The Research On Reactive Power Control Strategies And Loss Reduction Methods Of Cascaded SVG

With the development of the national economy and people's urgent need for efficient and clean energy,a large number of renewable energy and non-linear,unbalanced loads are connected to the grid,triggering a series of power quality problems.Problems such as reactive power,harmonics,voltage asymmetry and voltage fluctuation flicker are the most common,which Seriously affect the reliability and power quality of the power grid,and limit the ability of the grid to accept new energy.The static var generator(SVG)can compensate reactive power,maintain the stability of the system voltage,improve the power factor of the system,and improve the power quality.Compared with the traditional reactive power compensation device,SVG has obvious advantages and it is a research hotspot in the field of reactive power compensation.The topology of the cascaded H-bridge type can increase the capacity of the device,and is widely used in the field of medium-high voltage reactive power compensation.This article takes the cascaded SVG as the research object,and the main work is as follows:Firstly,the topology,working principle and carrier phase shift modulation strategy of cascaded SVG are studied.The mathematical model of cascaded SVG is established.Based on this,the SVG control method including overall voltage balance control,current decoupling control and phase voltage balance control is analyzed and designed.The feasibility of the control method are verified by simulation.Secondly,the research on star-cascade SVG reactive power control strategy under voltage asymmetry conditions is carried out.The zero-sequence component injection method is used to realize the star-cascade SVG phase-to-phase power balance under asymmetric conditions.Based on this,a reactive power support strategy with negative sequence current limiting function is proposed.By adjusting the ratio of positive sequence to negative sequence component in the SVG output reactive current,overmodulation can be effectively prevented and the output current quality can be guaranteed.Combined with the proposed methods,three reactive power control strategies,such as Active Power Oscillation Elimination(APOE),Reactive Power Oscillation Elimination(RPOE)and Balanced Positive Sequence Current(BPSC),are compared and analyzed from the perspective of voltage and current stress,providing a reference for the selection of power control strategies for SVG under symmetrical conditions.The simulation was carried out to verify it.Thirdly,the loss calculation method and the loss reduction method are studied for the power loss problem of large-capacity cascaded SVG.The loss calculation model is established and the calculation method of SVG loss under carrier phase shift modulation is analyzed.According to the influencing factors of loss,the distribution of SVG loss is given,and an optimized discontinuous modulation method based on adaptive zerosequence voltage is proposed.It clamps the modulated wave with the phase of the largest instantaneous value of the current to reduce the switching loss.The validity of the loss calculation and loss reduction method is verified by numerical analysis and simulation..Fourthly,for the development of 10kV/±2Mvar star-scale SVG device,the main circuit hardware design,control system hardware design and control system software design were carried out.Combined with theoretical research,a cascaded SVG prototype platform was built and low pressure and high pressure experiments were carried out.The experimental results verify the feasibility of the proposed SVG device development scheme and the control method.