To operate an electricity supply system you have to set-up a composition of the type of power plants to be used the next days according the the availability and demand (dispatching). So you give a schedule for each plant (scheduled power from time x to y). The power prediction is the possible output according to the weather forecast. To be on the safe side, you should fix the scheduled power lower than the predicted (depending on your level of confidence). As the name says, the available power is the power available without manipulation. The actual power could be the available power, but sometimes you have to throttle or curtail the available power due to grid overload.

Hello dear Prateek.

In response to your question: in scheduled issues, you need to forecast all devices production in season or time. for example if you have wind turbine as generation unit . and your objective function is minimizing operational cost, you need to predict its output power depend on that area wind velocity. in some of literature, researchers use weibull distribution  in order to predict wind velocity. in this prediction method weibull distribution separate to  sections and possibility of each section consider equal with  middle point of each section  . rated velocity is constant value which regard to environment historical data. but average wind achieved with your  prediction method like Monte Carlo or least square or other methods.

rated wind depend on your wind turbine construction and in order to use wind turbine in high efficiency use WT in region  which its historical data of wind close to  this value.

1) The available capacity can be different from the scheduled (in advance) due to forced outages (which, anyway, in the case of wind turbines are occurring with well lower frequency than the ones of thermal units)

2. Scheduled capacity and forecast capacity depend on the different  advance time on which the two types of forecasts are effected (in many cases such evaluations are made by the TSO in charge to keep the balance production-demand in the real time system behaviour)

3. Depending on the rules in the various markets, the schedule capacity can in some case be guessed if, as in many systems, it has a priority dispatch. In other cases long term contracts may exist. In other cases, the wind capaicty can be offered to the market: its amount  depend on the economic evaluations of the various producers. The TSO can also be allowed, by some regulations enforced by the Regulator,  not to use the whole wind capacity time -to time availble, should its amount conflict with some local system constraints.           

I agree with the analysis provided by Luigi Salvaderi. Available capacity is in terms of capacity of the units installed based on the thorough study of the wind details at the location of the plant.   Scheduled capacity is the one that is being assigned to meet the particular load at a particular day or time based on the availability of various plants including wind plants.   Scheduled capacity has to be calculated by the study of load generation balance for meeting the load at a given time with the available generation at that time.