## 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
When low-frequency oscillation occurs in the system frequency, energy storage dynamically adjusts its active output based on the frequency change of the system. Its control methods mainly include droop control, inertia control, and maximum power output. Figure 9 shows the control strategy of ESS to suppress low-frequency oscillations of the power grid in the wind storage system, which mainly includes synchronous generators, doubly fed fans, ESS, and an infinite grid. \(P_{\text{e}}\) is the electromagnetic power of the synchronous machine; \(P_{\text{W}}\) is the active power output by the doubly fed fan; \(P_{\text{ESS}}\) is the active power output by ESS; \(I_{\text{d}}\), \(I_{\text{q}}\) are the output currents of the d and q axes in the dq coordinate system, respectively; \(I_{\text{d}}^{*}\), \(I_{\text{q}}^{*}\) are the command values for the output current of the d and q axes, respectively [20]. The active output current \(I_{\text{d}}^{*}\) of ESS output is shown in Figure 9:  
\[I_{\text{d}}^{*}=(K_{\text{p}}+sK_{\text{d}})(\omega_{0}-\omega)=(K_{\text{p}} +sK_{\text{d}})\Delta\omega \tag{34}\]  
In the equation, \(K_{\text{p}}\),\(K_{\text{d}}\) is the gain of the PD controller in the speed loop. For low-frequency oscillations in the power grid, existing control strategies mainly include inertia  
Figure 9: ESS with additional POD control strategy to suppress low-frequency oscillations in power grids. control and droop control; \(\omega_{0}\) is the synchronous Angular frequency, and \(\omega\) is the measured Angular frequency of the power grid. Assuming that ESS only provides active support and does not consider the impact of reactive power on the system, the set value of reactive current \(I_{\rm q}^{*}\) is always zero. Expression for electromagnetic power increment of synchronous machines at the electromechanical time scale:  
\[\Delta P_{\rm e}=K_{\rm g}\Delta\delta-K_{\rm ESS}\Delta I_{\rm d} \tag{35}\]  
In the equation, \(K_{\rm g}\) is the synchronization coefficient of the power grid, representing the synchronous and stable operation ability of the power system itself; \(K_{\rm ESS}\) is a control parameter for energy storage, characterizing the impact of ESS on the dynamic characteristics of the power grid under the control strategy. So when the power grid oscillates, the electromagnetic power of the synchronous machine will change. If the active power injected by ESS into the power grid is taken into account, the damping coefficient and inertia coefficient of the power system are:  
\[\left\{ \begin{array}{l}T_{\rm D}=D+K_{\rm ESS}K_{\rm P}\\ T_{\rm J}=2H+K_{\rm ESS}K_{\rm d}\end{array} \right. \tag{36}\]  
In the equation, \(D\) and \(H\) are the inherent damping and inertia coefficients of the system. According to the above equation, the Mode of action of the P controller is equivalent to droop control, and the \(D\) controller is equivalent to inertia control. Therefore, droop control suppresses \(\Delta\omega\) by enhancing the positive damping of the power system, while inertia control mainly suppresses \(\frac{d\omega}{dt}\) by enhancing the inertia level of the power system. The above two types of control can directly or indirectly affect \(S_{\omega}\), thereby achieving suppression of low-frequency oscillations in the power grid. When the unbalanced power in the system is large, it is necessary for the ESS to release (absorb) enough energy \(P_{\rm max}\) in a short period of time, which may exceed the capacity setting of the inverter. At this point, even if the control parameters of inertia control and droop control are adjusted, it is difficult to meet the suppression of low-frequency oscillation, and it is necessary to enable the energy storage to output at maximum power in nonlinear control mode. When the power grid operates in a steady state, the mechanical power \(P_{\rm m}\) is equal to the electromagnetic power \(P_{\rm e}\).