FHMMC-HVDC Systen DC Fault Current Calculation Method And Suppression Strategy Research

How to design a fault ride-through strategy for a flexible DC system consisting of a half and full bridge Hybrid Modular Multilevel Converter(FHMMC)to actively provide reactive support to the AC grid while removing DC fault currents as quickly as possible is an important issue that needs to be addressed.This is an important issue that needs to be addressed.The current engineering FHMMC for often uses zero DC voltage control(i.e.zero-voltage control)to clear the fault current,but the zero-voltage control is not fast enough to clear the fault current and will subject the equipment to a certain time overcurrent.Therefore,to ensure equipment safety and operational stability under DC faults,it is necessary to accurately calculate the electrical component of the fault to lay the foundation for designing a more reliable fault ride-through strategy,but current fault current acquisition methods make it difficult to analyse the factors influencing the speed of zero-voltage control to clear fault currents.To this end,this paper investigates the fault current calculation method for the whole process of FHMMC zero-DC voltage control,analyses the fault propagation mechanism and the fault current characteristics after zero-DC voltage control,and designs an adaptive current suppression strategy based on zero-DC voltage control,which includes the following specific studies:Firstly,the effect of the FHMMC control strategy on the DC current is investigated and an FHMMC equivalent model is constructed to analyse the main factors affecting the fault current.The effects of conventional double-loop control during normal operation of the FHMMC and zero DC voltage control under fault on DC current are analysed;the RLC equivalent model of the FHMMC is constructed to analyse the characteristics of bipolar short circuit and single pole earth fault,and the effects of SM capacitance and bridge arm inductance on fault current are investigated.Secondly,a fault current calculation method for FHMMC transmission systems accounting for zero DC voltage control is proposed.The zero-voltage control process is accurately analysed in two stages,and the equivalence model and calculation method for each stage of control are constructed separately.In the fault current development stage,a controlled current source is introduced to characterise the effect of AC feed-in energy on the fault current based on the RLC equivalence model to improve the accuracy of the model;in the zero-voltage control stage,the effect of dynamic adjustment of the bridge arm voltage is analysed and the corresponding FHMMC equivalence model is constructed to improve the accuracy of the calculation.State-space equations are constructed to calculate the DC fault currents in the two phases,and the voltage return to zero caused by the capacitor voltage setting operation is accounted for by the sub-module throwing.The true bipolar FHMMC DC transmission system is built in the MATLAB/Simulink simulation platform,and the proposed calculation method is verified to have high computational accuracy at different fault locations and transition resistances.Finally,an adaptive current suppression strategy for synergistic zero-voltage DC control is designed.Using the proposed calculation method,the key factors affecting the fault current clearing speed are the resistance Req and inductance Leq in the loop.Based on the idea of reducing the equivalent inductance and increasing the equivalent resistance,a strategy of connecting the adaptive resistor RGk in parallel with the flat wave reactor is proposed,and the hardware architecture,internal parameters and control logic of RGk are designed to ensure that the LG in both underdamped and overdamped states can be approximately short-circuited,thus improving the fault current decay rate.The effectiveness of the proposed suppression strategy is verified by adding an adaptive suppression strategy to the original system built on the MATLAB/Simulink platform: the suppression strategy can accurately and adaptively input RGk resistance values according to the Req condition,and the clearance speed is improved by about 50% in the underdamped state and by about 25% in the over-damped state.