Imbalanced current on 3 phase system
Imbalanced current on 3 phase system
(OP)
I have a 3 phase 480v 2000a main lug only switchboard being fed from a wye secondary of a 1500KVA three phase oil filled transformer. The primary voltage is 2.3kv. There are three fusible disconnects on the switchboard feeding three soft starters each driving a three phase squirrel cage motor. The switchboard is about 6ft away from the transformer secondary and is fed by parallelled conductors in wire trough conecting to the 2000amp bus.The parallel conductors all appear to be of the same length,termination,etc. The soft starts are fed from the disconnects on the switchboard through cable tray using 1 single conductor per phase.Everything is relatively new including the transformer. I am having a noticable differnce in current on all three phases on all three of these motors. Example: if I measure the voltage with a load I read phaseA-B volts 480, phase B-c volts 472, and phase A-C 475, each phase to ground around 277. So my voltage looks good. What I am having trouble understanding is, my current readings look like this A-360 B-320 C-311amps.These readings were taken from a 300 HP motor running a screw compressor that was about 85% loaded from an air demand standpoint. The FLA of this motor is 345amps, so A phase is running above that.I am seeing this same realtionship on the two other identical machine/motors. This problem has been here since
the installation, so this is nothing new. I was wondering if this might have to do with the impedence of the windings of the transformer being different? We are not using 3 single phase transformers it is one unit. It is causing me some problems because if the machines have to run at full load for an extended period of time the soft starter will trip out on phase A overload. I understand the voltage is slightly different from phase to phase, but the difference between volts/amps isn't proportional. If impedence is an issue how can I prove it?
the installation, so this is nothing new. I was wondering if this might have to do with the impedence of the windings of the transformer being different? We are not using 3 single phase transformers it is one unit. It is causing me some problems because if the machines have to run at full load for an extended period of time the soft starter will trip out on phase A overload. I understand the voltage is slightly different from phase to phase, but the difference between volts/amps isn't proportional. If impedence is an issue how can I prove it?






RE: Imbalanced current on 3 phase system
RE: Imbalanced current on 3 phase system
Be aware that an induction motor develops a balanced back EMF or in other words acts as an induction generator. The back EMF is balanced both as to voltage and phase relationship. Any phase error or voltage unbalance in the supply will cause an unbalanced current to flow which is disproportionate to the voltage unbalance.
Bill
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"Why not the best?"
Jimmy Carter
RE: Imbalanced current on 3 phase system
Were your voltages measured at the motors or at the transformer? Are the two motors at the end of the same feeder or different feeders?
RE: Imbalanced current on 3 phase system
RE: Imbalanced current on 3 phase system
RE: Imbalanced current on 3 phase system
Voltage unbalance in motor feeds will cause a disproportionate unbalance in motor currents.
Bill
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"Why not the best?"
Jimmy Carter
RE: Imbalanced current on 3 phase system
Not bad. The worst phase to phase voltage is about 0.9% high. But if each phase is 277 V L-N. the angles don't quite add up.
Are there any capacitors on the 480V system anywhere?
RE: Imbalanced current on 3 phase system
RE: Imbalanced current on 3 phase system
RE: Imbalanced current on 3 phase system
Just a thought.Did you use a true RMS clamp meter to measure the given currents on the load side of the soft starter?It looks to me that the output currents of soft starter contains some harmonics therby giving a higher true RMS.So if your soft starter does not have thermal type overload relays then they would definitely trip depending on the setting.Otherwise if your soft starter has electronic type overload relays which might work on 60Hz wave then it may not trip.Therefore,to eliminate this type of situation is ther anyway that you can start and run one motor by-passing its soft starter?
RE: Imbalanced current on 3 phase system
For your voltages:
480, 472, 475
Average = 475.7
Max deviation from average = 4.33
Neg sequence fraction estimate =4.33/475 ~ 0.9%
For your currents:360, 320, 311
Average = 330.3
Max deviation from average = 29.7
Neg sequence fraction estimate = 29.7/330.3 ~ 9%.
So, the current unbalance (expressed as fraction of fundamental) is roughly 10* as much as voltage unbalance (expressed as fraction of fundamental).
Let 1, 2, 0 refer to positive, neg and zero seq.
V2/V1 = 9%
I2/I1~0.9%
(I2/I1)/(V2/VI) = 9%/0.9%~10.
What do we expect this ratio to be:
(I2/I1)/(V2/VI) = (I2/V2)/(V1/V2) = (1/Z2)/(1/Z1) = Z1/Z2
What do we expect the impedance ratio's to be. As a first estimate, we say negative sequence impedance is rougly the same as locked rotor impedance (there are good physics arguments to make this approximation since both conditions correspond to very high slip). What is ratio positive sequence? It is inverse of ratio locked rotor current to full load current which we know is around 7.
If we knew only the voltage unbalance and you asked me to predict the current unbalance, I would say 7 times higher. You actually measured 10 times higher. It is not such a big deviation (considering approximations in estimating unbalance and approximations in substituting locked rotor impedance for neg seq impedance). I would suspect that the current unbalance you are seeing is simply a result of the voltage unbalance you are seeing.
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(2B)+(2B)' ?
RE: Imbalanced current on 3 phase system
"What is ratio positive sequence? It is inverse of ratio locked rotor current to full load current which we know is around 7."
should have been
"What is ratio positive sequence impedance to negative sequence impedance~ locked rotor impedance? It is inverse of ratio locked rotor current to full load current which we know is around 7."
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(2B)+(2B)' ?
RE: Imbalanced current on 3 phase system
"What do we expect this ratio to be:(I2/I1)/(V2/VI) = (I2/V2)/(V1/V2) = (1/Z2)/(1/Z1) = Z1/Z2"
should have been:
"What do we expect this ratio to be:(I2/I1)/(V2/VI) = (I2/V2)/(I1/V1) = (1/Z2)/(1/Z1) = Z1/Z2"
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(2B)+(2B)' ?
RE: Imbalanced current on 3 phase system
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(2B)+(2B)' ?
RE: Imbalanced current on 3 phase system
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(2B)+(2B)' ?
RE: Imbalanced current on 3 phase system
Do the voltage unbalances and the current unbalances drop as more motors are added?
Bill
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"Why not the best?"
Jimmy Carter
RE: Imbalanced current on 3 phase system
It will be great if you can explain from where you get this approximation:
"The negative sequence as fraction of positive sequence can be roughly estimate from "unbalance" defined as max deviation from average over average".
Thanks in advance,
Milovan Milosevic
RE: Imbalanced current on 3 phase system
I didn't invent that. It is used in NEMA MG-1 as the basis for derating.
Here is a link.
http://users.encs.concordia.ca/~pillay/16.pdf
Note he calls the "true unbalance" to be V+/V-, and compares it to the NEMA maximum deviation from average in the figures. The agreement is good for small unbalances and generally starts to diverge as the true unbalance gets larger.
Also attached is a spreadsheet (requires analysis tookpak installed) which compares "true unbalance" = V+/V-, maximum deviation from average, and another formulation given by GE.
For the spreadsheet, you input the three magnitudes and the spreadsheet figures out the angles under the assumption that the three vectors sum to zero (which holds for currents flowing into a motor and also for phase to phase voltages).
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(2B)+(2B)' ?
RE: Imbalanced current on 3 phase system
it will be helpfull.
Milovan Milosevic
RE: Imbalanced current on 3 phase system