FPGA-Based Real-Time Simulation Of MMC-HVDC And Its Application In The Research On High-Frequency Resonance

Modular Multilevel Converter based High Voltage Direct Current(MMC-HVDC)transmission technology has the advantages of low harmonic content,high controllability and great expansibility.It has been widely used in the fields of large-scale new energy collection and transmission,power grids asynchronous interconnection,long distance and huge capacity transmission.MMC-HVDC achieves higher voltage level and larger transmission capacity by the cascade of sub-modules.With the system scale gradually expanding,the number of sub-modules gradually increases.The number of single-arm sub-modules of Chongqing Hubei DC Project has reached 500,causing a serious burden on the control and protection system,resulting in longer control link delay,worse system dynamic characteristics,and higher risk of high-frequency resonacne.At present,high-frequency resonance events have occurred in many MMC-HVDC projects from all over the world.Real-time simulation is an important tool for research and test of MMC-HVDC,which can effectively improve the efficiency of solving engineering problems.However,compared with the traditional power system,the real-time simulation technology of MMC-HVDC still has many problems,including the simulation accuracy of power electronic devices,the solution of system model and its frequency characteristic simulation accuracy,the hardware architecture and algorithm integration,which seriously limit its guiding role in engineering problems.This paper focuses on the research of real-time simulation technology of MMC-HVDC,builds the real-time simualtion platform of MMC-HVDC based on the Field Programmable Gate Array(FPGA),and studies the high-frequency resonance of MMC connected to AC grid.The main results are as follows:The fixed-admittance modeling methods of power electronic device with high simulation accuracy are proposed.Aiming at the problem of high power loss of fixed-admittance model,the mechanism of power loss is analyzed,and the influence of numerical integration method on loss is quantified.From the perspective of compensating power loss,a fixed-admittance modeling method based on negative resistance compensation is proposed.The power loss of the model is compensated by series negative resistance in the inductance branch.Based on the three-phase two-level voltage source converter(VSC)topology,the parameter optimization design method of the negative resistance compensation model is proposed from the perspective of minimizing transient error and maintaining the stability of the switched system.Aiming at the problem that the compensation effect of negative resistance compensation model is limited at high switching frequency,a fixed-admittance model based on additional branches is proposed from the root of power loss with the consideration of the effect of interpolation algorithm.By switching the additional branch according to the switch state,the model can still ensure low power loss without using interpolation algorithm.Based on offline simulation,the simulation accuracy of the model under multiple conditions and scenarios is verified.A parallel multi-rate simulation method for MMC-HVDC considering the simulation accuracy of high-frequency characteristics is proposed.Aiming at the high-frequency characteristic distortion caused by the network decoupling methods,a converter transformer decoupling method based on the Gauss elimination method is proposed to realize the error-free decoupling between the converter and the AC system.On the premise of ensuring the simulation accuracy of the MMC high-frequency characteristic,combined with the DC transmission line decoupling method and arm decoupling method,further realize the decoupling between the converters and inside the converter.At the same time,with the power electronic fixed-admittance modeling and arm equivalent modeling,the network division model of MMC-HVDC is constructed.Taking the converter transformer as the multi-rate interface,considering the difference of high-frequency characteristics caused by data interaction delay,the multi-rate data synchronization interaction mode is established,and the key factors affecting the simulation accuracy of MMC high-frequency characteristics in multi-rate simulation are analyzed.Based on offline simulation,the time-domain simulation accuracy and frequency characteristic simulation accuracy of the parallel multi-rate simulation model are verified.The real-time simulation platform based on FPGA for MMC-HVDC is built.For the parallel multi-rate simulation model of MMC-HVDC,the real-time simulation calculation of converter valve and the real-time solution of system equations are realized by using the parallel characteristic of FPGA.The calculation efficiency of valve model is improved based on pipeline architecture,the optimization of hardware resources is realized based on logic reuse,and the interconnection of converters is realized based on Aurora.The control function of single-terminal true-bipolar MMC is realized based on a single FPGA,the calculation speed of MMC control algorithm is improved based on the parallel design and timing optimization,the resource utilization of MMC controller is optimized based on fixed-point number operation and multiplexing sorting network.The high integration of circuit solution and control calculation of single-terminal MMC is realized based on the above.The simulation accuracy of the real-time simulation platform is verified by comparing with the simulation model in PSCAD/EMTDC under steady-state,dynamic-state and high-frequency resonance conditions.A suppression strategy for high-frequency resonance of MMC connected to AC grid based on passive filter is proposed.Based on the real-time simulation platform,the impedance characteristics of the AC/DC system are analyzed,the high-frequency resonance mechanism of MMC connected to AC grid is revealed,and the mathematical model of MMC high-frequency impedance is modified.The function mechanism and failure principle of the additional damping control strategies for high-frequency resonance are analyzed based on the impedance analysis method.The characteristic requirements of passive filter used for high-frequency resonance suppression are analyzed,and the suppression strategy based on parallel RLC second-order high-pass filter and its parameter design method are proposed,which can adapt to the change of operating conditions and effectively control loss.Based on the real-time simulation platform,the suppression strategies are digitally processed,and the effect of the suppression strategies are tested.