Flexibility study of the Greek power system using a stochastic programming approach for estimating reserve requirements

This paper presents a detailed flexibility and cost assessment study of the Greek power system for the time period 2021-2025 where for the first time the reserve requirements are determined based on the statistical analysis of the residual load variability and forecast error at hourly steps. In particular, the long-term uncertainties are modeled via forecast scenarios that incorporate different assumptions regarding temperature, load growth, renewable energy sources penetration and hydraulic conditions. In the operational time frame, the net load forecast errors are incorporated via scenarios and a two-stage stochastic unit commitment algorithm with balancing actions is implemented. Forecast error scenarios are generated with a non-parametric approach, using multivariate copula functions to capture the temporal dependencies. A Monte Carlo sampling is used to create uncertainty scenarios and a clustering algorithm to reduce their size. These scenarios are then used as input in the flexibility and cost evaluation study that is subsequently conducted. The study is carried out by solving the unit commitment problem in which all the required technical constraints are considered and then indicative reliability and flexibility metrics are calculated. The two-stage stochastic approach for estimating the reserve requirements is compared with the deterministic approach that has been used from the Greek Transmission System Operator for determining the reserve capacity. The obtained results present great interest, since in both cases the calculated reliability (LOLE) and flexibility metrics (IRRE, flexibility residual) take acceptable values, however in the stochastic approach there is a significant decrease in the system cost. Therefore, through this detailed flexibility study it is shown that the two-stage stochastic approach consists a useful alternative for assessing the reserve requirements of the Greek power system, since decreased operation costs is achieved while security of the system can be established.