Research On The Key Technology Of Grid-connected Converter Under Parallel Operation

The problem of energy is an eternal subject for mankind.In recent years,the energy consumption by human is increasing day by day with the rapid development of human society and a large amount of traditional fossil energy brings pollution to the environment and lead to global warming caused by the massive emission of carbon dioxide.It has been difficult to continue to rely on fossil energy for humain and wind energy and solar energy have been widespread concern due to the features of environment-friendly and sustainable.In the use of wind energy and solar energy,wind power and photovoltaic power generation technology has been greatly developed.As the core equipment of new energy power generation,the technical level of grid-connected converter plays an important role in the wide application of new energy.The technology of power electronics as core technology in the grid-connected converter is widely used in all walks of life in recent years.With the capacity of equipment increasing,the capacity of the converter is also growing.To connect multiple converters in parallel is one way to improve the capacity,but directing the AC and DC sides of the converter will lead to circulation.The presence of circulation will increase the loss of power devices,reduce the current control effect,and may lead to equipment failure.Therefore,it is very important to study the mechanism of the parallel circulation of the converter and propose the suppression strategy.In this paper,study the stand-alone operation control method of grid-connected converter and then the mathematical model of grid-connected converter is established,which includes the mathematical model of three stationary coordinate systems and two-phase synchronous rotation coordinate system.Based on the vectorcontrol strategies which rely on the grid voltage orientation,the phase-locked loop technology of grid voltage is introduced firstly.Then,the voltage outer loop and current inner loop of PI regulator are introduced according to the control model of the system.An improved deadbeat current regulator is presented to reduce the dynamic current error between multiple machines.Deadbeat current regulator has good dynamic characteristics and fast response,as well as a good current sharing effect in the dynamic process.Because of the close relationship between the circulation and the modulation strategy,three widely used modulation strategies are introduced,including traditional SPWM modulation strategy,saddle-shaped modulation wave SPWM strategy and space vector modulation strategy(SVPWM).In the case of asymmetric grid,the article introduces three control strategies according to the different application scenarios,(1)Balanced positive sequence control(BPSC)(2)Instantaneous active power control(IAPC)(3)Hybrid balanced positive sequence and instantaneous active power control(HBPSIAPC)In this paper,the model and definition of the circulation are given on the topology of the parallel converter.The circulation path of the parallel converter is given by the analysis of the topology.Then the mechanism of the circulation is analyzed for different modulation strategies.For the SPWM modulation strategy,the analysis of the circulation mainly starts from the synchronization of the modulation wave and the carrier,and the SVPWM modulation strategy is mainly analyzed from the synchronization of the target voltage vector and the principle of pulse generation.On the basis of analyzing the mechanism of circulation generation,the ideal parallel operation state of grid connected converter is put forward.Aiming at different modulation strategies,the corresponding circulation suppression methods are proposed,including SVPWM modulation strategy with nonzero vector modulation,circulation suppression strategy based on zero vector regulation,Carrier cycle double updated circulation suppression strategy and SVPWM equivalent saddle modulation wave Circulation suppression strategy.A circulation suppression strategy based on carrier synchronization method is proposed for the circulation caused by carrier asynchronous.Finally,the effectiveness of the method is verified by experiments.In the design of the experimental platform,the article gives the hardware design circuit and software design flow chart.In the hardware design,this paper gives the control circuit and the main part of the power circuit.The main modules in the control circuit include voltage and current sampling,PWM signal emission,communication circuit and DSP minimum system.In the design of the power circuit,this paper focuses on the principle of the power device selection and the common scheme of the driving circuit.At the same time,the common problems and it's solutions of the driving circuit are given.Finally,the dual pulse test method commonly used in power circuit is introduced,which can be used to detect the rationality of hardware circuit design.The software design part mainly gives the main flow chart of the program and the interrupt service function of the special event,especially the overvoltage and overcurrent event,which is very important during the initial debugging of the equipment.