isolated operation

[coolyaar]

hello guys,
we are operating a 40MW captive coal power plant in order to supply our own industrial load of about 20MW. our coal plant
is connected to grid system in order to obtain a startup supply fromthe grid and to export a surplus power of about 17MW.
the grid system is very unstable. often there is under-frequency and unplanned tripping which also trips our coal plant ( mostly on under frequency of generator protection) thus tripping all our industrial load.This has become a trouble for us as steady supply to our industrial load is important.
basically i am in search for solution to following two problems:
i. Can we install a Rocof (df/dt) relay to trip the grid breakers if the grid is becoming unstable? if so, how do we determine the settings of the Rocof relay? reference to some standars/codes/literature will be appreciated.
2. will our plant operate in isolated mode once we trip the grid breakers ( a load rejection of 17MW to grid will surely cause over frequency and over voltage at our turbine and generator)? i am of the view that specific equipment ( like auto load switching units) are needed to be installed at a coal power plant to operate it in isolated mode. i will appreciate if some reference material is shared regarding isolated operation of a coal power plant of such small size ( i.e. 40MW).
regards,

 

[waross]

If you are able to anticipate the loss of the grid, you may consider reducing your exported power when the grid is showing instability.
Review your protection philosophy. Consider tripping the grid tie breaker rather than the generator breaker when the grid causes low frequency.
When you trip off the grid, there will be an over frequency excursion. The effect may not be as bad as you fear.
As the frequency rises the connected motors will attempt to accelerate. The added load of acceleration will somewhat mitigate the over frequency by increasing the load on the plant until the governor is able to respond to the 17 MW load rejection.
Over voltage and over frequency together will tend to maintain the Volts-per-Hertz ratio and is not as bad as either one by itself.
You may be able to solve your problems with appropriate trip settings and a change from tripping the generator to tripping the grid breaker.
Can you provide a plot of the generator voltage and frequency excursions when the generator breaker trips?


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

 

[ScottyUK]

A coal station has a fair bit of internal house load in the mills, FD & ID fans, and BFW pumps etc, so the turbines aren't being faced with full-load rejection between house load and the 17MW process load. Your governors may be fast enough to catch a load rejection event without tripping - an electro-hydraulic system definitely has the capability if it's properly tuned, some mechanical types may be a bit slower to react. Regardless of the governor type, it needs to be tuned to have a fairly lively response without risking instability - a sluggish governor is the last thing you need. In principle you can do what you're wanting to do, but the specifics of governor hardware will determine the practicality.

A ROCOF relay is definitely an option, as is vector shift, but picking settings may get tricky. Have you considered a multi-stage frequency relay, tripping the grid tie first then a time-delayed turbine trip, and an instantaneous turbine trip at a lower frequency. That would be a lot simpler to configure.

 

[crshears]

What waross and ScottyUK said; the site load all on its own will have a significant amount of inertia, so the verticality of the rate of frequency step change on grid separation would not be as precipitous [no, that's not the right word; the plant frequency is rising, not falling...perhaps "steep" is a better choice...] as one might otherwise think.

ASSuming it's electro-mechanical and fully capable of islanded operation, there may need to be some adjustments to the turbine governor dashpots / orifices / linkages to ensure sufficient rapidity of response to ride through grid separations...but based on what you're telling us I wouldn't foresee any insurmountable problems, and mitigation of any that do occur will be greatly aided by capturing plots of frequency and voltage excursions during these events.

Let us know how it works out...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]