Key Technology Research For Converter Of Micro-grid

Micro-grid (MG) is a brand new form to construct network in which all kinds of Distributed Generation (DG) are utilized comprehensively. MG has been an important part of the public grid. It has been paid more and more attention and will be an indispensable part of the future intelligent grid. Power electronic converter is the core component of micro-grid. In this paper, theoretical and experimental studies were performed to study the key technology used in the converter control of micro-grid.Firstly, this paper studied the bi-directional DC/DC converter control applied to the energy storage unit in micro-grid. A general DG node containing energy storage system was established. In the hybrid energy storage system, the lithium battery and the super capacitor were directly paralleled and connected to the DC-bus of micro-grid through interleaved bidirectional buck-boost converter. Meanwhile the optimized configuration, analysis of the paralleled structure and equalization techniques were also studied. The experimental results validated that the phase-shift current sharing control strategy applied to bidirectional buck-boost converter can double carrier frequency and reduce total inductance current ripple.Secondly, control strategies of the three phase converter under grid connection mode were studied in this paper. Based on the mathematical model of the three phase converter, the current loop and voltage loop control blocks were deduced and the control parameters were designed, which can be the reference of the grid connection PQ control strategy for general DG unit. This paper also proposed a new software method to identify phase sequence quickly based on dq transformation phase lock loop. With this method the phase sequence can be adjusted automatically and quickly when three phase converters are connected to the grid. Moreover a new conversion method based on multi-function multi-use converter was proposed to improve the bad power qualities coursed by a number of nonlinear loads in micro-grid. In the proposed conversion method, a complimentary system consists of wind power and photovoltaic conversion can not only improve the efficiency of energy conversion but also compensate reactive power and harmonics. Simulation and experimental results were conducted to validate the effectiveness of this method.Thirdly, control strategies of the three phase converter under island mode were studied. Control systems of constant-voltage constant-frequency control and droop control were designed, which can be the design reference of the control of general DG unit under island mode. Meanwhile the preliminary synchronous control method was put forward to realize soft connection between the islanded DG unit and the grid. The practicability of this method was verified by simulation.Finally, the system-level control of micro-grid was researched. Master-salve control strategy and peer-control strategy were proposed and validated by simulation respectively.