House load operation steam turbine

[Vaibhav]

In the event of grid failure islanding mode of operation of the electric generation unit is known as House load operation. Under this condition turbine generator produce only auxiliary load which is required to keep electric power plant unit alive. After restoration of grid the unit can be quickly synchronized back & load could be increased.

I have following doubts about house load operation. Please share your knowledge & experience about this topic.

1. Have the machine supplied by major manufacturers in the world like Siemens, MHI, Alstom, GE, Skoda, Toshiba have House load capability?
2.Does the “House Load capability” of fossil fuel fired electric thermal unit has been demonstrated anywhere in the world? Has this test successful? If yes please provide unit reference where this test has been conducted.
3. How the boiler gets affected during house load operation of steam turbine generator?
4. What are the expected effects on the life of steam turbine generator of prolonged house load operation?
5. I believe for subcritical units under sudden large load rejection, unit gets tripped due to fluctuation of water level in drum. However for supercritical units drum gets replaced with separator. Can we say that house load operation can be easily demonstrated for supercritical units?

 

[CFDFEAGURU]

I can comment on question number 3.

The boiler is affected when the turbine load changes. For instance if the turbine is unloaded the main steam pressure will rapidly rise and if the fast acting bypass valves aren't fast enough the boiler steam drum safety valves will open to vent the excessive pressure.

As for a supercritical boiler, I cannot comment as I have zero experience with those boiler designs.

Thanks
Matt

 

[Vaibhav]

under this situation unit will trip. Is unit will come on house load?

 

[CFDFEAGURU]

Please provide more information.

Under what situation are you asking about?

Thanks
Matt

 

[Vaibhav]

A small disturbance in grid affects the machine which is connected to it. During grid failure machine shall be capable to sudden load through off & to operate on house load (just a small load which is required to run its auxiliaries like APH, BFP, CEP FANS & others). Auxiliary load is could be about 4% to 8% of rated capacity of generator.

For example for 600MW machine (with TDBFP) House load is about 30 MW.
Could it be possible to suddenly reduce the load from 600 MW to 30 MW?

 

[ank_gl]

For example for 600MW machine (with TDBFP) House load is about 30 MW.
Could it be possible to suddenly reduce the load from 600 MW to 30 MW?

There must be some way, though I don't know exactly what it is. I think the same situation is met while shutting down the unit. Excess steam has to be bypassed(or maybe vented, though unlikely) to prevent the over speeding of the rotor. I am not sure on how this is done.

I am working in a hydro company right now. During shut down or load rejection, governor automatically turns the guide blade & the kinetic energy of the fluid is dissipated in vertical shafts(known as surge shafts) situated generally at the start of pressure shaft.

 

[gmax137]

I can't really answer your questions, but I do know of one generating station where they have a number of diesel generators that can power the plant auxiliaries and allow the unit to "black start" without any power from the grid. So if the grid goes down, and the units go down, there is still a way to re-start the first unit an begin repowering the grid. This suggests that there is no need (at that site) to run the unit in what you call house load.

But I think you are more interested in (1) how exactly the plant is able to survive the 95% load rejection and then (2) how exactly do the operators run the 600 MW unit at 30 MW. I would think (for one example) that control of the feedwater flow at 5% might be unstable and hard to maintain for an extended time.

 

[ank_gl]

I talked to some people at work about this, though I am not sure about all plants. Plants are generally connected to the grid through an alternate transmission line which powers its auxiliaries, & have standby DG sets, so there isn't really any need for "house load operation" from the plant unit itself.

 

[Vaibhav]

I understand the concept of black start. Generally gas turbines units are used under black start condition. Gas turbine has very small aux loads which can be supplied by DG sets.
But bigger thermal units have very high start up power requirement. Typically for 600 MW unit for startup about 10 to 12 MW power is required. (for BFP, FD, ID fans etc). Out which electric motor driven BFP & ID fan takes maximum power.
It seems to uneconomical to keep stand by DG sets of such a large capacity inside the power plant. It is much better that the unit has house load capability of some minutes. (say 10th to 15th minutes). If grid become stable in the mean time unit could be synchronized back & load could be increased.

