Research On Mode Switching And Islanding Detection Technologies Of Energy Router AC Side

With the increasing energy crisis caused by the development of social industrialization and the increasing awareness of environmental protection in contemporary society,the maximization of the use of clean and renewable energy such as wind and light has become a research hotspot.Because of the intermittent and volatility problems of wind and photovoltaic power generation,the concept of energy Internet emerges as the times require in order to better solve the complex and diverse power quality problems.As the key equipment of the energy Internet,the energy router should have the ability to operate stably in the grid-connected and islanded modes.This paper studies the key issues of the stable operation of the energy router AC side.It mainly studies the grid-connected/islanded smooth switching and unplanned islanding detection technologies to enhance the stability and security of the energy router AC side.This paper presents a multi-port converter(MPC)energy router topology.The DC side includes a photovoltaic unit and an energy storage unit.The photovoltaic unit includes a photovoltaic module and a post-stage Boost circuit to provide energy for the system.The energy storage unit includes a battery module and a bidirectional DC/DC converter,which can realize bidirectional flow of energy and achieve peak-shaving and valley-filling purposes.The AC side is a three-phase T-type three-level converter that provides energy for the AC load and enables grid-connected and islanded operation.The working principle of energy router is analyzed in detail,and its basic functions are realized by simulation.Next,research is conducted on the energy router AC side smooth switching.This paper analyses the causes of impact during the switching period of energy router AC side.For the mutual influence of the phase and frequency existing in the traditional presynchronization method,an improved pre-synchronization method is used to solve the problem.The problems of the traditional PQ-U/f control grid-connected/islanded switching are analyzed,and the current command change and state follower control are used to solve the problem and achieve smooth switching.A control strategy based on disturbance observer is adopted to realize the smooth switching based on the droop control.The effectiveness of the proposed control strategy is verified by simulation.Then,the problem of unplanned islanding detection on the AC side of the energy router is studied.The problems of traditional passive detection and active detection methods are analyzed.Aiming at the problems of large non-detection zone of traditional phase-shifting method and frequency-shifting method in grid-connected inverters of energy router AC side based on droop control,the phase-shifting method and frequencyshifting method are improved by introducing offset.The simulation results show that the unplanned islanding detection of the system can be realized even when the detection difficulty is the maximum.Finally,for systems with less harmonic distortion,the phase angle generated by the droop link is improved to inject specific subharmonic into the output voltage of the AC side.The goertzel algorithm is used to measure the harmonic content in order to solve the problem of large computational complexity by the discrete Fourier method.An unplanned islanding detection method based on the combination of current harmonic amplitude and AC side equivalent impedance is designed.Simulation results verify the correctness and effectiveness of the proposed method.