Control And Stability Analysis For Small-signal Model Based Distributed Generation AC Microgrid

Renewable energy distributed generation has been explored and investigated:in practical projects and academia,which is an effective solution for the shortage of traditional energy and environmental crisis.AC microgrid can build a framework for integrating renewable energy units with energy storage elements to dispatch the local loads and interact with stiff grid.This dissertation focuses on the stability and operation control technique of the distributed generation AC microgrid.Based on the input-output stability criterion,the small-signal model is built for the hybrid multi-energy generation AC microgrid.Enhanced power flow control with stability improvement is proposed to achieve accurate and stable power dispatch.Moreover,inverter output impedance is explored as a clue to solve the power quality issues and stability problems.Low-frequency active damping methods are classified,evaluated and extended to illustrate the stability solutions.Firstly,the interfacing inverters are classified into the voltage-controlled inverters and the current-controlled inverters according to their individual output characteristics.The voltage-controlled inverters are modeled as the Thevenin equivalent circuit while the current-controlled inverters are modeled as the Norton equivalent circuit.Moreover,the system characteristic equations can be derived with Kirchhoffs current law and voltage law.The voltage-controlled strategy consists of droop loops and inner voltage-current loops while the current-controlled scheme includes power control loops and current loops.On the basis of the control objects and bandwidth of different control loops,the system characteristic equations can be divided into power equation and voltage-current equation to illustrate and distinguish the property in different frequency domains with the input-output stability criterion.In addition,the system parameters and advanced control algorithms can be optimized and evaluated with the proposed small-signal model.Secondly,the power flow control in grid-connected mode is investigated for conventional droop-controlled inverters to explain the impacts of grid fluctuation and illustrate the intrinsic tracking error in reactive power control.The feedforward of grid voltage magnitude and grid frequency can be introduced to track with the grid and enhance the power flow control.The feedforward items can be achieved through the synchronous reference frame phase-locked loop with the input of PCC voltage or the filter capactitor voltage.The implement with filter capacitor voltage can avoid measuring the PCC voltage or grid voltage in distributed generation.Meanwhile,accurate reactive power flow control can be realized by inserting the integrator into the reactive power droop loop with voltage magnitude control.The seamless mode transition feature is still reserved with the enhanced power flow control strategy.In addition,the droop controllers are modified to improve the control degrees of freedom and realize the low-frequency active damping.Thirdly,the equivalent output impedance of droop-controlled inverters consists of virtual impedance,inverter outptut impedance and grid-side impedance.The difference of the dominant frequency domains among these impedances is explored to illustrate the individual impacts.The influence of inverter output impedance on voltage unbalance,harmonics and dynamic response during load switching is analyzed and the relationship between inverter output impedance and low-frequency resonance is also revealed and verified.The inverter current feedforward control scheme is proposed to introduce negative impedance and mitigate the impacts of inverter output impedance without extra sensors for grid-side current.The negative impedance keeps approximately linear relation with inverter output impedance and the linear coefficient is determined by the feedforward gain.The design of negative impedance is simplified without the necessity of accurately measuring the line impedance.With proposed inverter current feedforward scheme,stability and dynamic performance as well as load adaptability in islanding mode can be effectively enhanced.Finally,according to the equivalent Thevenin model of droop-controlled inverters,the low-frequency active damping methods can be classified into two categories:source-type damping strategy and impedance-type damping strategy.The source-type strategies are realized by inserting supplementary control loops into the droop loops or modifying the droop controllers.The low-frequency control degrees of freedom are increased to suppress the low-frequency oscillation.The impedance-type damping strategies are implemented through introducing current feedforward.The low-frequency characteristics of output impedance are modified to realize active damping.The realization of active damping among the same type of damping methods is different with each other.Furthermore,the object of different types of damping methods is also disparate.Consequently,different damping methods can be synthesized to improve stability margin and realize performance complementation.