VFD DC bus extension

[thinker]

We plan to use a common bus VFD system (charging unit and 4 motor inverters). Usually, this would be a line-up with DC bus bars going along the line-up and dropping down connecting bars to respective inverters. In this specific case, the only possible location is a charging unit with radial connections to stand alone inverter cabinets. My question: is the difference between using bus bars or cables for these DC connections? Is there a limitation on the length of such a run? (For reference: each inverter has rated input current about 1000 ADC at 1100 VDC). To our surprise, the drive supplier can not provide a clear answer to these questions.

 

[SceneryDriver]

At 1kA, those will be some mighty thick cables/bus bars. As it's DC, you'll have no skin effect to deal with, so that's good. How much voltage drop can the inverters tolerate? How long are your cable runs? It's a simple application of Ohm's law to find out the resistance - and thus, cross section - of the needed conductors.

Other thoughts:
Ensure that your cables' insulation is rated for the voltage it will see.

At 1kA, methinks fusing is a good idea; I wouldn't want to see what an unprotected bolted fault would do to that bus. Remember that DC faults are harder for overcurrent protection to clear, and thus fuses/breakers need to be spec'ed and rated properly.


SceneryDriver

 

[mikekilroy]

Thinking 'easy answer,' I did quick search of various dc link sections of manuals on drives we use and found it is NOT written so difinitely after all! Thought I had seen like 4 foot type length as absolute max on these 40-400amp continuous (each) drives. but all I found quickly was cable size suggestions AND the expected need to go to SHIELDED CABLE over a few feet length.

Realize that your 1100vdc bus will have 2200V+ noise on it that will try to get into EVERYTHING else on your machine (and probably even your neighbors!).

We are working on some mining applications for 100kw 1500vdc bus drives, and leads here are also only 2-3 foot max. I would not recommend longer if possible.

We have some 7axis machines with 20-85 amp each drives on common dc link; during design phase drive designers decided we could not mount these in a way to make the dc link wires longer than about 2 feet max....

We recently did a multiaxis 460vac robot and customer tied the dc bus between 2 drives with 2 foot long untwisted wire that they pushed into the wire raceway with other wires on machine to look pretty: result was I had a 4 hour drive to help 'fix' the machine that would not run - computers and sensors all over the machine had so much noise they were not working right. removing this single set of wires solved the issues.

I suspect that if you search drive manuals from the likes of Bosh Rexroth and Siemens you will find the official guidelines you search.

So moral of my story is these link wires have probably the most horrendous high voltage and frequency noise on them in a machine so need real RFI thought and shielding.

 

[thinker]

Thank you for responses, colleagues. Scenary driver: We have no concern about the voltage drop, this can be easily compensated for. The concern of mikekilroy regarding noise is very important. Our drive supplier sugggests to run dedicated overhead cable trays (or cable bus systems)from centrally located charging unit to inverters,they plan for the length of each DC run about 70 ft! I was concerned about sanity of such solution and surprised by their optimism. I was always under impression that the length and construction of DC bus link is dictated by minimization of parasitic inductance and capacitance of conductors.

 

[electricuwe]

You should also consider that the DC-link capacitors and the line inductances of the connections will create quite some resonant circuits.

This usually will create the most severe limitation in such systems.

 

[oldfieldguy]

I'm thinking of the DC bus on the three Siemens GM-150 MV drives (one-7000, two-9000 HP). Siemens saw fit to make this bus very thin and wide to reduce the inductance since the firing of the IGBT's in pulse-width modulation produces some very interesting changes in current along the bus.

You might want to give some thought to that potential sticky point.

old field guy

 

[SceneryDriver]

I most definitely deal with lower voltages and currents than those brutes do! 9000HP is about 4,500 times larger than most of the motors I typically use. At those voltages, is it impractical to add enough capacitance to swamp any ringing that might happen due to IGBT-caused standing waves?

Could the design essentially treat the DC bus as a type of transmission line, and be designed accordingly? Since you would know the switching frequency of the IGBT's and thus the harmonics that would be created, could the DC bus be designed to not be resonant at those frequencies?

Machines that large are something I've only seen in pictures. Managing that much energy safely is impressive to say the least.


SceneryDriver