VFD DC-Link Quick Disconnect? 2

[WWWJD]

Hello all,
We have a little project where we're tying ten 5HP 480VAC drives together on a common buss setup. These drives are on portable tables, and I'm wanting to find a way to do a quick disconnect on the DC connections. 1kV, 5-6A, 2 or 3 pole, finger safe on both the male and female side. My other option would be to some up with some kind of safety interlocking via contactors, but I haven't really thought that through thoroughly. The risk is that operators will potentially be making/breaking connections to a hot DC link. The easy answer is "don't do that" but reality is it will probably happen at some point. Just trying to make sure the electrons stay on the right road.

Any thoughts or recommendations? I'd like to find a good fingersafe connector and be done with it. Not sure where to look first.

Thanks,

 

[jraef]

Look at Anderson Power Products. Their main business has been battery connectors but recently they have expanded into solar system connectors and some of them are rated for 1000VDC. But NOTHING in a connector like this will be rated for switching 1000VDC under load. DC is not like AC and it draws and maintains an arc for a long time (relative) as well as needing a long distance of separation to extinguish it. My suggestion would be to add a paclockable safety cover (like those used for Lock-Out/Tag-Out) over the connection, then hang the key for the locks begin the door of the disconnect so that they MUST kill power before they pull the connectors apart. Yes, they could re-energize it, but then you have proof for the lawyers later that they took additional action to make it unsafe.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

Consider contactors with all three contacts in series. This keeps everyone away from the 700V backed by capacitors. Unplugging a live connector would be foolhardy.

I think you need to go beyond here. If I thought people would be tempted to unplug these things live I'd make that impossible. Keep in mind even if they turn off the drives the DC links will retain lethal energy for a while. So even 'power down - then unplug' is still dangerous. I'd lock all the connectors in boxes and require a supervisor or some third party to be present to unlock the boxes and raise the care and warning aspect.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

I highly doubt the VFD would be happy being plugged onto a live DC bus. It sounds like it'll just cause VFD explosions to me. Does the manufacturer say this can be done?

 

[WWWJD]

All good points. Trying to prevent I.D.1.0.T. errors but I may inadvertently be inviting them in. Safest way would be to hard-wire everything in on the DC link side and force them into a position to where they need to bring the whole system offline prior to connection or disconnection.

Thanks for the sanity check, fellas.

 

[John2025]

Out of curiosity, what's the advantage to a common DC bus? A few less rectifiers and caps?

 

[stevenal]

Would a key interlock system fit the bill? It forces a particular switching sequence.KIRK

 

[jraef]

Out of curiosity, what's the advantage to a common DC bus? A few less rectifiers and caps?

The most common application is in a set of AC motor powered machines in which some motors will be motoring while others are braking or decelerating to where they go into regeneration. So rather than using Dynamic Braking resistors (or supplementing them) the regen energy from those decelerating motors is consumed as motoring energy on the others.

And he is not necessarily reducing the number of components, on some models you can tie the DC busses together of standard drives.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[John2025]

jraef,
Ahhhh,that makes sense. Thanks.

 

[itsmoked]

John; It works well, allowing regeneration without the large expense of actually laying out for regenerative drives. It's a poor-mans regen scheme. The only downside is it can only regen between the drives and not, say, the lights.

I've used it successfully on several servo-motion systems.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sreid]

http://www.gigavac.com/

 

[waross]

There are interlocked receptacles for use in EX areas. They are a disconnect switch/receptacle combination. The switch may only be closed when a proper ex proof plug is properly inserted into the receptacle. The plug is trapped and may not be removed until the disconnect switch is opened.
The challenge now is to find a switch or breaker with an acceptable DC rating.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Bill, I looked at Meltric, which has plugs and receptacles rated as disconnect devices, and Cooper Arktite, which make the type that interlock with a separate disconnect switch, but their DC rating (250VDC) is too low for a DC bus on a VFD.

Keith’s point about the safety aspect with regard to the DC bus hold-up time is very valid. The DC bus on a VFD can remain lethal for quite a long time after powering down the input (the only electrocution I have ever witnessed was from that scenario). Opening the DC bus circuit between drives with a disconnect or contactor is a must. Again, the solar industry has solutions; there are several suppliers (ABB, Socomec are two that I know of for sure) who offer a 1000VDC rated rotary thru-door interlocking disconnect switch that you could put on each VFD cabinet.

Not an endorsement, but here is a link to the Socomec version because they are not well known outside of the Solar industry, although you see their products brand-labeled by several other companies.

https://www.socomec.com/files/live/...gue/GB/LEAFLET_SOLAR_UL98B _DOC_163013(B).pdf

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

Nice to see there's a high voltage DC disconnect out there.
Though I thought I've seen some 1,200V solar arrays around here... ?


With regard to the OP's situation there are only two choices:

1) An expensive electromechanical system that unlocks connector boxes 5 minutes after a 'Disconnection Switch' is thrown. The systems even needs to work when all power is dropped to the entire table so someone doesn't defeat it by just tripping a breaker.

2) Get in writing that no one will disconnect anything without "Bob or Ed" present on pain of immediate dismissal. The operator can drop the power and then go find Bob or Ed and once they arrive they take a clipboard with all the drives numbered on it and go around and check off that each drive's 'charge present' light has gone out. Date and signs it and then plugs can be disconnected and re-arranged. (I'd note which ones - see below.)

