Research On Virtual Inertial Control Technology Of DC Distribution Network

In recent years,with the development of distributed power,energy storage,high-density DC load and the development of power electronics technology,DC power grids have attracted more and more attention.However,since the converter isolates the electrical part from the mechanical transmission part,the DC power grid loses the support of the AC large power grid to the DC side stability,and the mechanical kinetic energy stored in the rotating electrical machine is not well applied,resulting in a small inertia of the DC power grid.The voltage fluctuates greatly with the power variation,the voltage quality is not good,and the DC fault is especially serious.In order to solve the above problems,this paper studies the inertial control technology of DC grid,provides power support for DC grid,improves system inertia,suppresses DC voltage fluctuation,improves DC voltage quality,and mitigates the hazard of DC fault.The main work and achievements completed are as follows:(1)Establish mathematical models for wind power,energy storage units,and analyze their working principles.On this basis,a five-terminal toroidal DC grid system including wind power generation unit,photovoltaic power generation unit,battery unit,grid-connected converter unit and AC/DC load unit is built,which lays a foundation for the research of virtual inertial control technology of DC grid.(2)A virtual inertial control strategy for DC grids is proposed.Based on the inertia concept of AC grid,the concept of DC grid inertia is proposed,and the DC voltage variation is introduced into the control link to introduce the concept of virtual capacitor.The rotor speed and DC voltage of the coupled wind turbine are linked with the synchronous motor control variables and DC voltage,and a virtual inertial control strategy is proposed respectively.From the perspective of stable operation of the power grid,the virtual control strategy of virtual control is analyzed stably,and the reasonable range of virtual coefficients is obtained.(3)The key characteristics of DC-to-pole fault characteristics are analyzed,and the theoretical study on the influence of the introduction of virtual capacitors on t he fault characteristics is proposed.Introducing a virtual inertial fault control strategy at the battery end can significantly reduce the speed of DC voltage drop during faults,slow down the DC capacitor discharge speed,greatly delay the voltage zero c rossing,and even change the system damping.Get more time for fault detection and protection actions,improve protection effectiveness and accuracy,and reduce damage to the system caused by extreme faults.Finally,the DC grid system was built in MATLAB/Simulink and verified by simulation.The results show that the proposed control strategy can play a good power support role under different working conditions,improve system voltage quality,improve system inertia,and improve system operation stability.The simulation of DC fault conditions is also used to verify the proposed control strategy,which can reduce the peak current and rising speed of the fault current and reduce the damage of the inrush current to the converter.