| The micro-grid system is a solution to energy utilization and management of renewable energy with distributed characteristics,an intermediate form of transition from traditional power grid to smart power grid.The parallel inverter is a significant part of this system,and the droop control inverter has become one of the hot topics in the field of parallel inverter control due to its characteristics of parallel operation with no communication line,its ability to supply voltage and frequency support,and unified control structure in different operation modes.In this paper,the parallel inverter system control strategy,based on droop control,is studied in the aspects of islanded power distribution,islanded disturbance suppression,operation modes transfer and harmonic suppression of the injected grid current,with the effective solution to promote the application of droop-controlled inverter.This paper is organized into the following aspects.When a single inverter cannot meet the load power demand,the parallel inverter is usually used to distribute the load power reasonably.When the parallel system contains renewable energy such as photovoltaic(PV),the active power distribution principle based on the traditional droop control method will cause the waste of PV energy.In order to improve the utilization rate of PV cell energy,an improved droop control strategy of PV inverter is proposed.Based on the closed loop control method with PV maximum output power,the active power translation is generated and superimposed on the traditional droop control active power loop when the reactive power loop is not changed,and the stable interval of the corresponding parameters is provided.The algorithm can realize the maximum power output of PV cells in parallel system by shifting the active droop curve,and meanwhile can restrain the external disturbance of load and PV cell power fluctuation.With the traditional dual-loop control structure,the dynamic characteristics of the droop control inverter are limited by the bandwidth of droop control loop,and the output voltage dynamic response is slow and the disturbance suppression ability is poor due to the use of the linear controller when the external disturbance may trigger the system protection.In order to improve the dynamic characteristic of the output voltage and the disturbance suppression ability of the system,this paper analyzes the influence of the dual-loop structure and control parameters on the performance of the system according to the traditional dual-loop control structure based on quasi-proportional resonance control.On this basis,the disturbances suppression control strategy is proposed based on SuperTwisting algorithm for droop control,where the second order sliding mode Super-Twisting algorithm is used to design the novel output voltage controller,with the result to optimize reference signal generation of inductance current and improve the effect of output current control;and with this algorithm,the novel inductance current controller is designed to optimize reference signal generation of output voltage modulating signal,improve the effect of output voltage control and restrain external disturbance.There are two operation modes in micro-grid inverter: islanded and grid-connected.If the control strategies of the two modes are obviously different,the surge current or voltage overshoot will occur at the transient of modes transfer.The utilization of droop control can unify the control structure in different modes and realize seamless transfer.However,in grid-connected mode,the injected grid current can only be controlled indirectly,which is easily affected by the harmonic of grid voltage when the transient dynamic response is slow.In order to improve the power quality of the injected grid current,a seamless transfer strategy is proposed based on improved droop control.In islanded mode,based on the output voltage frequency in reverse,the output power reference is generated in the traditional inductive droop equation.And the direct current control structure is adopted in inner loop and the reactive power-voltage control method is superimposed to optimize current reference generation;the direct current control is used in grid-connected mode to ensure the unity of the control structure in the two operation modes and seamless transfer of operation modes.Based on droop characteristics,the method can take three control objectives into account: parallel inverter,seamless transfer of operation modes and direct control of the injected grid current.Harmonic content of the injected grid current is an important performance index for grid-connected inverter.The choice of controller influences significantly on harmonic control of the injected grid current.In this paper,the generation mechanism of harmonics of the injected grid current is analyzed,and the limitations are illustrated concerning traditional droop control-based grid-connected current harmonic suppression method in practical application,such as complex control structure,many control parameters and slow dynamic characteristics of harmonic extraction.In order to overcome the problems above,a novel harmonic suppression strategy is proposed.The port-controlled Hamiltonion model of the inverter is established,and according to the energy shaping control theory,the grid-connected controller is designed to simplify the structure of grid-connectedcontroller and the number of control parameters.On this basis,the harmonic extraction algorithm is designed by using linear discrete Kalman filter to optimize the dynamic of harmonic extraction.On the basis of the work above,a parallel inverter system experimental platform is built based on Danfoss converter and RT Box of real-time hardware-in-the-loop Platform,with the verification of the proposed power maximization strategy of PV,voltage disturbance suppression strategy,operation mode seamless transfer strategy and harmonic suppression strategy of the injected grid current. |
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