Redundancy N-1 and N+1

[Kani77]

Hi,
Can you please help me to understand the different between N+1 and N-1 redundancy. If the actual requirement is 75kW heater. What will be the combination for N-1 and N+1 redundancy system

Thanks

 

[waross]

For generators, N+1 means that one more generator than is needed is connected in parallel.
Thus if one set fails the remaining sets are able to carry the load.
Thus if the load requires 3 sets in parallel, N+! will be 4 sets in parallel.
I don't know what N-1 is used for.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Kinda the reverse...

Say you have a large nuclear generating station with a number of paths leading away from it to its receiving grid; the N-1 criterion states that the remaining circuits will not be overloaded upon the loss of the circuit with the highest export capacity.

Of course circuits are expensive and frequently politically controversial, so the alternative of having a Special Protective Scheme that will immediately trip selected generation at that station to eliminate circuit overload upon first contingency, while simultaneously tripping an equivalent amount of load out on the grid so as to maintain frequency stability, becomes quite fiscally attractive.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[jraef]

As it would relate to equipment like a heater, N-1 would mean the 75kW system is comprised of multiple units working to attain 75kW and if you lose one unit, the spare for it is already there, ready to be added in as a replacement to the failed component. So for example if the 75kW capacity of your heater was comprised of 3 x 25kW heater elements, N-1 would mean that you actually had 4 x 25kW heater elements, but normally only used 3. So if you lost one, you have a spare 25kW heating element on standby ready to connect to replace the failed component.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Kani77]

Thanks all for your replies.

For Jraef: if we are using 4x25W, this is N+1. Isn't it?

 

[argotier]

I wonder if both refers to the same, I've always known as "N-1 redundancy" the waross example (the same with jraef's).

 

[Kani77]

What i think n+1 shall have all units same. as per jraef ( 4 x 25kW). N-1 can be any combination. If any unit fails, non of other units shall be overloaded. example: for 75kVA requirement, we can use 40+40+35 three gen set in parallel. I think this is what Crshears means above. any comments?

 

[waross]

What is"N"?
I have seen examples where "N" referred to the number of units needed.
So, if 4 units are needed then N+1 = 5 units.
Others have seen examples where "N" referred to the number of units online.
So, if 5 units are online, N-1 would mean only 4 units can handle the load.
Both N+1 and N-1 may mean one more unit than is needed.
You have to check the context.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[argotier]

The N-1 redundancy I was taught in my time, stated that the "1" is the largest unit in the system that could fail.... wich is what crshears wrote above, now that I paid attention

 

[waross]

I Googled
"n-1 redundancy"
The hits were 5:1 for N+1
A hit for N-1 yielded this gem from Evidian.-

The SafeKit N-1 redundancy cluster is simply configured by deploying N mirror modules on N+1 servers.

Apart from Evidian, the terms are basically the same.
Some industries use N+1, some industries use N-1.
In the context of data servers Evidian manages to use both. This seems to be the lone exception.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Colloquially, N+1 seems to be referring to "the way we run things so as to be prepared for the MSSC" [ Most Severe Single Contingency ].

Conversely, N-1 seems to be "the situation we'll be into if the MSSC occurs".

Coins have two sides...yeah, yeah, I know I can't say that categorically, because, after all, "it depends."

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Kani77]

Thanks all!
According to your replies and my search, I came to a conclusion

System N+1: N is required units and +1 will be standby. If one of N fails, the standby unit will kick on.
75kVA Generator Example: 3x 25kVA online and one 25kVA is standby. or 40kVA+35kVA online and another 40kVA is standby.

System N-1: N is online units, and "N-1" means, if one of N fail, rest of units shall handle the load.
75kVA Generator Example: 2x40kVA+35kVA are online. any of fail, we still have 75kVA.

What do you think?

 

[waross]

N+1 means that the extra unit is already online.
All units producing less than maximum output.
If one fails there is available capacity to support the load.
Some industries use N+1, some use N-1.
Basically the same.
If one unit fails there is enough capacity to support the load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Install N+1 so that you can go into an N-1 situation with no disruption. This tread is a simple concept made difficult.