Are resistive load banks used as Dynamic Braking resistors for thermal power plant transient stability and for better house load operation? If yes, how commonly Dynamic Braking is used in Thermal Power Plants. I need some references of plants plants that use them.

 

Can you be more specific about how using Dynamic Braking could be employed to improve Power Plant Transient Stability?

 

In case of sudden load rejection turbine speed tends to overshoot, if the speed is not controlled by resistive brake insertion or fast valving or both, speed of turbine overshoots resulting in disconnection from grid and plant shutdown. Reconnecting to grid takes time and particularly Nuclear power plants take much more time to reconnect to grid compared with conventional thermal plants. Hope this helps.

 

I re-read the original post, and, personally, I've never seen such an arrangement nor heard of such an arrangement.

Sounds like an interesting concept. "The devil is in the details," though, as they say.

 

Look at page 22 of the following pdf document you will find one such system
http://phasors.pnl.gov/resources_standards/WAPS_WPRC04.pdf

 

I wondered what those towers were for at Chief Joseph sub.

Now I know.

Thanks.

Like I said, the devil is in the details. Without some of the Schweitzer equipment that would not have been possible. And, the breaker closing and opening times to support 0.5 sec of dynamic brake application are pretty amazing in themselves, considering how much current must be flowing and the voltages at which they are being operated.

Interesting stuff, that's for sure. If the public only knew how much effort went into ensuring the stability of their electricity supply!

Have you contacted Schweitzer for any other implementations they've worked on?

 

I will search about Schweitzer but since I am outside USA therefore it may not be possible for us to have their services. The Chief Joseph Dynamic Brake is huge (1400MW). The Power Plant of my interest is small and we may need around 20-30MW resistive load for transient stability enhancement.

For this load, compact small size resistive load banks (5MW each) with forced fan cooling are available and for switching, ABB makes bi-directional Thyristors each capable of switching about 5 MW of load within milliseconds. The reason for considering load banks is that these loads can dissipate energy for long periods without overheating and thus can also be used for adding load for safe and stable house load operation. House load operation at very low load can be unstable and detrimental for Turbines.

 

Hello Shal234,
A single shaft unit will NOT have an overspeed trip in case of a full load rejection. It is standard GE layout that the unit comes back to FSNL and can be resynchronized immediately (after having cleared the load rejection problem).

Regards, Hans G.

 

Hans thanks for responding. I must admit our plant may not be using state of the art technology. The plant has a history of shut downs on full and even partial load rejections. Plant uses wet steam turbines and sometimes its house load operation period has to be extended beyond recommended time. The grid is also not that stable.

Regards.

 

Shal1234, perhaps you don't have to use a "State of the Art Dynamic Braking System."

Have you done a transient-stability study to determine the turbine-generator's acceleration-rate?

Regards, Phil Corso ([email protected])

 

The Designers of the Plant must have done that. However Plant performance falls short of what was claimed. Transient-stability study to determine the Turbine generator's acceleration rate will have to be made again by us or third party to find the best possible solution.

A small Dynamic brake using Bi- directional Thyristors does not seem to me to be too difficult to implement or maybe I am missing something.
Here is a link of one paper on this subject:
http://eprint.iitd.ac.in:8080/dspace/bitstream/2074/2076/1/patelimp2003.pdf

Regards.

 

Hans G.,
You seem to be familiar with Shal234's site and turbines? (Long time no post, by the way!)

EVERY prime mover can overspeed if the control system doesn't respond appropriately to events, such as events that only open the generator breaker and don't trip the prime mover (what I call "breaker open events" as opposed to "unit trip events", which open the generator breaker and initiate an emergency shutdown (trip) of the prime mover).'

A "full load rejection" occurs at Base Load for; a "breaker open event" can occur at any load (a "load rejection") and if the control system doesn't respond appropriately the unit can overspeed.

For GE-design heavy duty gas turbines (not necessarily only single-shaft designs), a breaker-open event is accompanied by a very sharp reduction in fuel flow. The intent is to reduce fuel flow sufficiently enough to prevent shaft speed from reaching the overspeed trip setting(s), but also to prevent reducing the fuel so much that flame in the combustion cans is extinguished. This takes some tuning and testing, and the nature of the testing (load rejections!) makes it very uncomfortable for most utilities and grid operators.

By preventing overspeed trips *and* flame-outs, the unit will quickly settle at Full Speed-No Load (synchronous speed) and can be re-synchronized to the grid (presuming the condition which caused the breaker to open has been quickly resolved).

In my personal opinion, using resistive loads to stabilize unit operation at low loads for extended periods (in excess of 0.5 seconds!) is, well, wasteful. It would seem that there is some other problem or problems (tuning, etc.) that making it difficult to achieve stable output.

Based on what the originator has suggested as recommended reading, at least one of the problems is "grid" frequency. It's also not clear what kind of thermal power plant(s) the originator is referring to. And, "grid" might mean an isolated ("island") load, or it might mean a large national grid, but that's also not clear from the thread.

I, for one, am going to leave this thread to the venerable Phil Corso, P.E. Take it away, Phil Corso, P.E.!

 

The Plant is a small base load nuclear power plant and it is connected to national grid. As it takes long time to bring the plant online after trip/shutdown therefore, in case of grid fault, operators prefer to bring the plant on house load instead of tripping/shutdown. In case of grid fault operators operate it on house load in anticipation or by getting some information from the the Electric supply company regarding the fault clearing time.
The plant used to trip on full load or partial load rejection transients.
Plant engineers added a turbine speed sensor and connected it to a relay to bring the plant on house load in case of speed overshoot, the set point for this relay for brining the plant to house load was set at turbine speed slightly lower than the turbine tripping speed set point. Since there is a vendor support for the plant therefore, if plant transient stability study and tuning could improve by tuning existing controls, it would have been done. Based on this I think with existing control systems it may not be possible to improve plant stability and some measures as mentioned in previous posts will have to be taken.

During house load operation some steam is dumped anyway, so why not dump energy in resistive load banks instead of dumping steam in condensers.
Regards.