Research On Power Electronic Transformer Based On Virtual Synchronous Generator Control

With the rapid development of distributed generation and micro-grid technology, the capacity of renewable energy generation devices, storage energy equipment and the new type load represented by electric vehicle will be connected to the grid, cause the distribution network power energy control and management more and more complicated. Therefore the traditional distribution equipment fail to meet the needs of the power grid and the future electricity market, while the power electronic transformer (PET) which is integrated with power electronic technology and information technology can provide flexible electrical interface for all kinds of distributed generation devices and loads, and its interface converter has the ability of energy regulation and active control.However based on the power conversion circuit of PET, its interface converter has a fast response speed, but it does not have the mechanical characteristics of the traditional synchronous generator and the synchronous operation mechanism, so with the increase of PET device in the distribution network, the interface converters in energy conversion and power regulation process have influence on the power quality of distribution network. Among the interfaces, the high voltage (HV) AC interface easily lead to distribution network transient instability, even can cause the impact of grid connected current on distribution network due to lack of the inertia and damping characteristic. Thus, the HV AC interface cannot respond to distribution network voltage or frequency scaling due to lack of the synchronous operation mechanism. While the low voltage (LV) AC interface of PET in the process of responding to power regulation, can easily influence the system power quality due to lack of the network supported.Thus based on virtual synchronous generator (VSG) control, this thesis study the AC interface of PET. Firstly, according to the PET HV and LV AC interface with different application scenarios and functional requirements, the corresponding control strategy is proposed. According to the mechanical characteristics of synchronous motor rotor, the HV AC interface established active power-frequency and reactive-voltage regulation control strategy, enhanced the flexibility of HV AC interface, inhibited the grid current harmonic distortion rate, and reduced the impact of the distribution network. Meanwhile, the HV AC interface with automatic response distribution network voltage and frequency regulation ability, finally effectively enhance the stability of the distribution network. While the LV AC interface through the outer loop power frequency and reactive power voltage control strategy in Fusion VSG control technology, improved the LV AC interface power frequency response characteristics and reactive power regulation performance and effectively enhances the LV side of the system frequency and voltage stability. And with the application of PR controller in the inner loop control of voltage and current, it make the output voltage keep track of the reference voltage better, and adopt the current control loop as a compensation block to improve the response speed of power and maintain the dynamic stability of the low voltage side system. Then, the influence of relevant parameters on the performance of the PET is analyzed by establishing small signal model of VSG. Besides, the grid-tied and parallel operation characteristics of PET under the control of VSG are studied. Finally, the system simulation model based on PSCAD/EMTDC and prototype platform in the laboratory are established, the proposed control strategy is verified by simulation and experimental results.