Research On DC Voltage Control Strategy Of Single-phase Back-to-back Converter

The single-phase back-to-back converter system has the advantages of net-side current sine,energy been four quadrant flow,DC voltage control,etc.It is widely used in four quadrant AC speed control system,AC power electronic load,uninterruptible power supply and other power electronic devices.It uses a two-stage AC/DC/AC converter structure.the middle part is through the DC capacitor connection for transfering the energy.DC capacitor voltage control is usually completed by the PWM rectifier connected to the grid side,often using double closed-loop PI control strategy.However,when the load disturbance or access to reactive load,the traditional double closed-loop PI control can not quickly track,so that the DC voltage fluctuations,reducing the life of components such as capacitance,affecting system stability.People often used to increase the DC capacitor capacity to suppress DC voltage fluctuations in industry,resulting in increased costs,reduced power density,which do not help its promotion and application.In this paper,a new DC voltage control strategy is proposed based on the feedforward control method by using the method of theoretical analysis,digital simulation and platform test to suppress the DC voltage fluctuation and reduce the DC capacitance.Firstly,the mathematical model of single-phase back-to-back converter is established.Based on this,the traditional DC voltage double closed-loop control strategy is studied,and the control parameters are designed.At the same time,the power flow path in the system is analyzed in detail,and the conclusion is drawn that controlling power balance suppresses the DC voltage fluctuation.Aiming at the problem of DC voltage fluctuation caused by load disturbance,this paper first analyzes its causes.On this basis,considering the control delay,a DC voltage improvement control strategy based on DC current feedforward is proposed,and its feedforward Coefficient of the parameter design method are given.Based on the active power balance between the AC and DC side of the system,a single-phase back-to-back converter AC/DC unified model is established by using the small-signal analysis method.The effectiveness of the proposed feedback control strategy to suppress the DC voltage fluctuation is proved by the model.Aiming at the problem of DC voltage ripple caused by reactive current,this paper is inspired by the three-phase instantaneous reactive power theory.Through the instantaneous power balance analysis,the instantaneous power model of single-phase back-to-back converter in dq coordinate system is established and the input,The output reactive current relationship.Based on the DC current feedforward control,a DC voltage improvement control strategy based on instantaneous current feedforward is proposed to realize the input and output instantaneous power balance and suppress the DC voltage ripple.The simulation verifies the validity of the proposed control strategy.Finally,considering the role of feedforward control,a minimum DC capacitance parameter design method is proposed.The theory and simulation show that the feedforward control can effectively reduce the demand for DC capacitor and improve the economic benefit.A power hardware in-loop simulation platform is built,and its power interface adopts single-phase back-to-back converter as the main topology.After adding the DC voltage improvement control strategy based on feedforward control,the DC voltage fluctuation is effectively suppressed and the system stability is improved.