Control Design For Voltage-Controlled Interface Inverter Of Distributed Generation

Nowadays,voltage-controlled interface inverters have received more and more attention and developed due to their unique advantages,such as flexible switch ability between grid-connected and standalone modes,automatic power sharing among different inverters,voltage source characteristics similar to conventional synchronous generators and the ability to operate without PLL,etc.The local control of the voltagecontrolled grid-connected inverter is mainly composed of a two-layer control system and a grid synchronization system.The first layer is the inverter control layer,which realizes the zero-static-state-error voltage control of the inverter;the second layer is the application control layer which is used to realize the power control according to different functional requirements.The two-layer control system constitutes the local control function(or decentralized control)of the voltage-controlled inverter.The grid synchronization system is used to implement the pre-synchronization function before the grid connection.Consequently,to improve the control performance of the gridconnected application for the voltage-controlled inverter,this paper systematically studied the fundamental principles of the three-phase inverter and its modeling and control method,then designed the algorithms for grid synchronization,voltage control and power control.For the control methods proposed in this paper,the complex variables are introduced to reduce the system order and increase control freedom.Moreover,the state space model and time-domain optimization method are used to design the control parameters.The paper includes the following three parts:Firstly,for the grid synchronization algorithm,a complex variable model of threephase signal is established,and based on the state observation theory,a unified threephase voltage observer is designed.On the basis of the designed observer,the equivalence between the traditional real variable filter represented by the Second-Order Generalized Integrator(SOGI)and the real-coefficient complex variable filter represented by the Reduced Order Generalized Integrator(ROGI)are analyzed and demonstrated.Furthermore,a complex-coefficient complex-variable filter is proposed to extract the fundamental and harmonic components of the grid voltage,and make full use of the additional control degrees of freedom introduced by the complex coefficients to improve the dynamic performance of the filter.Based on the proposed complexcoefficient complex variable filter,a Frequency-Locked Loop(FLL)is designed to make the complex-variable filter adaptive to the frequency changes of the grid to achieve the grid synchronization.The grid synchronization algorithm observes the grid voltage,phase angle,and frequency which is necessary for subsequent voltage and power control.Simulation and experiments show that the proposed frequency-locked loop based on complex-coefficient complex-variable filter significantly improves the dynamic performance of the traditional frequency-locked loop,and can extract the harmonic components and frequency phase angle information of the grid voltage more quickly.Secondly,for the inverter voltage control of grid-connected applications,this paper analyzes the advantages and disadvantages of the traditional series dual-loop voltage control structure from the perspective of modern control theory.An inverter voltage control algorithm which has a good dynamic response is proposed on the basis of the complex-variable inverter model.The designed voltage control algorithm simplifies the control structure into single-loop control,separates the state feedback and feedforward control(reference feedforward and load current feedforward)which are nested in the conventional dual-loop control.An analytical parameter design method is proposed to determine state feedback and feedforward gain.By means of the free placement of the zeros,the slow dynamic process of the system is eliminated,so that the voltage control bandwidth is greatly improved.The zero dynamic performance is approximately achieved.The voltage control acts as the inner loop of the power control.The simulation and experimental results show that the voltage control method proposed in this paper has a very fast dynamic response,which effectively improves the performance of the power control.Finally,based on the designed dominate-dynamic-elimination voltage control,a novel power control is proposed to fulfill the grid-support function.Different from the previous publications,the instantaneous power is adopted as the control variable to remove the loop filter in the feedback loop which makes the loop design easier.For the different storage configurations,two power controls are proposed,in which the inertia support control is designed for the power-type energy storage(e.g.,supercapacitor),and the droop control is designed for the energy-type storage(such as the battery).In this condition,the inverter will output active power in a short time which is referred as inertia time when the grid frequency changes.For the energy-type energy storage,the droop control is designed to achieve primary frequency regulation.At last the proposed power control method is validated by the experimental results.To sum up,this dissertation systematically designs the control strategy of the voltage-controlled inverter for the grid-tied application.The proposed control strategy improves the performance of the whole system remarkably.The experimental results verify the effectiveness and correctness of the proposed control strategy.