Self-Synchronization of Wind Farm in an MMC-Based HVDC System: A Stability Investigation

The stability of an offshore wind power network connected through a high-voltage dc (HVDC) transmission line can be challenging since a strong ac collection (ACC) bus might not be available, when there is no rotating machine connected in that bus. In addition, the synchronization unit (phase-locked loop, PLL) has shown to have a significant impact in achieving satisfactory performance. To tackle this problem, this paper has proposed a wind energy conversion system (WECS) controller for such an ACC bus based on the synchronverter concept. A synchronverter—an inverter without PLL that mimics the synchronization mechanism inherent to synchronous generator—is introduced in the grid side of WECS voltage-source converter (VSC), where the wind farms are connected to ac network through a modular multilevel converter (MMC)-based HVDC system. In order to determine the stability of the interconnected system, an impedance-based stability method is adopted. The impedances of both the wind power inverter and the MMC-HVDC converter are analytically derived, and the analytical model is verified by comparing the frequency responses obtained from numerical simulation. The detailed analysis and the results presented show the benefits of this controller and its potential for stability. The results highlights the synchronverter's ability in keeping better performance compared to PLL-based dq-domain control in point of stability and control in integrating offshore wind farm through the MMC-based HVDC system, since the impedance of the synchronverter reflects a simple RL characteristic. On the other hand, the impedance of PLL-based dq-domain control impedance is inductive above 2 kHz and reflects composite characteristics below 2 kHz with different resonance points and with higher impedance magnitude at low frequencies, making it more vulnerable to voltage instability. Finally, time-domain simulation results are presented to validate the theoretical analysis and to show how the self-synchronization impacts on the system performance.