Current present in Earthing wire of 3 phase motor, what is an acceptable leakage current? 4

[_NoobNoob]

I've got 400kW motor with a soft starter that trips sometimes on an unbalanced current (larger than 15%), supply voltage on load is 591V-595V-603V in a delta configuration. corresponding currents are 353A-373A-422A. From what I'm reading is that a 1% imbalance in phase voltages contributes to a 10% imbalance in current. The 3 supply phases we're stepped (A-B-C -> C-A-B -> B-C-A) and everytime the imbalance follows the supply and not the motor, leading me to the conclusion that this is a distribution problem.

Measuring the earth wire that goes from the motor to the MCC measures about 16A. When the motor is witched off, the leakage current goes to 0. My question, what is an acceptable earth leakage for such a size motor?

In order to reduce the voltage imbalance I was thinking to move some single phase equipment over to the high voltage phase in order to try and balance the voltage phases.

Please share you're thoughts.

Update: added clarification

 

[Gr8blu]

1. The 1 percent voltage unbalance results in UP TO 10 percent current unbalance. How much the actual unbalance is at any given moment depends on the circuit impedance being fed.
2. I assume by "... phases were stepped ..." what is meant is that A-B-C connections were switched to C-A-B (i.e. changed terminals, but kept same sequence). And noticed that the unbalance followed the "A" connection, for instance. If that is indeed the correct assumption then yes - the most likely scenario is that the unbalance is happening external to your machine (or at least upstream of wherever you made the phase change).

Unbalanced loads on any phase will result in observable unbalance elsewhere in the distribution system. "Level loading" each phase is the ideal approach, but is seldom achievable in practice. The best we can (usually) accomplish is to limit the unbalance to an acceptable level.

Note that with a delta connection, the equipment is more susceptible to the effects of third-order harmonics as well (a wye connection tends to balance these out, whereas a delta allows them to circulate between phases).

Converting energy to motion for more than half a century

 

[LionelHutz]

16A during running is rather high. The ground current can be higher during starting due to capacitive coupling as the SCRs switch but it should drop to a low current once it's running.

 

[jraef]

Assuming the soft starter is bypassed on running.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[cranky108]

Also depends on the frequency of the leakage current. Different frequencies will tell you the unbalance reason for the leakage. Turn to turn faults will be at a different frequency than normal noise.

 

[_NoobNoob]

Assuming the soft starter is bypassed on running.

Yes it is.

Also depends on the frequency of the leakage current. Different frequencies will tell you the unbalance reason for the leakage. Turn to turn faults will be at a different frequency than normal noise.

I haven't thought about this. Excellent suggestion, I will investigate it and let you everyone know.

 

[RRaghunath]

Current measured in earth wire has no relevance to motor. This appears to be part of the stray currents circulating in the earth or of some other reason.
The unbalance in power supply voltage is truly a concern and a reason for negative phase sequence current trip of motor.
One way of mitigating the problem is choosing HV power supply from the distribution company and stepping it down to 600V within the facility. This would obviously a costly path, to be chosen if the distribution company fails to maintain the voltage balance following your request.

R Raghunath

 

[waross]

I have seen many instances where a primary distribution voltage imbalance and phase shift were transferred to a secondary system.
Many had dedicated transformer banks, some utility owned, some privately owned.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[_NoobNoob]

I have seen many instances where a primary distribution voltage imbalance and phase shift were transferred to a secondary system.
Many had dedicated transformer banks, some utility owned, some privately owned.

Like a phase shift where the 3 phases are not completely 120 degrees apart?

 

[waross]

"Like a phase shift where the three phases are not 120 degreess apart?"
Yes. This is a common effect when remote voltage regulators correct uneven phase voltages.
Often when correcting unequal phase to neutral voltages the phase angles and the phase to phase voltages are unequal.
This causes circulating currents in tertiary delta windings and motors.
The circulating currents act to partially correct the unbalances.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[_NoobNoob]

Yes. This is a common effect when remote voltage regulators correct uneven phase voltages.

WHAAAAT?! I was under the impression that it was always 3phase from hydro with a step down and no other interference (How ignorant am I?). You're breaking my brain here. I'll need to check the phasors on the power meter. We've got an Eaton PMX2260.

 

[waross]

Compare phase to neutral voltages with phase to phase voltages.
Compare phase to neutral angles with phase to phase angles.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

I have the following opinion for your consideration.
1. Based on the measured voltage 591V-595V-603V and current 353A-373A-422A , the Voltage unbalance 1.11 % and Current unbalance 10.28 % ; seems to be acceptable. Attention: This is a spot reading. It could be the Voltage and Current unbalance go far beyond these values, under certain circumstances, like unbalanced loading at certain time.
2. When measurement of unbalance Voltage either V[sub]LL[/sub] or V[sub]LN[/sub], it is understood that the three vector magnitude are NOT equal and their angle is NOT 120 deg.
3. "..the earth wire that goes from the motor to the MCC measures about 16A". If the motor frame is insulated from any other earthing and the only conductor to the MCC earthing bar having 16A is far too high.
3.1 Check a) insulation resistance with 500V DC.
b) dielectric test with 2kV AC.
Note: with these two tests, hopefully the root cause of high 16A is detected.
Che Kuan Yau (Singapore)

 

[LionelHutz]

Assuming the soft starter is bypassed on running.

Well, if it's full-on then it's the same bypass or not.

That'd only possibly apply if it was doing a NOLA type energy savings thing.

 

[Marke]

I have seen a high earth current in a number of installations of bore hole pumps with a steel riser.
If the cables down the well to the motor are not perfectly fed down the bore with the motor, you can end up with a partial turn of one phase around the riser and it acts like a short circuited winding around a steel core and causes significant current to flow in the earth conductor.

Mark Empson
Advanced Motor Control Ltd