## 1 Introduction
With higher penetration of renewable generation, the IBG units would eventually and inevitably take the responsibility of regulating the overall energy balance because of the consequent displacement of traditional SGs. Therefore, the flexibility of the power system operation requires the VSG algorithm to extend its application scope. The generation units can be divided into two components, the primary energy input, and the electricity generation. The electrical part mainly determines the external characteristics, but it is also restricted by the availability of primary power input. The mechanical power of traditional SGs comes from fossil fuels, hydropower, and nuclear power, which are abundant and fully controllable. On the contrary, renewable resources are time-varying and stochastic. In the view of transmission/distribution system operators (TSO/DSO), the renewable generation is irresponsible and undispatchable [6]. To maintain the overall power balance against unexpected deficits or surpluses, system reserves are indispensable. In some countries, the reserve is quantified as the capacity of the largest generator, which assumes any loss of generation greater than the largest infeed is very infrequent [20]. This reserve quantification method, which focuses on the forced outages of individual plants, is simple, but restrained by rules of thumb. Large-scale renewable integration has challenged this method due to its uncertain nature [20]. The reason for this is that the variations of renewables are continuous, and may be in addition to generator outages caused by discrete events (such as a lightning strike). To deal with this,some scholars have proposed reliability-based reserve quantification methods, which determine the reserve requirement based on reliability index, such as loss-of-load probability (LOLP) [7]. The loss of load happens if the power production and upward reserves are not able to compensate for the power deficits caused by the combined uncertainties of the load and renewable generation, as well as the outage of power plants. The renewable generation spillage happens when negative reserves cannot compensate for the surplus power from renewable plants [21]. Furthermore, the reliability-based reserve requirement is integrated with the economic dispatch model to realize joint scheduling of generation and reserve [22, 23]. It is noted that, in these approaches, the reserves are scheduled to the synchronous generators only, because the IBG is not schedulable. In recent research, the predetermined power profile and deloading strategies have been proposed for RG units to participate in power balance regulation. Firstly, the predetermined strategy modifies the power profile of RG units [24, 25]. When frequency fluctuations happen, the output of wind turbines is set according to a predesigned curve [26, 27, 28, 29]. It is noted that once the predetermined strategy kicks in, the RG output is irrelative to the system frequency. Moreover, this strategy may cause another issue called the second frequency drop (SFD). This is because when the response capability is exhausted, RG units must decrease their output power and restore initial states (such as rotor speed or DC bus voltage) by their own generation capabilities for self-stability, disregarding the actual system frequency conditions. Another strategy is the deloading strategy, which controls the RG unit to operate at a sub-optimum point in the normal conditions [30]. The generation availability is partly curtailed between the maximum power point (MPP) and the suboptimum point, and the output power can be adjusted between such intervals in different time periods. The deloading strategy enables the renewable units to function as long-term power reserves to provide not only PFR, but also secondary frequency regulation (SFR). For example, overspeeding and pitch control can control the power output in a wide range of wind speeds for wind turbines [30, 31].