Research On Optimal Operation And Key Technologies Of Grid-Connected Interface In Microgrid

With the deterioration of energy crisis and environment pollution,the utilization rate of renewable energy is getting higher and higher.Microgrid and distributed power generation have attracted wide attention at home and abroad.Distributed generation and is connected to the grid through power electronic devices,and microgrid is connected to the upper grid through devices.The design and control of grid-connected interface is the key technology in micro-grid.It is very meaningful to study it.In view of the above problems,this paper discusses and studies the stability of micro-grid operation,the topology and control of the grid-connected power electronic transformers,the grid-connected control strategy of distributed generators and the three-phase unbalanced control strategy of micro-grid.The specific work and innovation are as follows:The characteristics of control structure and its advantages and disadvantages of microgrid are analyzed.At present,droop control strategy is commonly used in parallel operation of distributed power supply in microgrid.Because of inconsistent line impedance,circulating current will occur in the line,and power distribution among distributed generations will be uneven.In order to improve the problem,virtual impedance is introduced into the control.However,the influence of virtual impedance on the stability of the whole microgrid system is still unknown.This paper establishes an equivalent circuit model for parallel operation of distributed power supply in microgrid,establishes a small signal model for droop control based on virtual impedance,draws the root locus of the small signal model of microgrid,which reflects the influence of different parameters in droop control on the stability of the system.The control strategy of distributed generation is studied.And the mathematical model of virtual synchronous machine control strategy is analyzed with the basic principles of active power and reactive power control.The DC side of virtual synchronous machine needs to be equipped with energy storage to provide energy for power fluctuation and damping inertia simulation.As the virtual synchronous machine control is a typical second-order model,the energy density and power density of DC side energy storage are analyzed in three cases: over-damped,under-damped and critical damped.A hybrid energy storage structure on the DC side is proposed.When power fluctuation occurs,the power support on the DC side is divided into high frequency variable power caused by frequency fluctuation and low frequency variable power caused by grid frequency deviation according to energy conservation,which ensures the smooth charging and discharging of energy storage battery and suppresses power fluctuation by supercapacitor.In order to meet the working requirements of power grid and make full use of the advantages of control flexibility of power electronic devices,the operation mode can be switched while simulating the external characteristics of synchronous generators.The active and reactive power control module of virtual synchronous machine is improved with the discrete control model.And the influence of different parameters on the control of virtual synchronous machine is analyzed.ADRC is introduced to optimize current tracking control.The micro-grid is connected to the upper power grid through the interface of power electronic transformer,and the topology and operation characteristics of power electronic transformer are analyzed in the paper.With modular multilevel converter structure as the front stage of power electronic transformer,an n+1 hybrid modular multi-level structure is proposed.A full-bridge sub-module is connected in series on the bridge arm of half-bridge sub-modules.With the number of sub-modules reduced,the AC voltage level is increased.The hybrid topological mathematical model is established,and the modulation method is studied.On the basis of keeping HMMC working stably,the switching loss of HMMC sub-module is reduced.Because the capacitance voltage of full bridge sub-module in HMMC is only half of half bridge submodule,the traditional charging control strategy can no longer be applied.The precharging strategy of hybrid topology is studied when the micro-grid is black-start.The capacitor voltage balance and maintenance of sub-module also affect the normal operation of the whole system.This paper proposes a capacitor voltage balance strategy for the full-bridge sub-module and half-bridge sub-module of hybrid topology,and analyses the mathematical model of capacitor voltage charging and discharging.The operation control strategy of microgrid under three-phase unbalance is studied.The characteristics of three-phase unbalance are analyzed.The main objectives of unbalance control are three-phase current balance on AC side,voltage stability of DC bus and reactive power balance.And the positive and negative sequence separation is separated by the complex coefficient filter.Aiming at the abnormal operation of distributed generation caused by three-phase unbalance in micro-grid,the electrical part model of virtual synchronous machine is improved to achieve two control objectives: current negative sequence component suppression and DC voltage ripple suppression.The one-cycle control is applied in the modular multi-level converter topology in power electronic transformer,and the control model is established.By tracking the reference waveform accurately in one cycle,the influence of integration error is eliminated,and the robustness of the whole system is improved.An improved one-cycle control is proposed to keep the transmission power constant,which makes the DC bus of power electronic transformer constant and restrains the double frequency ripple.The capacitor voltage of each sub-module is balanced by virtual loop mapping,and the voltage difference is controlled within a certain range.The block diagram of PWM control signal in traditional one-cycle control is improved to simplify the block diagram and improve the stability.