| With the increasing demand for renewable energy applications,distributed generation(DG)technology is widely used.In the DG system,the multi-converter grid-connected power generation(PG)systems have attracted more interest due to its advantages of large PG capacity and high device utilization.Currently,the research of this kind of system involves its power topology,control strategies,modeling methods and stability analysis.This paper studies the control strategies and stability of multi-converter grid-connected system based on H-bridge converters.In addition,since the multi-converter parallel grid-connected system based on H-bridge converter is a simplified model of multi-converter system based on photovoltaic power generation(PPG)unit,the control strategy and aggregation modeling of multi-converter systems based on PPG units are studied in this paper.The details are as followings:(1)The LCL-type H-brdige converter grid-connetced PG system,including power topology,the synchronous reference frame proportional-integral(SRF-PI)controller,the dual-current loop grid-connected control strategy based on hybrid coordinates,are studied in detail.The traditional s-domian modeling method is adopted to establish the model of the H-brdige converter grid-connetced system under the strong power grid.Based on the built system model,the zero-pole method is used to analyze the influence of the dual current loop control parameters on the stability of the stuided system.With the TMS320F28335 DSP,the experimental platform of the low-power H-bridge converter grid-connected PG system is built to verify the proposed control strategy and the results of the zero-pole analysis.(2)Stability analysis for the cascaded grid-connected PG system based on the H-bridge converter under weak grid conditions.The system composition is described in detail,which includes power circuit and control system.For ensuring the integrity of the analysis,small-signal analysis is performed on the hybrid modulation strategy.And then,the small-signal model of the hybrid modulation can be obtained.The complete small-signal model of the multi-converter cascaded grid-connected(MCCG)system is established by a simple step-by-step derivation method,and the impedance model and equivalent cascade circuit of the system are sequentially established based on the built small-signal model.Moreover,an improved impedance criterion is proposed to complete the stability analysis of the MCCG system.The proposed criterion can be used to discuss the effect of the phase-locked loop bandwidth,power factor angle and the reference amplitude of grid current on the stability of the MCCG system under weak grid conditions.The stability criterion equation,which includes phase-locked loop and dual-current loop,is derived,and with the equation,the influence of them on the stability margin of the system can be considered simultaneously.At the same time,a systematic design procedure for the optimal selection of the proportional-integral controller of the phase-locked loop is presented,which guarantees the steady-state performance and dynamic response of the multi-converter cascaded grid-connected system.Finally,the theoretical analysis of the impact of PLL on system stability are prvoved by the experimental results of different bandwidths of multi-level multi-converter cascaded grid-connected PG system test platform under strong and weak power grids.In addition,the effectiveness of the selected control strategy and parameter design method are verified by the steady-state and dynamic experimental results at different power factor angles(3)To deepen the understanding of the source of parallel-connected systems for multi-converters based on H-bridge converters,the low voltage ride-through(LVRT)and aggregate model problems of multi-converter photovoltaic grid-connected PG systems based on PPG units are studied.The control strategy based on voltage-outer loop and current inner loop is introduced in PPG unit.Then,the unloading circuit and its control method are proposed for the grid voltage fault,which can be used to suppress DC voltage swell and send reactive power to support grid voltage recovery.Aiming at the problem of low efficiency of simulation analysis of multi-converetr photovoltaic grid-connected PG system,an aggregate model method is proposed to built an euqivalent model of large-capacity system.Based on the model,the switching devices in the system can be reduced,which contributes to saving simulation time.In addition,a parameter equivalent calculation method is presented.Finally,the 500kW PPG unit grid-connected model,the actual model of the 3MVA multi-converter photovoltaic grid-connected PG system and its polymerization model are built under the PSCAD platform.The effectiveness of proposed control strategy and aggregation modeling scheme are verified by the simulation results of 50%symmetry and asymmetric drop of grid voltage.(4)The microgrid-type multi-converter photovoltaic grid-connected PG system model is simplified into multiple single-phase H-bridge converter parallel grid-connected PG systems,and then its modeling method is studied.The state-space method is used to establish the small-signal model of MCPGC PG system and the transfer function model of MCPGC PG system is established by using block diagram transformation and circuit equivalent method in control theory.When n converters are connected in parallel and the parameters of each converter are consistent,it can be seen that the mutual influence of the output currents of each converter can be equivalent to the equivalent admittance of the grid is reduced by n times based on the model and experimental results.With this condition,the influence of the output current of the converter and the control parameters of the parallel system can be obtained by analyzing the control parameters of a single unit.Finally,the multi-converter direct parallel grid-connected system experimental platform is used to verify its the transient characteristics,the changes of the grid impedance and control parameters. |
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