Paralleling two UPS

[davva]

Has anyone ever paralleled two UPS on the ac output? I realise it could be done quite easily if converted to dc but I was wondering if anyone has ever implemented two independent UPS in parallel synchronised on the ac output?

Thanks in advance for any replies.

 

[advidana]

Not a good idea. Check with the ups supplier for the reason. Out of phase and polarity may be the some of the reasons.

 

[itsmoked]

I'm sure you can do it with UPSi that are set up to run synchronously with each other. But two not specifically built for it.. I would say no!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

There are some invertors capable of running in series to produce 120/240 volts. They are provided with connectors so that a synchronizing cable may be used to ensure that they work together.
respectfully

 

[davidbeach]

It is done all the time in large installations. Paralleling outputs is the only way to get any redundancy.

 

[rcw retired EE]

As David said, it is done a lot on the large systems, but the units have to be designed for it. The transfer and synchronizing logic and equipment need a fair amount of engineering.

Another way of getting about the same reliability is to split the load on two UPS's and have a down stream static switch to swap loads for maintenance or failure of one UPS. One UPS can be used as the reference source for the other to keep them in synch. Again, check with the UPS vendor engineers to get the correct equipment.

 

[BJC]

davidbeach is right.
I have seen 3 paralled. The critical factor is the controlls, the inter-UPS communications and the vendors software.
Three can be set to share load and if one goes away the other two shift to 100%.
Check your vendor and ask for references to existing working installations. You can't do it with a UPS you buy at Wal-Mart.
It's a Mercedes Benze type thing, your not going to get it for Chevrolet prices.

 

[Kiribanda]

davva,

Yes, it is possible and done. Paralleling can be done for 1) Redundancy 2) Capacity.

Therefore, a software has to control the two outputs of the UPS.

Leading UPS manufacuturers like GE, MITSUBISHI and GUTOR have got these systems in service. You can contact them and with your required out put voltage and the rating.

Kiri

 

[slavag]

Hi Davva.
I think unpossible add somethings to previos posts
I would like only add several small points.
Possible option:
1. Hot stand bay connection.
2. Redandant parallel operation
3. Parallel operation.
and today very popular use second UPS as power source for first UPS bypass.

 

[ScottyUK]

If the two UPS units are either not intended for paralleling or are different then one option might be to feed the bypass of one from the output of the other. Not as good as true parallel operation but gives some of the benefits.


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[davva]

Just to confirm the intention is to have two UPS in parallel for redundancy requirements such that if one UPS fails there would be no loss of supply to the critical load.

I did a search on the internet before posting the topic and I only found hits on suppliers that build the redundancy in the inverter stage of the UPS. Our client is requiring total (n+1) redundancy for the UPS, which I take as basically being 2 UPS operating in parallel with synchronising controls between the inverter output.

I was trying to get a feel for if anyone has done this (which it seems they have) and how complicated it would be.

I am only writing a specification for the UPS system (not implimenting the detailed design). It is likely that the UPS will be procured in China and I wanted to ensure how feasible it would be.

Slavag ..I quite like the idea of using a UPS (No.2) as the bypass supply for UPS (No.1).

 

[rbulsara]

Hire consultants experienced in UPS system designs.

The two "independent" UPS system cannot be nor are they intended or engineered to be parallled like two generator sets.

However as davidbeach indicated all large UPS systems uses multi-module systems which are paralleld for either capacity or dedundnacy, but they are "engineered" systems.

You can visit websites of leading UPS mfrs such as Liebert and Powerware if in the USA. Beyond that hire consultants!!!

 

[ScottyUK]

GE are one vendor who have managed implement redundancy where no module is critical to system operation, i.e. there isn't a 'master' unit with a number of slaves. There are others too, but I know where the link is for the GE one:

http://www.geindustrial.com/publibrary/checkout/38652.30055.3474.36017/generic/GEA-D1005.pdf

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[rbulsara]

ScottyUK:

This may be a tangent on this thread; we just installed largest GE UPS system in the USA. We have much more experience with "other" systems. Suffice to say that GE is not the first one to do what they do plus their arrangement has its own issues vs. "classic" system cabinet and multiple module system. (Plus GE only acquired a Swiss mfr.). We ended up making a custom PLC controlled UPS output switchgear that will safely bypass the system with human action only at one location.

Even in “classic” multi-module systems, there is no master/slave arrangement. Both type of systems have a common "collector" bus. The key difference is that the "classic" parallel system has a common single static bypass switch and a common brain (system cabinet) whereas GE or Mitsubishi (who in fact led the "new" paralleling system without a system cabinet) has the static bypass switch in each module and they “all” must transfer in sync to get system to bypass, although each module has the “brain”. I believe MGE is following this method too.

Even one of the conventional multi-module system mfr. (in the USA) claims that when a module’s communication is disconnected from the system cabinet, the module will still stay in operation and parallel with others as long as it is connected to the load bus.

There is much more to this as you know, but I do not want to hi-jack this thread.

 

[ScottyUK]

Yeah, that was IMV's design prior to GE giving it a new paintjob and calling it the 'Signature' line (oh, and they replaced all the good quality components in the IMV design with GE ones instead )

What problems did you find with GE's design? We've had a few problems, mainly with hardware manufacturing problems which should have been designed out and a couple of QA issues. How big is this monster you've just done? Must be multi-MVA to win that title.


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[rbulsara]

ScottyUK:

Two(2)-2700KW systems and provision to add one more.

Its not bad, just different and you need to address some issues.

 

[ScottyUK]

Wow, that is big. I haven't seen a solid state UPS bank even close to that size before, only rotary units. Didn't GE have a 'UPS in a trailer' solution for multi-MW applications a few years ago? I think that was rotary-based but might be wrong. Sometimes I can't remember what day it is, let alone what I saw years ago!


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[rbulsara]

ScottyUK:
Yes, they (and some others too )had some '"trailer" solutios in hope for roping in business during the IT bubble around 2000 but it burst miserably. Never seen or investigated one. We only get involved with regular data centers and most of systems are large, 1500kW and above.

 

[ruggedscot]

I quite like the idea of using a UPS (No.2) as the bypass supply for UPS (No.1)

Hmmm not such a great idea as it does not give you true n+1

If your 1st UPS has a catastrophic failure then it will affect both the UPS units and you will be left without protection. So no I would avoid doing it this way.

For n+1 to exist you have to keep both UPS units seperate and then ensure that you have electroncis to support the network should you have an issue with one of the UPS units. Also if your UPS has an integral bypass function this may operate if your UPS has an issue taking out the other UPS unit. So there has to be alot of communication between the UPS units. Customer also has to be educated so that they know that although they have say two 500kW UPS units together they only have a support factor of 500kW. Ive seen places where they start to eat into this and then complain when the maintenance is required and they find out that they have to run on raw utility as the one UPS wont support the load. Process managers get really jittery when they are on raw utility !

Best way is using these two UPS units configured in parallel redundant mode and then you will be covered for all eventualities.

Its not a complicated thing actually and there are plenty of suppliers out there than can design and install a functional system for you to meet with your requirements.

rugged

 

[ScottyUK]

rugged,

You are assuming that this is a new installation. The option presented was a means of adding some redundancy to an existing system with no parallel operating capability. But yes, it definitely has some flaws if the primary unit has a bad day. A few manufacturers were offering this as a standard solution a few years ago - I know Merlin Gerin was one - before true parallel operation became more commonplace.

You are a brave man to tell a customer that "you will be covered for all eventualities" with the new UPS. If you commit that to writing then you are plain daft because the lawyers will have a field day when it goes wrong (and it will).



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