Research On Power System Flexibility Dispatch And Its Assessment With Large Scale Renewable Energy Sources

Under the dual pressure of environmental crisis and energy depletion,the renewable energy industry has developed rapidly,and the integration scale is continuously expanding.With the gradual increase in the penetration rate of renewable energy,the uncertainty and fluctuation of its output have a huge impact on the safe and stable operation of the power system.Flexibility of power system refers to its ability to cope with power fluctuations.The traditional dispatch models usually only consider the net load information at one period,and ignore the fluctuation of the continuous power balance during the adjacent period.Under the impact of strong fluctuation renewable energy,the problem of insufficient power system ramping ability is increasingly serious.The idea of flexibility provides a fresh angle for solving the operation optimization problem of large-scale renewable energy integrated power systems.To this end,this dissertation focused on the flexibility of power system operation.The main research work involves:Accurately grasping the volatility and uncertainty of renewable energy output is of great significance for studying the impact of large-scale renewable energy intergration on the system operation.For this purpose,this paper establishes a volatility model of renewable energy output considering the relevance,uncertainty and volatility of renewable energy output comprehensively,defines the fluctuation of renewable energy output as the power difference between adjacent moments,and a fluctuation model of renewable energy output considering the spatial and temporal correlation is proposed based on the analysis of a large amount of historical data.Firstly,statistical characteristics of the renewable energy output fluctuation and the impact of correlation on the fluctuation are analysed from historical data.The analysis results show that the correlation of renewable energy output has a significant influence on its fluctuation characteristics.Therefore,correlation must be considered properly.Secondly,a general generating function algorithm considering correlation is introduced to calculate the conditional probability model of the total renewable energy output fluctuation.Thirdly,combined with the prediction sequence and error information,a probabilistic model of the time-series output fluctuation of renewable energy is established.Finally,the simulation results show that the proposed model can restore the fluctuation characteristics of the renewable energy output,and the research results provide a model basis for subsequent research.Aiming at the quantification of the regulation capacity required by the renewable energy integrated system,a novel flexibility based day-ahead generation-reserve bilevel decision model is proposed.In the upper level planning.the relationship between reserve and flexibility established by the lower level decision is converted into constraints,and a unit commitment(UC)plan is formulated based on the predicted load and wind power output.In the lower level,taking into account various factors of uncertainty and fluctuation,e.g.,wind power ramping,load ramping and random failure of conventional units,the ramping probability distribution of an equivalent system is obtained by the universal generating function method,then the quantified relationship between operating reserve and flexibility is established ultimately.If the unit commitment scheme gained from the upper level could not provide sufficient reserve,a feedback for the correction of the upper level is needed.The examples show that the proposed model can fully reflect the power fluctuations in the system,and the obtained generation-reserve coordinated scheme can ensure that the system has sufficient flexibility on the basis of meeting the load demand.The conventional dispatch model is difficult to fully consider the power fluctuations between adjacent periods in the real-time balancing process.Therefore,a novel day-ahead flexible dispatch model for renewable energy power systems is proposed to better coordinate the system flexibility resources.Firstly,the value at risk theory is introduced to assess the system's net load ramping risk,and the system's operational flexibility requirements are quantified by the correlation-based renewable energy power plant system's net load ramping probability model.Then,minimizing the sum of flexibility service cost and conventional unit operating cost are as the objective function,considering the flexibility requirements constraints and network transmission constraints,a flexible dispatch model is proposed.The case studies show that the flexibility dispatch model fully considers the fluctuation in the continuous power balance process,and is more suitable for the power system with renewable energy.The flexible scheme can coordinate flexibility resources and achieve both economic and flexible operation of the power system.In the aspect of evaluating the operation flexibility of power systems with renewable energy,a set of evaluation index and methods for operation flexibility of dispatch schemes are proposed.It includes six indices:node upward/downward flexibility margin,node insufficient upward/downward flexibility probability,and node insufficient upward/downward flexibility energy.The upper and lower limits of node net load are obtained by an improved continuous power flow method,considering network transmission constraints,conventional unit regulation capacity constraints,and node voltage constraints,so that flexibility margins are obtained;then the remaining flexibility indices are calculated by the node net load fluctuation probability model.The examples show that the proposed indices can describe the node flexibility in detail,as well as the potential load loss or renewable energy curtailment,and help the operators to identify weak links in operation.The proposed indices and methods provide a theoretical basis for the optimal dispatch of flexible resources.