Modeling and stability control of power-electronic-dominated power systems

Inspired by the target of carbon neutrality, an increasing number of renewable energies has been integrated into modern power systems. Due to the fluctuation and randomness of such renewable energies, power electronic converters are widely employed to interface power grids with renewable generators, such as solar photovoltaics (PV) panels and wind turbines, to produce stable and grid-compatible power outputs. Consequently, the traditional synchronous-generator-dominated power systems are evolving towards the power-electronic-dominated paradigms with a more complicated structure, thus giving rise to new instability problems. In light of this background, several critical stability issues are highlighted in this thesis, including the frequency stability associated with the lack of inertia, the power interaction stability of islanded microgrids, and the stability regarding constant power loads (CPLs) in power-electronic-dominated power systems. For power converters with a commonly used cascaded control architecture, the stability issues mentioned above are dominated by different control stages of power converters, with typical response times varying from seconds to dozens of milliseconds. First, the frequency instability in power-electronic-dominated power systems is focused on. Opposite to conventional synchronous generators, which have rotating components with mechanical inertia to support frequency stability naturally, power electronic converters are strictly static with no rotating masses, leading to the lack of inertia in power-electronic-dominated power systems. To deal with the lack of inertia problem, a virtual inertia control scheme based on grid-connected converters (GCCs) has been presented in previous studies by utilizing the energy stored in DC-link capacitors to provide inertial support for power grids. However, due to the coupled relationship between grid frequency and DC-link voltage, the DC-link voltage cannot be restored automatically even after providing inertial support, thus deteriorating DC-link capacitors' lifetime and losing the capability to offer multiple inertial support during cascading frequency events. A high-pass filter is applied in this thesis to settle this issue, which extracts out only high-frequency components from the grid frequency for inertial emulation during frequency events. As a result, the DC-link voltages of grid-connected converters can be restored autonomously after releasing inertial power during each frequency event and thus avoid the abnormal working conditions of power converters. Second, the stability issue in droop-controlled islanded microgrids is highlighted. For islanded microgrids employing droop control to achieve power-sharing among multiple power generating units, a large droop gain benefits the accurate power-sharing at the expense of sacrificed power interaction dynamic stability. In light of this problem, a converter-based power system stabilizer (CBPSS) is proposed. By generating an additional damping torque with the developed CBPSS, the stability and dynamic performance of the islanded microgrid can be improved in the face of disturbance. In addition, an eigenvalue-mobility-based approach is also presented to guide the selection of the optimal installation location for the proposed CBPSS in islanded microgrids, and thus the design work of the proposed CBPSS could be further simplified. Then, the instability caused by CPLs for power-electronic-dominated power systems is studied. In addition to the application on the power generating side, power converters are also widely employed to interface loads with power grids. The tightly regulated power converters on the load side behave as CPLs, featuring negative incremental impedance and introducing adverse impacts on the stability of power-electronic-dominated power systems. Moreover, with the increase of the power level of CPLs, such adverse effects also grow, and finally, it may trigger instability, threatening the security and reliability of the whole system. In light of the instability problem resulting from CPLs, two control schemes, e.g., a damper-based scheme and a stabilizer-based scheme, are proposed to handle the instability issue. In summary, the overall research objective of the thesis is the analysis and control of several critical stability issues in the emerging power-electronic-dominated power systems. The stability issues focused on in the thesis include the frequency stability associated with the lack of inertia, the power interaction stability of islanded microgrids, and the stability issues regarding CPLs in power-electronic-dominated power systems. Different control stages of power converters dominate these stability problems mentioned above, with their typical response times varying from seconds to dozens of milliseconds. Causes for these stability issues are analyzed in-depth, and corresponding stability control schemes are also developed in this thesis. With such effort, the stability and security of the emerging power-electronic-dominated power systems can be enhanced.Doctor of Philosoph