Research On The Key Techniques Of Power Quality Improvement Of Microgrid

From the perspective of the development of the energy revolution,renewable energy represented by solar and wind energy will become the mainstay of the third energy revolution.In order to support the solution of the large number of renewable energy consumption problems,scientists have proposed a small power generation and distribution system that integrates distributed power(devices that convert renewable energy into electrical energy),energy storage,load,monitoring and protection devices,which is also known as microgrid.The microgrid has two operating modes: on-grid and offgrid.Regardless of the mode,the power quality of the microgrid determines whether it can operate stably and economically,and limits the penetration rate of renewable energy.Therefore,how to ensure the power quality of the microgrid is one of the key issues to be solved urgently.The traditional method of ensuring power quality is to incorporate power quality conditioners such as active power filters and static reactive power compensators into the system.Although this method can ensure good power quality,it often requires a variety of equipment to cooperate with each other,occupying a large physical space,and the initial investment and subsequent maintenance costs are high.In fact,the distributed generators in the microgrid(the core device is an inverter)and the power quality conditioners have a similar topology.On the basis of active power,it has the hardware foundation to provide power quality improvement services,thereby reducing the required capacity of the traditional power quality conditioners and the cost of the system.Therefore,the research of the key technical issues of inverters participating in the power quality management of the microgrid has extremely important theoretical and engineering value for the construction of a low-cost,high power quality microgrid,and is of great significance to promoting the development of renewable energy in China.This dissertation is finished by financial support of the key research and development plan of Jiangsu province(BE2016184): Research on Key Technology and System of Multi-Time Scale Coordination and Optimization of Intelligent Microgrid Power Quality.Aiming at the microgrid with multiple inverters,our research focuses on(1)the problems of harmonics,reactive power and threephase imbalance of the public grid-connected point current in on-grid operation mode;(2)the problem of power sharing accuracy and the frequency and amplitude adjustment of output voltage in off-grid operation mode.The main work and innovations of this paper are:1.In the on-grid operation mode,a distributed cooperative control of micro-grid power quality based on multi-functional grid-tied inverter is proposed.The detailed process of the algorithm is: when the total remaining capacity of the microgrid is sufficient,each inverter communicates with neighbors to share the power quality improvement task according to the ratio of its remaining capacity to the total remaining capacity;When the total remaining capacity in the microgrid is insufficient to complete the power quality task,based on the leader-follower algorithm,the active power output of all inverters is reduced by the same proportion,so as to make room for the power quality improvement task.This method introduces multi-agent consensus theory into the field of microgrid power quality improvement,and provides new perspective for power quality management.2.In the off-grid operation mode of microgrid,an improved P—V roop method based on virtual complex impedance is proposed.The equivalent output impedance of the inverter is shaped as purely resistive through the virtual complex impedance strategy,which realizes the decoupling of active power and reactive power.In the droop control equation,the active power and reactive power are no longer calculated by the capacitor voltage and grid-side current,but the virtual power calculated from the voltage and current of the virtual voltage source is substituted into the droop control equation to ensure the accuracy of power decoupling.In order to reduce the active power sharing error caused by line impedance mismatch,a P—V droop method is proposed,which can reduce the active power sharing error to a certain extent.In order to further improve the precision of power sharing,a proportional coefficient is introduced into the voltage droop equation,which not only speeds up the transient adjustment speed of power sharing,but also improves the precision of steady power sharing.This method effectively improves the power sharing accuracy of the lowvoltage microgrid and improves the power quality of the microgrid.3.In the off-grid operation mode of microgrid,a secondary control strategy based on static/dynamic event trigger mechanism is proposed.On the basis of the improved droop control,in order to further improve the accuracy of power sharing and voltage quality,a secondary control strategy can usually be adopted.Unlike the traditional secondary control strategy,which requires periodic communication,this dissertation first puts forward a secondary control strategy based on static event-triggered mechanism(SETM).The SETM introduces a constant term in the event-triggering condition.Only when the inverter's status error is greater than the trigger condition,the inverter will send its own status information to the neighboring inverter,which greatly reduces The burden of communication.But this scheme can only guarantee bounded stability.In order to further improve the accuracy of power sharing and the speed of voltage regulation,a secondary control based on dynamic event-triggered mechanism(DETM)is proposed.During the dynamic regulation,the threshold value of the event-triggering condition is adjusted to a larger value,which greatly reduces the communication burden in the transient state.In the steady state,the dynamic variables of the event-triggering condition converge to zero.The scheme based on the DETM can ensure consistent gradual stability.The proposed methods effectively improve the power quality of the microgrid and lower down the bandwidth requirements for communication facilities.