Backup Generator Reliability

Suppose I need a backup generator in the 1-2 MW range, output voltage about 4 kV.  It would be used very rarely (and then only for a day or a few days), but would need to be very reliable. The fuel choice doesn't matter.


1)  Is there any data on reliability of such generators -- gross numbers such as overall failure rate (/hr) or MTTF (mean time to failure)?  Would you include such data in your reply, please? Even approximate -- order of magnitude -- data would help, if nothing else is available.

2)  Is finer data available also:  most important (frequent) failure modes of such generators, the MTTF or failure rate for each main failure mode and the MTTR (mean time to repair) for each main failure mode?  If so, can you quote it?  Do the data in 1) and 2) strongly depend on manufacturer?

3)   Suppose the anticipated usage of this generator is very rare, as in, hopefully, never. Suppose that I don't even have it on my site (I think I have plenty of backup power, but then there are those rare events), but if I need it, it's very important that I get it. Would this be something that is in stock somewhere in the country and can be flown to the site and installed in a few hours, in an emergency, (I do have several hours lead time), or is this something that must be ordered ahead of time, with long lead times to delivery?


Thanks for the answer. 


Re: Backup Generator Reliability


 I am assuming that 4kv range is 4160? as for availability of a rental gen, that is going to depend on where you are located. if you had a step up transformer (480 to 4160) for emergency generator tie in, the chances for finding a rental gen in the 1 to 2 MW range is greatly improved.

 In response to the reliability factor , being a standbye unit the greatest cost incurred is usually maintenence and periodic testing common failures and estmates on repair time and cost are all a swag (scientific wild a@# guess)at best. Common failures are usually caught with a proactive maintenance program (such as blockheater, batttery failure, batt charger problems, leaks, fuel maint.)

 Just for general knowledge, do you have multiple utility grid service entrances that give you redundancy to minimize your exposure to power failures?Or is the grid that stable where you are?

 Hope this helps some.


Re: Backup Generator Reliability

Thank you, Tech 1.  You're right, it's 4,160 V.  There is also a 480V network on site, which gets its power from 4,160 V, whether or not this makes it amenable to backfeeding through the 480 V network I don't know, I'll have to check.  And yes, there is a great redundancy in the electric grid connections, and there is backup diesel power.


In some area-wide events (e.g., hurricanes) the electric grid will be down (and for a long time, too, several days probably), and we'll be down to the backup diesel power. My question deals with, what happens if that fails too.  (It may take some time to fix the diesels, typically more than the few hours that we can afford to be without power, and it may be more challenging fixing them in blackout conditions and other prevailing conditions in the event). 


I can find reliability data on the on-site backup diesel generators, as they have to meet stringent criteria, and we have accumulated extensive data on those kinds of units in our industry.  But no such luck with off the shelf "retail" generator units, thus my question.  Cost is no concern; at this point I am just calculating probabilities of chains of events, and in any case in an actual situation cost would be of no concern.  Preventive maintenance probably doesn't come into play either:  we would put the retail unit into operation upon diesel failure (or a few hours thereafter), and it would be required to run for several hours or a few days, until we can restore backup diesels.


So I am looking to find probability of failure to start and probability per hour of failure to keep running (if we can get data on main failure modes and time to repair, that would be good too, but coarse overarching FTS and FTR data would be higher priority right now).  Am not looking for "precise" data -- order of magnitude data would be good enough at this point, but just wild guesses would not.  Order of magnitude means, for example, failure to start probability "on the order of 0.01", whereas the true number may be 0.007 or 0.03.   So if perfect records don't exist, or if there's a variation between vendors, that is OK.  That means based on some record somewhere, or somebody's actual extensive experience with such units. 


Of course, I realize that any data acquired in a forum like this would be "soft" data; at this point I am interested to know if any such data has been compiled or exists at all; if not, then I can say to others, sorry no data for these units (at least I didn't get an answer in this forum), if you're interested you'd have to find a way to compile such.  If on the other hand I get an answer, even a very coarse one, then I can say, OK there is some data out there, it's "soft" (and maybe it makes sense compared to the data we have for our backup diesel power), it should be developed further, but at least we have some starting point, and based on this soft data this is the orders of magnitude we are looking at for our situation.


Re: Backup Generator Reliability

one more clarification, if anyone has documented experience with such units:  for example probability of failure to start is:


An example: I (a user of such units) had the occasion to start such units about 1,000 times in my career (if you kept such records).


