Isolation Transformer for AFE drive 1

[jimmy2times]

Is it necessary to have a drive isolation transformer for an active front end drive? There is no requirement to step down the voltage for the drive input.

The VSD would be LV (440V) and the power system is unearthed.

 

[LionelHutz]

I would consider one because of the unearthed power system. A trend we have noticed is that a VFD is more likely to fail on a floating power system then a grounded Y system. So, to give it the best chance create a balanced grounded system for it.

 

[Skogsgurra]

I have seen many AFE drives on ungrounded 500 V systems.

The only problems I know about are that the insulation monitors (Bender and such devices) sometimes have a problem with the idling AFE, seems to be some kind of sampling effect that tricks the Bender device to believe there is a true resistance to ground. Loading the drive makes the problem go away. Which is unexpected, but can be explained.

The other problem is more severe. An AFE (or any PWM drive) will usually introduce high PWM common mode voltage residues on the ungrounded system. This sometimes causes problem in other systems connected to the same grid. DC motors, for example, are known to have heavy bearing EDM on such a grid.

But, as I said, many systems work quite well on ungrounded grids. There is an issue with EMC filters, though. Your supplier can guide you there.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dpc]

Just to show my ignorance - what's "AFE"?

 

[ScottyUK]

Active Front End - draws a pretty good sinusoid from the mains using an active switching input stage instead the harmonic-rich waveform of a rectifier.


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

 

[Drivesrock]

Jimmy2times, I would suggest you follow the AFE drive manufacturers recommendations for the transformer or mains supply for the AFE acquired or being purchased by you/your company. Some AFE VSD manufacturers insist on an earthed neutral of the supply whereas others have a DC link neutral point connection to earth and the transformer neutral should not be connected for the AFE. I have successfully applied AFEs for these 2 systems.

So, depending on what mains supply you have and your AFE or drives type could define if you need an isolating transformer to either earth or not the neutral.

But then there is the issue of where you are applying the drive - industrial or domestic environment? In an industrial environment I believe you would not have to have an isolating transformer (unless your mains is neutral earthed but you AFE cannot have that supply or visa versa). In a domestic environment - I'm thinking of maybe a lift AFE VSD in a block of flats - then you may need an isolating transformer and/or PWM/EMC filters as mentioned above.

I am intrigued as to why you have a floating supply - and do you already have VSDs on that power system? And then of course the questions are why and what is the manufacturer of the AFE?

I would agree that VSDs work much more reliably on earthed neutral power systems or dc link neutral point earthed. The DC link voltage - in your 440V case approx 620Vdc across DC+ to DC- rails - will then be 310Vdc from either rail to earth and should always be so. This means that the DC capacitors and power transistors and other internal components should always be at the same potential to earth - ie not floating. It's the floating around voltage that these components could have to earth that can reduce their working lives or causes a failure in VSDs.

Drivesrock.

 

[Skogsgurra]

Drivesrock said: "I am intrigued as to why you have a floating supply"

I have seen floating grids in a few places. Last week, in Norway, for instance. The whole country has an ungrounded three-phase system.


Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Or at least they think it's ungrounded.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

I see what you mean. I thought so, too. But the ones (two of them) I checked really were.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jimmy2times]

Thanks for the responses to date.

Sorry, the application is a ship, hence the 3 phase input voltage is derived from an IT isolated neutral system, i.e. the generator neutral point is left floating.

(IT is european blurb for Insulated Terre or isolated from earth).

It is interesting to note the problems found with the Bender insulating monitoring equipment as this is equipment that we use on our switchgear for earth fault detection.