| The scale of photovoltaic power generation as a clean and renewable energy source is expanding year by year.Considering the reverse distribution of energy resources and load centers in China,how to improve the capacity of photovoltaic energy consumption is a key issue that needs to be solved.Compared with AC pooling,the use of DC pooling in photovoltaic power plants is the preferred way to achieve high controllability and high-voltage large-scale power delivery.With the development of integration and scale of power electronic devices,the cascade of multiple converters can form more flexible and complex power electronic devices.However,the difference in impedance characteristics of each converter often results in system stability variety.Therefore,this paper studies the impedance stability of cascaded converters in photovoltaic DC boost collection system under different lighting conditions,sets up three working modes to compare and analyze the differences in system stability,discusses their causes and influencing factors,and adopt active damping method to improve system stability.The specific work of this paper is as follows:(1)This paper investigates the network architecture of full DC cascaded systems and independent input,series output cascaded converters in photovoltaic DC boost collection system,the maximum power point tracking control and over voltage regulation of DC/DC converter,and voltage and current double closed-loop control strategy of DC/AC inverter.The mathematical model is established by analyzing the topology,working principle and control strategy of the converter,and the system is divided into 3 working modes according to the lighting conditions: uniform MPPT mode,single-module OVR mode and dual-module OVR mode.(2)Aiming at the problem of the difference in the stability of operation in multiple working modes,the small signal model is established for each converter under various control modes firstly,the output impedance at the micro source side and the input impedance at the load side are derived,and the stability and influencing factors of each control mode are analyzed,then the equivalent impedance model of the system under 3 working modes is obtained.Finally,the stability differences between the operating modes is compared through the impedance cross-section criterion,the Nyquist stability criterion,and the phase angle-amplitude margin.The results show that when the DC/DC converter is switched from MPPT mode to OVR mode,the system stability performance will be reduced and the stability will be significantly reduced with the increase of the number of switching control mode modules.(3)A voltage loop virtual resistance control and a modulation loop virtual impedance control strategy based on the OVR control mode are proposed.The active damping method is used to increase the closed-loop output impedance of the micro-source-side converter to improve the stability of system and the circuit design steps of the two controls and the selection principle of control parameters are given.Among them,the virtual impedance control strategy of the modulation loop can effectively make up for the defect of DC/DC converter output voltage drop caused by the virtual resistance control of the voltage loop,while improving the stability of the system,maintaining the original voltage and power output of the system.(4)A photovoltaic DC boost collection system of three 0.5MW photovoltaic unit cascaded boost converters and DC/AC inverters are built on MATLAB/Simulink.The simulation results verifies that the system can operate stably in 3 working modes and smoothly switch between modes,and the stability decreases with the switching between MPPT mode and OVR mode;the effectiveness of the proposed virtual impedance control strategy of the modulation loop is verified,which can reduce the ripple of DC bus voltage and photovoltaic output power,and the active damping method is used to improve the system stability and power supply quality. |
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