Out of those 1,000 times, there were 10 failures from all causes.  Thus, roughly probability of failure to start is 10/1,000=0.01


Similarly for failure to run:   cumulatively, my units ran for about 10,000 hours. There were about 10 failures, FTR probability is 10/10,000-hr = 0.001/hr


If no experience with units of these particular sizes, is there experience with units of similar sizes?  Would different rating units have drastically different reliability -- can we use data from different rating units as a coarse starting point, or are they totally different? Which user group (industry) would be most likely to be worried about reliability of these units and thus keeping some records?


Re: Backup Generator Reliability

If you can wait a few days I will go thru our customer data base and look at a few of our long time customers with 1MW and larger gens,and compile some numbers as far as callouts and maint issues that have arisen, realizing that the majority of these units are 480 volt, I will check the 4160 volt units we take care of.

 As far as opinion goes I believe CAT probably has the best large industrial gens on the market thier controls are solid and the way thier generator is made  has made them top dog in this industry.



Re: Backup Generator Reliability

Thank you again, Tech 1.  Yes, I can wait, take your time.
Cat Dealer

Re: Backup Generator Reliability

3)   Suppose the anticipated usage of this generator is very rare, as in, hopefully, never. Suppose that I don't even have it on my site (I think I have plenty of backup power, but then there are those rare events), but if I need it, it's very important that I get it. Would this be something that is in stock somewhere in the country and can be flown to the site and installed in a few hours, in an emergency, (I do have several hours lead time), or is this something that must be ordered ahead of time, with long lead times to delivery?


I work for a CAT dealer, but I am not in the rental department. If there is a widespread calamity, and everyone is scrambling for generators, the people that have a contract already in place will get first priority. For a group of banks locally, we have 2 MW units on a monthly retainer specifically for them. When a large scale disaster occurs, almost everything locally will be gone. Then all the local dealers will start pulling equipment from around the country, if this is a large enough calamity, there may not be enough equipment to go around. During Katrina, even our rental fleet here in Chicago was sent down to Louisiana, and we are talking over 60 MW of equipment.


So my suggestion is to check with your local rental equipment supplier on their contracts and terms now, before you get into a situation where you need it.



Tony Yang
Systems Engineer
Patten Power Systems
Regular Contributor

Re: Backup Generator Reliability

Dear Wilkinson,

The topic that you have raised is a real issue that is going around in many Forums.. I have not come across any conclusion on this so far.. However just wanted to share some of my opinions on this based on field experience...

The challenge for a Diesel generator set is the number of components that go to make up the unit and also it cuts across both Electrical and Mechanical reliability. This makes it quite a nightmare to provide MTTF and MTBR and reliability numbers..

If we were to go as per the Operation and Maintenance manuals of the generator set we have the oil change as the period between repairs.. Technically we can run the sets continuous during this period without any failure or shutdown.. Unfotunately this is an ideal situation.. The Mechanical component failure rate within this period will be extremely low, but the challenge has been on the Electrical side (i.e. Generator, controls etc).. We have Rotating diodes, fuses, relays, etc which would dictate the time between failures based on their designed MTBF..

Hence for getting overall reliability we need to electrically select the Generator and controls portion to get the required reliability. From experience there are far more failure of DG sets due to Electrical components than Mechanical failures..

 The other important but often ignored component is the Generator controls.. These contribute to more than 80% of the Electrical failures of DG sets.. We generally tend to pick Market available controls and try to adapt them to the genset.. It is important to have a reliable controls to go with the generator set if we need a reliable backup power. If the controls underprotect then we will have failures and if we provide wrong protections we will have nuances trips and hence will impact the backup power reliability.

The best way to start would be to get a feel of the component MTBF for the Generator and controls as a base and take it from there.

Hence the opinion is that we would need to analyse the total syatem for reliability rather than the component, Generator set..

Hope this will be of help..

Regular Contributor

Re: Backup Generator Reliability

You are describeing our standard installation. We currently have over 25 Cat 3516b units installed as emergancy/ peak shavers around the state.  1825 to 2000 kw usally at 4160 or 12470. Several are actually 480 volt machines steped up to 4160 distributaion voltage.  Beyond that many other Uitilities in the upper Midwest operate this same equipment in the same way. It has really become a standard design with little or no problems encountered in several years. We have encountered no major technical issues in the past several years they have only operated 1 or 2 hours a month as excersize.....
New member

Re: Backup Generator Reliability

Have you found the satisfied generators? If you haven't, here is a website that will be useful to help you choice the backup generator. It provides various brand generator set and the genset style is also varied. Good luck to you.