Research On Topology And Control Strategy Of The Hybrid AC/DC Microgrid

Hybrid AC/DC microgrid is an important part of the future smart grid,compared with individual AC microgrid and DC microgrid.Hybrid AC/DC micrgrid has the following advantages: 1)AC and DC microgrid can provide power support for each other in order to improve the reliability of power supply system;2)the conversion links using power electronic device can be reduced so as to lower the construction cost;3)the cutting flexibility and selectivity of loads can be enhanced and the reliability of the power supply for important load can be improved.Therefore,to promote the research and application of hybrid AC/DC microgrid is an effective way to solve the high penetrating integration of renewable energy.In this paper,the key technolo gi es such as topology design and operation control of hybrid AC/DC microgrid are researched in detail.The concrete work are as follows:Firstly,aiming at the shortcomings of current hybrid microgrid topology design,this paper discusses the influence factors of microgrid topology design,including microgrid scale,characteristics of renewable energy and penetration of di stributed generation,load distribution and type,reliability of power supply,and the necessity of interconnection between microgrids.Based on the influence factors,this paper puts forward four principles of microgrid topology design,including principle of partition,principle of stratification,prnciple of power supply quality assurance and principle of interconnection between microgrids.Based on the proposed four basic principles,three kinds of new hybrid AC/DC microgrids are designed,including one-to-many type,many-to-one type and many-to-many type.Taking a university as an example,a case study of hybrid AC/DC microgrid topology is presented,and the applicability of the proposed design principles is verified.Besides,the proposed topology is analyzed and compared from the aspect of reliability,and the influence of the line,converter,and AC/DC circuit breaker on the reliability of the system is studied.Secondly,in view of the difficulty of power exchange between different types of microgrid,this paper proposes a control strategy for interlinked converters in hybrid microgrid.Four typical operation modes of hybrid AC/DC microgrid are defined,including AC grid-connected while interconnecting,both off-grid while interconnecting,AC grid-connected without interconnection,and both off-grid without interconnection,and the power balance relationship in each operation mode is studied.The droop control applied to the hybrid AC/DC microgrid has been improved to make it suitable for low voltage microgrid application.According to the droop characteristics of the microgrids on both sides of the hybrid AC/DC microgrid,the power control strategy applied to the interlinked converters between the hybrid microgrids is proposed.The simulation results show that the proposed power control can realize the active power balance and maintain the voltage stability of AC and DC sides.Thirdly,due to the diversity of the operation modes and the complexity of the switching control of the hybrid microgrid,the paper analyzes the switching relationship between different operating modes,and defines two basic concepts of adjacent operation mode and relative operation mode.For the problem of the timing cooperation between software switching,hardware switching and control strategy switching,the paper proposes the switching principle: software switching is prioritized when the microgrid is switched to islanding mode while hardware switching is prioritized when the microgrid is switched to grid-connected mode.On this basis,a switching timing method of the slack bus is proposed.The voltage and current reference compensation algorithm is introduced to solve the problem of sudden change of the output of PI regulator in the controller in the switching between PQ control and V/f control,constant voltage control and voltage-power control.On the PSCAD/EMTDC simulation platform,the proposed switching principle and the switching timing method of the slack bus are verified,and the results show that smooth switching can be realized among the four operating modes.Finally,in view of the inadequate study in fault mechanism and protection scheme in the hybrid microgrid at present,this paper analyzes the fault propagation mecha nis m in the hybrid microgrid,and puts forward the improvement scheme of hybrid AC/DC microgrid cooperative protection.The mechanism of the influence of AC fault on the voltage of DC side is studied,and the propagation mechanism of the frequency and double frequency component of the voltage in the hybrid microgrid is analyzed theoretically.Based on the above analysis,the protection scheme of the hybrid AC/DC microgrid is improved.The specific improvement is: when protection is provided for DC lines,the protection scheme based on amplitude of voltage and current should better be adopted;when setting the rate of voltage and current in the protection of the DC microgrid,the power harmonic content is required to be introduced as the criterion of DC protection delay action;when configuring the protection for DG,it is needed to use the protection based on the rate of voltage and current change,or use DC harmoni cs protection(50Hz or 100 Hz protection)directly for DGs.The simulation results verify the correctness and validity of the proposed fault mechanism analysis and protection improvement scheme.