Research On AC-AC Grid-connected Control Technology Based On H-MMC Direct Drive Permanent Magnet Wind Power Generation System

Driven by the "dual carbon" goal,wind power is one of the current research hotspots.As the voltage and power level of wind turbines are getting higher and higher,the traditional two-level converter is limited by the withstand voltage level of the switching device and has gradually withdrawn from the stage of history,replaced by multi-level converter technology.This paper studies the permanent magnet direct drive wind power generation system based on hexagonal modular multilevel converter(HMMC).Compared with traditional back-to-back MMC,H-MMC saves the DC bus link.,directly through the AC/AC conversion of the six bridge arms to realize the wind power grid connection,the topology structure is simple,and the sub-module capacitor voltage ripple is smaller at low frequencies.Therefore,H-MMC has obvious advantages in the field of wind power.This paper studies wind power system modeling,pre-charging and grid-connected control technology,fault detection and location,and redundant faulttolerant control.The specific research contents are:First,this paper introduces the topology and mathematical modeling of the directdrive permanent magnet synchronous wind power system,and analyzes the different working modes and charging and discharging conditions of the full-bridge sub-module.According to the mesh current method and KVL,the mathematical expressions of the bridge arm current and the bridge arm voltage are deduced,and the power components of different frequencies on the bridge arm are calculated based on this.The capacitor voltage ripple of the back-to-back MMC and H-MMC sub-modules is compared under the same conditions,and the superiority of H-MMC at low frequency is verified.The two modulation modes of H-MMC and their corresponding voltage regulation control are introduced.Then,based on the working mechanism and mathematical model of H-MMC,this paper proposes an AC-side pre-charging technology for H-MMC,analyzes the characteristics of the uncontrollable charging stage and the controllable charging stage,and proposes a method based on proportional resonance(PR)current controllable constant power charging mode.In terms of wind power grid-connected control,this paper adopts the maximum power tracking control with the best tip speed ratio to achieve the maximum utilization of wind energy.At the same time,a power balance control is proposed to maintain the H-MMC output power equal to the input power.In order to stabilize the capacitor voltage,a layered control strategy for capacitor voltage is also proposed in this paper.According to the difference of the current inner loop,the traditional quasi-dual PR control method is introduced in this paper.The effectiveness of the proposed method is verified.Finally,according to the output characteristics of the full-bridge sub-module,the detection process and positioning method when the IGBT is short-circuited and opencircuited are proposed.The cold standby and hot standby redundancy methods are introduced when the faulty sub-module needs to be removed,and on this basis,an improved redundant fault-tolerant control is proposed.effectiveness.