The large-scale development of renewable sources, with particular reference to wind, in the generation mix might introduce several problems for a secure management of the power system, due to a higher generation uncertainty, thus requiring careful investigations by the responsible for planning and the involved system operators. In this context, CESI has developed a thorough methodology to evaluate all aspects associated to uncertainty and intermittency of RES (Renewable Energy Sources) generation, particularly in the planning process. More specifically, our procedure addresses the cross-relationships between non-programmable renewable productions with reserve margins, network constraints and storage devices. The methodology aims at ensuring the security, reliability, operational integrity, and efficiency of the power system in presence of a remarkable share of non-programmable RES generation.
At first, the analysis consists of assessing the maximum amount of RES generation, as a global value for the whole power system, in compliance with the operating reserve needs to assure the security of the systems. The operational reserves must consider the performances of the generation fleet of the specific electric power system under examination, particularly concerning the additional reserve due to non-programmable RES productions. In fact, the need for additional upward and downward reserve shall match the flexibility of the conventional generation fleet and its capability to cope with fluctuations and unpredictability of the renewable generation. The combination of these technological factors with climatic and geographical factors, which affect the wind and solar variability, plays an important role to assess the need for reserve in relationship with the RES penetration level. Therefore, a statistical analysis of variability of wind generation and of the load shapes is performed to evaluate the additional reserve needed to maintain the required quality of service and to maximise the RES penetration. Only after this phase the network characteristics are taken into account to identify the necessary grid reinforcements and the best connection points for each RES power plant: static and dynamic analyses are carried out considering both the system steady-state operating conditions and the fluctuations of the main electric parameters (voltage, frequency and power flows) caused by intermittent production of RES (mainly wind).
In order to complete the evaluations of RES impact on the transmission network the reliability analysis is worked out using a probabilistic approach based on Monte Carlo method. The probabilistic approach allows to assess the reliability of the whole system to fulfil power demand highlighting the risk of wind curtailment due to network elements overloads, lack of interconnection between regions or countries as well as limits due to the minimum requirement of conventional generating units operating in low load conditions. At the same time the reserve margins necessary to preserve the static reliability and security of the system and the marginal gains on overloaded network elements can be verified. The benefits related to network reinforcements or additional storage devices can be calculated along with investment costs maximising the renewable available productions.
Practical application to a Mediterranean country is presented to confirm the robustness of the methodology.