# Improved Control of Forest Microgrids with Hybrid Complementary Energy Storage
## 3 Hybrid Complementary Energy Storage System
### Control of the Energy Stored in the Wind Turbine  
In the wind power generation system, the wind turbine can store or release rotational kinetic energy through the change of its rotational speed, which makes it \"the third energy storage unit\" in the microgrid. However, in a converter-based microgrid, the direct connection between the disturbance in DC side and the change of rotational kinetic energy in the wind power system is isolated by the turbine-side converter. Generally, the wind power unit operates in maximum power point tracking (MPPT) mode, in which the output electric power is independent of the change of DC voltage. Therefore, auxiliary control is needed to establish the correlation between the kinetic energy of the wind turbine and the DC voltage, so that the mechanical rotation system can provide power support for the microgrid under conditions of significant change in DC voltage, to enhance its capability of fault ride-through. Figure 6: Predictive control model of an energy storage system adopting adaptive droop control in the outer loop. Figure 7: Control block diagram of the hybrid energy storage system. When disturbance of DC bus voltage occurs, the charge or discharge power of the DC side capacitor in parallel with the wind power converter is calculated as follows. \[\Delta P_{\text{dc}}=C_{\text{w}}U_{\text{dc}}\frac{\text{d}U_{\text{dc}}}{ \text{d}t}, \tag{10}\]  
where \(C_{\text{w}}\) is the DC side capacitor of the wind power system converter. The variation of the output power caused by the change of the generator speed in the wind power unit is shown in Equation (11). \[\Delta P_{\text{e}}=\frac{\text{d}E_{\text{k}}}{\text{d}t}=J\omega_{\text{r}} \frac{\text{d}\omega_{\text{r}}}{p^{2}\text{d}t}, \tag{11}\]  
where \(\Delta P_{\text{e}}\) is the power variation of the wind power unit, \(E_{\text{k}}\) is the rotating kinetic energy of the generator, \(J\) is the rotating inertia of the synchronous generator, \(\omega_{\text{r}}\) is the angular speed of the synchronous generator, and \(p\) is the number of pole pairs of the generator. As the wind turbine and the permanent magnet generator are directly coupled, the generator speed is equal to that of the wind turbine. As the mechanical energy of the wind turbine is much larger than the energy stored in the capacitor, variation of the wind turbine speed is much smaller than that of the DC voltage when bearing the same amount of power disturbance. To quickly restrain the fluctuation of DC voltage with urgency, it is necessary to transform the power unbalance on the DC side into the change of rotational kinetic energy of the wind turbine through a control method. Therefore, fluctuation of DC bus voltage can be assumed by the wind turbine. The following equation is available by combining Equations (2) and (3). \[J\omega_{\text{r}}\frac{\text{d}\omega_{\text{r}}}{p^{2}\text{d}t}=C_{\text{w} }U_{\text{dc}}\frac{\text{d}U_{\text{dc}}}{\text{d}t}. \tag{12}\]  
The relationship between the variation of the wind turbine speed and the change of DC voltage under the same power disturbance is obtained by integrating and normalizing both sides of Equation (12) simultaneously. It is assumed that the voltage is kept at its rated value \(U_{\text{dc}\text{,N}}\) before occurrence of the disturbance. \[\omega_{\text{r1}\text{,pu}}^{2}-\omega_{\text{r0}\text{,pu}}^{2}=\frac{\frac{ 1}{2}C_{\text{w}}U_{\text{dc}\text{,N}}^{2}}{\frac{1}{2^{p^{2}}}J\omega_{ \text{r}\text{,N}}^{2}}(U_{\text{dc}\text{,pu}}^{2}-1)=k_{\text{reg}}(U_{ \text{dc}\text{,pu}}^{2}-1), \tag{13}\]  
where \(\omega_{\text{r0}\text{,pu}}\) and \(\omega_{\text{r1}\text{,pu}}\) are the per-unit values of the angular speed of the generator before and after disturbance, respectively; \(U_{\text{dc}\text{,pu}}\) is the per-unit value of DC voltage after disturbance; \(\omega_{\text{r}\text{,N}}\) is the rated speed of the generator.