Enhanced Frequency Control Capability from Wind Turbines and Farms

Wind energy is becoming an increasingly significant electricity supplier in modern society. Wind energy provided 16% of the electricity consumption in the twenty-seven EU countries and UK in 2020, and the total installed capacity is supposed to reach 318 GW by the end of 2025. Solar energy, in form of photovoltaic arrays, is also being integrated into power systems at an increasingly speed. Both of these two types of renewable energies are inverter-basedresources (IBRs) nowadays. The integration of increasing IBRs make traditional generation units decommissioned and thus decreases the total inertia of the power system. As inertia plays an important role in limiting the rate of change of frequency (RoCoF) during frequency events, it is becoming more challenging to maintain the frequency stability of power systems. Reduced inertia can result in larger RoCoF, which can cause generators to disconnect. Generators will also have to comply with stricter grid codes on the RoCoF withstand capability. Lower inertia also contributes to lower frequency nadirs during under-frequency events, which may trigger undesirable load shedding. Therefore, this frequency stability issue needs to be carefully addressed in power systems with increasing IBRs.

To address the challenge, grid-following frequency control methods (GFL-FCMs) for wind turbines (WTs) and wind plants (WPs) were proposed at an early stage. This type of methods usually include virtual inertial response, which is proportional to RoCoF, and frequencyactive power droop control. Such methods rely on PLLs to estimate the frequency, which consequently causes time lag and induces noise to the controls that may lead to instability. Furthermore, synchronization instability is prone to occur for grid-following (GFL) controls with increasing number of paralleled inverters or in weak grids.

Afterwards grid-forming (GFM) control was proposed, which enables converters to work as an ideal voltage source with a given amplitude and frequency. GFM control is supposed to be able to overcome the disadvantages of GFL control abovementioned. Within the scope, virtual synchronous machine (VSM) control becomes a popular type of GFM control.

In this dissertation, a reduce-order-VSM-based frequency controller is proposed. Such a controller allows WTs to help regulate the system frequency automatically and includes virtual inertia to help limit RoCoF. Compared with other methods, the controller achieves satisfactory frequency support capability with considerable simplicity.

In addition, based on the proposed controller and a classical GFL-FCM in the literature, the dissertation provides a comparison, which illustrates the differences between GFL-FCMs and GFM-FCMs in providing frequency support in a power system. They were not only compared separately in identical systems, but also when they coexisted in the same system. Hence, interactions between them during frequency support are also revealed. The work shows superiority of GFM-FCMs over GFL-FCMs in frequency support, and gives insights on frequency stability of power systems in the transition from GFL to GFM frequency support from wind power.

Last but not the least, the influence of the VSM-based frequency controller on the mechanical dynamics of WTs during frequency support is investigated. While its performance in terms of frequency support is relatively good, torsional oscillations may be triggered in the drivetrain of WTs by the controller when supporting frequency. The phenomenon is illustrated, and methods for damping the torsional oscillations are introduced. Two damping methods are compared and combined to derive an improved one, which provides better damping performance.

In summary, this PhD work contributes to enhancing the frequency support capability from WTs and WPs for the benefits of both WTs themselves and power systems. With the control schemes proposed and analysis work accomplished, the dissertation provides valuable information, methodology, and support for power systems to be more capable of maintaining frequency stability with less inertia, and therefore be able to integrate more renewable energies in the future.