Short Circuit Ratio Analysis And Optimization Planning Of Large-scale New Energy Grid Connected System

The new energy power plant is connected to the AC power grid through power electronic converters,forming a new AC network with a high proportion of access to new energy.Compared with the traditional AC Power Grid dominated by synchronous machine,this new power network has the characteristics of high flexible control performance and fast response.However,at the same time,due to the lack of mechanical inertia of synchronous machine and poor anti disturbance performance of power electronic devices,the electrical strength of AC power grid connected with new energy becomes weak.Especially when a large-scale new energy power plant is incorporated into the AC power grid,complex coupling relationships will occur between new energy units and AC power grid and between new energy units,which will lead to a series of oscillation problems.These problems will lead to the offline of new energy units,damage the electrical equipment in the power grid,bring property losses and potential safety hazards,and hinder the development of new energy power generation.Therefore,how to scientifically evaluate the electrical strength of the new power network is an urgent problem to be solved.Short circuit ratio is widely used to describe the static stability of DC system.Now it is extended to analyze the stability of new energy grid connected AC system.Through the analysis and calculation of the short circuit ratio parameters of the parallel node,the electrical strength of the parallel node can be evaluated.In this dissertation,the method of impedance analysis is used to study the short-circuit ratio,the optimization method of grid connected capacity limited by the stability constraint critical value of short-circuit ratio is proposed,the calculation program of short-circuit ratio is compiled,and the small interference stability experiment is carried out to verify the scientificity of using short-circuit ratio to analyze the instability risk of grid connected system.The main research contents and achievements of this dissertation are as follows:In view of the current situation that the new energy grid connection system is prone to instability problems,investigate and summarize the research progress of this problem at home and abroad,understand the main technical solutions at this stage,and summarize different solutions.Focus on the stability of China’s new energy grid connection system in the new stage.Aiming at this problem and the existing solutions,this dissertation establishes the main work content and significance of this dissertation.Firstly,the impedance analysis method is used to establish the impedance model of new energy grid connected system.According to the control model of grid connected inverter,the impedance expression of grid connected inverter is derived.Secondly,the concept of short-circuit ratio is introduced to evaluate the instability risk of new energy grid connected system,and the effectiveness of using short-circuit ratio to analyze the system oscillation risk is verified from the perspective of impedance.For the case that multiple stations are connected to the same AC power grid,the concept of equivalent short-circuit ratio of multi infeed system is introduced,the coupling relationship between units and between units and power grid is taken into account,and the equivalent short-circuit ratio is calculated.The short-circuit ratio of multi infeed system is equivalent to that of single infeed system,which is used to analyze the stability of multi machine grid connected system.Then,the single machine grid connected system and multi machine grid connected system are modeled and analyzed.By analyzing the system stability in the critical state,the critical value of short-circuit ratio stability constraint of the system is determined.The maximum grid connection capacity shall be determined on the premise that the conditions for new energy control and access to the power grid are given.It is concluded that the short-circuit ratio can be used as an index to measure the stability of the system.Finally,taking the critical value of short-circuit ratio stability constraint as the constraint,and considering the influence of fan simultaneity rate,whether there is a condenser and other factors on the grid connected capacity,the calculation program is compiled to calculate the grid connected capacity of new energy system.Finally,the planning and optimization scheme of grid connected capacity of new energy source considering the constraint of short-circuit ratio is given.