Parallel combination of VSD

[maypot]

Hi,

We need to install one 1500 amps @ 400 V variable speed drive for a variable torque application.
We have two options in hand-one with two 700 amps VSD in a parallel combination that is two single VSD units in parallel sharing a common DC bus.
The other option is a standard one with a main rectifying unit,one DC bus and a main inverter module.
I have no basic experience of parallel drives and, as technical support is miles away, I would be pleased to have inputs from forum members.

Bob

 

[Skogsgurra]

If you are going to hard wire them in parallel, a prerequisite is, of course, that switching is synchronised or that one inveter is slaved to the other on gate drive level. Another option is to have a motor with two windings driven by separate inverters. Or even two motors working on one shaft.

Output reactors will be good for you so that static and dynamic load sharing is OK. Paralleling has some advantages - and disadvantages. Advantages are that you have two smaller units and that you (if load permits) sometimes can run with reduced load if one of the inverter sections fails. Keeping spares for two smaller units sometimes is cheaper than for one big unit. It depends on what IGBT are used. Disadvantages are that most people are less familiar with such inverters and that the might be a psychologic "wall" to break.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

Hi skogs,

A project I worked on a year or three ago deliberately sequenced four parallel switching modules so they switched in quadrature, thus raising the ripple frequency of the combined bank four-fold and allowing a much smaller switching filter to be used. Obviously they had to be synchronised, but there was no requirement for coindicident switching: indeed this would have defeated our intent completely. The bank was part of a fairly large (MW-level) active harmonic filter built for a water company who were upsetting their DNO with massive harmonic pollution. A number of banks were ganged to provide the required power rating.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[Skogsgurra]

That's good engineering! But I think that you have left the "support is miles away" situation when you have an installation like that. It is not something that the common field support techie would tackle, I think.

Gunnar Englund

http://www.gke.org

 

[torslum]

Agree with Gunnar; make sure the gatesignals are synchronized or in practice that the vendor you use have done this kind of paralleing before. Some vendors have so-called master-slave applications where gatesignals are not synchronized, but loadsharing is taken care of on a controller-level (reference). Works well if you have several motors parallelled to one shaft, but not if you simply parallell inverters.

Even if one use double-winding motors, one can experience problems if there is no phase-angle between the two windings or if there is not enough inductance before the common coupling. Been there

My advice; ask your vendor for good references.

torslum