Assessment and Augmentation of Power Converter Control Towards Enhanced Power System Stability

Ambitious global carbon emission targets are driving a substantial increase in renewable energy. Most of the renewable generations are wind and solar, which utilizes a power converter-based interface for integration into the power system. In addition to renewable generation, the power converters are also the building blocks in other technology solutions such as High Voltage Direct Current (HVDC) transmission systems, Battery Energy Storage Systems (BESS), Flexible Alternating
Current Transmission Systems (FACTS), and battery electric vehicles. Such massive integration of power converters into the system is transforming the power system dynamics. In contrary to synchronous machines/generator (SG), where dynamics and responses are aected by its physical characteristics, the control of the power converters directly determines the dynamics and responses within the hardware limits. As a consequence, the power converter control plays a significant role in power system operation and stability. The power converter control is generally classified as grid following control with a current source characteristic and grid forming control with voltage source characteristics. While grid forming converter (GFC) possesses superior response and dynamics over grid following converter (GFL), particularly in weak grid conditions, the majority of the converters integrated into the system are GFL due to their mature technology. Consequently, the power system will in the future constitute a mix of GFC, GFL, and SG. Hence, it is now imperative to investigate the impact of GFC and GFL on system stability. This thesis develops a comprehensive assessment of the GFL and GFC using time-domain, eigenvalues, and impedance-based analysis to address potential instabilities. Design considerations are derived for both converters to avoid instability originating due to the converter control derived. In addition, the interaction of the converter with the synchronous machine is assessed based on a simplified system model. Besides the small-signal approach, a transient response evaluation of the GFC and GFL is conducted with focus on the synchronization block in GFC and GFL. In this thesis, an improved PLL model, (synchronization block in GFL) is proposed. Furthermore, a compensation method that improves the PLL dynamics during large voltage transients is also proposed. Likewise, for GFC, the response of the synchronization block during the transient events is investigated. The phenomena of loss of synchronization in GFC during dierent frequency, phase, and transient voltage events are demonstrated analytically and illustratively. The impact of the current limiter of the GFC’s on the system synchronization stability during transient disturbances is presented, and a method is proposed for improvement. All the proposed solutions are validated in power hardware in the loop simulation.