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Discussion: Are Variable-Frequency Drives used in Grid-Tied Wind Turbines?

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Boloar

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I'm mostly curious - I'm not fully familiar with the details of the subject.
Since a variable-frequency drive works from mains power/frequency and can drive a motor at any power level with greater efficiency, are they used in the reverse fashion - i.e. working with the variable power from a wind turbine - to connect to the grid? I've been reading up on MPPT used in solar panels and wind turbines, and this thought occurred to me.
I'm afraid I haven't found much information on it.
 

Many recent wind turbines use inverters to connect to the grid. A standard VFD has however a rectifier input stage that can't source AC power. A possible topology has two back-to-back connected inverters with a DC link.
 
As FvM said there is a DC intermediate level to isolate the turbine input and the mains output. BUT the mains output has to be able to phase lock to the actual mains frequency (50 HZ +- .5 HZ) else the output stage could go into current limit and only deliver real power when the actual mains frequency and phase suits the inverter output frequency.
Frank
 
Boloar said:
are they used in the reverse fashion - i.e. working with the variable power from a wind turbine - to connect to the grid?
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Theoretically a regenerative VFD (example: ABB's ACS800-U11) can work in the reverse fashion. But practically these drives are not designed for continuous high reverse power. Inside, they are similar to back-to-back two inverters.
 
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    FvM

    FvM

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Do you know a specification that restricts the ACS800 continuous power in generator mode? Technically I don't see why it should be limited, inverter efficiency will be almost identical in active and passive quadrants.
 
Lack of complete specification about generator mode implies these VFDs are may not suitable for continuous generator mode at full power.
Possible limitation only I see is in cooling system. Working as drive with regenerative braking, most of the heat will be produced by drive output side IGBTs. As braking is not a continuous operation so average heat produced by line input side IGBTs is much lower. But working as generator, both side IGBTs(drive output and line input side) will produce heat and cooling system of the drive may not able to handle the heat.
 
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    FvM

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I don't agree with the assumptions. My experience tells that the sum of module power dissipation (IGBT + diode, forward + switching losses) won't change more than e.g. 10 percent when switching between motoric and generatoric mode. The detail behaviour is bit more complicated because also modulation index (actual motor voltage) and phase angle affects the individual losses.

We can also assume that grid and motor inverter are (at least partly) sharing a common heatsink, in so far the module overtemperature will be further balanced.

In case of doubt, it's of course better to ask ABB about possible limitations in regenerative mode.
 
I also don't see reason why an inverter shouldn't be able to handle equal power in both directions. Any bridge suitable for a VFD should also be suitable as a rectifier for DC-AC conversion, assuming the control is done correctly.
 

Interesting.
Conceptually, of course, it seems simple enough (but naturally not in practice).
MPPT in a wind turbine is basically power-factor correction at variable frequency, if I understand it correctly. And it seems that it would only be economically viable for power levels greater than the kilowatt range.
Do correct me if I'm wrong.
 

And it seems that it would only be economically viable for power levels greater than the kilowatt range.
Click to expand...
Why? "Micro" solar inverters are working in a low kilowatt range, too.
 

Lol, my bad. I should clarify: Using a VFD for wind turbine MPPT, I think, would be too costly for the low kilowatt range. In the low range I assume one would use a buck/boost converter as is currently used in solar panels.

I'm working on a MPPT controller design, in fact, inspired by Tim Nolan. But I'm using a TI buck/boost IC (LM5118) to manage the switches so that I don't have to deal with the coding (plus the TI chips have automatic undervoltage, overcurrent, etc. protection). Since the output is connected to a battery (and therefore holds itself at battery voltage), MPPT is achieved by varying the controller's voltage set-point to adjust the current flow.
 
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