Multi-Functional Grid-Connected Inverter For Microgrid Application

Accompanying with the continual energy crisis and environmental pollution globally, solar and wind energy resources have caught common attention recently. As important supplements of traditional large-scale wind/solar plants, distributed generation systems (DGSs) have attracted worldwide attention. As a special kind of DGS, the microgrid is a key linkage between distributed renewable energy resources (DERs) and the utility and has been widely favored because of its high reliability and flexibility advantages. However, the traditional grid-connected inverters in a microgrid is too stiff and just generate purly active current, so much more flexible and multi-functional grid-connected inverters (MFGCI) or distributed generators (DGs) are emerging challenges to utility-friendly interface DERs into utility and simultaneously provide much more ancillary services, in which power quality compensation, autonomous operation, and harmonic resonance damping are the most expected. Therefore, a utility-friendly grid-connected inverter with much more functionalities and flexibilities is an innovative and challenging research field.Face to the increasingly desired ancillary services of micro-grids, some detailed and deep studies on the flexible and multi-functional grid-connected inverter have highlighted in this dissertation by using mathematical modeling, insightful analysis, numerical simulations, and experimental results. Some novel and important conclusions are drawn as follows.1. On the ancillary service provision of power quality enhancement of a grid-connected inverter, the MFGCI combined power quality conditioning service can generate and compensate harmonic, reactive, and unbalanced current components of local loads are well addressed in this dissertation.On one hand, taking the capacity limitation of a MFGCI into consideration, to strategically utilize the limited capacity margin, an objective-oriented optimal compensation control scheme for MFGCI application is presented and can be employed to customize the power quality of the microgrid. In cases of three H-bridge converter and three-phase two level converters, simulated and experimental results from a microgrid demonstration have been displayed to verify the performances of a MFGCI on the provision of power quality ancillary service. Besides, it is also confirmed that the feasibility of a MFGCI to optimally customize the micro-grid’s power quality based on the given objectives.On the other hand, to organize and coordinate the multiple MFGCIs in a microgrid, some useful controllers without communication among MFGCIs to share the power quality issues are also presented. Firstly, a "capacity-compensation" droop scheme is proposed. According to the detected apparent capacity of harmonic and reactive load current flowing through the MFGCI, each MFGCI can decide to partially generate the harmonic and reactive current for compensating by its local information. Then, a cooperative control scheme based on the conductance and susceptance limitation of a MFGCI is also proposed to partly compensate the harmonic conductance and reactive susceptance of local loads in the microgrid. In such a way that, the MFGCIs in the microgrid can autonomously and independently work as power quality conditioners without communication wire. Experimental results performed on a microgrid demonstration are indicated to show the capability of the proposed schemes on coordination control of MFGCIs in a microgrid. Because no communication wire is needed, the proposed approaches can meet the requirements of microgrids on plug-and-play, hot-swap, etc. Additionally, the comparison of the two control schemes is also given, so the users can make a flexible choice.2. On the autonomous operation of MFGCI s in a microgrid, virtual synchronous generator (VSG) is addressed to make a MFGCI to emulate the synchronization and power swing of traditional synchronous generator (SG), so that the MFGCI can provide virtual inertia and damping to utility as ancillary services. Besides, like a SG, the VSG also can regulate its terminal voltage and frequency to support the stability of the utility. Full mathematical model of the VSG is built and discussed in detail, including the rotor moving, governor, and exciter models. In addition, some useful discussion on the influences and settings of VSG’s parameters are contained, too. Besides, some texts on the strategic installment of energy storage devices of a VSG are also embedded in. As a result, a VSG-based MFGCI can be much more flexible and utility-friendly to interface renewable energy resources into the utility. Finally, experimental results from a VSG prototype are also demonstrated to verify the feasibility and validation of the proposed VSG model and control scheme. 3. On the harmonic resonance damping in a microgrid, a control law is presented to reshape the output impedance of a MFGCI, so the impedance of the microgrid can be controlled. Firstly, the mechanisms of the harmonic resonance of microgrids are well investigated with the aid of open-loop and close-loop equivalent models of multiple grid-connected inverters. By reshaping the output impedance of the MFGCI, much more resistive impedance elements can be introduced to the microgrid, and the harmonic resonance of the microgrid can be well suspressed. Insightful analysis, simulated and experimental results are displayed to confirm the performances of the resonance damping control.Aim to provide more ancillary services to a microgrid by using some advanced control schemes of grid-connected inverters, many attempts to improve the multi-functional and flexible features of an inverter-based DG are carried out in this dissertation. The MFGCI combined power quality conditioning technology focuses on the compensation of harmonic and reactive load current components. And the VSG approach tends to emulate the dynamics of a SG in1Hz scale. But the impedance reshaping technology aims to change the impedance in several kHz and damp the harmonic resonance in the microgrid. In summary, all of these advanced control schemes can be integrated in the DSP-based control unit of a grid-connected inverter. Furthermore, with the upgrading of controller in software, the traditional grid-connected inverter can be given much more functionalities and flexibilities as provision of ancillary services. Of course, much more ancillary services can also be considered into the grid-connected inverter in a microgrid scenario and/or utility scale. Thus, further surveys on such framework for ancillary service provision of grid-connected inverters should be carried...