Current Imbalance

[Allenation]

Good Evening Everyone. our company is one of the authorized dealer of Grundfos Pumps here in our country Philippines particularly here in Kidapawan City, about a hundred kms away from Davao City, Mindanao. i have this one issue that has been unsolved for the last two years. we supplied a 125HP Franklin electric Submersible Motor, 3ph, 460Vac driven by a ABB Softstarter with Sub monitor from Franklin Electric Products here in Metro Cotabato Water District, Cotabato City. actualy, i already sent a additional photos to the Franklin Electric Representative in asia. and as of the moment while waiting for the Franklin peoples evaluation, this Engineer here in local water district ask me to put "FILTER" just to address the issue of current imbalance. looking at the AIM of Franklin, the Allowable Current Imbalance for Submersible Motors is only at max of 5%. previous test we have initiated the Submonitor and the Softstarter always trip and indicated a Imbalance trip. now, the MCWD called on the attention of the Local Power Company to Conduct such test. after the Test using their Scanners, according to National Code in my country, it registers about 1.5% which means, the Voltage imbalance is acceptable. shown in the Picture.

Now, in Franklin Electric Submonitor and ABB Softstarter, the Current Imbalance should not exceed at 5%. in our previous test, we use the ABB Softstarter and Interchanging the Lead Wires of the Motor in 3 position: 1st: Y-R-B, 2nd: B-Y-R, 3rd: R-B-Y. we suspect that there is something wrong with the ABB Sofstarter. so as per Franklin Electric suggestion to have it tested again in Direct Online with the Submonitor and bypassing the ABB Softstarter, the Tripping occurs still the same: Current Imbalance. heres the Photos in my last testing:

if you notice in the display, the Voltage Imbalance if we are going to compute, did pass in -+10% Imbalance.. but take notice on the Current Rating Displayed in Submonitor.

on the 3rd test, i also took pictures of the Clampster Rating output in 3 legs.

would it help to address the Current Imbalance Issue by using "FILTER"? thank you for your time reading my concerns, il be glad to hear any help from all of you. thank you

 

[Allenation]

Also, this engineer also insisted that our Motor and Controller should adapt to their old system. by the way, the old system are composed of 4units 150HP, 460Vac Vertical Turbine Line shaft Pump.

 

[waross]

A wye/delta transformer bank is often problematic, but it may find a use here.
A 480 Volt wye to 480 Volt delta transformer with a four wire primary connection may do the trick.
This will go a long way towards balancing the voltages.
Once the transformer or bank is energized, a contactor should be arranged to open the neutral connection and leave the wye point floating. This will avoid the over voltage switching transients associated with a wye/delta transformer with a floating wye point.
You should include loss of phase protection.
Again, energize the transformer with the wye point connected to the system wye point and then immediately disconnect the neutral connection and leave the wye point floating.

The action:
Unequal phase to phase voltages imply unequal phase angles also.
An three phase induction motor acts as an induction generator generating a back EMF. The back EMF is equal in voltages per phase and equal in phase angles.
When the equal voltages and phase angles of the back EMF meet the unequal voltages and phase angles of the grid, something has to give.
The motor draws extra current from the high voltage legs and less current from the low sides. In extreme cases of unbalance, the currrent may reverse on the lower voltage leg as the motor attempts to balance voltages and phase angles. The excess current is limited by the impedances of the motor and of the source.

A wye delta transformer wants to have balanced voltages and phase angles on the secondary, delta winding.
If you draw the vectors for a delta with unequal voltages and/or unequal phase angles, the delta will not close. That gap will represent an EMF that will drive a circulating current around the delta. The current is limited by the impedances of the transformer and of the source.
A wye/delta transformer bank with the primary neutral connected to a source with unbalances will have a delta secondary that does not close.
If the primary wye point is left floating, the delta secondary will force the best fit that it can for the least circulating current.
The balancing action of the wye/delta transformer reduces the unbalances by finding the best fit of voltages and phase angles. The unbalances at the delta secondary are much less than at the primary. This is partly due to the balancing action and partly due to the heat generated by the circulating current.

The motor is now supplied with much better balanced voltages and phase angles. There is still some unbalance but the resulting current is limited by the impedances of the motor and the transformer. The transformer impedance is much greater than the grid impedance which is an infinite source compared to the motor.
The wye/delta bank can not completely cure unbalance conditions but it is capable of reducing the effects.

Bill
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"Why not the best?"
Jimmy Carter

 

[Allenation]

Noted sir Bill. thanks for the Input. we'll let our customer read your post here for their evaluation.

this is my last question, aside from your suggestion sir, someone told me about "Line/Load Reactors" can also help resolving the issue of Current Imbalance. i'd like to know how Load Reactors can help or does it really can help in correcting the Current Imbalance? our Controller is only ABB Softstarter and as what i have heard, Line/Load Reactors has been used in VFD Controllers. we cant afford to buy brand new VFD and Load reactor since the Designed Controller is Softstarter with by pass contactor. is Load reactor is applicable too with Softstarter? thank you.