VFD on step-up transformer problem
- Thread starter tom kanzler
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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
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But Sahib is correct none the less. And I myself want to know for the future. :thumbsup:
mbrooke
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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
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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.
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- Electrical Engineer
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.
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Putting the question in a different way, will a K-rated transformer have a lower %Z than a non K-rated?
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I find, if there are no Line to Neutral loads, K-rated transformers are rarely worth the special price versus simply oversizing the transformer.
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- Placerville, CA, USA
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- Retired PV System Designer
+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.
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.
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- Placerville, CA, USA
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- Retired PV System Designer
Nice to see that the engineering caught up to the gut feelings.
I've done it on some 780kW drives. The core was gapped to prevent saturation.
Worked fine.
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- Placerville, CA, USA
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- Retired PV System Designer
Sounds like a pulse transformer to me.
Not one where you'd want to get your sums wrong.........Sounds like a pulse transformer to me.
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.
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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?
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- Placerville, CA, USA
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- Retired PV System Designer
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.
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.
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