switching between power supplies

[john russell]

is this possible?
i have a customer has has a large factory, he has 2 x 800amp supplies coming into the site and would like to switch between suppliers dependant on which is offering the cheapest rate at the time
he has automated software that will dictate when he wants switch between supplies
i need to be able to remotely control a plc that will switch the supplies
he has a ups system and generators as back up
i was thinking of using 2 x safety contactors on each supply controlled through pilz safety relays

any thoughts would be much appreciated

 

[A_G]

Need to determine when is the equipment in a safe state to actually perform the switching of the power supplies. In other words, determine what interlocks you need before switching. For example, the machine has to be down/not running production I assume. Also need to figure out how the PLC will know that the switching was successful. Maybe a couple PLC inputs, one indicating that power is coming from one power supply, another PLC input indicating that power is coming from another power supply. If the power supplies have error/fault diagnostics, try to use those as PLC inputs also.

 

[john russell]

yep it really needs to be tied down so that the supplies are kept well apart

 

[I_Automation]

I do this to run my air conditioner compressor when the utility shuts off the controlled, cheaper, meter. I go to the always on (as always as the electrical utility in Detroit provides) higher priced electricity.


Since this is single phase and the compressor isn't supposed to be started when pressurized I have a time delay to restart, similar to the compressor saver setting on my thermostat.


Keeping that in mind I wonder about the phasing of the 3 phase electric (unless these are 800A DC supplies)


If the phasing doesn't exactly match, a quick changeover could be a phasing problem for running motors, and could trip faults in VFD's and servo controls.
EDIT: I have a customer that has a shop with 240V single phase power and runs a 3 phase converter when he needs it for a large saw. his air compressor's VFD will run off single or three phase, but if it's on when he starts or stops the the 3 phase converter the VFD faults.


If the shop can go dark for 5 seconds then power back up on the other service it could work, but no shop is going to accept that 5 seconds when they are open.

 

[Levi G]

I would look at Asco Automatic Tranfer Switches. You wouldn't even notice the changeover.

 

[Steve Bailey]

Can you be sure that the lower cost source is synchronized with the higher cost source? You should probably duplicate the existing infrastructure for transferring between the generator and the grid.
Then when you decide to switch between sources, you first transfer the plant to the generator, disconnect the generator from the first source, sync the generator to the second source and transfer the plant back from the generator to the second source.
Regardless, the two utilities involved will likely have a lot to say about what you have to do.

 

[ASF]

You need an automatic transfer switch (ATS). It's a specific assembly designed to do exactly what you're proposing. Although usually it's to switch between mains supply and generator backup, rather than mains 1 and mains 2.

I agree with Steve that (a) you need to be really confident about synchronisation between supplies, and (b) you might be better doing a three-way switch via a generator.

 

[GaryS]

ThatÂ’s interesting, I would like to see how they implement that system with multiple sources supplying power to the same consumer
I canÂ’t imagine multiple sets of power wires throughout country, what a mess that would make. It would look like the pictures I have seen of the old telephone system in the cities in the US what a mess.
Here where I am at, we have has the ability to select who we purchases the power from but itÂ’s delivered by the local provider. We donÂ’t have the ability to switch at will we have to set that up with the provider and the delivery company. Here just about every power generation company is connected to the national grid and send power into it, the consumer receive their power from the grid. There are a few plants that have a dual dedicated lines ( I have work on some of those systems) they have their own substation. With 2 feed lines to the substation at 66KV 200 Amp tap to the substation if a line need shut down to be worked on they switch to the other supply line. Bothe feel line are phased match so when they switch the phase rotation is the same.
It would be great if we had a picture or a drawing of the proposed connections to help us better understand what you have
When you say 800 amp feeds I assume that you mean at your local standard 240v 3 Phase 50 HZ these are important details so people can understand and help you. For you application I believe you are switching between the local grid and an on site generated power a transfer switch will work but you will have to deal with the transfer delay and of course the safety requirements

 

[Saffa]

What is the total site load? Is it less than 800A? Seems excessive having two 800A supplies if your total load is less than 800A.

The fact that there are also generators complicates things. If it was just a case of "pick one of the two 800A supplies" then this is trivial using mech and elec interlocked MCCBs.

3 or more sources is less common. I know this is possible to do 3 source interlocking with Schneider NW ACBs, but it's not possible with their NS or NSX range.