Research on key technologies for distributed power generation based on virtual synchronous generator

Abstract

The shortage of non-renewable energy and environmental pollution have accelerated the development and utilization of renewable energy (RE). However, traditional grid-connected inverters lack moment of inertia and oscillation damping which makes it challenging for them to actively contribute to frequency and voltage regulation. As a result of it, the Virtual Synchronous Generators (VSG) concept has been proposed to control the converter that interfaces the distributed generation (DG) and power grid.
Although the emulation of the characteristics of synchronous generators (SG) enables VSG-based grid-forming converters (GFC) to provide virtual inertia and oscillation damping under the VSG control, it remains challenging to replicate the short-circuit behaviors of SGs under the grid voltage sag/recovery conditions. During a grid voltage sag or recovery, the GFC either shifts from voltage control mode to current control or applies an incorrect voltage under the control of VSG. Therefore, the dimensional analysis has been carried out to investigate the I-V characteristics of the VSG during the grid contingency. A novel approach based on the I-V characteristics of VSG (IV-VSG) is proposed in the thesis to enhance the low-voltage ride-through (LVRT) capability of the converter under a variety of fault conditions. Additionally, the transient stability is rigorously analyzed to ensure that the synchronism is maintained between the VSG and the grid during a severe grid disturbance. During asymmetric fault situations, a negative-sequence current control is used to eliminate negative-sequence voltages and achieve balanced output current. As a result, fault currents are limited while power synchronization is maintained and the need for a large GFC and current limiters is eliminated.
Though the power decoupling process is crucial for the LVRT capability of the VSG, the conventional power decoupling control is not perfect because it relies on small signal models. To address this limitation, a graphical model is developed to analyze transient power transfer and the assessment of the impact resulting from significant changes in power angle and grid voltage. Furthermore, a voltage compensation mechanism is designed to mitigate the effects of power angle perturbations. The dynamic performance of power decoupling is demonstrated to be enhanced through the control strategy based on the large signal analysis.
The proposed LVRT control can be implemented when a power grid fault occurs. However, there are numerous faults that may occur inside the working load bus (WLB) with DGs based on VSG. Since most of these are asymmetric faults, a composite criterion based on the waveform distortion for DGs is presented in the thesis for identifying them by comparing the current waveform with the historical waveform. In the event of a single-phase short-circuit fault on the WLB, the original overcurrent protection for the static electrical switches cause deficiencies for the accuracy protection of the WLB with DGs. Therefore, a WLB protection combined with LVRT control based on the differential current with voltage blocking is designed in the thesis for identifying and isolating the fault.
Extensive simulation and experiments have been carried out to verify the effectiveness of this design. The simulation and experimental results demonstrate that the LVRT control based on the IV-VSG, the dynamic power decoupling for the large signal disturbance and the WLB protection combined with LVRT control are feasible.

Date of Award	Jul 2025
Original language	English
Awarding Institution	University of Nottingham
Supervisor	John Xu (Supervisor) & George Chen (Supervisor)