Transformer for a 6 pulse rectifier 5

[edison123]

Need some advice for a transformer-rectifier for electroplating application.

Quantity - 2 nos.

KVA - 600

Voltage - 11 KV / 100 V

Configuration and usage - Both of them connected to the same 11 KV bus on the primary side. One transformer or both the transformers may be used depending on the load requirement.

Requirement - Minimum harmonics

Given the above, what is the best transformer winding connection that would give the minimum harmonics and circulating currents (if any)

1. Both transformers Delta / Delta

2. Both transformers Wye / Delta

3. One transformer Wye / Delta and the second one Delta / Delta



Muthu

http://www.edison.co.in

 

[davidbeach]

#3. Of the options listed, that's the only one that will allow a 6-pulse rectifier to operate. I'd probably go with #4 though, delta on the high side with wye and delta on the low side.

 

[edison123]

Thanks David. Any reason why 1 & 2 would not work ?

Muthu

http://www.edison.co.in

 

[ScottyUK]

1 & 2 would both work, I think David has confused the 3-phase supply required for a 6-pulse design with the 6-phase supply required for a 12-pulse design.

That said, I agree that #3 is probably the best solution to minimise harmonics if both units are in service and operating at similar loads and with similar delay angles. Change the firing delay of either unit and the harmonics might actually get worse if they start to add in phase rather than cancelling.

If these units are independent maybe you should think about a passive filter to suppress the 5th and 7th harmonics?


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If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

Although I would also avoid wye on primary side.


Rafiq Bulsara

http://www.srengineersct.com

 

[edison123]

Thanks Scotty. The supplier also wants the third option.

When operated individually, which one will need those 5th and 7th harmonic filters ? Wye connected or Delta connected ?

And do they have to be bypassed when operating in parallel ?

Muthu

http://www.edison.co.in

 

[wolf39]

Muthu:

Option #1 and #2 with bridge-connected rectifiers does result in a 6-pulse system.

Option #3 does result in a 12-pulse system with minimized harmonics. It is of paramount importance, however, that you either stick to wye OR delta on the low voltage side of both transformers. If you select wye on the first transformer and delta on the other you most likely may not be able to match the number of turns for 100 volts.

Reason: Wye number of turns = 0.5774 * delta number of turns.

Regards

Wolf

http://www.hydropower-consult.com

 

[davidbeach]

Sorry, posted that too late at night. Scotty is right, I was thinking 12 pulse. So strike the bit about

that's the only one that will allow a 6-pulse rectifier to operate

but the rest stands. Grounded wye on the utility side, delta on the load side as a load serving transformer is undesired. If a customer really, really wants one of those we will require it be protected as though it were a generator since it is a ground source.

If you go with #3 or #4 you have to have two independent rectifiers sharing a common dc bus, you can't have the transformers directly in parallel (you probably know that already, but maybe not everybody else). When you are running both transformer/rectifier sets you will have a 12-pulse rectifier and your first harmonics of note will be the 11th and 13th, the 5th and 7th having canceled at the high side of the transformers.

 

[ScottyUK]

My interpretation was slightly different to some of the others - perhaps Edison could clarify? I envisaged two independent 6-pulse rectfiers fed from a common 11kV source, but trying to achieve some of the benefits of a 12-pulse system in terms of harmonic cancellation. In this case good harmonic cancellation would only occur if the two rectifiers were of similar design and with reasonably well matched output currents, otherwise the low order harmonics from the more heavily laden rectifier would be apparent as cancellation would be imperfect.


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If we learn from our mistakes I'm getting a great education!

 

[edison123]

Sorry, I was not clear. The DC output of these rectifiers will not be connected to a single dc bus. Only the primaries of the transformers on 11 KV side are connected to the same 11 KV bus. DC outputs will be in two different bus which will not be interconnected either.

In such scenario, which is the best primary winding combination that result least harmonics on the primary ?

Muthu

http://www.edison.co.in

 

[davidbeach]

I'd still go with option #4, or if forced to do so, option #3. Options #1 & 2 (or their corresponding options with delta high side) won't do anything about 5th and 7th harmonics. #3 and #4, with both rectifiers running, will leave you some uncanceled 5th and 7th if the rectifiers aren't matched, but the resulting level of 5th and 7th will be the difference between the contribution of the two rectifiers.

 

[edison123]

Thanks David. # 4 is out since the supplier is not willing do it. If I go with # 3, how does operating only one unit at a time affect the harmonics ?

Is there any online reference for such harmonic study ?

Muthu

http://www.edison.co.in

 

[rbulsara]

Muthu:

As an example see this. Read the first half of the first page.

http://www.hammondpowersolutions.com/upload_files/hpsi-ta2.pdf

You can google more, perhaps.

Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

Operating only one unit provides no reduction of harmonics. With both unit in service, the high side of each transformer also still has full harmonic content. But where the transformers are tied together the will be cancelation of the 5th and 7th. Due to the transformer phase shifts the 5th and 7th from one transformer will be shifted 180 degrees relative to those from the other transformer.

 

[edison123]

Thanks Rafiq for that link. Gotta to brush up my google skills. I've been finding my solutions at ET too easily.

Thanks again David for staying with this Luddite. I'll go with option 3 - one trafo delta and other wye.

Thanks you, scotty and wolf. Appreciate your inputs.



Muthu

http://www.edison.co.in

 

[thinker]

There is additional advantage of using 5th and 7th harmonic LC filters (tuned or detuned): at fundamental frequency they would serve as a source of reactive power to compensate a lagging and reduced power factor of the 6-pulse converter.

 

[electricuwe]

I'd recommend to use a totally different approach:

Buy a standard MV transformer 11 kV/400 V or 11 kV /690 V and 2 separate 12-pulse plating rectfiers operating from Low voltage.

(Plating rectifier: unit combining transformer, rectifier and control, designed by several smaller suppliers according to customers specifications)

Designing plating rectfiers is a quite special thing as you have to be aware that plating always needs a kind of voltage control. Currently there is no statement how you plan to do this. Unless you choose the classic variable transformer, voltage control will cause varying harmonics and variable reactive power demand which is both quite difficult to deal with at MV for such a (low) power level.


See:

http://www.munk.de

http://www.rgb-heythekker.de

http://www.befeld-systeme.de

You will find a link to several other languages on all of these sites.