K-Factor Rated Utility Transformer

[LMAO]

Proposed set up for our new testing facility: 2.5MVA 480V utility transformer feeding our 2.5MVA 600V transformer (utility does not offer 600V transformers) to power our loads including 1800kW of non-linear VFDs loads. These VFDs create typical harmonics (about 35% I THD, 8% V THD). My understanding is that utilities don't offer transformers with k-factor rating.

1. Does oversizing the 480V transformer compensate for its lack of k-factor rating?
2. How about passive harmonic filter to mitigate the harmonics? Filter can have a contactor that closes at higher loads to avoid creating leading power factor on utility transformer.

Thank you for any input, thoughts, comments.

 

[petersonra]

It seems to me this is a question to be asked of the utility.

 

[jim dungar]

K-Rated transformer do nothing to mitigate harmonic problems. They are simply able to tolerate the heating associated with the harmonics, pretty much the same way an oversized transformer would.

Personally, I do not recall ever being able to cost justify a K-rated versus standard constructed transformers.

 

[ron]

I think IEEE Standard C57.110-1998, UL 1561 and UL 1562 all describe how to specify k-ratings when there are significant non-linear loads.

For some reason, in larger distribution transformers, it isn't done much.

Essentially a k-rating just oversizes the transformer and terminals / grounded conductor and terminations for triplens.

I just arm wrestled with ABB for a K-13 3MVA FR3 transformer because it was foreign to them. I was feeding 100% non-linear load pretty close to 100% continuous load relative to the rating.

Avoiding an overheating condition is what you need to consider whether it be k-rating or oversizing relative to your load.

 

[LMAO]

Thanks guys, I understand that a k-factor rated transformer does not filter out the harmonics, it is rated for handle the heat. We have existing similar test setup and I am being told they used passive harmonic filters. I think we'll go that route.

 

[Jraef]

A: Triplen harmonics on 3 phase non-linear loads are not a big problem, triplens are a big problem when you have single phase non-linear loads fed from a 3 phase system. If most of your non-linear loads are VFDs, a K-factor transformer wouldn't do much for you. Utilities don't botheer with them because they would rather just take the risk of an overheated transformer anyway; for them it's a cost/benefit analysis that favors not using K factor rated.

B: A Passive Harmonic Filter tuned to the fifth harmonic will take your I-THD down to around 10-12% in most cases, if you get one tuned to the 5th and 7th you can get down to around 6-7% I-THD. The contactor to take them off-line when not being used is an excellent idea, I don't recommend using Passive Filters without that now, I've seen too many issues when they are left on-line all of the time.

C: If I had 1800kW of VFD loads on a test bench situation, I would be looking at using the newer Low Harmonic Active Front End drives now, especially if you can have one AFE converter feeding multiple smaller inverters for different loads. That would get your harmonics down to less than 5% I-THD.

 

[LMAO]

A: Triplen harmonics on 3 phase non-linear loads are not a big problem, triplens are a big problem when you have single phase non-linear loads fed from a 3 phase system. If most of your non-linear loads are VFDs, a K-factor transformer wouldn't do much for you. Utilities don't botheer with them because they would rather just take the risk of an overheated transformer anyway; for them it's a cost/benefit analysis that favors not using K factor rated.

B: A Passive Harmonic Filter tuned to the fifth harmonic will take your I-THD down to around 10-12% in most cases, if you get one tuned to the 5th and 7th you can get down to around 6-7% I-THD. The contactor to take them off-line when not being used is an excellent idea, I don't recommend using Passive Filters without that now, I've seen too many issues when they are left on-line all of the time.

C: If I had 1800kW of VFD loads on a test bench situation, I would be looking at using the newer Low Harmonic Active Front End drives now, especially if you can have one AFE converter feeding multiple smaller inverters for different loads. That would get your harmonics down to less than 5% I-THD.

There is just one enormous load, and AFE is not an option due to cost and size.

Another question that has come up is regarding 600V transformer configuration. Assuming a 480V/600V Δ/Y: obviously grounding the neutral makes it safer because OCPD will trip in case of a ground fault but what would be the advantage of not grounding the neutral of Y?

 

[jim dungar]

...what would be the advantage of not grounding the neutral of Y?

Check with the load manufacturer. They may require a grounded connection.

IMHO there is no reason to not ground it.