# Multi-Timescale Control of Variable-Speed Wind Turbine for Inertia Provision
## 1 Introduction  
The increasing penetration of renewable energy resources (RESs), such as the photovoltaic (PV) panels and wind turbines, has fundamentally changed power system characteristics. These RESs are usually integrated into the power grid through power-electronic converters, which, naturally, do not possess the attribute of mechanical inertia. Conventional synchronous generators (SG) are gradually being replaced by inertia-less power converters (low-inertia systems), which will result in a reduction in power system inertia to a large extent [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. The decrease in the power system inertia, on one hand, will result in a higher rate of change of frequency (RoCoF). On the other hand, the reduction in the power system inertia can cause a low-frequency nadir [2]. As a consequence, frequency stability issues are more likely to happen [3, 4]. One recently reported incident is the 2016 Australia blackout, which affected millions of people and caused considerable economic loss. It is believed that a lack of power system inertia is one of the reasons behind the blackout [5]. Therefore, it is of importance in many countries to increase power system inertia and enhance the frequency stability of future power-electronics-dominated power system, where more and more renewable energy resources are adopted. To achieve this objective, a number of research activities have explored the possibility of using power-electronics converters to provide grid inertia support [6, 7, 8], in addition to the use of extra energy resources, e.g., batteries. Among them, the virtual synchronous generator (VSG) technique stands out as a promising solution. The VSG control strategy enables the power converter to emulate the rotor swing equation of a conventional SG. As a consequence, the dynamic of the converter is similar to that of the SG. Nevertheless, the successful implementation of the VSG control strategy usually requires a bulky and costly energy storage system to provide active power support. Alternatively, the electrical energy stored in DC-link capacitors can also be utilized to provide inertia support [10]. To achieve this objective, large DC capacitors are normally required, which may not conform with the power-electronics converter design principle. In [11; 12], the voltage across a supercapacitor was proportionally linked with the AC power grid. Through this effort, the supercapacitor can be controlled to behave similarly to a virtual synchronous generator to support the power system frequency response. Alternatively, the inertia emulation effect can be equivalently achieved by demand-side management techniques. In [13], the power consumptions of certain noncritical loads, such as electric heating, lighting, and cooling, were adaptively regulated to realize synthetic inertia and suppress the power grid RoCoF and frequency deviation. The inertia emulation target can also be achieved by PV generation systems. Conventionally, the PV inverter operates under maximum power point tracking (MPPT) mode, which harvests the maximum solar energy and does not participate in grid frequency regulation [14]. Recently, power reserve control (PRC) and active power curtailment control were developed to intentionally reduce the PV output power and spare a certain amount of active power for inertia provision [15; 16; 17; 18]. Through this effort, the requirement of energy storage units, capacitors, and supercapacitors becomes unnecessary and largely alleviated. Accordingly, an event-triggering control strategy was developed to switch the PV system between different operation modes to optimize the frequency support from the reserved power [19]. A coordination scheme was further proposed to achieve the virtual inertia and frequency-damping effect in [20]. These advanced control algorithms have made PV generation systems more grid-friendly and actively contribute to power system frequency regulation. In comparison with PV generation systems, wind turbines have even greater potential to provide inertia support.