[18] proposed an SSCI damping control strategy based on the multiple-input multiple-output state-space approach and pointed out that the configuration of the SSDC in the rotor-side converter has a better damping performance with a more significant effect than the inclusion of the GSC. The control methods proposed in References [12; 13; 16; 17; 18] incorporate adding damping controllers only in the grid-side or rotor-side control system on one side. This paper presents a control strategy that introduces a sub-synchronous damping controller (SDC) simultaneously on both the grid and rotor sides, with experimental verification showing superior suppression effectiveness. References [14; 15] presented parameter optimization strategies. This paper utilizes the genetic algorithm to optimize the parameters of the SDC, achieving parameter optimization across all operating conditions. Furthermore, the primary objective of the method proposed in this paper is to enhance the SSO damping of the DFIG itself, particularly the oscillations between the DFIG and series compensation. When other types of wind turbines are connected in the vicinity, the improved damping of the DFIG itself can still effectively suppress the oscillations. There are primarily three existing methods to suppress SSO: The first is to disconnect series compensation, but this can reduce the transmission capacity of wind power. The second method involves utilizing an SVG (Static Var Generator) or STATCOM (Static Synchronous Compensator) to add supplementary damping control for oscillation suppression, although this approach is constrained by the capacity of the SVG or STATCOM. The third method involves modifying the control of wind turbines, mainly by altering the strategies for stator and rotor control. This paper focuses on the third method and, building upon previous research, is the first to address the optimization of controllers on both sides. The research results demonstrate that the proposed method achieves better damping effects. At present, research on the mechanism of SSO has been relatively sufficient, but there is no effective, efficient, and convenient method for the optimization and transformation of doubly fed wind turbines and the inhibition and solution of SSO problems. Based on the research of scholars, this study analyzes the factors influencing SSO through frequency domain impedance modeling and time domain simulation analysis, proposes an optimal suppression strategy based on the sub-synchronous damping controller (SDC), and optimizes the parameters through a large number of experiments to achieve the best suppression effect to improve the stability and reliability of the overall operation of the system. The main contributions of this study can be summarized as follows:  
* For the first time, the cooperative damping method for the grid-side and rotor-side control systems of the DFIG is proposed to solve the SSO problem of doubly fed wind power via a series-capacitor grid-connected system. * The parameters, including the gain and phase shift of the proposed SSO damping method, are optimized using the genetic algorithm to achieve the best performance for all variable operation conditions. * The proposed optimized SSO damping method is verified with the established model under the RTLAB platform. The structure of the paper is as follows: The second part establishes the mathematical model and frequency domain impedance model of the doubly fed wind turbine grid-connected system via series compensation, and deduces the impedance function of the system, to reasonably speculate the influence of the parameters on the SSO. The third part designs the SDC, which is added into the control system of the doubly fed wind turbine, and the fourth part of the article utilizes the genetic algorithm to obtain the optimal parameter settings.