Research On Key Techniques Of Multi-functional Grid-connected Inverters

With the increasing shortage of fossil energy and the environmental problems caused by it,more and more attention has been paid to clean energy such as photovoltaic and wind power.As the key equipment for renewable power connecting to the main grid,the control technology of grid-connected inverters has been greatly developed.With the continuous improvement of power electronic device performance and the enhancement of controller computing ability,people are no longer satisfied with the control of grid-connected inverters simply injecting the fundamental sinusoidal active current into the power grid,and then turn to the research of multi-function grid-connected inverter.Multi-functional grid-connected inverters can theoretically provide auxiliary services for the power grid in the wide frequency domain.This can contribute to the stability,high quality operation and lower cost of the power grid,without use of dedicated equipment.This paper focuses on the fundamental and the hundreds Hz range of reactive current,three-phase unbalanced current and low-order harmonic current compensation for power quality control.Then two aspects of research are carried out,one is the power control in the frequency range of 0.1 to 2.5Hz,another is the simulation of synchronous Generator swing equation to provide virtual inertial support for Power Grid.Finally,some key technical problems in the realization of auxiliary functions of the multi-function grid-connected inverter are discussed.At first,the different functions of the multi-function grid-connected inverter were analyzed and discussed from the perspective of theoretical analysis.The mathematical model of the grid-connected inverter was established and its control strategy was analyzed.The control idea of the inverter was introduced from the current loop direct control to the voltage loop,and the power outer loop control.The unified control model of voltage,impedance and current control loops for current-quality control and voltage quality control multi-function grid-connected inverters were analyzed.The physical characteristics of the multi-function grid-connected inverter with virtual inertia function were analyzed.It was pointed out that the virtual inertia was mainly reflected in the control of the power outer loop.Then the paper compared the droop control with the virtual synchronous generator control,and introduced the several common realization methods of virtual synchronous generators control,providing the theoretical basis for the analysis of the full text.Then,we focused on the specific implementation methods of the current quality control type multi-function grid-connected inverter and multi-objective optimization problems.Considering the limited installed capacity of a single grid-connected inverter,it is worth paying attention to coordinate the capacity required for power generation and power quality control to maximize its benefits.Combined with a comprehensive energy assessment method based on mutation decision theory,taking into account the limited installed capacity of grid-connected inverters,a multi-objective optimal control strategy was proposed to optimize the integrated energy index and to minimize the compensation capacity.In terms of harmonic current detection,in order to manage the various power quality indicators separately better,a harmonic and reactive current detection scheme based on FBD power theory was adopted,and detailed theoretical analysis and implementation methods were given.And the idea of Pareto optimal solution was adopted to solve the multi-objective optimization problem.The multi-objective optimization problem was analyzed from the three-phase group and three-phase two-level topology structure.The results of simulation and experiment proved the feasibility of the proposed scheme.Then,the method of realizing the reasonable allocation of the compensation capacity of multi-function grid-connected inverter under distributed hierarchical control was discussed.Aiming at the problem of multiple parallel-connected multi-function grid-connected inverters working collaboratively to improve the current quality at the grid-connected point,a hierarchical multi-machine parallel management structure was selected to divide the local controller of the inverter into a tracking layer and a compensation layer and a communication layer,the detailed design process and implementation methods for each layer's functions were provided.The inverter only communicates with the neighboring inverters and achieves a distributed control with consistent and finite average time,so that each multi-function grid-connected inverter can reasonably assign the current to be compensated according to the remaining capacity without a central controller.In the limiting control,in conjunction with the formulation of the sub-harmonic compensation scheme,a control principle that preferentially compensates the worst harmonic current was selected.The simulation model of multi-machine parallel connection is built in MATLAB,and the redundancy and effectiveness of the proposed decentralized hierarchical control are verified from a variety of different working conditions.Finally,the multi-function grid-connected inverter with virtual inertia function is analyzed and improved.The mathematical model of the virtual synchronous generator,the power sharing characteristics during parallel operation,the off-grid switching process and other control methods were analyzed in detail.The small-signal model of the virtual synchronous generator was deduced.The relationship between different inertias and the power output of the inverter was discussed,and the method of determining the key control parameters was given.By analyzing the relationship between the transient process of the synchronous generator power change and the rotor inertia,it was pointed out that the transient process can be divided into different sections according to the changing trend of frequency and active power,and the corresponding rotor inertia can be set.Taking full advantage of the flexibility of the multi-function grid-connected inverter controller,a scheme of self-adaptive adjustment of virtual inertia was given,and the specific method of parameter design was given,which improves the characteristics of virtual synchronous generator in the transient process.Experimental results show that the oscillation peaks of active power and virtual angular frequency are suppressed at the same time.