Frequency Stability Analysis And Control Of Power Grid Considering Emergency DC Power Support

China’s vast territory and uneven energy distribution make HVDC transmission technology widely used in recent years,including the operation of more and more DC asynchronous networking projects.The DC asynchronous networking makes the large power grid gradually divided and reduced into multiple asynchronous small grids.The system spare capacity and inertia are gradually reduced.This makes the system frequency stability problem caused by large disturbances such as large capacity generators falling off,large load fluctuations and regional grids tie line faults more significant.Therefore,frequency stability evaluation and control for the DC asynchronous interconnection power grid is very important.This paper considers that the existing frequency stability analysis method is mainly for the pure AC grid,and its application to the complex AC-DC grid will have problems in accuracy and applicability.In addition,the emergency power support capability of the HVDC transmission system provides a new frequency stability control method for the DC asynchronous interconnected power grid,which has the advantage of fast action compared to the traditional under frequency load shedding,and does not bring the power supply reliability issue to the grid as same as automatic load shedding control.Therefore,this paper firstly studies the frequency stability analysis method applicable to complex AC/DC power grids.Then,based on DC emergency power support capability,the frequency stability control methods for HVDC and multi-HVDC asynchronous interconnection grids are studied.Therefore,the main research work of this paper includes:(1)In this paper,the detailed LCC-HVDC and VSC-HVDC transmission system models are considered based on the existing AC grid linearization frequency stability analysis method,and its application is extended to complex AC/DC power grids.Firstly,a steady-state frequency prediction method for AC/DC hybrid power grid after disturbance is proposed.This method takes into account the power equation,control equation and network equation of HVDC transmission system in the existing AC grid steady-state frequency prediction model.Using wide-area measurement data,the steady-state frequency after AC/DC hybrid power grid disturbance can be quickly obtained.Secondly,a frequency dynamic prediction method of HVDC asynchronous interconnected grid is proposed.The method establishes the post-disturbance state equation of the asynchronous grid containing the HVDC transmission system model.By directly solving the state equation with the wide-area measurement data,the post-disturbance frequency dynamic characteristics of the AC grid at both ends of the HVDC can be obtained at the same time.(2)For the HVDC asynchronous interconnected power grid,the steady-state frequency and extreme frequency stability control methods based on DC emergency power support are proposed respectively.The methods can respectively calculate the DC emergency power support amount according to the given post-disturbance steady state frequency and the extreme frequency control target value.Considering that the AC bus voltage level at both ends of the LCC-HVDC system will affect its operation mode,thus affecting the actual emergency power support capability of the HVDC system.Therefore,this paper further expands a frequency stability control method that can account for the limited emergency power support of the HVDC system.The method can determine the operating state of the HVDC system by the AC bus voltage at both ends,and calculate the power modulation command value of the HVDC system required to achieve the desired emergency power support in this state.(3)For the asynchronous grid with multi-HVDC links,the steady-state frequency and extreme frequency stability optimal control methods based emergency power support are proposed.The proposed methods first evaluate the grid frequency stability by predicting the steady-state frequency and the lowest frequency after disturbance.For the case that the steady-state frequency and the extreme frequency exceed the accepted security range,the emergency power support of the multi-HVDC links are considered to participate in the frequency stability control.The nonlinear optimization models are employed to formulate the proposed optimal controls respectively,which can not only restore the post-disturbance steady-state frequency and the lowest frequency to the target value,but also enable the node voltage,the line transmission power and the frequency deviation of the interconnected power grids to meet the security requirements.By solving the optimization model,the optimal emergency power support scheme for frequency stability control can be obtained.(4)Considering the time-effect advantage of machine learning method in power system security and stability online control,this paper proposes a SVM-based frequency stability control method for the HVDC asynchronous interconnected power grid.By establishing a multi-layer SVM model,the method can quickly complete the post-disturbance frequency stability prediction,the frequency stability control mode selection and the frequency stability control scheme formulation by using the WAMS data,and realize the comprehensive utilization of DC emergency power support and load shedding control.Coordinated participation in frequency stabilization online emergency control.The proposed method can realize the comprehensive utilization of emergency DC power support and load shedding control to participate in the frequency stability online control.