## 3 Control Strategy of Wind Storage System to Suppress Low-Frequency Oscillation
### Analysis of the Influence of Different Installation Positions of POD on Low Frequency Oscillation  
The inverter mainly includes the installation of POD on the RSC side of the doubly fed fan and the installation of POD on the energy storage side of the inverter. The dual PWM controller can achieve bidirectional energy flow. When the doubly fed fan operates in a subsynchronous state, the energy flows from the grid side to the rotor side of the doubly fed fan; When the doubly fed fan operates in a super synchronous state, energy flows from the rotor side of the doubly fed fan to the grid side. RSC mainly realizes decoupling control of output active and reactive power, while GSC mainly realizes control of input power factor and DC voltage stabilization [17]. Figure 7: Structure diagram of POD controller. According to the control object, POD can be divided into active control, reactive control, and active/reactive hybrid control. POD can achieve different functions and different oscillation suppression effects when installed in different positions, and can suppress oscillation amplitude when installed in a rotor side converter (RSC); The grid side converter (GSC) installed on the grid side can speed up the Rate of convergence speed of oscillation. Usually, LFO is more sensitive to \(P_{\mathrm{s}}\) than \(Q_{\mathrm{s}}\), \(U_{\mathrm{d}c}\), or \(Q_{\mathrm{g}}\) and installing reactive power control in the GSC circuit can reduce voltage quality to a certain extent [18]. The control block diagram corresponding to the mathematical model of the doubly-fed windmill converter described above is shown in Figure 8 below [19]:  
\[\Delta\omega=\frac{s}{1+T_{f}s}\Delta\delta \tag{30}\]  
\[P_{\mathrm{POD}}=\frac{K_{\mathrm{POD}}}{1+T_{\mathrm{D}s}}\frac{T_{\mathrm{h }}s}{1+T_{\mathrm{h}}s}\frac{1+T_{\mathrm{d1}}s}{1+T_{\mathrm{d2}}s}\frac{1+ T_{\mathrm{d3}}s}{1+T_{\mathrm{d4}}s}\Delta\omega=G_{\mathrm{POD}}(s)\Delta\omega \tag{31}\]  
\[\Delta P=P_{POD}+P_{s}^{*}-P_{s} \tag{32}\]  
\[\Delta P=\frac{K_{\mathrm{POD}}}{1+T_{\mathrm{D}s}}\frac{T_{\mathrm{h}}s}{1+ T_{\mathrm{h}}s}\frac{1+T_{\mathrm{d1}}s}{1+T_{\mathrm{d2}}s}\frac{1+T_{ \mathrm{d3}}s}{1+T_{\mathrm{d4}}s}\Delta\omega+P_{s}^{*}-P_{s}=G_{\mathrm{POD 2}}(s)\Delta\delta+P_{s}^{*}-P_{s} \tag{33}\]  
Renewable energy has become a feasible solution to the problems of global energy security and environmental degradation. Due to the mature technology of wind power  
Figure 8: Control Structure Diagram of DFIG with Additional POD. generation and low investment cost, wind power has developed rapidly, and the amount of wind power generation has been increasing exponentially year by year. However, like other renewable energy sources, wind energy is uncertain and uncontrollable. This uncertainty has brought adverse effects on ensuring grid stability, reliability and power quality. Existing methods for mitigating wind power fluctuations can be divided into two categories, namely power smoothing methods without ESS and methods using ESS. The cost of pitch angle control and inertial kinetic energy control in the first method is low, but its adjustment speed is slow; it is limited by wind speed and wind power penetration rate, causing the adjustable range of fan output power to be small. In the second type of method, ESS is added to the wind power generation system, which can not only stabilize the fluctuation of wind power, but also quickly make up for the system power shortage with the maximum output power in a short period of time; it can also reduce the limitation of wind turbine capacity and increase the output power adjustable range. Due to the technological development of ESS, it is widely used in alleviating wind power fluctuations and suppressing system frequency oscillations. This paper adopts the second method, and ESS can quickly and massively contribute to make up for the power shortage caused by the sudden increase in system load. Energy storage suppressing low-frequency oscillation provides positive damping for the frequency oscillation of the system.