Converting 3-phase to single phase with resilience

[Caecilius]

I suspect this isn't possible, but I thought I'd ask just in case...

I've got a large office with a 3-phase electrical supply. But I don't have any 3-phase equipment; I split the 3 phases to 3 x single phase sub-mains feeding CUs on each floor.

The original building has a 3x60A supply, but that was upgraded to a

3x100A supply about five years ago. I don't think our total draw ever exceeds 100A, so in theory we could operate from just one of the phases.

We have regular power outages, all of which are acknowledged by UK power networks and normally involve some sort of problem at a substation. But strangely they don't always affect all three phases. Hence my question:

Is there a way to convert a three-phase supply to some sort of redundant single-phase supply? I wouldn't want something that involves power-loss, as the 3-phase supply comes into the basement and I don't have any easy way of getting rid of heat from there; but even some sort of "throw the switch Igor" switchover solution would be better than I've got at the moment when I end up losing a random floor (and typically the CH boiler as well, which means we need to close the office when it gets too cold).

[Harry Bloomfield]

Caecilius laid this down on his screen :

It should be possible to have (3x) interlocked contactors installed, so that only one could be active and providing the entire supply, at anyone time and to the entire building. That should make it possible for it to sequence, through the phases should one drop out. I have never seen or heard of it being done, but it is certainly practical to do.

There would be a short interruption in the supply, during the switch over.

Best talk to the technical department of your supplier about the possibilities.

You would still be paying for the 3x supplies, despite only using one at any time.

[tabbypurr]

Relays or rather contactors could do this easily. But do you need (to pay for) a 3 phase supply?

NT

[Brian Gaff]

Sounds like you need a very big changeover switch break before make three way! I cannot see how you could easily make this automatic or combine them considering the phase shift involved. Brian

[Brian Gaff]

It does seem strange to me that power outages are often enough to be a problem. Maybe the infrastructure where you are needs some serious work. Might be an idea to see how many locals this affects and get on to the power network people to fix it properly rather than messing around with the supply. After all in my road the phases are fed to alternate houses so if this did happen a lot every third house would lose power, and they don't. Brian

[harry]

All you need is three contactors and a single pole, three way (rotary?) switch.

It would be ideal if they were also mechanically interlocked.

[harry]

A reversing switch for a three phase motor could probably be modified.

[Harry Bloomfield]

harry used his keyboard to write :

I would suggest both electrical and mechanical interlocking are essential.

[Nightjar]

How about three double throw contactors, each powered by one phase and wired so that it passes that phase through to its associated CU when powered, but switches to another phase when the power fails? It should work completely automatically, with minimum down time, and they shouldn't draw enough power to cause you heat gain problems.

If you want to allow for the possibility of two out of three phases failing, a second contactor in series after the first and powered by the output from the first would allow you to switch in the phase that isn't connected to either pole of the first if both of those fail.

[Caecilius]

Strange it may be, but it keeps happening. The power was out for the best part of two days this week in two seperate outages.

The UK power networks engineer said that they'd finally worked out the problem, which was apparently caused by two substations being linked when no such link was shown on the map.

But we've had promises of "it's all fixed now" for a few years, and each year there's one or two months with outages.

[Caecilius]

Probably not, but I beleive it's just a higher standing charge which isn't a big expense for the business. And having 3-phase adds flexibility in case I ever need more than 100A (unlikely) or want to sell off a leasehold for one floor.

[charles]

That could be a problem with copied maps, when the copying was done by tracing the old one. Short links did get misses. When I was with SESEB (getting on for 60 years ago), one customer wanted to upgrade to a 3 phase supply - but the maps showed he had no supply at all.

[Caecilius]

Thanks. That sounds like it might work. I'll sketch it out and see if it's workable.

[0345.86.86.888]

I currently have 3-phase issues at the shop.

1 phase is down on voltage, the 2nd which we were connected to started browning out randomly a few years back so we were switched to the 3rd phase which has now started breaking down giving intermittent voltage drops.

A fault has been identified as 5m from shop but to fix it means knocking out all the shops/flats in the row which they aren't keen on doing but it's now been escalated to the fat-controller (I think that's who he said) for planning apparently..

[Harry Bloomfield]

Brian Gaff submitted this idea :

Its no biggie to arrange, phase shift has nothing to do with it as only a single phase is needed at any one time.

[ARW]

I like that idea.

[Harry Bloomfield]

It happens that Caecilius formulated :

Be very aware that contacts can and do sometimes weld up, so mechanical interlocking is essential.

[Nightjar]

What should be interlocked? The whole idea of the setup is that each contactor operates separately from the others.

[Chris J Dixon]

The potential hazard is that if the main contacts on one of the contactors weld closed, then the system closes another contactor, it thus creates a potential phase-phase connection.

If the contactor has auxiliary contacts, so long as their state mirrors the main contacts if they are welded, then the safety interlocking can be electrical.

Chris

[Harry Bloomfield]

Chris J Dixon formulated the question :

Exactly! I would also want to provide some mechanical interlocking too, to be certain it never could happen. I have come across mechanical interlocking on star/delta motor starter systems, so I know it can be done.