VFD on step-up transformer problem

Sahib

Senior Member
To check whether harmonics is the issue, a K-rated transformer of 300 KVA instead of present ordinary 400KVA Tr may be used. It may also be taken as a policy decision (by your company) to use only K-rated Tr in conjection with VFD.

Jraef

Moderator
Staff member
To check whether harmonics is the issue, a K-rated transformer of 300 KVA instead of present ordinary 400KVA Tr may be used. It may also be taken as a policy decision (by your company) to use only K-rated Tr in conjection with VFD.
He already solved the problem, he was just wondering why it worked.

Sahib

Senior Member
He already solved the problem, he was just wondering why it worked.
1)Neither the OP nor we are sure whether the issue was due to harmonics.2)Simply providing higher capcity Tr without designing for harmonic currents may not be a proper solution.

mbrooke

Senior Member
He already solved the problem, he was just wondering why it worked.

But Sahib is correct none the less. And I myself want to know for the future. :thumbsup:

Sahib

Senior Member
mbrook:We have a case of economy. Providing a 300KVA K-rated Tr is probably less costlier than 400KVA ordinary Tr in OP's case.

mbrooke

Senior Member
mbrook:We have a case of economy. Providing a 300KVA K-rated Tr is probably less costlier than 400KVA ordinary Tr in OP's case.

Good point, it certainly would be cheaper. and the fact a larger unit has more inrush? I will remember this for the future in case I run into such.

mike_kilroy

Senior Member
But Sahib is correct none the less. And I myself want to know for the future. :thumbsup:
SO.

Tom K., original poster: we all want to know why also. Since you would not check spec requirements of your Siemens vfd, let us.

Please give us the complete part number of the Siemens drive used and we will check if your issue is one of not paying attention to the input impedance requirements and thus solving it by simply oversizing the transformer (so its effective Z at the lower load is less and perhaps in spec for the Siemens drive). Your Siemens vfd may also be a line regen unit, which makes this Z even more important. We must not guess.

Jraef

Moderator
Staff member
To check whether harmonics is the issue, a K-rated transformer of 300 KVA instead of present ordinary 400KVA Tr may be used. It may also be taken as a policy decision (by your company) to use only K-rated Tr in conjection with VFD.
Despite this having become a moot point in this case, I also don't agree with the concept of using a K-rated transformer for the purpose of not having to up-size it in this situation where it's possible that the lack of current from the transformer is affecting the VFD operation.

The purpose of using K- rated transformers on non-linear loads such as VFDs is because the harmonics cause excess heat and circulating currents that concentrate in the neutrals on Wye-Delta transformers. The K rating is based on the additional heating this causes and the ability of the transformer to dissipate this extra heat at the SAME load as a non K-rated transformer of the same size heat without degrading the insulation life, as well as having over sized neutrals. K-rated transformers are therefore a good idea for sure, compared to over sizing a transformer for HEAT DISSIPATION alone, because as mentioned, it also increases the fault current. But the K rating does NOT increase the capacity of the transformer. A 300kVA K-rated transformer is still rated for 300kVA, it cannot put out as much current as a 400kVA transformer.

GoldDigger

Moderator
Staff member
Putting the question in a different way, will a K-rated transformer have a lower %Z than a non K-rated?

jim dungar

Moderator
Staff member
...having over sized neutrals. K-rated transformers are therefore a good idea for sure, compared to over sizing a transformer for HEAT DISSIPATION alone,...
I find, if there are no Line to Neutral loads, K-rated transformers are rarely worth the special price versus simply oversizing the transformer.

