## 1 Introduction  
With the continuous expansion of power systems, the structure of the power system is becoming more and more complex, and technological disturbances are gradually increasing with the growth of renewable energy. The traditional electromechanical oscillation of the synchronous generator (SG) dominated power system is becoming more frequent and complicated, which seriously affects the safe and stable operation of power grid [1; 2; 3]. The analysis and control of power oscillations on the electromechanical time scale has always been a hot issue in power systems, and scholars at home and abroad have undertaken extensive research in this field. Traditional oscillation suppression methods include power system stabilizers (PSS) and flexible AC transmission systems (FACTS), which can effectively suppress the electromechanical oscillation to a certain extent. In recent years, power systems have taken on higher requirements for grid operation, and traditional methods cannot meet the system operation requirements in terms of suppression response speed and the control of low ultralow frequency oscillation [4; 5]. With the development of power electronics, FACTS devices have been widely used in the power grid. The rapid and effective suppression of grid power oscillation can be achieved with the help of fast-response FACTS devices and advanced control strategies. The technical methods of FACTS devices to suppress power oscillation can be roughly divided into active power-based methods and reactive power-based methods. In [6], the electromechanical oscillation of a power system is suppressed by controlling the active power of the ESS. The ESS can directly and effectively suppress grid power oscillation, showing excellent technical performance. However, when the power system is large, the capacity of ESSs will increase dramatically, so the cost of pure active power-based methods is very high. To overcome the cost issue, distributed generation, such as wind power and photovoltaic power, were utilized to suppress the power oscillations in [7; 8; 9], however, distributed generation has great volatility and randomness, and works in the maximum power tracking mode, so it is difficult to effectively suppress the electromechanical oscillation. Compared with pure active power-based methods, the pure reactive power-based scheme shows great cost advantages. In [10], a control strategy based on a genetic algorithm was proposed, which uses the unified power flow controller to suppress the oscillation. In [11], a flexible transmission system is used to equivalently change the impedance of a power system, which can improve the system damping characteristics and suppress power oscillation. By controlling the output reactive power of STATCOM, the oscillation suppression can be indirectly realized [12; 13], which makes full use of the fast response of STATCOM. In [14; 15], the physical mechanism of STATCOM suppression of power grid electromechanical oscillation was investigated. The effects of different control modes on power oscillation suppression were also studied in terms of system inertia, damping, and synchronization effects. By optimizing the inertia and damping parameters of the virtual synchronous generator, the transient response process of the power system can be effectively controlled [16; 17; 18]. However, the existing oscillation suppression devices adopt control schemes with fixed parameters, which cannot automatically adjust the system inertia and damping according to the operating status of the specific power system, so better oscillation suppression performance cannot be achieved. Therefore, this paper fully combines the cost advantage of reactive power-based methods and the technical advantage of active power-based methods, so that the ESS can work mainly in the reactive power suppression mode, and its active power output is mainly used to compensate the power loss and to ensure DC side voltage stability.