n+1

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mshields

mshields

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I think of a typical n+1 UPS configuration as 2 UPS. Each capable of handling the entire load if one goes down. But what precisely is the "n" part of n+1 and what precisely is the +1 part?
 
mshields said:
I think of a typical n+1 UPS configuration as 2 UPS. Each capable of handling the entire load if one goes down. But what precisely is the "n" part of n+1 and what precisely is the +1 part?
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"N" is the required quantity, "1" is the number of failures or errors that can be tolerated by the system as designed. The "1" could become a "2" (or even more) if the system were required to tolerate the failure of more than 1 component.
 
Mike,
Sometimes a N+1 is two units, if for example the load was 675kW.
675kW corresponds to a standard unit for most manufacturers. So if you provided two 675kW, it would be N+1.
Not to confuse the issue, but that example could also be considered 2N, since the total load (N) could be picked up by the standby unit. Normally in a 2N configuration each UPS carries half the load at any given time, but can assume the whole load if one unit fails.
 
A UPS (singular) is always (at best!) N.

N+1 is usually used in relationship to modular UPS systems, or paralleled UPSs, where there is one more module or UPS than is required, and so if that fraction of the working system fails, you've still got the ability to supply full power. So for example, a 200KVA load may be serviced by three 100KVA paralleled UPSs, any two modules will do, one left over. If you had four 100KVA modules, you would have N+2, as any two could fail and you would still be OK. Note there is an assumption here that the UPS "system" can fail in parts whilst still operating as a whole. Sometimes this works, sometimes the whole thing dies. I generally don't like N+ systems as I've had to clean up after a couple of these things have failed. A failed N+ system is a dead system.

Better than N+1 modular or paralleled UPSs is 2N, where you have two separate UPSs feeding two separate distribution systems (note this requires each item of load equipment to be dual fed also), so the entire chain can fail, and the load is still supplied. For your hypothetical 200KVA load, you thus need a pair of 200KVA UPSs.

Note the difference in nomenclature between N+(some number) and (some number)N - they are very different beasts. You can do N+ for a Tier 3 facility, but to get Tier 4, you need the compartmentalization offered by the 2N approach. This may seem like a mountain and a molehill, but its a very big (and expensive) deal.
 
Chlorine Rectifiers

Chlorine Rectifiers

We have a Rectifier that is used for Chlorine production. It consists of 4 housings each having 2 sections. Each section has 11 modules. Each module is rated for 1000 Amps. This gives the total Amperage available as 88,000 Amps. Our cell line-ups really only need about 72,000 Amps. We operate in an "N+2" scenario. If two modules fail in a given section, the Amperage of that section drops to a maximum of 9,000. We can operate with two failed modules per section before we have to take a shutdown to replace the faulty modules.

Eric Stromberg, P.E.
 
Having worked with both UPS and DC rectifier battery plants for a couple of decades here is my spin.

N=the number of modules to carry the rated load, plus enough excess capacity to recharge the batteries in 24-hours or less. So +1 or more is for redundancy in the event one of the modules fail.
 
Jim H said:
Most cars have (4) wheels and (1) spare in the trunk.

N+1 :D
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I know you're kidding around, but that really shows the difference between true N+1, which is a hot standby, and not exactly N+1.

Real N+1 is no interruption in service with a single failure. For a car what you need is 2N -- each wheel has a hot standby because there's no mechanism by which you could have a single spare tire cover all four independently operating tires.
 
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