Research On Control Strategy Of Active Power Filter Based On Modular Multilevel Converter

With the development of power electronic technology, our industrial production and daily life are becoming more and more efficient and energy saving. However, various nonlinear loads using power electronics are also deteriorating power quality greatly by injecting non-negligible harmonics and reactive currents into the grid. Among all the technologies of harmonics and reactive power compensation method, active power filter(APF) is the most widely used. As a new topology, modular multilevel converter(MMC) has a highly modular structure, which makes it easily to be extended to higher voltage level. Besides, it has an excellent output quality due to its multilevel output waveforms. Therefore, MMC has a bright application prospect on occasions where high voltage and large capacity devices are needed to compensate for harmonics or reactive power. Up to now, the research of MMC used in the field of power quality control is still in the initial stage, which makes this paper's research of great significance as it applies MMC to harmonic compensation.This paper firstly introduces the generation and harmfulness of harmonic currents as well as the influences of reactive power. Then the development history of APF and its research status is investigated. By comparing the advantages and disadvantages of various multilevel converters, the benefits of using MMC to compensate for harmonics and reactive power are pointed out. Then the circuit structure and working principle of MMC-APF is introduced. Through mathematical analysis and deduction, mathematical models of MMC-APF under abc coordinate system and dq coordinate system are obtained. The current inner loop model is obtained by introducing feedforward voltage and cross decoupling control.The design of control system in all aspects for MMC-APF is introduced in details. Based on carrier phase shifted sinusoidal pulse width modulation(CPS-SPWM), output waveforms of N+1 level modulation and 2N+1 level modulation are realized respectively with different phase-shifted angle. In order to realize a smooth startup of the experimental system, precharging process is divided into uncontrolled and high-frequency rectification statge. Hierarchical control strategy including three parts is introduced to get a balanced capacitor voltage working condition. Multiple synchronous rotating coordinates are introduced to get the command value of harmonic compensation. As for the control method of inner current loop, after analyzing the limitation of PI controller in tracking harmonic currents, repetitive controller is introduced as it has complementary advantages and disadvantages with PI controller, so this paper achieves the tracking of periodic harmonic currents by combining PI control and repetitive control in the current loop.To verify the correctness of the control strategies mentioned above, simulation model of a three-phase MMC-APF with 12 submodules is established in Matlab / Simulink, and an experimental prototype with the same parameters is builded. The results of simulation analysis and experiments show that MMC-APF can compensate for harmonic currents and reactive power effectively.