Precharing of VFDs on a common dc-link 1

[torslum]

Dear colleagues,

I am doing some brainstorming on how to complete a VFD-system with several large inverters connected to a common dc-link and with one rectifier to supply the DC.

First thing on my mind is how to precharge the capasitors which are already in the inverters. I am, ofcourse, aware that I could make some kind of resistors in series to reduce the peak-current, but I would then need a contactor to short-circuit the resistors after the charging is done. As I am thinking very large power-drives, I do not believe that this is a good solution, since the contactors would become quite expensive also. Any good, practical solutions? I've tried to search the web for any information on the subject, but it seems quite hard to find.... How is this being done by ABB, Siemens and the other guys? A small, low voltage transformer particulary there for this reason and being disconnected at a certain level? The reason why I'm doing this, apart from the advantages with a common dc-link itself, is that I want to use a specific brand of inverters while I want to choose the rectifiers independently.....

Another thing bothering me, is that I do not have very much experience with high power DC. Anything in particular that I should watch out for while doing the design, apart from the regular issues as for AC (thinking of the total design with the busbars, protection, distance et.c)?

Appreciate your replies.

tm

 

[ScottyUK]

Instead of a contactor, have you considered an IGBT switch? What voltage is the DC link and what rating are the drives?

A few thoughts on designing with high power DC:

Fuses are usually de-rated for DC. Switches and breakers have severe voltage de-rating and / or require multiple series poles for DC duty. DC arcs are powerful and not are not inherently self-extinguishing. Treat link inductors with utmost care.


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

 

[Skogsgurra]

You were very close to one solution when you said "A small, low voltage transformer particulary there for this reason and being disconnected at a certain level?"

You may do it even simpler. Just wire a rectifier (simple single phase bridge will do) and a resistor to charge the capacitors. Then - when voltage is up to 80 or 85 percent - close the big switch. The precharge rectifier and resistor can be left in circuit.

The nice thing with this approach is that you do not need DC contactors since you switch the three-phase supply.

Another technique is to use controlled rectifiers to supply the DC link. The charging can then take place at a constant current. This usually needs an inductor in the DC link but since there is almost always an inductor in there from the beginning, it is not a problem.



Gunnar Englund

http://www.gke.org

 

[Barry1961]

The way Hitachi recommends is to run the negative to the negative terminal and the positive to the AC input terminals, jumper AC inputs. This way you still have the current limiting resistors in circuit. I would guess it would work for most drives.

Barry1961

 

[electricuwe]

I'd like to warn you to follow the Hitachi approach cited by Barry1961. If you use AC-inputs to supply DC you will loose the advantages of a common DC-Link because the rectifier diodes will not allow fo feed back energy from one inverter to the other.

Further some inverters might rely on AC for auxilliary supply.

(here the term inverter is used for the complete electronic part of the VFD like it is commonly used today. The more precise wording, used in the remainder of my post is:
Converter or Rectifier: for AC-DC conversion
Inverter: for DC-AC conversion )

torslum, I do not understand why you want to combine recifiers and invertes from different manufaturers. Many suppliers of drives sell rectifier and inverter units to combine as a drive system with common dc-link. Usually the rectifier units provides a lot of "house-keeping"-features to the inverters with the precharging of the dc-link capacitors beeing only one of them. If you combine recififers and inverters from different manufacturers you may have lots of trouble with that.

 

[torslum]

Thanks for your valuable replies, everyone.

I am aware of the derating for regular breakers and fuses, and the increased risk of arcing due to DC. Any good suggestion on equipment that is particularly well suited for such applications?

 

[ScottyUK]

DC contactors tend to be the 'old-fashioned' bar and shaft design with a magnetic blowout. Good ones I have used are available from Telemecanique, ABB's 'R' series, and Siemens. In my experience standard AC contactors pushed into DC service have short violent lives, even if the manufacturer claims otherwise in their spec sheet.

What voltage and current are you considering in this application? Once you get above 250V DC most MCCBs will require multiple poles in series.


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

 

[Drivesrock]

torslum,

Some inverter manufacturers have on-board pre-charging circuits IGBT switches or Thyristors and resistors for ac and/or dc connection - Siemens, Alstom, for example, but could depend on model and kW rating or options selected. Why not pre-charge the complete dc link and all connected drives thru your one rectifier and a common pre-charge circuit? Do you have to have the capability of switching a 'dead' drive onto a live dc link? I suppose this will depend on your application.
What is the rating of your inverters?

It would be interesting to hear about your application but I have done projects with a common dc link precharging circuit and individual drives with the capability of switching on to the live dc link - it depended on what the drives were doing and what the customer wanted. Remember that you wouldn't necessarily want to go into a live (or be allowed to for that matter!) dc fed cubicle suite to work on a diconnected drive! I wouldn't be so happy with the idea of dc contactors - large and eat up cubicle space, could open under load if the coil fails and other reasons. If you want to switch a drive on to a live dc link then choose one that has on-board precharge capability from a manufacturer that has as a proven solution.

Drivesrock