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Generator Redundancy Operating Philosophy (N+1)

07/04/2012 9:40 PM

Hi Guys,

I always come across N, N+1,N+2 when it comes to designing of genset system.

What does all these terminology mean?

Is there such thing as a N-1?

As my client now is asking for a N-1 genset system design.

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#1

Re: Generator Redundancy Operating Philosophy (N+1)

07/04/2012 10:02 PM

Liukawa,

N+1 refers to the redundancy built into the design.

'N' = Maximum load or rated loading for the site. Say 2 X 1.5MVA Diesel Generators for example.

'+1" = enough generator capacity to support that load if one unit fails. In this example that would be a third 1.5MVA Diesel Generator

So if you require 6 Generators then you install 7 for an N+1, for N+2 you you need to install 8 in this example.

As for N-1...Translates to lights out.... If you need 6MVA and you have less generator capacity than 6MVA, the generators will all trip out on overload.

The only way an N-1 could work would be load-shedding, where non-critical and non-important system are not covered by the Generators and allowed to remain powered off until the mains power is restored.

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Sapper

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#3
In reply to #1

Re: Generator Redundancy Operating Philosophy (N+1)

07/05/2012 12:08 AM

Instead of N+1,won't it be economical to have N/2+N/2+N/2,with one of them as standby?.

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#4
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Re: Generator Redundancy Operating Philosophy (N+1)

07/05/2012 5:31 PM

No, because then you would need 4 generators instead of 3, 5 instead of 4 and so on.

It is better to go with the minimum number of larger generators so that the Generators are running in the minimum load sweet spot (35-45%) rather than having more generators and they are all running at 25% well below the minimum.

As per previous discussions, diesel generators will have a minimum load requirement to prevent glazing of bores, damaged seals etc.

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Sapper

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#5
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Re: Generator Redundancy Operating Philosophy (N+1)

07/05/2012 10:34 PM

You don't need 4,3 would be ok because most probably only 1(N/2) may breakdown & the standby set will take over. Meanwhile you repair the defective one and keep it ready as standby.

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#9
In reply to #1

Re: Generator Redundancy Operating Philosophy (N+1)

07/06/2012 9:46 AM

Sapper,

I must respectfully disagree with portions of your statement. N-1 is standard terminology for determining the reliability of a system in contingency analysis, where 'N' is the total number of installed units. With N-1 design, the lost of the most critical (usually the largest) single component of a system will not cause any loss of function.

For generation, a system designed to N-1 reliability should be able to supply all loads within voltage and frequency tolerances after losing the largest single generator. For a transmission grid, loss of the most heavily loaded line will not cause overloading of any other line or a voltage deviation beyond design tolerance. Critical data centers are often designed to N/2 level, meaning loss of 50% of any particular component (generator, chiller, server, UPS, etc.) will not cause loss of data or reduce processing capacity below unrestricted operational requirements.

Here are some references which may help explain:

http://www.ea.govt.nz/industry/transmission/code-regulations/grid-reliability-standards/

http://www.nerc.com/files/concepts_v1.0.2.pdf

http://www.nationalgrid.com/NR/rdonlyres/8B8287EC-4976-4051-B690-4BD5D789C911/33402/2GridReliabilityStandardsNewZealand.pdf

http://www.idahopower.com/pdfs/aboutus/planningforfuture/projectnews/tvep/infoarchive/example.pdf

http://www.ieor.berkeley.edu/~oren/pubs/Economic_Analysis_N1_2009.pdf

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#10
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Re: Generator Redundancy Operating Philosophy (N+1)

07/06/2012 11:30 AM

You say 'tomahto' I say 'tomayto', aren't the two concepts the same (assuming we're talking about identically sized units)?

As others has said, if the customer wants to talk N-1, then speak to them in their "language". Other customers could very well want to talk N+1.

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#12
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Re: Generator Redundancy Operating Philosophy (N+1)

07/08/2012 5:49 PM

pwr2thepeople,

After reading the links you provided, seems I learnt something today... Thanks.

In data centres, redundancy is only referred to as "N+1,2,3etc"

In Power generation and distribution systems, seems that redundancy is defined as either "N+" or "N/" or "N-" depending on perspective, design and possible where you got your higher learning from.

Always a good day when you learn something!

Regards,
Sapper.

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#13
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Re: Generator Redundancy Operating Philosophy (N+1)

07/08/2012 10:47 PM

Computer data or voice or video could be recorded or stored using standby power but electrical energy storage is the biggest challenge requiring spare capacity or standby.

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#14
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Re: Generator Redundancy Operating Philosophy (N+1)

07/08/2012 11:18 PM

Not talking about the Data.

This is about Data Centre Infrastructure (Power, HVAC & Mechanical) and Utility Distribution.

Regards,
Sapper

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#2

Re: Generator Redundancy Operating Philosophy (N+1)

07/04/2012 11:08 PM

If I were you, I'd do a quick search for "N-1 generators". You won't find any, but you may find an explaination of "N" numbers as they relate to multiple generators.

You should know this for your next client, in case you are pressed for time.

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#6

Re: Generator Redundancy Operating Philosophy (N+1)

07/05/2012 10:40 PM

I wrote in another thread that Rule 1 is The Client is always right and if in doubt refer to rule 1. Obviously I'll have to revise rule 1!!!!!

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#7

Re: Generator Redundancy Operating Philosophy (N+1)

07/05/2012 10:52 PM

N-1 is common when referring to electrical redundancy in transmission and distribution sysstems. I would read the customer as meaning that they want N+1 mechanical redundancy.

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#8

Re: Generator Redundancy Operating Philosophy (N+1)

07/06/2012 8:24 AM

N-1 or N+1 terms have different references. When the operations engineer says N-1 design, he is meaning that the system is stable with trip of one of the machines.

When the designer says N+1,he is meaning that if N machines are required to supply the load, one additional (N+1) machine has been included in the design.

Thus, we can say for Operation security under N-1 contingency (loss or trip of one of the machines), the system needs to be designed with N+1 (required no. of machines to meet the load + 1 extra) machines.

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#15
In reply to #8

Re: Generator Redundancy Operating Philosophy (N+1)

01/22/2016 3:14 AM

Hi raqhun,

I would like to ask how I will size an equipment with "N+1" or "N-1"? Is there any criteria or standard to use?

Is there any standard to be used in choosing of "N" contingency considering it is not Client preferences.

thank you in advance.

Regards,

Emer2830

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#11

Re: Generator Redundancy Operating Philosophy (N+1)

07/06/2012 11:17 PM

N+1 is a redundancy descriptor. ie N in operation and 1 in standby

N-1 is a capacity descriptor where One faulty unit/component doesn't lead to a following overload, fault or outage of another unit/component in the regarded network

eg

two synchronised generators each capable of supporting the full load and thereby running at 50% each under normal operation and any one at 100% in the event of a single failure. would be N-1.

one generator operating at 100% with a standby unit of the same capacity waiting to take the full load in the event of the first croaking would be N+1

You could have N-1 capacity with N+1 redundancy. eg two 50% synched generators with a 50% standby.

(%s for illustration only)

I think I got this right......

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Brave Sir Robin (1); cristle (1); Emer2830 (1); gckent (1); lyn (1); pnaban (3); pwr2thepeople (1); raghun (1); Sapper (4); Wal (1)

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