Research On High Quality Current Modulation,and Control Technology For Parallel Inverters In Renewable Energy Dominated Power Systems

The development of renewable energy is an effective way to address the issues such as energy depletion,environmental pollution,and ecological imbalance.Nowadays,building a new type of power system with renewable energy as the main body has become a substantial national strategy.Accordingly,the renewable energy industries have been developing rapidly in China,with wind and photovoltaic installed capacity both ranking first in the world.In addition,the proportion of electrical energy consumption and the degree of power electrification are constantly increasing.However,the development of renewable energy dominated power systems is generally accompanied by serious power quality issues,threatening the safety,stability,and economic operation of the system,and thus have attracted more and more attention from academia and industry.Among them,the inverters to achieve DC/AC energy conversion have particularly prominent power quality issues such as current harmonics and leakage currents,which should be carefully addressed.The power quality issue is dependent on the modulation and control strategies of the inverters.Since the system had a small capacity and fewer parallel inverters in the past,the relevant problems have been solved to a certain extent.However,at present,it becomes an important trend to construct large-scale renewable energy industries and further expand the scale of the power application.Therefore,the large-scale and diversified inverters in parallel have also become an inevitable demand,and there will be some new issues in the power quality with increased control complexity.The power quality issues are mainly considered in three aspects:total output current harmonics,leakage current,and low-frequency circulating current.Specifically,it includes:(1)Suppression of total output current harmonics.The existing modulation methods are limited by the synthesis way of voltage vector and the number of vector levels,resulting in complex implementation and poor scalability,and cannot be applied to parallel inverters with different structures and topologies.That results in linear accumulation of total output current harmonics,and makes it difficult to meet operating standards.(2)Suppression of leakage current.The common mode voltage and differential mode voltage of inverters are coupled.Unfortunately,the prior-art methods are difficult to balance the suppression of differential mode current harmonics and common mode leakage current.As a result,the quality of the total output current will be degraded,and the leakage current will be increased,which threatens the safe operation of the system.(3)Suppression of low-frequency circulating current.The previous strategies for reducing the low-frequency circulating current are mainly designed for certain working conditions,and strongly rely on communication,resulting in poor control performance,environmental adaptability,and low system stability.Therefore,this thesis takes the parallel inverters in the renewable energy dominated power systems as the research objective,focusing on the suppression of the output current harmonics,leakage current,and low-frequency circulating current.With the proposed control strategies,the parallel inverters can achieve low harmonic,safe and reliable operation to provide support for the expansion of the renewable energy dominated power systems.The main contributions of the thesis are as follows:(1)To realize harmonic suppression of total output current in parallel inverters,this thesis proposes a current ripple suppression modulation method to satisfy the diverse topology requirements of renewable energy dominated power systems in China.According to the different equivalent output levels of the parallel inverters,this thesis conducts two tasks,namely:① A current ripple suppression modulation method for parallel inverters with equivalent three-level output voltage is proposed.This thesis fully explores the equivalent three-level modulation mechanism of parallel inverters.The vector sequences are selected based on output voltage characteristics to simplify the model of current ripple.Based on this,a vector sequence and action time solving strategy combining offline solution and online look-up table is proposed,which significantly reduces the solution for the current ripple model.The novel method fully utilizes the advantages of three-level output of parallel inverters to reduce the current ripple and simplify the implementation.② A current ripple suppression modulation method for parallel inverters with equivalent multi-level output voltage is proposed.This thesis presents the standard design of a multi-level vector decomposition strategy,which reduces the dimensionality of the multi-level ripple optimization problem to a three-level one.Based on the output voltage characteristics,the vector diagram is divided into two regions:carrier in-phase region and carrier inversion region.For the above two areas,error vector geometry optimization strategies and three-level ripple suppression modulation methods are proposed/applied to optimize the vector sequence and action time,effectively suppressing the total output current harmonics.This technology provides important technical support for low harmonic operation of parallel inverters with various topologies.(2)To address the difficulty of leakage current control,a leakage current suppression method for parallel inverters with carrier phase angle self-correction is proposed.This thesis designs a carrier phase angle correction strategy for the inverter bridge arm,which decouples the output phase voltage and common mode voltage control,and ensures the quality of the output current and the leakage current suppression.A perturbation and observation based carrier synchronization method is developed to ensure accurate equalization/correction of the carrier phase angle between different inverters.In addition,this method replaces the traditional triangular carrier with a sawtooth carrier wave,and associates the carrier phase angle with the modulation waves.That realizes the online update of the carrier phase angle,which simplifies the control algorithm.This technology provides essential technical support for the safe operation of the parallel inverters.(3)In terms of the low-frequency circulating current suppression in parallel inverters under balanced and unbalanced inductance conditions,an inductance parameters adaptive lowfrequency circulating current suppression strategy for parallel inverters is proposed.This thesis employs a virtual reference inverter unit to the average model of parallel inverters,and designs a low-frequency circulating current deadbeat suppression strategy.That achieves low-frequency circulating current suppression without additional communication,and improves the stability of system.Subsequently,an online identification strategy for inductance parameters based on the recursive least squares method is presented,which provides key parameters for the deadbeat suppression strategy and further improves the circulating current suppression effect under balanced and unbalanced inductance conditions.This technology provides essential technical support for the stable and reliable operation of the parallel inverters.In summary,this thesis mainly tackles the three significant challenges,i.e.,total output current harmonic suppression,leakage current suppression,and low-frequency circulating current suppression,in parallel inverters of renewable energy dominated power systems.Modulation and control technologies system for the parallel inverters has been formed,including total output current ripple suppression modulation method,leakage current suppression method with carrier phase angle self-correction,and inductance parameters adaptive low-frequency circulating current suppression strategy.The above research can effectively improve the power quality of parallel inverters in renewable energy dominated power systems,ensuring low harmonic,high safety,and high-reliability operation of the parallel inverters.The technical achievements of this thesis provide essential technical support for the large-scale development of renewable energy dominated power systems in the future.