Research Of Three-level APF Based On Non-harmonic Detection Control

The research content of this paper is a sub project of Shanxi Key Projects of Coal Based Science and Technology “Technology Development and Research of Large-scale Intermittent New Energy Grid Connection''(MD2014-06),aiming at power quality management.With the development of economy and society in China,the number and types of electrical equipment connected to the power grid are increasing,which makes the harmonic pollution of the power grid more and more serious.Therefore,it is an important issue for power grid development to control harmonic pollution and improve the power quality.As a new harmonic control equipment,due to its advantages of small output voltage harmonic content,high DC voltage utilization,and low voltage on switch,the three-level active power filter(Active Power Filter,APF)has a broad application prospects in the field of medium and lowvoltage power supply.However,there are still many technical problems to be solved in the current three-level APF,such as the complex control process caused by the increase of the switch tubequantity,the exacerbation of the dead-zone effect,and the imbalance of the neutral-point potential,etc.Therefore,it is of great significance to carry out research on the theory and practice of the three-level APF for the improvement of its compensation performance and its further application.APF can be divided into two typesaccording to its control methods: harmonic detection and non-harmonic detection.The existing three-level APF is mostly based on harmonic detection,but the calculation amount of harmonic detection is large,and the calculation process has different degrees of delay,so that the effect of compensation is susceptible to the detection accuracy.The APF control system based on non-harmonic detection does not need harmonic detection and extraction,simplifies the compensation algorithm,improves the compensation accuracy,and only detects the voltage and current of the power supply,reduces the hardware cost,and thus has received increasing attention.Applying the non-harmonic detection control method to the three-level APF can integrate the advantages mentioned above to further improve the APF performance.In current research,there are many active power filter control strategies based on the judgment of the compensation current,such as weaken the dead-zone effect and adjust the midpoint potential based on the compensation currentdirection.However,the APF without harmonic detection control limits the application of the above method because it does not detect the compensation current.In order to solve this problem,this paper proposes a compensation current calculation method for three-level APFundernon-harmonic detection control,and designs a Cascade Delayed Signal Cancellation(CDSC)filter to improve the accuracy of the operation.Thereforethis paper obtains the compensation current output by APF without affecting the original control effect,and then adjusts the APF modulation signal according to the compensation current to achieve the object of compensating the dead-zone effect and adjusting the midpoint potential.Thusthe control effect and compensation performance of the three-level APFunder no harmonic detection are improved.The specific content of the study includes:1.Summarized the development history of APF,compared the advantages and disadvantages of different types of APF on the basis of previous studies,discussed several typical three-level topologies,compared their characteristics and applications,and presented the simplified circuit and mathematical model of three-levelAPF.Taking into account the characteristics of various three-level topologies,the APF with a neutral-point clamped three-level topology is considered as the research object,and its operating characteristics are discussed.2.Based on the instantaneous reactive power theory,the typical harmonic detection method and the corresponding harmonic detection control APF model were analyzed.From the perspective of system power balance,the energy flow problems on the AC side and the DC side of the APF were analyzed.From this,it is deduced that the harmonic detection control does not need to detect the positive sequence active current,so that the positive sequence active current detection loop were saved so as to obtainthe basic model of APF based on non-harmonic detection.The feasibility,advantages and disadvantages of the non-harmonic detection control method are discussed.3.Based on the principle of Space Vector Pulse Width Modulation(SVPWM),the voltage space vector distribution of three-level topology and the calculation method of the action time of each vector are analyzed.Seven-segment SVPWM modulation is used to rank the voltage space vectors in different regions.4.According to the operation characteristics of the three-level APF under the non-harmonic detection and control,a calculation method of the compensation current was obtainedand a CDSC filterwas designed to improve the accuracy of the operation.The dead-zone effect was weakened based on the calculated compensation current,and the hysteresis adjustment of the neutral-point potentialbased on the direction of the calculated compensation current was performed.5.Based on the simulation study,a set of three-level active power filter experimental system was built and the proposed dead-time effect reduction strategy and neutral-point potential adjustment method were experimentally studied.The experimental results verified the effectiveness and feasibility of the proposed method.