 

[gmax137]

It seems to uneconomical to keep stand by DG sets of such a large capacity inside the power plant.

You may be correct, nevertheless, at the plant I am familiar with, there are five 'black start' diesel generators, each approximately 2 MWe (totals 10 MW as you correctly estimated). I suspect that the particular plant's location (weather conditions, such as hurricane frequency) and grid characteristics, etc. play into how likely a black start condition really is, and whether installation of the DG sets is 'economic.' Also - it may not be necessary for every unit on the grid to be capable of the black start (once the first units are running they can power up the remaining units). So you need the diesels only at a few sites.

 

[abhiks4u]

1. Have the machine supplied by major manufacturers in the world like Siemens, MHI, Alstom, GE, Skoda, Toshiba have House load capability?
Reply: yes major turbine manufacturers (OEM) machine have capability to run under house load, however each OEM specifies their maximum limitation of time to run machine under house load operation.

2.Does the “House Load capability” of fossil fuel fired electric thermal unit has been demonstrated anywhere in the world? Has this test successful? If yes please provide unit reference where this test has been conducted.

3. How the boiler gets affected during house load operation of steam turbine generator?
Reply: Boiler runs at its minimum generation / stable load. the extra steam generated by boiler than demanded by Turbine is being dumped to condenser via HP/LP bypass. this is called Parallel operation.

4. What are the expected effects on the life of steam turbine generator of prolonged house load operation?
Reply: in addition to point 1 reply:- prolonged house load operation more than recommended duration of OEM leads to overheating of machine. also there are many process constraint like maintaining condenser back pressure/ condenser neck overheating)

5. I believe for subcritical units under sudden large load rejection, unit gets tripped due to fluctuation of water level in drum. However for supercritical units drum gets replaced with separator. Can we say that house load operation can be easily demonstrated for supercritical units?
Reply:: it depends on bypass capacity any boiler sub/supercritical have provision of runback. Thereafter bypass parallel operation controls the house load operation.

 

[tobyvn]

I understand the concept of black start. Generally gas turbines units are used under black start condition. Gas turbine has very small aux loads which can be supplied by DG sets.
But bigger thermal units have very high start up power requirement. Typically for 600 MW unit for startup about 10 to 12 MW power is required. (for BFP, FD, ID fans etc). Out which electric motor driven BFP & ID fan takes maximum power.
It seems to uneconomical to keep stand by DG sets of such a large capacity inside the power plant. It is much better that the unit has house load capability of some minutes. (say 10th to 15th minutes). If grid become stable in the mean time unit could be synchronized back & load could be increased.

dear friends,
I am new member, could you tell me where state/stipulate time period for house load test as you said (10th - 15th min.)?

Our plant is a Coal Fired Power Plant (2x622.5 MWe); and now, we are on negotiation meeting and hoping you can help me!

thanks so much!

 

[muaz_aburass]

for house load if the the unit load from 600mw to 20-30mw ,what the time exactly the unit can be stable for this situation, imean what about pressure, level and other mechanical parameters.?
second one :some one told me that there is (FAST CUT BREAK(FCB)) built in the DCS,what about time stamped ,if anybody heard or have information about that ?

 

[abhiks4u]

for house load if the the unit load from 600mw to 20-30mw ,what the time exactly the unit can be stable for this situation, imean what about pressure, level and other mechanical parameters.?
second one :some one told me that there is (FAST CUT BREAK(FCB)) built in the DCS,what about time stamped ,if anybody heard or have information about that ?

immediately generator load will come to house load of say 20-30 MW from 600 MW. Turbine tends to over speed ( around 106% for 2-3 seconds thereafter 102% for around 2-3 minutes) & boiler goes on run back ( say 50% within a minute or twoo)...thereafter load further reduces in controlled manner to minimum boiler stable load (30% / 40%)with or without oil support as per plant specific design ( total time around 5-10 minutes ); in the mean time steam generation difference of boiler & actual steam consumed by turbine ( flow, pressure & temperature) is being managed by HP/LP bypass valves which is self content to maintain steam upstream pressure & steam downstream temperature.
last but least house load operation has time limitations generally TBN manufacturer recomment <2 hrs of house load operation in a stretch.
Regards,
Abhishek