[marconi]

Mmmm - again.

I was writing on imbalance in phase and line voltages AS A RESULT OF the SIGNIFICANT DIFFERENCE IN CURRENTS (as reported) CAUSING UNEVEN VOLTAGE DROP ALONG EACH LINE!!!!!!!!!! That it, as your fourth sentence explains. Not unbalanced voltages causing the reported significant difference in phase currents which is how you chose to comprehend the sentence.

As my second para and references go on to say, voltage imbalance can cause problems to motors, etcetera.

I was providing some advice for the case he is preparing for the landlord.

 

[westward10]

I would say the issue of the OP having tripping triple pole breakers is due to overload on L1 because there is a mindset that adding a single phase circuit to a triple pole board it must to L1.

 

[Lucien Nunes]

I know you know what you're talking about Marconi - I just misread it the first time, so I tried to amplify, not very successfully!

 

[Jim90]

We have put our monitor on for another week and connected it in to monitor voltage. Unfortunately it’s not at the load end but might give us an indication.

Cheers guys. Landlord is a bit tight but gets advice from others. All I’ve got really is that it doesn’t give much allowance for expansion!
We have narrowed it down to 5 x 40A circuits split across different sub mains on one phase (L1) they are pretty much flat out, feeding fully racked out data cabinets. Would explain that the current is constant.

 

[Lucien Nunes]

Good that a lot of the load on L1 is within a few circuits, hopefully they can be transferred to other phases easily. Not so convenient if they are on SP+N submains.

 

[westward10]

See it so many times, must be on L1.

 

[rapparee]

See it so many times, must be on L1.

It's funny

We managed to take out one of our whole factory sites, when we tried to start two presses at once.

The L1 fuse was the first to pop

It was rated at 1250A

The NIE engineer said it is always the L1 fuse which blows.