Alternatively just don't hook up the DC-Links ever. It will use a bit more money over time but it avoids the hazards completely. Safety usually costs money anyway, pay this one over time.

The upside of 2) is that likely, with time, it will be learned which DC-Links in the setup need to be switched and which never need to be switched. At that point remove the switched ones and permanently wire the not-switched ones and stop the fiddling on a safety/time based optimization.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Socomec are quite well-known over here in the Old World. Their switch-disconnectors and switch-fuses are generally well-regarded compared to the competition. I'm surprised they're seen as a niche market manufacturer in the US.

 

[jraef]

ScottyUK,
That's because their initial foray into our market was by selling their products as brand-labeled through Schneider, Siemens, Bussman, Littlefuse, Rockwell and other smaller suppliers, leaving them with little to no marketing of their own until relatively recently. What put them on the map here was the Solar industry, because the other players (other than ABB) were slow to catch onto that market potential. I met with the marketing Mgr for Socomec US in around 2010 when I had a need for about $5 million worth of 1000VDC 1000A disconnect switches for a big solar PV farm and we talked about his North American woes over a few drinks in San Francisco.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Yes, I understand that the DC bus is lethal.
But, I understand that this will be one DC bus powering several drives. The drives will be with neither front ends nor DC bus.
Once the drive is shut down, there will be no current from the DC bus to the drive.
If it is proven that the drive is off, then the connector may be disconnected without any current.
Yes there will still be a lethal voltage present, but if the hot side of the plug is a finger safe female device,- but with a receptacle-plug arrangement it should be safe to disconnect.
The problem or issue is to prevent a disconnection when current is flowing. No current and a dead plug will be better, but this will be between the DC bus and the inverter side of the drive.
Jeff; Your item #2; "

2) Get in writing that no one will disconnect anything without "Bob or Ed" present on pain of immediate dismissal. The operator can drop the power and then go find Bob or Ed and once they arrive they take a clipboard with all the drives numbered on it and go around and check off that each drive's 'charge present' light has gone out."
Yes it will be safe but time consuming and I wouldn't trust it. I'm not going to bet my life on Ed not having a bad day and checking the wrong drive by mistake.
I understand that we will be breaking the connection between the common DC bus and the inverter side of a partial drive. It may not be feasible to power down the entire line to disconnect one motor and drive. And as for administrative solutions, rtemember the adage; "The Push is on And Safety's Gone."
I believe that isolate and interlock is the way to go.
I can't count the number of times that I have seen an electrician carefully put a lock on the wrong circuit and then go cheerfully to work.
On one project I estimate that the mistakes were in the hundreds.
If I have misunderstood the issue then I apologize.
Let's make sure we define the problem.
Over the years I have seen some solutions applied to solve a misunderstood issue that solved the issue as it was understood, but as far as the real issue were actually ineffective and foolish. Some were/are downright dangerous.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

If I have misunderstood the issue then I apologize.

A) I said #2, it not Jeff.

B) I never saw that this was a bunch of DC-Linked ONLY drives. Seems to be regular drives sharing at the DC-Link but maybe you're right.

Seems to me "isolate and interlock is the way to go" would likely be larger and cost as much as the drives since you'd need two special disconnects for each connection. Might be cheaper to use contactors and a switch.

Note AMTRAK de-powers the entire train even though they have thousand dollar connectors that have longer pins that drop the Head End Power as the plugs are disconnected.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Rereading the OP's posts it seems that I did misunderstand.
I apologize.
Keith, I have seen too many gung-ho supervisors take a short-cut with the paper work and seen too many mistakes to ever trust a "clip board" safety solution.
I was once instructed by a supervisor to work on a 4160 HP motor WITHOUT A LOCK ON THE SWITCH. IN FACT I WAS THREATENED WITH DISMISSAL IF I INSTALLED A LOCK.
I suggested that millions of dollars and many lives could have been saved on the Piper Alpha disaster by a $2 lock. The supervisor told me I was wrong, and gave me a URL to a report on the disaster.
(I underestimated. 3.4 BILLION dollars and 167 lives.)
Back to the report.
Page one, paraphrased.
"This report will deal with the domino effect of the disaster AFTER THE INITIAL ACCIDENT. It is outside the scope of this report to consider the root cause of the accident."
I stand by my statement.
A $2.00 lock on the switch would have avoided the disaster..
You can't change the culture of the Oil Industry.
Back to another statement of mine;
"I suggested that millions of dollars and many lives could have been saved on the Piper Alpha disaster by a $2 lock."
"I'm not going to bet my life on Ed not having a bad day and checking the wrong drive by mistake."


I accept your suggestion to power down the entire line.
Apology repeated.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

The "Kirk" lock method may be worth investigating, as long as there are no spare keys.
I was responsible for installing a set of Kirk locks. The locks come with spare keys to facilitate installation and the stern warning;
"After Installation Destroy All Spare Keys."
The supervisor gave me the package and the instructions to return to him all spare keys after the installation was finished.
He wanted to make sure that the keys were destroyed, right?
Wrong! He wanted to have the option to circumvent safety in the future if it was convenient.
We had had a couple of instances of safety short-cuts already.
My suspicions were confirmed by the look on his face when I returned the keys as instructed, with an unrepairable 90 degree bend in each key.
Sometimes I think that I have spent too much time in the field, resolving what happened with what should have happened.
I used to be the guy that got called in when things didn't work as planned, or when all the on-site experts were stymied.

Bill
--------------------
"Why not the best?"
Jimmy Carter