**4. Overall Control of the Forest Microgrid**

to fill the power gap and avoid load shedding.

**4. Overall Control of the Forest Microgrid**  The basic control and the improved control with hybrid complementary energy storage of the microgrid in different working modes are summarized in Figure 10. In free mode, the supercapacitor is supplementary to the main control of the biomass unit to smooth the DC voltage and enhance the power quality. In emergency mode, the battery and the supercapacitor take in charge of stabilizing The basic control and the improved control with hybrid complementary energy storage of the microgrid in different working modes are summarized in Figure 10. In free mode, the supercapacitor is supplementary to the main control of the biomass unit to smooth the DC voltage and enhance the power quality. In emergency mode, the battery and the supercapacitor take in charge of stabilizing

complementary energy storage.

high frequency and low frequency disturbances, respectively. In addition, as a special energy storage unit, the wind turbine can absorb superfluous energy when the DC voltage is higher than the upper limited value, and release its huge kinetic energy when the voltage decreases to the lower threshold to *Appl. Sci.* fill the power gap and avoid load shedding. **2019**, *9*, 2523 12 of 19

*Appl. Sci.* **2019**, *9*, 2523 12 of 19


**Figure 10.** Comparison diagram of the basic control and the improved control of hybrid complementary energy storage. **Figure 10.** Comparison diagram of the basic control and the improved control of hybrid complementary energy storage. **Figure 10.** Comparison diagram of the basic control and the improved control of hybrid

As the control of the microgrid is based on the variation of DC voltage, operation mode of the system is primarily determined through the voltage hysteresis, as shown in Figure 11, where, *u*t1 and *u*t2 are the threshold voltages at mode switching points, which are selected to be 0.02 and 0.05, respectively, in this paper. Variables *S*0 = 1, 2, and 3 indicate that the system is operating in free mode, emergency mode, and power limiting mode respectively. Moreover, voltage hysteresis control is adopted to avoid frequent switching of operation modes of the converters. As the control of the microgrid is based on the variation of DC voltage, operation mode of the system is primarily determined through the voltage hysteresis, as shown in Figure 11, where, *u*t1 and *u*t2 are the threshold voltages at mode switching points, which are selected to be 0.02 and 0.05, respectively, in this paper. Variables *S*<sup>0</sup> = 1, 2, and 3 indicate that the system is operating in free mode, emergency mode, and power limiting mode respectively. Moreover, voltage hysteresis control is adopted to avoid frequent switching of operation modes of the converters. As the control of the microgrid is based on the variation of DC voltage, operation mode of the system is primarily determined through the voltage hysteresis, as shown in Figure 11, where, *u*t1 and *u*t2 are the threshold voltages at mode switching points, which are selected to be 0.02 and 0.05, respectively, in this paper. Variables *S*0 = 1, 2, and 3 indicate that the system is operating in free mode, emergency mode, and power limiting mode respectively. Moreover, voltage hysteresis control is

adopted to avoid frequent switching of operation modes of the converters.

**Figure 11.** Operation mode judgment through voltage hysteresis. **Figure 11.** Operation mode judgment through voltage hysteresis. **Figure 11.** Operation mode judgment through voltage hysteresis.

The overall control structure of the islanded DC microgrid in forest area is presented in Figure 12, in which hybrid complementary energy storage is appreciated for power quality improvement and fault ride-through enhancement. *S*, *S*1, *S*2, and *S*3 are the switches of the convertors for transferring among different operating modes. The overall control structure of the islanded DC microgrid in forest area is presented in Figure 12, in which hybrid complementary energy storage is appreciated for power quality improvement and fault ride-through enhancement. *S*, *S*1, *S*2, and *S*3 are the switches of the convertors for transferring among different operating modes. The overall control structure of the islanded DC microgrid in forest area is presented in Figure 12, in which hybrid complementary energy storage is appreciated for power quality improvement and fault ride-through enhancement. *S*, *S*1, *S*2, and *S*<sup>3</sup> are the switches of the convertors for transferring among different operating modes.

*Appl. Sci.* **2019**, *9*, 2523 13 of 19

**Figure 12.** Overall control of the islanded DC microgrid in a forest area. **Figure 12.** Overall control of the islanded DC microgrid in a forest area.

## **5. Simulation Analysis 5. Simulation Analysis**
