Analysis and Optimization of VSG Stability in Microgrids under Large-Signal Disturbances

With the increasingly prominent environmental and energy issues, the large-scale application of renewable energy sources（RES） is imperative. Especially, the large-scale offshore wind power plants (OWPPs) are gaining extra attention due to their increased generating capacity and low environmental impact. However, RES connected to grid via inverters could lead to low or even zero inertia in the grid system. large offshore wind power systems controlled by conventional control algorithms could contribute to short-term power imbalance and suppressing frequency variation, even triggering loss of synchronization. Virtual synchronous generator (VSG) control strategies are being applied to power generation systems that use power electronics as the interface to the grid. The VSG can emulate the inertia and frequency regulation characteristics of synchronous generators to improve the voltage and frequency support to the system. VSG control strategies can assist the compatibility between distributed power sources and the grid and are an effective way to ensure the reliable operation of large offshore wind power plants, making future power system development more flexible.

However, the VSGs have several limitations. When a fault occurs in the grid, a large inrush current will inevitably be generated. In this case, if the inverter remains in grid-connected operation, it will exceed the capacity of the power electronics and cause permanent damage to the device. On the other hand, if the current limit is directly triggered, the VSG will be accidentally disconnected from the grid; and it can have a devastating effect on the grid. In the most severe cases, this could even result in permanent damage to the inverter itself and the collapse of the grid.

Current research on VSGs is mainly focused on the operating conditions for standard grid voltages. Only a few papers refer to the operation of VSGs under large signal disturbances. The aim of this PhD thesis is to analyse the stability of VSGs under large signal disturbances and to come up with an evaluation method for indicating system stability margins. In addition, novel control strategies to improve VSG stability will be discussed.

In order to verify the validity of the above research, a VSG model will be built using MATLAB/Simulink. For experimental validation,  the results of the project will also be implemented on a real-time control and monitoring platform (RTDS) at the AAU Microgrid Laboratory facility.