AC backfeed protection

[elektrinis]

Not building anything, just testing out an idea.
Let's say there are several remote users with unreliable common AC line, which also drops low if overpowered.
The idea is to install a battery solution with bidirectional AC-DC converter to assist the AC line when voltage drops below set level and recharge when AC voltage is high. This would simply tap in to existing line. Imagine it as simply plugging a device in your AC socket...

It also must work as emergency backup power source in case of AC outage.
But here comes a problem: when AC is out, it is out for several users, but we want to protect only that one user. Mainly because, if AC is down, the power will backfeed to several users and they will overpower the battery.

So I think we can split the question to:
1. Is there a jellybean part, like an active rectifier or something, to prevent AC backfeed?
2. Should there be some kind of different solution? Like power measurement on primary side and cutting line off if it is negative? And then switching back on if AC levels back to normal.
3. I know there are similar off the shelf battery solutions out there, one in the UK. They market it as plug in AC socket at your home and it takes care of everything. Anyone know how it works?

[Gregg]

To answer your questions:
1.   Uninterruptible power supplies fit the application and there are many to choose from.  Get a name brand and known reliable model.  Some of the older ones are more long term reliable than the newer ones.  Many focus on bells and whistles that are of little use to your application and are mainly marketing tactics for up-selling.  Make sure it is true sine wave and not some partial sine substitute.  Most good UPSs bypass the inverter section until the input power falls below a specified value; however the inverter is always running and wastes some power.
2.   You could always roll your own, but unless you call it a learning experience your time and efforts would probably not be cost effective.  A good used unit may be the cheapest option but most likely would need batteries.  Lead-acid batteries are usually used in UPS units and with a little ingenuity one could substitute the stock small and expensive batteries for some larger ones housed separately.  You could even charge the batteries with solar.
3.   The device you mention is probably a small cheap household marketed UPS with less than stellar design or components. 

[Jeroen3]

1. A dual conversion UPS.
2. Reverse power protection relays.
3. See 1.


Where bypass is for maintenance only.

[David Hess]

Grid-tie inverters do what you describe but generally do not provide backup AC power in the event of a complete AC power loss.  There is nothing preventing this and some do.

[elektrinis]

Active UPS is not the way to go, due to 10% or so energy going to waste.
So the only way is to monitor input power and cut it off it negative numbers show up?

[NiHaoMike]

What is the load? If it's electronic like most critical loads are, DC might be the way to go.

[Bratster]

How come you don't want to use a standard UPS? Seems like that is the optimum solution to me. But I'm still not 100% sure exactly what you're trying to do.

A standard UPS will just sit there float charging its batteries, until there is power cut or the power line goes out of spec and then it will switch over to powering your loads. When the power is good again it will switch back to charging its battery.

The standard UPS (unlike a dual conversion/online UPS) is only going to be using power to keep its battery charged, the AC line will pass straight through it until there's a power issue.


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[Red Squirrel]

I don't know about the legalities of this but I suppose a grid tie inverter with a battery charger could work. Essentially when the grid is reliable, the batteries are on float being charged with the charger (charger can simply be a fixed voltage psu). If grid voltage drops, the charger turns off and batteries power the grid tie inverter.  I imagine those allow you to limit the current, so you could limit the current and slowly add more current until the grid voltage goes back to normal.   If everyone on the grid had this setup it would basically act as a capacitor for the grid to smooth out brownouts.    In fact IMO they should do this at large scale, as it would make green energy more viable. 

From an individual standpoint concerned about your own equipment, it makes more sense to just use a UPS though.  Dual conversion is the most reliable way to go as there is no delay and if designed right can run on a very wide input voltage range. 

Basically:  AC mains -> Rectifiers outputting a fixed regulated voltage like 13.5v (for 12v system) -> batteries -> inverter -> loads.

The AC mains can be super whacky or drop or anything, and the rectifiers will continue to do their best at outputting a clean 13.5v.  But should that output stop, the batteries will just continue to provide it as they are in parallel with it.   This is basically how the telcos do it but a 48v system with 54v float. You end up with super reliable power.  You can easily switch from AC mains to a generator as well.  Most small central offices will do that, they'll have 8+ hours of battery run time, and when voltage starts to get too low a generator is dispatched and plugged into the building.  The bigger COs will have standbys.

[jbb]

The double conversion UPS would be very effective in this case. The nice thing is that the rectifier only rectifies, so considerations like anti-islanding don’t apply.

If you want to clean up the power line, an Active Power Filter is a possibility. However, these are specialised kit and you would need to talk to the lines company.