K Factor Transformers and 3 Phase Non-linear Loads

[jtinge]

In one of the DOE's facility engineering standards, there is the following guidance statement for the use of transformers feeding switched mode power supply loads. "K-factor rated transformers should never be used for three-phase non-linear loads such as motor drives, three phase UPSs, or any three-phase device with SCR phase-control or static-diode input circuits."

There is no footnote to explain the rationale for this requirement other than that it was lifted from a NAVFAC 16400 specification requirement. Can anyone explain the rationale for this requirement? I have an application where a K-13 transformer is proposed for single-phase computer loads but a large 3 phase UPS is also proposed to be fed from the K-13 transformer. Based on the guidance statement above, is there a problem feeding the UPS from the K-13 transformer?

 

[tortuga]

I have an application where a K-13 transformer is proposed for single-phase computer loads but a large 3 phase UPS is also proposed to be fed from the K-13 transformer.

I dont think you'll have any electrical issue, its more of a cost issue.
I think the reason they lifted that is product manufacturers have in theory have lowered the THD that those power supplies apply.

 

[steve66]

K factor transformers handle the larger neutral currents single phase harmonics cause.

Three phase loads may still have harmonics, but they don't result in neutral currents.

So if you have a mix of single phase and three phase loads, you should be able to use a lower K rating than would be needed if all the loads were single phase.

 

[jtinge]

Thanks you all for the replies. They basically support my suspicion that there would not be a problem with the mixed loads. However, I still can't figure out why the guidance was so specific about three phase UPS's or any three-phase device with SCR phase-control or static-diode input circuits. Is there a technical operational characteristic of these devices that don't make them suitable for use with K-factor transformers? I'm thinking possible lower transformer impedance and negative operational performance, not harmonic related.

 

[steve66]

Thanks you all for the replies. They basically support my suspicion that there would not be a problem with the mixed loads. However, I still can't figure out why the guidance was so specific about three phase UPS's or any three-phase device with SCR phase-control or static-diode input circuits. Is there a technical operational characteristic of these devices that don't make them suitable for use with K-factor transformers? I'm thinking possible lower transformer impedance and negative operational performance, not harmonic related.

Nothing that would be a problem that I'm aware of. Its probably just a waste of money to use a Krated xformer on a three phase non-linear loads.
One feature K rated transformers usually have is:

• Neutral bus size configured to accommodate at least 200% of the rated current compensates for increased neutral currents found in non-linear loads, thus reducing heat

That's kind of a waste for VFD's and UPS inputs that don't even use the neutral.

 

[d0nut]

Nothing that would be a problem that I'm aware of. Its probably just a waste of money to use a Krated xformer on a three phase non-linear loads.

I would argue that it is almost always a waste of money to use a K rated transformer. It is extremely unlikely that the level of harmonics in a typical installation would reach a point of damaging a standard transformer. And, if the harmonics are that bad, it would most likely be better to remove the harmonics with filters, better drives, or phase shifting rather than allowing them to propagate through the system.

 

[synchro]

In one of the DOE's facility engineering standards, there is the following guidance statement for the use of transformers feeding switched mode power supply loads. "K-factor rated transformers should never be used for three-phase non-linear loads such as motor drives, three phase UPSs, or any three-phase device with SCR phase-control or static-diode input circuits."

There is no footnote to explain the rationale for this requirement other than that it was lifted from a NAVFAC 16400 specification requirement. Can anyone explain the rationale for this requirement?

That statement appears to be from the Square D "Low Voltage Transformers Selection Guide" from around 25 years ago which said:

"Selection Criteria K-Rated Transformer for Non-Linear Load:
K-rated transformers should never be specified for three-phase non-linear loads such as motor drives, three phase UPSs, or any three-phase device with SCR phase-control or static-diode input circuits. K-rated transformers are evaluated only for the heating effects of harmonic currents, not for the thermal and mechanical stress of drive loads. These transformers have double- size neutral terminals and, therefore, are intended only for use in high 3rd harmonic-single phase non-linear loads. For transformers specifically designed for high 5th and 7th harmonics and current pulse stress of three-phase converter loads, see Note 1rive Isolation Transformers."

 

[tortuga]

So they are saying the 'k' rated thing only helps L-N loads with THD?
Interesting.
So syncro if we have 'delta' connected loads that are converting 3 phase AC to DC like drives or even 240V servers or HPS/LED lights connected 240 or 480 delta, do we need to consider these non-linear L-L loads when ordering a xfromer?

 

[synchro]

So they are saying the 'k' rated thing only helps L-N loads with THD?
Interesting.
So syncro if we have 'delta' connected loads that are converting 3 phase AC to DC like drives or even 240V servers or HPS/LED lights connected 240 or 480 delta, do we need to consider these non-linear L-L loads when ordering a xfromer?

The transformer manufacturers say you do.
https://americas.hammondpowersolutions.com/en/resources/k-factor-vs-h-factor-in-drive-applications

The delta connected non-linear loads you mentioned will have higher order harmonics like the 5th, 7th, etc., and so apparently the "drive" transformers targeted for them have their construction tailored to further reduce eddy current losses, etc. at the higher harmonics. But I suspect that using a standard transformer, especially one with a lower temperature rise spec, might be sufficient in many cases.

Drive isolation transformers also reduce THDi on feeders to the transformer. Therefore they produce less voltage harmonic distortion upstream that could affect other equipment. One way they do this is to introduce additional impedance like a line reactor does. In the OP's case, however, both L-L and L-N nonlinear loads are fed by the output of one transformer with a shared impedance. In that case a higher transformer impedance is not necessarily better because it would do the opposite of what I mentioned above: it would tend increase the harmonic voltage distortion on the various loads.
I'm sure that Jraef would be able to provide much more input about drive isolation transformers than I could.

The Op mentioned a "large" 3-phase UPS. What percentage of the total load on the transformer would it present?
What is the THDi spec on the UPS? If it's relatively large could line reactors be considered to reduce harmonic levels at the transformer?

 

[jtinge]

Thank you synchro for identifying the source the guidance was based on. That was exactly what I was looking for. As far as your question about percentage of total load the UPS presents to the transformer, that's TBD. I have a meeting today with our GSA contractor to discuss their proposed design details. If past experience holds true, their proposed transformer rating will be 2x to 3x larger than the actual load. Has been a source of angst on several previous building projects where they grossly oversized transformers and generators over my objections.

I will be looking into the THDi specs for the UPS as well as the comparative load to the transformer as you suggest to see if there may be a need for a reactor.