Generator operating hour penalty for starts

[rmw]

Does anyone have any document-able or even anecdotal information about the equivalent operating hours attributable to generator starts? (Large) motor information on this topic might also be useful.

I am a ME and I am very familiar with the factor fired hour approach for combustion turbine maintenance interval determination and even have some reliable figures for steam turbines. While I recognize that a generator might not go through the same wide range of temperature excursions that some turbines do, I believe that they do in fact go through some transition where iron expands at a different rate than copper and different than insulation, blocking bracing, etc, during each start-up/shut-down cycle and that multiple starts soon has to have an influence on recommended maintenance intervals over and above a purely operating hours based recommendation.

Anyone got any information that might help?

rmw

 

[waross]

In the range of sets that I am familiar with, up to about 1.5 MW, I think that it is more an engine issue than a generator issue. I have seen sets were the rebuild interval for the engines was 15,000 hrs. to a minor overhaul and 30,000 hrs. to a major. Some sets ran over 100,000hrs. with no maintainance to the generator end other than dust cleaning when the engine was being rebuilt.
respectfully

 

[rmw]

Thanks Bill, but I was going more for the average CT range which is from 40-150 MW. And as regards the engines, they are low hour but high start count. Sometimes as many as 3 starts/day with less than 3 hours total run-time for that same day.

I'd still like to know if anyone has anything definitive.

rmw

 

[ScottyUK]

Siemens Westinghouse haven't implemented anything on generators yet as far as I know. Their turbines certainly have equivalent baseload hours & equivalent starts calculations derived by the control system from running conditions and trip conditions respectively.


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

 

[asekhar]

I have not seen any such factor put for generators to account for number of starts. Basically because in case of generators, the starting does not give it any serious thermal shock. I have experience with power generation from 12 MW upto 110 MW machines, & in none of them the vendors have recommended any factors for number of starts.

Anand Sekhar

 

[slavag]

Hi.
I'm also don't know about some vendors recommendation.
But Rmw's case remaind me some story from side:
I worked before half year on CCPP and mechanical team repaired rotor of generator 250MW ( 7-8 years old). I asked vendor, what is a problem and he explained me that the reason of fault is number of starts of generator, same case, 2-3 times per day. Vendor is very serios company and I'm not customer, then, I think it's true.
Regards.
Slava

 

[NCTHAI]

Comparatively Electrical engineering technology is matured and advanced than mechanical engineerig technology. Generators are designed to match or extend the maintenance intervals specified for turbines (or other prime movers) this has been proven in practice over the year. Exception to this would be early inspections recommendation by OEMs. However this is preventive in nature if operating conditions are good, no abnormalities are reported, they can be skipped with consultaions with OEM (and insurance companies).

To answer original question by RMW, the best approach would be to refer the OEM manuals. I know that GE does not have EOH approach to generators similar to combustion turbines but Siemens have. Have worked on both OEM machines but do not have manuals on hand to quote specific reference or formuales.

Further, to eloberate little further, Generator maintenance program can be build for two major items. A) Generator auxiliaries e.g. coolers B) Insulation system and Core Core. The three important elements of a thorough program are maintenance frequency, electrical testing and visual inspection.

A maintenance program should reflect the level of acceptable risk for the unit. That will probably vary
from unit to unit and plant to plant, and will change over time as the importance of the unit to the power system
changes. In addition, new technologies are constantly being developed to improve unit reliability, performance,
monitoring & inspection equipment, and otherwise provide more cost effective means for maintaining
the generator

With reference to SLAVAG's input regarding, number of starts affecting the generator, one need to carefully examine the operating cycle. Generators are designed to tolerate normal switching of gen breakers. However, Operational events that should be carefully noted are
• Errors in synchronizing
• Under- or overfrequency operation
• Lightning surges or other transients
• Faults (such as short circuits) on system or adjacent machines
• Single-phase or out-of-phase operation
• Unbalanced load
• Overload
• Overvoltage or undervoltage
• Loss of field

These events cause stress to generator insulation system. Also you need to look at auxilairies - coolers and excitation system. Effective cooling and the way excitation system function can effect the absolute temeprature rise and rate of change of temperature. The temperature has direct effect on insulation. One need to review complete operation history to determine the root cause of failure.

Regards,

 

[burnt2x]

Hi,
No information on equivalent operating hours attributable to the duty of generators!
My experience tells me the generator maintenance usually jibe with turbine/engine maintenance. All maintenance measurements results did not indicate generators having problems earlier than the drive equipment. Some of our problems were related to cooling/ fouling and lube (tropical installations), but those are not related to generator starts. Have scanned lots of OEM specifications and did not find any attributable to gen starts.
We follow operating hours recommended by the OEM and other operational procedures defined and made known to all our insurers. As long as your insurers do not find your maintenance program deficient (to factor in the gen starts), you're pretty well covered.

 

[slavag]

Hello.
What is normal operation of generator ( from 25MW up to..)?
One , two starts per year?
We talk about 2-3 starts per day.
I think, generators mnf. don't take it in account.
Regards.
Slava

 

[rmw]

Slavag,

You and I are on the same page and this is what this is all about; a beef with a generator manufacturer. They have their first maintenance scheduled for 8K hrs which for a machine that runs 24/7 and starts a couple of times a year about one year's running.

On the other hand I have a client (one among a class of similar clients) who is a peaker and who has 1500 hrs in 5 years and something like 400 starts in that 1500 hrs. If he waits until 8K hrs to do take his first look at this generator, he and I and probably you will be retired before that happens.

I hold that every time this generator starts iron and copper expand at differing rates and things rub and scrape and chafe in both the rotor and the stator as well as blocking and bracing on the end windings of both the rotor and stator that goes through the stress range at every start.

Meanwhile the turbine guys have an elaborate formula that makes the customer do his hot gas paths, combustion inspections and majors based on factored hours in which starts, and especially cold starts play a big part.

The Generator OEM on the other hand blissfully holds to their 8K, 25K, 50K and 100K hr schedules as if the machines run all year with a minimum number of starts.

I just can't seem to find any literature for the generator like;

http://www.gepower.com/prod_serv/products/tech_docs/en/downloads/ger3620k.pdf

especially figures 23 and 24.

I stated above that the generator temperature excursions were not as severe as the hot gas parts of a CT, but there is still a consideration that should be made for the high number of starts IMHO.

So far I am losing the case with the OEM. There doesn't seem to be much out there. I appreciate all who have contributed.

rmw