# 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
\(k_{\text{reg}}\) is defined as the speed regulation coefficient, and varying degrees of regulation of the DC voltage can be achieved by setting different values of \(k_{\text{reg}}\) to ensure smooth transition of the microgrid during major disturbances. Figure 8 is the switching principle of the improved MPPT curve with speed response of the wind turbine when disturbance occurs in the DC voltage of the microgrid. As shown in this figure, \(P_{\text{opt}}\) is the maximum power point tracking curve, which determines the output of wind power unit under normal conditions. In addition, the curves \(P_{\text{opt}\text{,max}}\) and \(P_{\text{opt}\text{,min}}\) are the upper and lower limits of the output power, respectively. Usually, the microgrid is stable and the wind power unit operates at point A, with the proportional coefficient of MPPT curve being \(k_{\text{opt0}}\). When a steep fall of the DC voltage occurs due to a sudden power vacancy, a large amount of energy is needed to reduce the change rate of DC voltage so that load shedding can be avoided. At this time, the curve switches to \(P_{\text{opt}\text{,max}}\) and the operating point moves from A to O, with the proportional coefficient rising to \(k_{\text{opt}\text{,max}}\). The wind turbine, which connects directly to the generator, slows down due to the fact that the output electromagnetic power is larger than the mechanical power captured by the wind turbine, and the operation point decreases to B along \(P_{\text{opt}\text{,max}}\). With the recovery of DC voltage, the proportional coefficient decreases gradually from \(k_{\text{opt}\text{,max}}\) to \(k_{\text{opt0}}\), with the power tracking curve cutting back  
to \(P_{\text{opt}}\) slowly and smoothly. Therefore, the wind turbine recovers and operates again at point A. Similarly, the switching process of the operation points can be analyzed when DC bus voltage surges. Point A and point B in Figure 8 correspond to the rotational speed \(\omega_{\text{r0-pu}}\) and \(\omega_{\text{r1-pu}}\), respectively. When the range of speed regulation is not wide, the output power of point A and point B is approximately equal, as shown in Equation (14). \[k_{\text{opt}}\omega_{\text{r1-pu}}^{3}=k_{\text{opt}}\omega_{\text{r0-pu}}^{3}, \tag{14}\]  
where \(k_{\text{opt}}\) is the proportion coefficient of power tracking curve after adopting the improved speed control. \(k_{\text{opt}}\) can be obtained by introducing Equation (13) into Equation (14). \[k_{\text{opt}}=\frac{\omega_{\text{r0-pu}}^{3}}{\left[\omega_{\text{r0-pu}}^{ 2}+k_{\text{reg}}(l_{\text{dc-pu}}^{2}-1)\right]^{3/2}}k_{\text{opt0}}. \tag{15}\]  
In order to adjust the inertia response of the wind power unit according to the change rate of DC voltage in microgrid, the speed regulation coefficient \(k_{\text{reg}}\) is modified by the change rate signal to be \(k_{\text{reg}}=k_{\text{w}}\left|\text{d}U_{\text{dc}}/\text{d}t\right|\), where, \(k_{\text{w}}\) is a constant. By utilizing the proportional coefficient \(k_{\text{opt}}\) calculated in Equation (15) instead of the fixed proportional coefficient \(k_{\text{opt}}\), adaptive speed response of the wind power system can be achieved to quickly adjust the output power by using the energy stored in wind turbines. The block diagram of the improved control strategy for the wind power system is shown in Figure 9, where \(k_{\text{opt-min}}\) and \(k_{\text{opt-max}}\) are the limit values of the proportional coefficient.