load Shedding

We are doing load shedding for Refinery having 2 Gas turbine Generator(25 MW each) for critical loads and a Public grid supply (10 MW)for non critical loads
We are going with PCS7 for this project.
I have read many load shedding schemes like Max. Demand based load Shedding,Data Management Load Shedding,Under frequency based load Shedding,Generator Status based load shedding but couldn't get a clear picture.
Can anybody provide me details structure for various load shedding scheme or algorithm for the same who has done similar kind of projects


Thanks in advance

The one system I have seen was based on overload and under frequency.
If either one occurred the designated loads would be turned off. The loads would then be re-added sequentially ( if the overlaod and underfrequency corrected it self). If during the re-add an overlaod or underfrequecny occurred again a re-add lock-out and all loads would then be turned off again. Someone would have to correct the problem before any loads could be turned on again.

Whenever I've done load shedding it was based on demand kW. You simply identify non-critical loads like HVAC fans and heaters. I'd usually divide the loads into two or three groups, and when demand exceeded the set limit I'd start dropping the first group off, then the second if needed, etc. It may be a good idea to have a time limit on the shed loads, and after the delay elapses then shed the second group, alternate the groups, and allow the first group to come back on. This would be similar to alternating pumps or such, but instead you are alternating groups of loads. This keeps things like ventilation from getting completely screwed up during the load shedding process.

When demand drops below the limit plus the shed loads you bring the groups back on one at a time. You would also alternate the groups as you bring them back on. That keeps the same grouping from always being first to suffer.

I guess you would have about 4 possible levels of power supply (60 MW, 50 MW, 25 MW, and 10 MW). You could make a list of the best loads to be run at each of the 4 power levels. Then set up your load shedding for each level. A complete accurate list of all the loads in the refinery will be the place to start. You can find a lot of that on electrical single-line drawings. Also if you have KWHour recorders in the plant, get copies of the electricity used for the past year.