## 1 Introduction  
Renewable energy, such as photovoltaic and wind energy, has been widely developed and utilized in power systems during the past few decades, which has been prompted by the urge to tackle environment crises and sustain economic development [1]. According to the 2018 BP energy outlook, more sustained support for renewables will be largely phased out by the mid-2020s, leading to a strong growth in the share of renewables reaching 25% of total energy consumption in an evolving transition scenario by 2040 [2]. The integration of renewable energy generation is also one of the central demands of any future smart grid, yet there are still many issues and challenges. Regarding the characteristics of their generation profiles, renewable energies such as wind and photovoltaic vary largely when compared to conventional generation such as coal and nuclear. The output of wind turbines and solar panels fluctuate with random and intermittent meteorological conditions, while conventional power plants can be regulated precisely and continuously. Moreover, wind and solar generators, which are integrated by power converter interfaces with almost zero inertia characteristics, cannot provide spinning reserves to the system [3]. As fossil energy depletes, renewables will eventually take the place of traditional power sources, while high-penetration renewable energy generation would severely degrade the reliability and stability of such traditional power system. Many studies have been published that address the issues of renewable energy generation integration. In [4], the classification of power converters in alternating current (AC) microgrids is proposed, in which converters are generally divided into three functional categories as grid-forming, grid-feeding, and grid-supporting. However, the functional conversion of power converters is constrained by the current state of renewables; as a result, the faults of key converters may cause entire systems to break down. Such concerns also exist in multilayer architectures such as master-slave structures in which converters are parallel operated in such a way that allows only a few key converters to regulate the whole system [5]. In [6], communication and information technologies were harnessed for renewables in both demand-response and energy storage units. Such a grid-wide information technology (IT) architectural framework enhances the maneuverability, efficiency, and resiliency. However, the entire grid could fall if the communication network fails or a cyber-attack happens. Also, the management of IT infrastructures for distribution networks with a huge quantity of renewable distributed generations (DGs) and micro grids (MGs) involves further issues. In [7], the planning of renewable energy generation considers mainly the whole life cycle cost with maximum energy extraction, which seems to be irresponsible for the upper grid, as the regulation cost and efficiency of reserved conventional generation are left out of consideration. In [8], in the operational scheduling for power systems with renewable power plants, only the participation of conventional sources (coal and pumped storage plants) is considered. The scheduling would be better if it feasibly took the participation of renewables into account. In [9], the load management strategy and autonomous demand-side response in smart buildings were illustrated. However, the load was maneuvered by the power access control in on-off mode. Note that the key point of the evolving transition from conventional energy to renewables involves coordinating heterogeneous renewable energy generation into the present power system, which is dominated by synchronous machines, and maintaining system reliability and stability. Thus, finding the most economic and compatible way to integrate renewable energy generation is the most evolutionary way for future power systems. The VSG control strategy utilizes the flexible controllability of converters to mimic the behavior of traditional synchronous machines [10; 11; 12; 13].