GoldDigger

Moderator
Staff member
I find, if there are no Line to Neutral loads, K-rated transformers are rarely worth the special price versus simply oversizing the transformer.
+1 as long as you count balanced non-linear wye loads as lune to neutral loads.
Since the affected neutral wire is just a short one from terminal to wye point of windings it will not affect impedance much, so VD would be unchanged.
Oversizing the transformer gives you both VD benefit and harmonic tolerance in same unit.
Space limits would be the biggest decider in favor of K IMHO.

topgone

Senior Member
We're having some trouble with VFD powered temporary equipment running through a 300kVA step-up transformer, from 208 to 480V. Machine specs shows 354A peak, 257A rated. Constant motor start/stop, with limited run time in between (hoist). Control system faults out when starting the hoist. We changed the transformer out for a 400kVA unit, and no more problems.

What I can't get straight is if the transformer size is actually causing undue voltage drop, triggering the fault. I wouldn't think there should be a problem at 282kVA (at 354A), and the transformer should be able to absorb an overload anyway. What I don't yet have is historical data on actual current and voltage at the drive, but I'm working on that.

Can the non-linear effects cause this? We have to use a grossly oversized generator when running this equipment, but that's for other reasons. This application is on utility power.

I don't want to use a blanket recommendation for an 'oversized' transformer without some justification, other than one bit of success, as the customer has to supply and connect the mains and transformer. And I can't be sure the transformer was the actual cure.

I've searched the web and this site, and found nothing to suggest that VFD's, as a general rule, require special treatment in transformers. Can anyone shed any light on this?
I'd say it' better late than never! Here are the reason/s:
Here's the usual harmonics content of a 6-pulse VFD without a line reactor:
fundamental = 100.0%; 5th harmonics = 80.0%; 7th harmonics =58.0%; 11th harmonics=18.0%; 13th harmonics=10.0%; 17th harmonics=7.0%; 19th harmonics=6.0%; 23th harmonics=5.0%; 25th harmonics=2.5%

If you'd care to compute, you will arrive at a K Factor = 21.4353 (per UL 1562) or a Factor K = 1.434 (BS 7821, assuming a transformer winding/freq constant n=1.7 and an eddy current loss e at fundamental frequency as 10%). This would force you to overrate your 300 kVA by 1.434. You'll need to have a 300 x 1.434 = 430kVA! The nearest size being a 400kVA. Crystal!

IMHO, a line harmonics filter could have solved the problem instead of upsizing the transformer to 400 kVA. But if they have a 400 kVA spare, it's more economical.

GoldDigger

Moderator
Staff member
Nice to see that the engineering caught up to the gut feelings.

Besoeker

Senior Member
In general there is no way that any load on s transformer secondary can cause core saturation. That can only result from applying too high a voltage to the primary.
Putting a transformer after a VFD is, on the other hand, likely to expose the primary to high voltage pulsed DC, which it may not handle well at all.
I've done it on some 780kW drives. The core was gapped to prevent saturation.
Worked fine.

GoldDigger

Moderator
Staff member
The core was gapped to prevent saturation.
Worked fine.
Sounds like a pulse transformer to me.

Besoeker

Senior Member
Sounds like a pulse transformer to me.
Not one where you'd want to get your sums wrong.........

Sahib

Senior Member
Putting the question in a different way, will a K-rated transformer have a lower %Z than a non K-rated?
No. So filters are still required. However higher capacity Tr only solution may not work in the long run, practically speaking: loads have a tendency to grow and the Tr unless it is K rated is liable to be seriously overloaded.

iwire

Moderator
Staff member
However higher capacity Tr only solution may not work in the long run, practically speaking: loads have a tendency to grow and the Tr unless it is K rated is liable to be seriously overloaded.
You state the loads may grow. OK

In most cases a higher capacity transformer is a good way to deal with growing loads.

Can a K rated transformer of the same rating provide the capacity for a growing load?

Sahib

Senior Member
Can a K rated transformer provide the capacity for a growing load?
May be.

Can an ordinary transformer of the same rating provide the capacity for a growing load?
May not be.

GoldDigger

Moderator
Staff member
If the K rated transformer is sized to the calculated load and is used to mitigate harmonic problems it will not have room for additional calculated load.
If an oversized transformer is used to mitigate harmonic problems OR to reduce the VD, it will probably have some headroom for additional "normal" loads.