Battery Charger AC Isolation

[yesterlectric]

Is there a good consensus on when dealing with higher voltage battery chargers which are high voltage DC and powered by 3 phase systems, if and when someone may need to consider the possibility of DC voltage back feeding through to the AC side when planning safe isolation for the AC side? I recall basic theory of how chargers work. And I was taught backfeed isn't possible. But that was with simple 1 phase chargers that charge at 12/24 volts DC. Often, some of the larger equipment is more complex than the simple stuff we learned of in trade school.

 

[hbiss]

Is there a good consensus on when dealing with higher voltage battery chargers which are high voltage DC and powered by 3 phase systems, if and when someone may need to consider the possibility of DC voltage back feeding through to the AC side when planning safe isolation for the AC side?

Not saying that that can't happen, but it isn't an issue.

-Hal

 

[jaggedben]

Is there a good consensus on when dealing with higher voltage battery chargers which are high voltage DC and powered by 3 phase systems, if and when someone may need to consider the possibility of DC voltage back feeding through to the AC side when planning safe isolation for the AC side? ...

I would be concerned if the charger isn't listed. And I would not be concerned if it is. And I would never, ever install a charger for that voltage that wasn't listed.

(Granted I'm saying this without actually knowing what the standards say about it. But still.)

 

[yesterlectric]

I would be concerned if the charger isn't listed. And I would not be concerned if it is. And I would never, ever install a charger for that voltage that wasn't listed.

(Granted I'm saying this without actually knowing what the standards say about it. But still.)

I wouldn't base my thinking on if it's listed. It's the technology not whether someone paid for a UL sticker.

 

[jaggedben]

A UL sticker is not received merely by paying UL, you have to show them that you met their standard, and usually pass some tests. The technology is essentially a black box to me. We rely on third-party listing to evaluate the technology. So does the code.

No offense but if you are asking the question in the first place, then (like most of us) you don't have the expertise to evaluate the technology yourself. Your best substitute is to rely on listings.

 

[yesterlectric]

A UL sticker is not received merely by paying UL, you have to show them that you met their standard, and usually pass some tests. The technology is essentially a black box to me. We rely on third-party listing to evaluate the technology. So does the code.

No offense but if you are asking the question in the first place, then (like most of us) you don't have the expertise to evaluate the technology yourself. Your best substitute is to rely on listings.

I’ve seen enough UL listed equipment with problems show up on job sites. It doesn’t necessarily mean as much as UL makes it out to be. Certainly it provides some safety but it’s not as great as they say. And no you don’t just decide whether or not you need to isolate something based on whether it’s listed.

 

[jaggedben]

Then I will just say that in about 10 years of installing non-isolated inverters I have never seen a listed charger or PV inverter (same fundamental issue) backfeed high-voltage DC to the AC side. See Hal's post #2.

The point isn't that UL is perfect. The point is that not being listed would be a red flag, at least for me. And the listing provides you a pretty decent level of liability protection as an installer, as compared to installing unlisted equipment.

 

[retirede]

I’ve seen enough UL listed equipment with problems show up on job sites. It doesn’t necessarily mean as much as UL makes it out to be. Certainly it provides some safety but it’s not as great as they say. And no you don’t just decide whether or not you need to isolate something based on whether it’s listed.

How do you decide, then? Ask some strangers on the internet?

 

[jaggedben]

Also I think if you really want to understand the consensus methods or criteria, the UL standard for the equipment is probably the best place to start. Don't let the perfect be the enemy of the good.

 

[yesterlectric]

How do you decide, then? Ask some strangers on the internet?

This wasn’t really the point of the post. I think UL versus not UL was a bit of a sidetrack. Sure it should be listed. Sure, if it’s not it’s reasonable to be suspect. But the general understanding of the technology is a different topic.

I’ve worked at companies that require LOTO on both sides of an inverter and others that don’t. It’s sort of a similar thing when it comes to big chargers. And yes I am asking strangers on the Internet who have been somewhat albeit minimally vetted on the forum. Because I wanted to see what the consensus is elsewhere.

 

[jaggedben]

Sorry, I totally did not get that you were asking about LOTO, as opposed to equipment design or system design.

If you are just working on something on the AC supply side, I would say that disconnecting the DC side couldn't hurt but is paranoid with newer technology. (Especially if listed! Still gonna go there.) Most of these things these days have either relays or power electronics that either literally or effectively disconnect the two sides when not operating. I've never told anyone that they had to turn off DC in order to be be safe doing things on the AC side of equipment (leaving aside systems with battery backup that still output AC to some circuits).

If you are working on the charger equipment itself, then I would say disconnecting both sides should be standard operating procedure.

