High neutral current on 480V substation

[westgbazo]

A meter on a 480V substation is reporting 233A of neutral current. 'A' phase current is low compared to 'B' and 'C'. Any ideas for what might be causing this?

The substation feeds air compressors, chillers, boiler feedwater pumps, etc.

 

[jim dungar]

How is I_n being determined?
Is there a dedicated CT on the Neutral?
Is the common point of the 3 phase CTs wired into the meter input?
Is the meter calculating the neutral current based on the summation of the phase CTs?

 

[David Castor]

Is this display giving the current in rms or fundamental? Could be triplen harmonics, depending on how it is being measured and computed. Max phase imbalance is about 8%. Any single-phase loads? Adjustable frequency drives?

 

[synchro]

This is a WAG, but perhaps the 480V is feeding a wye-wye transformer with a 3-leg core (perhaps to obtain 208V or 240V) and there's an open fuse on the phase A input. The voltage for phase A would then be regenerated on the secondary as noted on pg. 2 (pg. 3 of the pdf) in the document at the link below, and so the transformer could still power 3-phase loads fairly well as long as they do not exceed about 50% of the transformer rating. This is because with a 3-leg core, the leg of the core for phase A is the only return path available for the components of magnetic flux passing through legs B and C that are in-phase with each other, and therefore cannot flow between legs B and C. However, transformers with 4 or 5 leg cores provide additional paths for such flux and hence the voltage would not be regenerated on a missing phase to any extent. But with a 3-leg core and with the phase A feed to the transformer open, the neutral will conduct substantial current in order to power all the loads on the wye secondary from only the B-N and C-N primary inputs.

And so in the situation above, the current on 480V phase A would be reduced because of the open fuse on phase A.
And the neutral current would be high in order to feed the neutral terminal of the 3-leg wye-wye transformer with an open circuit on its phase A input.

https://selinc.com/api/download/124320

It's just a guess, but it's something that could be checked out.

 

[westgbazo]

How is I_n being determined?
Is there a dedicated CT on the Neutral?
Is the common point of the 3 phase CTs wired into the meter input?
Is the meter calculating the neutral current based on the summation of the phase CTs?

According to the drawing, there is no neutral CT. The common point of the 3 phase CT's is connected to the "I_n" of the meter, so the meter appears to be calculating the neutral current.

Is this display giving the current in rms or fundamental? Could be triplen harmonics, depending on how it is being measured and computed. Max phase imbalance is about 8%. Any single-phase loads? Adjustable frequency drives?

RMS. There are a few lighting panels but most are motor loads with VFD's. This substation feeds half utility loads (chillers, AC's, pumps, etc.) and the other half are process loads (conveyor motors, accumulators, labelers, etc.). I'll check the harmonic data from this meter to see if triplen harmonics may be the cause.

This is a WAG, but perhaps the 480V is feeding a wye-wye transformer with a 3-leg core (perhaps to obtain 208V or 240V) and there's an open fuse on the phase A input. The voltage for phase A would then be regenerated on the secondary as noted on pg. 2 (pg. 3 of the pdf) in the document at the link below, and so the transformer could still power 3-phase loads fairly well as long as they do not exceed about 50% of the transformer rating. This is because with a 3-leg core, the leg of the core for phase A is the only return path available for the components of magnetic flux passing through legs B and C that are in-phase with each other, and therefore cannot flow between legs B and C. However, transformers with 4 or 5 leg cores provide additional paths for such flux and hence the voltage would not be regenerated on a missing phase to any extent. But with a 3-leg core and with the phase A feed to the transformer open, the neutral will conduct substantial current in order to power all the loads on the wye secondary from only the B-N and C-N primary inputs.

And so in the situation above, the current on 480V phase A would be reduced because of the open fuse on phase A.
And the neutral current would be high in order to feed the neutral terminal of the 3-leg wye-wye transformer with an open circuit on its phase A input.

https://selinc.com/api/download/124320

It's just a guess, but it's something that could be checked out.

There are few smaller transformers for lighting panels. We're going to have the electrician meter the MCC's and PDP's downstream to try and figure out where the current imbalance is coming from.

Jim/David/Synchro, thanks for your replies. It is much appreciated!

 

[jim dungar]

If the neutral point of the CT is wired to the I_n input terminal the meter is reading the unbalanced current from the three line CTs, it is not 'calculating' the neutral current.

Have you performed manual calculations using your line currents and measured angles to see if the displayed result is correct?
This is one way to rule out CT wiring errors.

 

[wwhitney]

FWIW, computations by hand with a basic calculator:

Ia = 958.36 < 37.5 deg = (760.32, 583.41)
Ib = 1077.71 < 145.6 deg = (-889.23, 608.87)
Ic = 1078.09 < 265.8 deg = (-78.96, -1075.19)

Ia + Ib + Ic = (-207.87, 117.09)
|Ia + Ib + Ic| = 238.58

That differs from the displayed neutral current of 233.294, but only by about 2%. I assume that can be attributed to harmonics?

Cheers, Wayne

 

[westgbazo]

If the neutral point of the CT is wired to the I_n input terminal the meter is reading the unbalanced current from the three line CTs, it is not 'calculating' the neutral current.

Have you performed manual calculations using your line currents and measured angles to see if the displayed result is correct?
This is one way to rule out CT wiring errors.

Yes, I get 238.6A for the neutral current.

FWIW, computations by hand with a basic calculator:

Ia = 958.36 < 37.5 deg = (760.32, 583.41)
Ib = 1077.71 < 145.6 deg = (-889.23, 608.87)
Ic = 1078.09 < 265.8 deg = (-78.96, -1075.19)

Ia + Ib + Ic = (-207.87, 117.09)
|Ia + Ib + Ic| = 238.58

That differs from the displayed neutral current of 233.294, but only by about 2%. I assume that can be attributed to harmonics?

Cheers, Wayne

Thanks Wayne. I'm getting 238.6A as well. I should have mentioned this earlier but the two pictures from the meter may not be from the same exact moment in time. That would explain the difference between the 238.6A calculated from the phase angle data and the 233.294A shown on the other picture.

Now I need to figure out where the imbalanced load is coming from.

I was able to get a little more info from the meter: THD for Phase-A is showing 1.04% while B/C are at 0%.

Alarm triggers are also listing some voltage sag issues on Phase-A in the past.

 

[synchro]

While not common, if the CT on phase A is damaged and this is causing the lower current readings on phase A, then the wye connected CT's would cause a neutral current to be indicated even without any L-N loads.

 

[Julius Right]

I don't know if the results [measurements] are correct but, however considering the data as corrected, the total neutral current calculated is 238.58 A [233 A as it will be for a 2% error [good precision, in my opinion].