paralleling UPS's

[saltukhan]

hello, ? have a critical load (8 operations room in hospital) and want to feed this load with 2 40 kVA UPS (parallel operation).Maximum load is 80 kva or so, now what if i do this instead of 1 80 kva ups? ups type is online and if i use 2 x40 kva ups it will cost less and i think this will be more reliable, if it is really so it means as i divide the load between 3 4 5 .... ups it will be more reliable and cheaper?

 

[ykee]

If you've got an 80kVA load, two 40kVA UPSs are not more reliable than one 80kVA UPS -- if one of the 40's goes down, you are still down. I can't comment directly on the qaulity of a 40kVA vs a 80kVA, but my gut tells me that the 80kVA will be better. Remember that you usually get what you pay for, especially when it comes to quality.

 

[ScottyUK]

If you can separate your loads then if one 40kVA UPS fails then you only lose half your loads. Whoever is in one of the 4 ops rooms when the power goes out is still in deep 'stuff', but I guess four lawsuits are better than eight. If you want redundancy then 2x 80kVA paralleled or 3x 40kVA paralleled are both reasonable approaches.


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If we learn from our mistakes I'm getting a great education!

 

[saltukhan]

if there are 2 40 kva ups and if one of them is gone and the load is 60 kva it means the load will be transferred to the mains,if the load is less than 40 kva it means we can replace the broken ups without interruption,
well what if the single 80 kva ups is gone?? the load will be transferred to the mains, so i have nothing in my hands, at least the 2 ups option sounds good with loads less than 40 kva.
am i right or just a nonsense?? also the more ups gets bigger cost of unit kva gets bigger, so what is the drawback of using 2 ups instead of a big one???

 

[ScottyUK]

Well, either your load either needs conditioned, clean, continuous power or it doesn't. If it doesn't then let the load take its chance on the utility supply and whatever generation you have. If it does need UPS power then it needs it all the time. 3x 40kVA gives you redundancy. So does 5x 20kVA. I am surprised that an 80kVA unit is more than double the price of 2x 40kVA units if they are otherwise equal. Are the battery capacities identical? Are their electrical performance specs similar?


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If we learn from our mistakes I'm getting a great education!

 

[VTer]

On another note if the two 40kVA UPS’s are paralleled and tied to a common bus feeding a 80kVA load, loosing one will overload the other and trip out the CB protecting it. There is no redundancy with 2 40kVA UPS’s and 80kVA load.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[itsmoked]

I'd go with eight 10kVA units.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[saltukhan]

what i cant understand is that if someone needed clean and conditioned power and used ups does it mean this ups will never bust ? i am speaking of the worst case namely busting of one ups(in both cases one of them fails).
so for the loads above you are right, there is no need for dividing the load to two ups, but for loads below 40 kva per ups it means u can change the busted ups without interruption isnt it so?
i think u might trying to tell me that increasing the ups number will fold the failure probability ? am i right?

 

[itsmoked]

Certainly! More UPS equal more chances of failure. If more than one paralleled is needed to reach the required size, you are seriously reducing the reliability as either one failing takes it ALL down.

Servicing one unit also takes it ALL down.

If you had 8 units you could always service one unit and only take down one bay. This often means you could find a bay down for a day or part of a day. If you have to have four bays down that is going to be a bigger issue.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sibeen]

Maximum load is 80 kva or so, now what if i do this instead of 1 80 kva ups?

If the load is approximately 80 kVA then you certainly shouldn't be using a UPS rated at 80 kVA. Normally about 80% is the maximum amount of steady state load that you want to apply to a UPS - so in this case a UPS rated at 100 kVA would be a better option.


ups type is online and if i use 2 x40 kva ups it will cost less

No it wont, not if both the 80 kVA and the 40 kVA units are of the same technology and with the same battery autonomy (back-up time) specified. As the UPS size increases, the cost per kVA decreases.

Also the installation costs for two UPSs rather than one will be considerably higher.

and i think this will be more reliable,

No. If it takes both UPSs to run the load then the failure of one UPS will mean that the load will transfer to the bypass (reserve) supply. Effectively you are halving your mean time between failure (MTBF). So in effect, under this scenario, your reliability is halved.


