He already solved the problem, he was just wondering why it worked.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.
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.
SO.But Sahib is correct none the less. And I myself want to know for the future. :thumbsup:
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.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.
I find, if there are no Line to Neutral loads, K-rated transformers are rarely worth the special price versus simply oversizing the transformer....having over sized neutrals. K-rated transformers are therefore a good idea for sure, compared to over sizing a transformer for HEAT DISSIPATION alone,...
+1 as long as you count balanced non-linear wye loads as lune to neutral loads.I find, if there are no Line to Neutral loads, K-rated transformers are rarely worth the special price versus simply oversizing the transformer.
I'd say it' better late than never! Here are the reason/s: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've done it on some 780kW drives. The core was gapped to prevent saturation.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.
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.Putting the question in a different way, will a K-rated transformer have a lower %Z than a non K-rated?
You state the loads may grow. OKHowever 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.