Two 250 VDC 1200A rectifiers feeding multiple crane rails - is it OK to put the outputs in parallel ?

[bdn2004]

A new installation is being put into this Plant that is a copy of another just like it: as described.... (2) 300 kW rectifiers feeding the power to multiple crane rails.
The rectifier vendor is not recommending this saying:

"You cannot connect two of these rectifiers’ outputs in parallel. They will not share current equally. Please confirm."

The other system just like it has been there for decades. Is the vendor correct ?

 

[waross]

So despite what the vendor is saying, do you really want to depend on advice from an anonymous source on the internet?
Wasn't there recently a discussion of a massive grid failure in Australia due to two rectifiers being in parallel?
Let us know how it turns out for you.
OR
Ask the vendor or manufacturer if the rectifiers may be modified so as to work and play well together.
There are control schemes that naturally lend themselves to paralleling seamlessly.
There are other control schemes that are not suitable for parallel operation.

 

[AG-PPI]

I am assuming the rectifiers are standard 3-phase AC to DC?

I'm a little rusty but I believe you can connect the DC outputs in parallel to share the load, but it is not as simple as shown in your diagram
The AC input on one rectifier must be delta, the other must be star. They cannot both be delta as shown in your diagram, and will need to be fed from a 6-phase transformer.
One major consideration is whether or not you intend to use an IPT on the positive outputs or not.
This will affect the DC voltage output as well as AC input current.

This method would require a control board for such operations.
I would personally recommend the FCOG1200 6-ph firing board from Enerpro in the US, but it would be well worth discussing specific needs with them directly. They are usually very helpfull and open to custom board modifications. (Company I work for uses them a lot, always been very happy with them).

 

[bacon4life]

Do you need the load split equally? Or are these located at opposite ends of building crane rail to provide reduce voltage drop?

 

[waross]

I'm a little rusty but I believe you can connect the DC outputs in parallel to share the load

Anonymous advice.
Simple rectifiers may be connected in parallel.
A star/delta configuration reduces ripple.
However, a power supply may be more than just a rectifier.
Many power supplies incorporate a voltage control feature.
The more sophisticated the voltage control, the less likely that units may be operated in parallel without one unit hogging the load.

 

[AG-PPI]

Anonymous advice.
Simple rectifiers may be connected in parallel.
A star/delta configuration reduces ripple.
However, a power supply may be more than just a rectifier.
Many power supplies incorporate a voltage control feature.
The more sophisticated the voltage control, the less likely that units may be operated in parallel without one unit hogging the load.

Hi waross,

Regarding voltage control, this is why I suggested the use of that control board. It has many control features, one being phase-angle control which I believe reduces RMS voltage on the output. That control board is designed to work in exactly these kind of situations.
However, this is assuming the rectifiers are using thyristors and not diodes.
Afaik, I don't believe I've ever seen a parallel bridge rectifier using diodes before. Not saying it's not possible, but I've never seen one.

 

[waross]

I am assuming the rectifiers are standard 3-phase AC to DC?

I have to assume that what the poster considers a rectifier is a complete DC power supply that incorporates voltage control and possibly current limiting.
Each "rectifier" will already have a control board.
And, these two rectifier units are not at the same location so it may be impractical to control both from one board.
Your suggestion is good for someone building a power supply from scratch and who wants to parallel SCRs to increase capacity.
This is an issue with two existing power supplies.

 

[bdn2004]

We are trying to work this out with the Vendor right now. Attached is the detailed drawing of the existing unit that is desired to be paralleled with a new one.
There is a mistake on the one line - the existing is a WYE-DELTA transformer as shown on the wiring diagram below. They are diodes. And the two units will be located 100's of feet apart from each other.

 

[bdn2004]

A little more information... below is the connection drawing of the existing installation. There are three rectifiers running in parallel. This proposed project is to separate the two on the left from the one on the right. Then add another one to the one on the right as a separate system.