There should only be two scenarios where parallel UPS systems are used:

1. For redundancy. The failure of one module doesn't effect the system as a whole and the site remains on UPS power (N+1).

2. Upgrading. On sites where the existing UPS has reached its limit another UPS can be placed in parallel to increase power.

 

[raithrovers1]

Don't know if I'm missing something here but I wouldn't want to be operated on in one of these theatres with 2 x 40kva UPS's in parallel and have to rely on mains power if one UPS fails.
I would think that an operating theatre would need as much "belts and braces" as possible (you would have to look at worse case scenario which would be one UPS failing, losing mains power and then the emergency generator not starting").
The only way to go in my opininion is 2 x 80kva UPS's in parallel or the 8 x 10kva as mentioned before.

UPS engineer

http://www.powerups.co.uk

 

[fraserjim]

Actually, competent UPS manufacturers give you the capabily of paralleling more than two UPS modules.

I don't have a pricelist handy, but if you really need N+1 redundancy, looks at two 80kVA or three 40kVA modules.

Don't forget a method of completely manually bypassing your UPSs, if there is a problem with the paralleling gear (usually a panelboard with a main breaker and four branch breakers). The branch breakers are feed from the output of the UPS modules. The main is feed from the utility. There are output lugs that feed to your load panelboards.

Don't forget high quality TVSS (now called SPD surge protection device by UL). This will be to protect the front end of the UPSs from multiple catastrophic utility hits and protect your loads if you are in bypass.

Fraser Jim

 

[SyncCheck]

Hello saltukhan,

Paralleling UPS's where either one cant handle the load by them self is a bad idea and a really bad idea in a hospital.The point of paralleling UPS's as some people have already noted is for redundancy and redundancy only when the units are physically separated,the point of Paralleling UPS's is if one UPS gos down the next UPS picks up the load with no problem.

Remember the UPS is the most important device in an uninterruptible power system,it is the first device that takes action to keep the load up so the ATS and generator(s) can start and pick up the load.

I would make shure that the UPS is capable of supplying 125% overload and also has an external bypass.

I have designed hospital backup power systems before and I have used a UPS per each operating room,because if their is some sort of a disaster like a flood that takes out your electrical room you still want the operating room to be up regardless...

 

[saltukhan]

so as regards operating rooms, what do standarts say? is it reasonable to feed op.rooms seperately or just via a single big UPS? is there any commonplace idea or standart?

 

[MikiBg]

Saltukhan,
Be aware that N+1 paralleling isn't only option to achieve true redundancy. Consider "hot standby" connection (my favorite). That means one unit output connected to other unit bypass input. No problems with communication and load sharing between units. By some calculations, this is a topology with highest availability (MTBF/(MTBF+MTTR). Consider 80+80kVA, or 100+100kVA (depends on load reserve) hot standby as option for centralized protection. The rest is matter of B/C analysis.

 

[ScottyUK]

Hmm, burn-down of the first unit tends to have a fairly negative effect on the availability of the second. I've only seen this once, and the facilities manager at the London data centre was more than a little unhappy that one UPS failure had taken down his operation when he had believed that he had a redundant system. We as consultant engineers were brought in to assess the system by the company's insurers who were equally unhappy. My old boss spent a bit of time in and out of court rooms as expert witness.

Most modern architectures are moving away from this topology because of the inherent dependence on the static switch of one UPS for the partner UPS to function. In my opinion 'true redundancy' is pretty much what it doesn't offer.


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If we learn from our mistakes I'm getting a great education!

 

[MikiBg]

Of course, experiences are different. It is always at least a little bit different in practise. I had problems with parallel connections. Communication fault or one unit internal fault transfered both units on bypass, i.e. exposed a load directly to grid. Static bypass of first unit is a week spot of hot standby, but at least you can minimize down time by transfering load on second unit via manual bypass. In case of parallel connection, coming back on inverter could be more complicated.