Study On Several Key Technologies Of Hybrid AC/DC Microgrid

Eleetrical energy is the foundation of the national economic development and people’s lives. How to reasonably develop and utilize the renewable “green” energy has become the common concern for all countries in the world. However, Microgrid consist of micro power sources and local loads can realize that the power generated from micro power sources is used locally, it has become the core of integration technology of distributed generation and energy supply system. Especially, the hybrid ac/dc microgrid has the advantages of ac and dc microgrid at the same time, will become the main grid structure for development and utilization of distributed renewable power in future.Aiming at the key theory and technology of hybrid ac/dc microgrid, based on the financial support of the key project of national natural science fundation of China and graduate student research innovation project in hunan province, the optimized allocation of renewable micro power sources and energy storage equipment capacity, the effective operation and control technology of bidirectional power converter connected between ac and microgrid, the circulating current suppress technology and the power quality improvement technology of hybrid ac/dc microgrid have been in-depth study in this dissertation. The emphasis and aehievement of the pape rmainly manifests in the following aspeets:(1) The optimized allocation method based on the optimal expected output power characteristics is proposed for the capacity of renewable micro power sources and energy storage equipment. Aiming at the total output power instability problem caused by random fluctuations and intermittent of the output power of renewable micro power sources in hybrid ac/dc microgrid, the output power characteristics of the photovoltaic and wind power generation and energy storage device are analyzed. The calculation method of expected output power characteristics of renewable micro power source and the system consist of renewable micro power sources and loads and energy storage equipment is expounded in detail. The optimized allocation method based on the optimal expected output power characteristics is proposed for the capacity of renewable micro power sources and energy storage equipment capacity. It can improve the stability of the output power characteristics of renewable micro power sources and hybrid ac/dc microgrid.(2) The control tasks and operation characteristics of bidirectional power converter under hybrid ac/dc microgrid operated in connected grid operation mode and island operation mode is discussed respectively in paper. Aiming at hybrid ac/dc microgrid operated in connected grid mode, the direct power control strategy based on Clarke transform is proposed to avoid the output current waveform and power control precision of bidirectional power converter is affected by the precision and delay of phase lock loop, which does not need the phase-locked loop in the control system of power converter. At the same time, the dc bus voltage control method is presented based on power feedforward, and effectively improve the fluctuation amplitude of the dc bus voltage.(3) Aiming at hybrid ac/dc microgrid operated in island operation mode, the automatic current sharing control method based on the improved virtual impedance is proposed for bidirectional power converter. This paper analyzed the principle of power sharing error caused among parallel running bidirectional power converters, revealed the relations between virtual impedance and current sharing error and the output active power of power converter respectively, presented virtual impedance technology based on load current sharing error, and realized the precise current sharing among bidirectional power converters. Meanwhile, the proposed automatic current sharing control methods can ensure the operating independence and power output capacity of bidirectional power converter at the same time.(4) This paper proposes deadbeat zero-sequence circulating current control method based on compensating zero-sequence voltage deviation. For suppressing the zero-sequence circulating current between parallel converters with common dc bus, the principle of zero-sequence circulating current caused between parallel converters was analyzed and the mathematical model of zero-sequence circulating current was establised in this paper, and the zero-sequence circulating current suppression principle and deadbeat zero-sequence circulating current control method are proposed based on compensating the zero-sequence voltage deviation, which effectively restrained the zero-sequence circulating current between parallel converters with common dc bus. The proposed circulating current control method provided micro power source and industrial converter power supply with parallel converter structure with a simple and practical theory and technology.(5) This paper presents a power quality comprehensive improvement technology for hybrid ac/dc microgrid. Through analyzing the power quality characteristic of hybrid ac/dc microgrid, bidirectional power converter with distribution grid are put forward as the power source system, and loads with other ac micro power sources based on inverter are put forward as the governed "load". A power quality comprehensive compensator is put forward for the improvement of harmonics, reactive power and unbalanced three-phase load of hybrid ac/dc microgrid. Meanwhile, aiming at the characteristics of the high frequency harmonic suppression requires rapid current tracking and three-phase unbalanced compensation requires rapid dc-side capacitor voltage regulating, a double deadbeat-loop control method is proposed for the power quality comprehensive compensator, and its stability is analyzed in this paper. At last, the power quality comprehensive compensation device is successfully developed.