Using 336V HVDC at home [closed]

Question

I have a 336VDC source and have been connecting it to an AC extension lead (UK, 3 Pin Sockets) and then connecting various AC devices (which have no DC input specifications on them where the power requirements are listed) such as TV's, phone chargers, network switches, NVR's etc. All of the devices work fine as presumably they all use some form of full bridge rectifier followed by a step down converter to operate.

To install this in a home or otherwise what are the safety implications or regulations of using this as opposed to AC? Obviously certain devices that use AC may not function, but in theory most modern devices should work?

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The 336VDC source is from a receiver unit from a company called VoltServer. It outputs 336VDC and takes input from one of their transmitter units.

Answer 1

Most fuses in 230VAC appliances are not rated for 336VDC. Some 230VAC fuses are only rated for 28VDC. Arcs are not blown out on the zero crossings with DC because there are no zero crossings. This means that the internal fuses could just keep arcing getting very very hot when a fault occurs. You can get larger more expensive fuses that are rated for DC that can be relied to blow when a fault occurs.

Answer 2

Looks like someone has a Tesla battery lying around.

A lot of hidden obstacles and dangers here.

1. AC-rated devices may look like working, but fail after a while (hour, day) because of some subtle part of their circuits depending on the input being AC (as in thyristor-based phase regulation) or the input being 240V RMS (see p.2).

2. 240V-rated devices that use resistive heating of some sort generating twice the intended heat. You may or may not notice at first, until something melts down.

3. AC-rated fuses and mechanical switches are generally not good for DC. DC-rated ones are much bulkier. Especially bad in regard to fuses because it is the fuse that separates the mild failure mode from the major fire.

Of course, everything explicitly depending on AC like transformer-based or capacitive-divider-based power supply or an AC motor is not going to work. Transformers and motors will blow spectacularily.

An universal cure for all these difficulties: an inverter. It will convert the DC into 220-230-240V AC that will be good for everything.

Answer 3

Accessories specified for use in the UK are rated for 250V AC. Plugs, switches, thermostats and relays may arc excessively on DC. Their life may be a lot shorter than normal.

If you use standard plugs and sockets, nothing will stop people plugging in devices that are not capable of running on DC. Anything with a linear power supply, a synchronous motor, or resistive components such as incandescent lamps may fail very rapidly on DC. Possibly with smoke and flames.

Answer 4

All of the devices work fine as presumably they all use some form of full bridge rectifier followed by a step down converter to operate.

Well, it's not that simple because not all the home appliances or the devices-supposed-to-run-from-mains are designed to operate in DC even if they have a bridge rectifier at their inputs to convert the incoming AC to DC. The bridge allows you to apply a DC voltage to the input regardless of its polarity but there's an important thing to consider.

^{simulate this circuit – Schematic created using CircuitLab}

When a DC is applied to the input, the whole current will flow through only two of the diodes (Either D1-D4 pair or D2-D3 pair). Depending on the design of the device, the input current may exceed the average current rating of the rectifier diodes. So they may burn eventually or instantly. This is not a problem when operating from AC because the polarity changes 50 or 60 times a second.

ADD: Luckily, supply from DC will not be a problem if there's a PFC pre-regulator in the device. Most of these devices' PFC pre-regulators generate an ENABLE signal for the post regulator (or in other words, for the rest of the circuit) when it sees the 90% of PFC boost voltage (typically 385-400VDC). So when they are supplied from 390VDC directly, this will not be a problem. But still, the main issue that I tried to express above still persists.

Answer 5

Electric railway mechanic here. This is a bad, bad idea.

Higher voltage DC is a mean drunk.

Here's a nice arc display from an inherently current-limited source - series-connected solar panels.

Here's about twice your voltage. Note the car is energized the whole time, and the arcing restarts - twice!

That could be your house.

AC crosses zero volts and zero amps 100-120 times a second. That is extremely useful for snuffing arcs. Inductive loads resist changes in current and will spike voltage to do so, and if they're doing that across a switch, in a few milliseconds it will be up against an opposing current.

Whereas in DC, absolutely nothing stops an arc. It can’t be bargained with. It can’t be reasoned with. It doesn’t feel pity, or remorse, or fear. And it absolutely will not stop, ever - unless you kill it or it destroys its own current pathway.

Like that poor tram.

Arc detection and suppression

If you ever have unhooked speakers "live", you're heard the crinkle-crunch sound of arcing (the waveform is the same on the wire). Better trolley substations have a "Rate of Rise" detector (AFCI basically) to use digital signal analysis to "listen" for that "sound", and trip power. You need something like that. The Odessa tram system didn't have it, as you can "hear" from the audio.

Now, on trolley cars, DC switches (even for small loads like a vestibule light) have two essential parts. First, the switch has a "snap" mechanism that throws the contacts far apart when they open. Second, there is a "blowout" mechanism that re-directs the arc into an "arc chute", made of ceramic or hard-board asbestos. The blowout is typically 2-3 turns of magnetic coil; the collapsing magnetic field pulls the arc into the chute.

So you need your switches and circuit breakers to be rated for interrupting DC. Your breakers need to be rated to interrupt a bolted dead short - AC power breakers are typically rated 10kA or 22kA. Further, you need a disconnect switch! You can't rely on pulling the plug, because if the fuse in a British plug blows, it will just arc across the opened fuse, and much like the poor tram, burn up the plug entirely, and even spread the arc to the melting socket. At which point you'll never get it out.

You see in Peter's video here (from comments).

DC ratings are much lower on average equipment

And if you read your equipment's spec sheets, you'll see that their DC ratings are lower than AC by as much as 90%. This is because of the sheer difficulty of snuffing a DC arc. For instance a switch rated 250V might be rated only 28 volts DC.

I remember seeing a light rail vehicle where they just could not obtain a 700VDC rated contactor. They used a 3-phase contactor rated for 2000VAC and wired all three phases in series.

Rearrange the battery pack for a lower voltage.

Split the pack and re-stack it in series-parallel for a lower voltage. Aim for voltage around 36 volts, although you'll be limited by the sub-pack sizes of course. Don't go much over 48 volts, even that is starting to head into DC's dark side.

Anything you do with DC of even 36 volts needs to be properly BSI/TUV/UL listed equipment. None of that "cheap Chinese" stuff, get gear from quality electronics supply, or use AC mains gear that is cross-rated for DC. For instance Schneider rates some of its consumer-tier lines for DC up to 48V (120V available on specialorder).

Answer 6

None of these answers seems to have checked up on VoltServer, and have gone off the assumption that this is an uncontrolled DC rectifier or a battery. I think we're all in agreement - a custom power system for 336 V DC has to be designed carefully by someone who knows what they're doing, AND checked by someone else who does as well.

Now VoltServer claims to be someone who knows what they're doing, and claims to create a DC circuit of a type called Class 4 Power-limited FMPS circuit, which seems to be digitally monitored power and which can stop power supply the moment any arcing or other anomalous condition is detected.

So overall, it seems to be fine and reasonably safe to use this 336V system because of the seemingly certified fault management inbuilt into the VoltServer power supply. Many Active PFC devices officially support DC input (95-400 V typically) in their spec sheets. The ones that don't, might have issues relating to overheating of 2 of the 4 diodes though.

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Now I'm not from the US, and I am not familiar with NEC - so I myself have several questions on this new power class - such as why it reads like company marketing material, or why it needs to be digital (a DC RCCB should do nicely as a quick analog device).

Here's one article on what the NEC regulation on Fault Managed Power System (FMPS)s apparently says.