## 3 Control Strategy of Wind Storage System to Suppress Low-Frequency Oscillation
### Design of Power Oscillation Damper  
Power oscillation damper (POD). In the nonpeak load stage, some SGs are replaced by wind power, and the original stable control strategy of SGs may not be feasible. To achieve this [13], POD can be installed on DFIG to achieve similar functions as SG-PSS, and damping can be increased by controlling DFIG output. The control structure of POD is shown in Figure 7, which consists of four modules. The first module is the POD amplifier gain unit, the second module is the isolation link, and the third and fourth modules are both lead modules or both lag modules. The phase compensation link provides the necessary lead (or lag) characteristics to compensate for the phase lag (or lead) between the slip  
Figure 6: Dynamic characteristics of SG rotor speed during low-frequency oscillation in the power system.  
input and the electrical torque of the generator. This paper uses two first-order links. In practice, more first-order links or second-order links are needed to achieve the ideal phase compensation effect. Among them, the gain unit is used to enhance mode damping; The isolation link is used for filtering [14], filtering out steady-state components, and the POD output is zero when the signal changes to steady-state; The lead-lag module is used for phase compensation, compensating for the phase shift of frequency measurement and filtering links at low-frequency oscillation frequencies, ensuring that damping control plays a positive damping role without the need for external power grid information [15].  
The feedback signals for suppressing oscillation modes in POD mainly include two types: the rotor speed (or grid frequency) and the power angle of the generator. This article combines two types of feedback signals, using the \(\delta_{\mathrm{A}}\) change in bus voltage phase angle information of the coupling node port of the wind storage system, and obtaining the \(\Delta\omega\) change in synchronous electrical angular velocity through a filter, replacing the traditional SGs \(\omega_{\mathrm{in}}\) as the input of the POD. This reduces the synchronization problem of the wide-area measurement system receiving speed signals, shortens the response time of the modulation output active power, and unifies the POD feedback signals of the wind turbine and energy storage.  
The mathematical expression for the power oscillation damper is [16]:  
\[P_{\mathrm{POD}}=\frac{K_{\mathrm{POD}}}{1+T_{\mathrm{D}}S}\frac{T_{\mathrm{h} }s}{1+T_{\mathrm{h}}s}\frac{1+T_{\mathrm{d1}}s}{1+T_{\mathrm{d2}}s}\frac{1+T_ {\mathrm{d3}}s}{1+T_{\mathrm{d4}}s}\Delta\omega=G_{\mathrm{POD}}(s)\Delta\omega \tag{27}\]  
\[\alpha=\frac{T_{1}}{T_{2}}=\frac{1-\sin\varphi}{1+\sin\varphi} \tag{28}\]  
\[T_{2}=\frac{1}{\sqrt{\alpha\omega}} \tag{29}\]  
where \(K_{\mathrm{POD}}\), \(T_{\mathrm{h}}\), \(T_{\mathrm{d1}}\), \(T_{\mathrm{d2}}\), \(T_{\mathrm{d3}}\) and \(T_{\mathrm{d4}}\) are control parameters, \(T_{\mathrm{D}}\) is the sensor time constant, \(s\) is the Laplace operator, \(G_{\mathrm{POD}}\) is the transfer function of POD, and \(\varphi\) is the angle of phase compensation.  
The gain coefficient \(K_{\mathrm{POD}}\) of the POD controller directly affects the damping effect of the controller. When selecting, the output compensation power capacity constraint should be considered to ensure that it does not exceed the total output of the wind storage system. At the same time, the impact of the controller output power on the synchronous machine terminal voltage should be considered to ensure that its fluctuation value does not exceed the limit. Therefore, under these two constraints, the control coefficient with the most obvious damping effect can be selected through frequency domain analysis.