Effects and Impact of Cyclic operation on 700MW Generators

[Bekza]

Hi all,

Our power plant was initially designed for base load operation but due to some reason we were then required to change operating regime to two shifting (cyclic operation). This meant increased barring hours, increased number of stop starts which had bad impact to our rotors and all rotor were rewound after reaching certain allowable starts and during inspection i have seen lots of defects on rotor windings, associated insulation, overhangs, cracking of connectors and D-leads the list is endless. However i am now doing a reaserch on the effect of cyclic operation on the stators. I have searched for related report from other contries to see if there was a stator failure due to excessive cyclic operation.

Any other information or comments will help as i must compile a risk assesment to address impact of extending generator outages. Our generators are water cooled(copper windings)and hydrogen cooled( rotor and core).

Thanks,

 

[wolf39]

Water cooled stator windings are running at about 35 to 40 degrees centigrade at the inlet portion of the cooling circuits (stator bars). The outlet temperatures at the end of the cooling circuits (stator bars) reach only 60 to 70 degrees centigrade (at full load). Thermal expansion matters are therefore of less importance, compared with the rotor windings. Because of the low temperature level, thermal degradion of stator bar insulation is negligible.

Regards
Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[byrdj]

for the field, copper dusting from the free floating bars must be adressed. even the units that have added springs to restrict bar movement while on barring also have strict guide lines for operation to limit barring time.

for the stator, the inspection of blocking and wrapping of end turns and lead extensions should be increased do phasing in stresses. again operations guide lines for angle and potential mismatch during phasing should be stricter.

 

[waross]

I would investigate the possibility of keeping the units running at a loading of about 10%. Even if the revenue is less than the expense of running lightly, the reduced maintenance cost may offset possible running losses.
The best solution may not be just an engineering solution but a compromise of engineering, economic and political considerations.

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

 

[ScottyUK]

Bill,

10% output is likely at or below the minimum stable loading - on a steam set you get into all sorts of prime mover problems at low loads, the most onerous being if water starts to condense in the final LP turbine stages. It gets expensive real quick.


----------------------------------

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

 

[waross]

Hi Scotty. Can I change "10%" to "Minimum stable loading"?? Is the minimum stable loading liable to be low enough to be useful?
Thanks.
Bill

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

 

[byrdj]

the effects on the steam turbine will be very stressfull and usually requires internal modifications. determining minimun stable load is an intersting challange. some stations playing with inlet steam and even back pressure in an attempt to get as low as possible (minimun stable boiler and coal handling)

as for turbine damages, not only will there be the effects of pumping condensing vapor in the last stages, there will be the pulsing of the first stage if it is 4 nozzle block design

 

[edison123]

I don't see why generator rotor winding would so badly be damaged due to cyclic operation unless the the original winding was poorly done.

I don't see much problem with stator also with cyclic loading unless the full load was picked up in a very short time resulting in differential expansion between copper and iron.

 

[rmw]

Some of those big units will get down lower than their original designers ever imagined. I have seen some 850 MW units operated down to 160 MW (newly established minimum stable loading point) on a nightly basis. Sliding throttle pressure modifications work wonders.

Just because they can do it doesn't mean that the equipment doesn't suffer. The same units have had all the HP and IP FWH's changed because they weren't designed for cyclic duty. The replacements are, however. I don't know if any detrimental effects on the turbine/generator (G3's). The furnaces catch heck, but they just attribute that to the ravages of coal firing.

rmw

 

[itsmoked]

How about running closed loop heating to keep everything at a constant temperature when not running so there is no thermal expansion cycling.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

This is a question, not a suggestion. What would be the effect of reducing the condensor cooling as the load and the steam flow was reduced? Would the back pressure mitigate condensation in the turbine or make it worse?

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

 

[rmw]

Often when the units are backed down in load and especially when the cooling water source is very cold such as during winter or early spring weather, those condensers that have dual flow paths will have one circuit shut down at low loads.

Every turbine has not only a high back pressure limit, (normally a hot summer day issue) but a low back pressure limit as well, definitely a cold cooling water source issue. So whatever you are doing with respect to modifying condenser operation, you are working within a rather narrow range, say 1.5" hga. to 5.5" hga (specific numbers that I remember from the 780 MW unit whose heat balance is depicted in the book Steam by B&W - that is a real unit at a real plant and I have spent a real lot of time working on and in and under it.)

So if the operating parameters permit, reducing the condenser vacuum (increasing the back pressure) is a beneficial option. It is kind of a pain in the butt, however, as the circ water pumps are large pieces of equipment, as is the valving in the circuit so this isn't something that you do on a whim. The size range of the circ water piping for a unit this size is in the upper double digit to low triple digit range on an inch scale.

rmw

 

[waross]

Thanks for the information rmw
Bill

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

 

[wolf39]

Bekza was asking what effects cyclic operation might have on his water-cooled generator stator windings.

As edison123 hinted, the temperature difference between stator winding and stator core (slot portion) is of importance. Temperatures of a water-cooled stator winding reach their maximum value within minutes after application of full load. The stator core temperatures reach higher values but will approach their maximum at a later stage. Measurement of stator winding cooling water temperatures, slot RTD temperatures and stator core temperatures will be important to determine the effects of thermal expansion.

Duration and number of load changes per day will also be important factors. Stator cores keep their temperatures for quite a time compared with water-cooled stator bars.

Regards
Wolf

http://WWW.HYDROPOWER-CONSULT.COM