Research Of Multi-functional Grid-Connected Inverter And Its Control Strategy In Micro-grid

As a special distributed generation system, micro-grid plays an important role in associating the distribution network with renewable energy. It can effectively reduce the impact of renewable energy grid unit to the distribution network and improve the power quality and reliability of power supply. Meanwhile, as an equipment of converting DC energy of distributed power into grid-used AC power, the grid-connected inverter plays a crucial role in the stable operation of micro-grid. However, distributed generation unit of the DC side of grid-connected inverter tends to have intermittent and random characteristics and the grid-connected inverter of micro-grid mostly only can rigidly convert distributed DC power into pure sine wave AC, make the grid-connected inverter mostly does not work at rated capacity, which leads to dynamic surplus capacity. Once the distributed power generation capacity is insufficient to grid-connecting, the grid-connected inverter has to be disconnected from the micro-grid, and this frequent state switch might brought significant impacts to micro-grid. How to make the grid-connected inverter continuously connected in micro-grid and use the changing power margin of grid-connected inverter to do such as, reactive current compensation, harmonic current compensation and negative-sequence current compensation to micro-grid, is becoming one of the great research value research in micro-grid research area.In this paper, the technological demands in adding auxiliary functions to grid-connected inverter under the condition of dynamic surplus capacity in micro-grid are investigated. The following researches: the harmonic governance strategy, harmonic detection methods, and harmonic control methods in micro-grid.1、 Studies on the harmonic current governance strategy based on grid-connected inverter in micro-grid. By analyzing the three types of typical traditional harmonic current governance objectives in grid, which are control the grid-side current waveform proportional to the voltage waveform, control the grid-side current waveform to be symmetrical sinusoidal and eliminate part of harmonic component of the grid-side current waveform, we point out that these three single harmonic governance objectives cannot cope with the dynamic changes of the non-linear loads access and exit and the remaining capacity of grid-connected inverter. We present a flexible harmonic current management strategy of full use of dynamic residual capacity of grid-connected inverters and adapted dynamic load changes:(1)Take full governance of harmonic current when there are sufficient residual capacity while govern part of the harmonic component when there are limited remaining capacity;(2) When the harmonic current caused by load is greater than the harmonic current caused by voltage, taking all governance(or partial secondary treatment) of harmonic current, and only taking governance of those caused by harmonic voltage when opposite.2、 A fine-grained harmonic detection method based on phase compensation is proposed. In order to achieve the flexible management strategy, each component of the random harmonic current(including the fundamental positive sequence component, the fundamental negative sequence component, selective harmonic positive sequence component, selective harmonic negative sequence component) is further subdivided into active components and reactive components. Harmonic current is applied to synchronize coordinate transformation on the phase compensation, completing the active ingredient and reactive component testing of each harmonic current component in the dq coordinate system. The simulation results show that the method can accurately extract the active and reactive ingredient of each component of the current, and meet testing requirements of fine-grained random harmonic with high precision and good real-time characteristics.3、 A current control circuit of grid-connected inverter is proposed to realize the flexible harmonic control. The power tracking control circuit of multi-functional grid-connected inverter is improved to control the fundamental positive sequence active current and calculate current remaining capacity. The control structure of part harmonic current and select the number of current compensation based on the remaining capacity. Through theoretical analysis, it is proved that it is not necessity for governance of any harmonic current component when the d-axis coordinate in the dq coordinate system is greater than zero. On the basis of this conclusion, the zero value detector of the comparator is designed to adaptively determine which the active ingredient of each component of current harmonic current are necessary to compensate. Simulation results verify that the grid-connected inverter used this control circuit can achieve the flexible harmonic current management strategy.The harmonic detection methods on fine grained and improved control circuit structure proposed in this thesis can simultaneously complete the auxiliary functions of basic functions of active outputs and fine harmonic current compensation, and provides low-cost and flexible ways to provide power quality governance in micro-grid.