| Vrious distributed generation units,energy storage units,and DC loads connect with the AC grid through voltage source converters(VSCs).With the increasing integration of VSCs in modern power systems,the inertia and damping of the whole system are reduced significantly,which may threaten the dynamic performance and stability of the power grid.Meanwhile,the coexistence of multiple micro-sources and multiple types of loads in the microgrid may lead to the dynamic interaction of"source-grid-load";the interaction between the devices with different characteristics will cause dynamic response characteristics different from the traditional power system,and induce the problem of frequency and voltage stability.Research on the adaptability of grid-connected operation and off-grid operation and the parallel control technology is not only related to the safety and reliability of the virtual synchronous generator(VSG)itself,but also has great theoretical value and practical significance for the construction of autonomous power system.The purpose of this study is to improve the frequency stability and the accuracy of dynamic and static power allocation for VSG-based microgrids from two aspects.On the one hand,the power-frequency dynamic characteristics of VSC are described by constructing a unified modeling method applicable to different VSC control strategies,in Chapters 2 and 3 of this paper.On the other hand,the improvement of VSG control strategies and the configuration of control parameters are studied,in Chapters 4,5,and 6 of this paper.The main contents are as follows.1)For the VSG-based microgrid,it is easy to cause transient power circulation and power-frequency oscillation between VSGs after various disturbances.However,the existing small-signal analysis models do not adequately describe the impact of the types of disturbance on the power-frequency dynamics.Accordingly,the rotor dynamic model of a synchronous generator(SG)is analogized to the mechanical network according to the electromechanical analogy principle.Moreover,the torque-frequency dynamic modeling method based on the mechanical admittance is proposed for the first time.The mechanical admittance model of a single SG/VSG is established and it is further extended to the mechanical admittance model of the parallel VSGs.The extended model can fully describe the power-frequency response characteristics of the multiple-VSG system under different conditions such as parallel connection,grid-connection,and pre-synchronization.When the parameters of each VSG are matched,the mechanical admittance is proportional and two key conclusions are further derived.In the case of load disturbance,the active-power response of VSGs directly enters the steady state process,and the angular-frequency dynamic characteristic is the same as that of any a VSG in the single off-grid mode;in the case of the reference input,the active-power dynamic characteristic of VSGs is the same as that of any a VSG in the single on-grid mode,and the angular-frequency dynamic characteristic is determined by the response of any a VSG in the off-grid and on-grid mode.The simulation and experimental results are consistent with the theoretical analysis,so the key conclusions are verified.2)To further extend the application range of the mechanical admittance method,a unified mechanical admittance model applicable to a variety of VSCs was constructed.The classical control strategies of voltage-source controlled VSC and current-source controlled VSC(VSG control is one of these strategies)are summarized,and the mechanical admittance model parameters of them are listed.The accuracy and effectiveness of the model are verified from simulation and experimental results,and the simulation system is a modified 3-machine 9-node test system.Particularly,in the case of the control structure and parameters of VSC are unknown,the mechanical admittance model can also be obtained by the identification method.Accordingly,an identification method of the active power-frequency transfer function(i.e.,mechanical admittance model)of VSC is proposed,which combines the frequency domain discrete excitation method and the complex curve fitting method.Taken the VSG control as an example,the identification method is verified.In the practical application of the identification method,the transfer function is obtained through the hardware-in-the-loop experiment and the complex curve fitting method.The identification method provides a new idea for the construction and validation of VSC mechanical admittance model.3)The VSG control scheme of existing second-order model is not clear in terms of orientation and differentiation of functions of primary frequency regulation and rotational speed oscillation damping.It shows up as damping coefficient affecting VSG steady-state power/frequency deviation and active power-frequency regulating coefficient affecting VSG oscillation damping performance.In this study,the control model is improved with high-pass feedback of rotational speed damping power and low-pass feedback of frequency regulation power to differentiate primary frequency regulation from rotational speed damping functions.According to the frequency domain characteristics of electromagnetic transient,electromechanical dynamics and steady-state response of SGs,the VSG regulators are divided into three action frequency bands:high,medium and low.Then,the influence of active power-frequency regulating coefficient and damping coefficient on VSG frequency domain characteristics is analyzed by using the mechanical admittance model.Experiments demonstrate that the proposed scheme can achieve both steady-state power allocation and speed oscillation suppression,and these two objectives correspond to the active power-frequency regulating coefficient and damping coefficient respectively.4)The existing control strategies are difficult to realize the coordinated control between the constant power control and the inertia and damping support of grid-connected VSGs when the power grid frequency fluctuates.To solve this problem,based on the comparative analysis on the power loop model of VSG and the phase-locked loop(PLL)model based on virtual power,the power loop model of VSGs is designed to be a second-order system with Zero to implement the steady-state constant power control and the dynamic inertia support under grid frequency disturbance while keeping synchronizing mechanism.The smooth switchover is achieved between constant active power control and frequency droop control by the parameter design and the switch between proportional-integral(PI)link and inertia link.The constant active power control mode and the frequency droop control mode are compared and analyzed from the two aspects of input and disturbance responses.Finally,the stable operation and smooth switchover performance of two modes are simulated and experimentally verified.Considering the constraint of state of charge(SOC)for the energy storage unit of VSG,the constant active power control mode can be considered as an alternative to the existing frequency droop mode.The VSG switched to the constant active power control mode will no longer participate in the frequency regulation.5)The large-scale integration of photovoltaic(PV)power generation results in the decrease of frequency regulation capacity of the power system.Although the current scheme of active power reserve(APR)can meet the needs participating in frequency regulation,frequently switching between the maximum power tracking mode and APR mode will affect generation benefits and system stability.Accordingly,a frequency regulation strategy of PV system based on the coordinated control of variable power point tracking(VPPT)and supercapacitor energy storage is proposed.The primary frequency regulation is accomplished by the means that VPPT responses frequency's up disturbance and energy storage equipment responses frequency's down disturbance,so that the PV system can operate at the maximum power point under normal operating conditions.The proposed scheme can reduce energy storage capacity by half without losing power generation benefits.The proposed scheme and the APR scheme are compared from the aspects of economy and control effect.The results show that the proposed scheme is more economical and technical. |
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