 

[retirede]

This wasn’t really the point of the post. I think UL versus not UL was a bit of a sidetrack. Sure it should be listed. Sure, if it’s not it’s reasonable to be suspect. But the general understanding of the technology is a different topic.

I’ve worked at companies that require LOTO on both sides of an inverter and others that don’t. It’s sort of a similar thing when it comes to big chargers. And yes I am asking strangers on the Internet who have been somewhat albeit minimally vetted on the forum. Because I wanted to see what the consensus is elsewhere.

OK…I think we were all thinking you were asking about how isolation is designed into the charger. LOTO is a different discussion.

 

[garbo]

Is there a good consensus on when dealing with higher voltage battery chargers which are high voltage DC and powered by 3 phase systems, if and when someone may need to consider the possibility of DC voltage back feeding through to the AC side when planning safe isolation for the AC side? I recall basic theory of how chargers work. And I was taught backfeed isn't possible. But that was with simple 1 phase chargers that charge at 12/24 volts DC. Often, some of the larger equipment is more complex than the simple stuff we learned of in trade school.

We were taught that diodes & rectifiers are similar to a check valve in a pipe system ( water, oil, etc ) in that they only allow flow in one direction. For three phase they use 6 diodes to convert AC to DC

Is there a good consensus on when dealing with higher voltage battery chargers which are high voltage DC and powered by 3 phase systems, if and when someone may need to consider the possibility of DC voltage back feeding through to the AC side when planning safe isolation for the AC side? I recall basic theory of how chargers work. And I was taught backfeed isn't possible. But that was with simple 1 phase chargers that charge at 12/24 volts DC. Often, some of the larger equipment is more complex than the simple stuff we learned of in trade school.

.I have PM'ed & troubleshoot 480 volt VFD'S up to 250 HP. Most times had to leave power on. Numerous times while troubleshooting a 480 volt drive would disconnect motor leads and measure to see if drive had at least 650 volts DC Buss voltage. Would loto the line side and use a meter to see if there was power on the input terminals. Not once did I have the DC Buss voltage backfeed back to the AC input. Some VFD'S can take up to 20 minutes for large capacitor bank ( connects directly to rectifier output ) to discharge and yes several times I have been hit with it rushing to get production equipment back on line. Even PM'ed numerous 18 pulse drives. They use 3 separate DC rectifiers ( 18 diodes ) and hope I get this right each of the three groups go thru a transformer ( ? ) to what was told supplies three out of phase AC voltages to three separate sets of rectifiers.

 

[kwired]

Is there a good consensus on when dealing with higher voltage battery chargers which are high voltage DC and powered by 3 phase systems, if and when someone may need to consider the possibility of DC voltage back feeding through to the AC side when planning safe isolation for the AC side? I recall basic theory of how chargers work. And I was taught backfeed isn't possible. But that was with simple 1 phase chargers that charge at 12/24 volts DC. Often, some of the larger equipment is more complex than the simple stuff we learned of in trade school.

Regardless of voltage they still use same logic to rectify from AC to DC - a diode or a network of diodes for three phase. Only way you get back feed through the diode(s) is if you exceed break down voltage of the diode, which for typical power rectification will be well over the design operating voltage.

 

[LarryFine]

Regardless of voltage they still use same logic to rectify from AC to DC - a diode or a network of diodes for three phase. Only way you get back feed through the diode(s) is if you exceed break down voltage of the diode, which for typical power rectification will be well over the design operating voltage.

That's why automotive alternators don't need to open the battery wire like DC generator regulators had to.

 

[synchro]

That's why automotive alternators don't need to open the battery wire like DC generator regulators had to.

Yeah, they're not cut out for doing that.

 

[garbo]

Regardless of voltage they still use same logic to rectify from AC to DC - a diode or a network of diodes for three phase. Only way you get back feed through the diode(s) is if you exceed break down voltage of the diode, which for typical power rectification will be well over the design operating voltage.

On drives that I have PM'ed & troubleshoot when the DC Buss voltage got too high the drive would shut down. The few times that I came across this was when two usually over 75 HP AHCU's on common ductwork was running and the only single return fan was trying to restart. Had to place both supply fans on hand and ramp the speed down to maybe 25% in order to restart return fan. We had over 500 drivers some over 15 years old and over 120,000 running hours. We had spare rectifiers but never had one go bad.

 

[drcampbell]

Regardless of voltage they still use same logic to rectify from AC to DC - a diode or a network of diodes for three phase. Only way you get back feed through the diode(s) is if you exceed break down voltage of the diode, which for typical power rectification will be well over the design operating voltage.

Or if one of those rectifiers (often SCRs) fails short.

 

[Tbrzezicki]

After working on Industrial For Lift chargers for 21 years I can say Always cut the AC when working on them. If it's a Ferroresonant charger it can sort through the diodes, if it's SCR's they can do the same so always cut the power