# Dispatchable Substation for Operation and Control of Renewable Energy Resources
## 2 Requirement for Dispatch-Able Substation
### Forecast Accuracy  
The TSOs can predict the expected generation from renewable sources in specific areas using the weather forecast data. Wind velocity and solar irradiance are the key factors for determining the generation from wind turbines and PV generators, respectively. The accuracy of the forecast information declines with long-term prediction. Therefore, a DIR operator needs to combine multi-scale forecast data and predict the uncertainties in generation until the next dispatch signal is determined. When multiple RESs are combined, the accuracy of forecasting of the overall generation varies depending on the type and combination of the renewable generators. With combined forecast information, the DIR calculates an affordable range of generation within a certain period and reports it to the TSO. Then, the TSO calculates the optimal power flow (OPF) and dispatch signal for each generator without violating the security constraints. In case the security constraints are violated due to surplus generation of RES, the TSO may decide to limit or curtail their output. Note that the OPF and security constraints should be managed by the TSO. A DSS is required only to provide accurate forecast information and firmly follow the dispatch order from the TSO. Therefore, a DIR or DSS requires only guaranteed output prediction with fewer errors. An ESS is an efficient solution for decreasing uncertainty from RES, because it can supply or absorb active power to compensate forecast errors [7]. For this purpose, an ESS can be assessed based on the manner a violation penalty is calculated and also based on the capital cost of these facilities. In terms of reliability, the capacity of the required ESS can be assessed based on the uncertainty of the interconnected RES. Because the stochastic nature of the RESs differs depending on the generator type, the configuration of an ESS should be assessed differently depending on the type and combination of the RESs. Several differences exist between PV and wind generation in terms of their pattern and forecasting method. For periods of less than 30 min, the persistence forecast method works fairly accurately for wind turbines [4]. This is a prediction method based on the most recent output values from wind turbines; because wind plants usually have large mass and inertia, their output does not change instantly. However, for extended forecast periods, e.g., beyond an hour or day, this method becomes inaccurate quickly. The state-of-the-art forecasting method for wind turbines combines the results from several prediction models and data, and develops a complex dynamic space model to increase the forecast accuracy [18]. Doppler radars or wind speed sensors can provide information regarding the wind field around an area and increase the forecast accuracy when they are installed in sufficient numbers. Changes in solar irradiance could be more drastic and noisier, compared with the variability of wind. For instance, a small group of clouds passing over a solar panel can cause variations in the irradiance level on the PV generator, thereby rapidly reducing the overall generation. Therefore, the formation of clouds and the movement of clouds and the sun are important factors in the forecasting of solar energy. Similar to wind forecasts, meteorological data and prediction can provide long-term forecast information, such as day-ahead prediction or much longer. For short-term forecasts, satellite images of clouds can provide accurate information about the time and location a shadow would cover an area. For a more accurate forecast, a high-resolution wind-angle camera can be installed in PV plants to find and track nearby clouds [19]. The system operator can put together multiple pieces of forecast information for an accurate prediction and report it to the TSO for dispatch. The forecast accuracy of PV and wind power generation plays an important role in determining the range of dispatch for the next few minutes (before receiving the next dispatch order).