Megawatt override

[redlinej]

Hello guys,
In most power plant I worked, we use a feature called megawatt override this is programmed in the Woodward 723 plus.
This featured is use when operating the plant in base load and if the frequency drop to 59.5 or rise to 60.5 the Woodward control will trip to droop to assist in regulating the frequency.

I would like to know, why these limits? The reason I rise this question is because all most all grid inverter manufacturer use these limits too to trip their inverter off line.

 

[Parchie]

redlinej,
There are two words that you have to understand well when operating a generator. 1) isochronuous and 2) droop.
The first means operating at constant speed, whatever your settings will be. If your load is not yet topped and the frequency goes beyond the rated value, you have to set a limit on the speed of your unit in order not to go over the rated frequency, even if your load is still low. The units operation will then revert into an "isochronuous" mode to prevent overspeeding.

The second means something is drooping or slowing down. When you operate in parallel with other generators you have to put limits on your unit to protect it from damage. You will take into consideration its load rating as well as its frequency. When your load is topped, you cannot load any more even if the frequency drops. The mode of control will not be isochronuous anymore but "droop" mode, meaning, your unit takes only the rated MW output no matter how the frequency drops.
Crystal!

 

[FreddyNurk]

Most of the grid inverter manufacturers would also trip off outside of nominal frequency as they're not supposed to operate outside of nominal areas, largely in order to assist in preventing islanding.

Setting the plant to droop on frequency deviation would likely assist if the plant is capable of being islanded, whereas to remain in baseload may result in an unacceptable speed event if the network was to be lost.

EDMS Australia

 

[redlinej]

@Parchie- Thank you.I do understand the operating modes of generator and they are basically [highlight #CC0000]three modes- Isochronous, droop and fixed KW[/highlight]. my question was not about the operating modes[highlight #EF2929][/highlight] but the limits.

@ Freddynurk- Thank you for you explanation its is correct, but not what I am looking for.It seems that the frequency limits are universally used and I just wanted to know, if they were arbitrary selected or they where set by one of the regulatory bodies.I did some research, but could not find anything.

 

[FreddyNurk]

redlinej, I wouldn't have thought they were arbitrary, but you're probably not likely to find a lot of difference in those values.

Certainly there is a lot of deviation in terms of frequency settings for trip off, mostly around the size of plant. Where I used to work a deviation of up to 5Hz was typical, but for the network protection people 0.2Hz was seen as cause to trip. The fact that in your case its not a trip, rather a change in operating mode makes it harder to make a valid comparison.

Using those typical 'large grid' settings for grid connected inverters in smaller networks would be expected to cause issues, albeit in smaller networks they're much less likely to allow for grid connected renewables to connect in the first place.

Some utilities will specify values for such systems feeding into their networks, others will rely on consultants to provide the numbers.

EDMS Australia

 

[bacon4life]

Those seem awfully wide settings since the system would be in pretty serious trouble when frequency declines to 59.5 Hz. Within the western USA, under frequency load shedding starts at 59.5 Hz and generators are allowed to trip for self protection at 60.5 Hz. Per NERC/WECC guidelines, generators around here should be set with a droop and a 0.036Hz deadband. Have you checked if you settings are the defaults from the Woodward instruction manual?

 

[redlinej]

@ bacon4life
In system we have, other generators are controlling the frequency. These (megawatts override limits) settings are for the generators operating in base load that would trip to droop in the event of a disturbance on the grid.This operation with try to restore the frequency before the under frequency scheme start to operate.this mode of operation is working okay.

The issue is since the grid tie inverters are set by their manufacturer at the same limits,they trips just before the grid recover the frequency.that the reason for the question.

 

[catserveng]

On several systems I have worked on we had a similar feature called "baseload override", sounds like the same thing. Most of the units I worked with were diesel units 1-3 MW on isolated networks for applications like mines, large drill sites and quarries. The 723 governors were used with Woodward DLSC controls. The feature had the same default settings as yours, but could be adjusted to suit particular site needs. We used the feature on sites that had multiple units on line and could see large transients, such as large drill sites or quarries. I can't think of any sites we used this feature that was in parallel to a utility grid. Maybe knowing some more about your particular applications could get you some better answers.

The 723 platform had several different part numbers depending on application, yours could be either one of these standard setups, or a custom setup done for a particular manufacturer, in my case we had custom 723 programs for certain Mak engines.

On the 723Plus there are usually three part numbers, the system, the hardware and the software application, if you have that info you can ask Woodward for the specifics of what the features were intended to do. But that assumes no one came after it was installed and modified the software application.

MikeL.

 

[bacon4life]

The old IEEE 1547-2003 standard requires small distributed resources to isolate from the grid above 60.5 Hz and below 59.3 Hz in an early attempt to prevent unintentional islanding. Once the new revision of 1547 finally gets published, hopefully distributed resources will be able to stay fully connected down to 57 Hz and just reduce real power output above 60 Hz. It is kind of crazy we are still installing inverters that don't support the grid during either voltage or frequency deviations.