## 1 Introduction  
Wind power generation is the most developed renewable energy technology. Wind power has attracted considerable interest worldwide because it is clean and pollution free [1]. The most widely-used wind turbine technology is the doubly-fed induction generator (DFIG) [2]. The volatility and unpredictability of wind power and the high penetration of wind power to the power grid are among the factors contributing to low-frequency oscillation (LFO) in power systems. However, DFIG can be controlled flexibly and can affect system damping by disrupting the power flow or the coupling between generators and power systems to suppress LFO [3]. In some countries, guidelines on wind power grid stipulate that wind turbines should provide (or reserve) damping control functions are based on basic control. Therefore, improving the control strategy of the wind turbine and increasing its ability to suppress the power oscillation of a system are of great significance to the safe and stable operation of grids with high wind power penetration. A DFIG equipped with a power oscillating damper (POD) is suitable for handling power system oscillations [4]. On the one hand, it can increase damping performance when used in place of synchronous machines, which are equipped with power system stabilizers (PSSs) for the utilization of wind power. On the other hand, it increases the number of power system damping control points and expands the control range, thereby improving the damper electromechanical oscillation capability of a wind power system. In recent years, DFIGs with additional damping controllers have been extensively researched. In [5], a traditional PSS in the synchronous machine in the control loop of the DFIG was used to suppress the low frequency oscillation of a power system. Initially, the input signal of the additional damping control of the DFIG is local. A wide-area measurement system (WAMS) enabled by the dispersed deployment of phasor measurement units (PMUs) can capture the dynamic information of a power system--i.e., voltage, current, angle, and frequency--at a high resolution and in near real-time. Owing to the development and advancement of the measurement system, a wide-area signal can be selected into the additional control and thus the damping characteristics of the DFIG can be further improved. It can be divided into active modulation and reactive modulation according to the type of DFIG output signal. In [5; 6; 7], an additional damping controller within the active power is proposed. The APM method can improve system damping by adjusting the rotor-side converter variable reference value. However, the damping of a DFIG-wind turbine shaft mode evidently decreases when APM is applied [8] and reduces the life and reliability of a drive shaft system. Given that DFIG can control active and reactive power independently, RPM through an additional control attached to the reactive power control circuit can damp power system oscillations [8; 9; 10]. This method does not increase the torque oscillation amplitude of a wind turbine drivetrain shaft. However, the effectiveness of RPM in suppressing inter-area oscillation depends on the reactive power injection position and voltage characteristics of the load. Therefore, in some cases, relying on RPM may not achieve the desired system damping [11]. A single active modulation cannot balance shaft torsional oscillation, and a single reactive modulation cannot match the desired system damping. In view of the modulation characteristics of the two additional damping controllers, a two-channel damping control strategy, which includes APM and RPM, is proposed in this paper, and the particle swarm optimization algorithm is used to optimize the controller parameters. This method allows the DFIG to maximize suppressing system oscillations and reduce the adverse effects on the shafting oscillation stability of the wind turbine. This paper is organized as follows: First, the modeling of DFIG is described in Section 2.