Stability Analysis And Active Damping Control Of Bipolar DC System Based On IPVB

For the purpose of realizing the inherent requirements of carbon peaking and carbon neutrality,in the "14th Five-Year Plan for Modern Energy System",the National Energy Administration clearly pointed out that it should actively promote the grid connection with large-capacity and high-density new energy sources and build a new type of power system.The new power system includes a large number of distributed energy sources,energy storage systems and DC loads,making the DC system with high conversion efficiency,large power supply capacity and high power quality an important development direction in the future power field.DC system can be divided into unipolar DC system and bipolar DC system.Compared with unipolar DC system,bipolar DC system with two voltage levels not only can effectively improve the power supply efficiency of DC system,but also can adapt to the access of distributed energy and energy storage system with multiple voltage levels.However,due to the large number of closed-loop control converters in the DC system,it can be regarded as constant power load with negative impedance characteristics,which can greatly reduce the stability margin of the system and cause voltage oscillation in the DC system.At the same time,the bipolar DC system has asymmetric operating conditions,which makes its stability analysis method very different from the unipolar DC system.The research on the stability and damping control of the bipolar DC system is important for the safe and stable operation of the new power system.Therefore,the main research work carried out in this thesis for the stability problems and damping control of bipolar DC systems is as follows:In this thesis,a bipolar DC system based on Interleaved Parallel Voltage Balancer(IPVB)is studied.Firstly,the research status of bipolar DC system structure,voltage balancer structure and DC system stability analysis and control is explained,the topology and working principle of seven types of voltage balancers are given,and the structure and ripple current of IPVB are analyzed.Next,the impedance characteristics of IPVB and constant power load converter cascade structure in bipolar DC system are analyzed,and the transfer function expressions of converter cascade structure are derived;the stability analysis method of bipolar DC system is given based on Middlebrook criterion;then,the constant power load equivalent small signal model is derived using the constant power load large signal model,and the IPVB small signal model is built.The stability analysis of the IPVB-based bipolar DC system is carried out by using Middlebrook’s stability criterion.Based on this,a passive damping control method with output capacitor in parallel with a resistor is applied to the IPVB-based bipolar DC system,and the simulation is verified.Finally,considering that the access of the actual resistor of passive damping will bring additional power loss to the system,an active damping controller is designed for the IPVB-based bipolar DC system by constructing a virtual resistor in the system to realize the damping effect of the actual resistor.To verify the feasibility and effectiveness of the active damping control method,an IPVB-based bipolar DC system simulation model is built in the simulation software MATLAB/Simulink to verify the effect of the active damping controller in suppressing voltage oscillations.The simulation results show that the active damped controller can effectively suppress the voltage oscillation caused by the negative impedance characteristics of the constant power load while taking into account the dynamic response speed of the bipolar DC system compared with the suppression effect of the passive damped control method.