*5.3. Simulation Analysis with Fluctuating Loads*

was greatly improved.

*5.3. Simulation Analysis with Fluctuating Loads*  The islanded microgrid in forest is generally vulnerable to stochastic fluctuating loads, such as electrical machinery with characteristic of frequent start-up and shut-down, especially when the micro-gas turbine of the biomass unit has encountered fault or energy exhaustion. The following simulation presents the above extreme condition, with corresponding figures shown in Figure 14. In Figure 14a, the dashed lines and solid lines represent the simulation results when the basic method of sectional coordinated control and the improved method of hybrid complementary energy storage control based on prediction model were adopted, respectively. As is shown in the figures, when the basic method was utilized, although the energy storage system responded quickly, the fluctuation of DC voltage was large and the lowest point of voltage was below 0.95 pu. When the improved method was adopted, the hybrid energy storage system could rapidly increase the output power according to the voltage change rate, so as to provide rapid power support for the system. At the same time, the wind power system released its rotational kinetic energy and provided an intermittent energy supply The islanded microgrid in forest is generally vulnerable to stochastic fluctuating loads, such as electrical machinery with characteristic of frequent start-up and shut-down, especially when the micro-gas turbine of the biomass unit has encountered fault or energy exhaustion. The following simulation presents the above extreme condition, with corresponding figures shown in Figure 14. In Figure 14a, the dashed lines and solid lines represent the simulation results when the basic method of sectional coordinated control and the improved method of hybrid complementary energy storage control based on prediction model were adopted, respectively. As is shown in the figures, when the basic method was utilized, although the energy storage system responded quickly, the fluctuation of DC voltage was large and the lowest point of voltage was below 0.95 pu. When the improved method was adopted, the hybrid energy storage system could rapidly increase the output power according to the voltage change rate, so as to provide rapid power support for the system. At the same time, the wind power system released its rotational kinetic energy and provided an intermittent energy supply to smooth the power peak. With the joint efforts of the battery, the capacitor, and the wind

to smooth the power peak. With the joint efforts of the battery, the capacitor, and the wind turbine,

turbine, the lowest point of DC voltage was raised to about 0.97 pu, and the quality of the DC power supply was greatly improved. *Appl. Sci.* **2019**, *9*, 2523 16 of 19

When the hybrid energy control (with only battery and supercapacitor) based on traditional PI regulation was adopted, as shown in the solid lines in Figure 14b, the output power of the hybrid energy storage unit increased slightly compared with the basic control, and the corresponding fluctuation amplitude of DC voltage was between the value using the basic control and the one utilizing the proposed method. When the hybrid energy control (with only battery and supercapacitor) based on traditional PI regulation was adopted, as shown in the solid lines in Figure 14b, the output power of the hybrid energy storage unit increased slightly compared with the basic control, and the corresponding fluctuation amplitude of DC voltage was between the value using the basic control and the one utilizing the proposed method.

**Figure 14.** Performance of each unit in the DC microgrid with fluctuating load: (**a**) The solid lines were obtained by using the improved method of hybrid complementary energy storage based on prediction and the dotted lines were obtained utilizing the basic coordinated control; (**b**) The solid lines were obtained by using the improved method based on PI control and the dotted lines were obtained utilizing the basic coordinated control. **Figure 14.** Performance of each unit in the DC microgrid with fluctuating load: (**a**) The solid lines were obtained by using the improved method of hybrid complementary energy storage based on prediction and the dotted lines were obtained utilizing the basic coordinated control; (**b**) The solid lines were obtained by using the improved method based on PI control and the dotted lines were obtained utilizing the basic coordinated control.

### *5.4. Simulation Analysis with Simulated Natural Wind Speed Similar to the Actual One 5.4. Simulation Analysis with Simulated Natural Wind Speed Similar to the Actual One*

To simulate the actual wind speed, the superposition of basic wind, gust wind, and random wind was adopted as wind time series. The basic wind is a constant value, which was 7.5 m/s in this simulation. The gust wind *v*z and the random wind *v*n were obtained through the following Equation. To simulate the actual wind speed, the superposition of basic wind, gust wind, and random wind was adopted as wind time series. The basic wind is a constant value, which was 7.5 m/s in this simulation. The gust wind *v*<sup>z</sup> and the random wind *v*<sup>n</sup> were obtained through the following Equation.

$$v\_{\rm z} = 0.5 V\_{\rm zmax} \left[ 1 - \cos \frac{2\pi (t - T\_{\rm I})}{T\_{\rm g}} \right] \tag{16}$$

$$v\_{\rm n} = V\_{\rm nmax} \cdot \text{unifrnd}(-1, 1) \cdot \cos[2\pi + \text{unifrnd}(0, 2\pi)],\tag{17}$$

where *V*zmax and *V*nmax represent the maxima of the gust wind and the random wind, respectively; *T*<sup>I</sup> and *T*g are the start time and the cycle of the gust wind, respectively; and unifrnd (a, b) is a function that generates uniformly distributed random numbers with a and b as upper and lower bounds. where *V*zmax and *V*nmax represent the maxima of the gust wind and the random wind, respectively; *T*<sup>I</sup> and *T*<sup>g</sup> are the start time and the cycle of the gust wind, respectively; and unifrnd (a, b) is a function that generates uniformly distributed random numbers with a and b as upper and lower bounds.

voltage-sensitive loads in forest areas, such as parameter detection devices, and reduce the quality of output power. When the hybrid energy storage method proposed in this paper was adopted, under

As is shown in Figure 15, the output of wind power presented fluctuating characteristics. When

As is shown in Figure 15, the output of wind power presented fluctuating characteristics. When basic coordination control was adopted, DC voltage was disturbed by the fluctuating power. Although the voltage could still be maintained above 0.98 pu, fluctuating voltage will adversely affect voltage-sensitive loads in forest areas, such as parameter detection devices, and reduce the quality of output power. When the hybrid energy storage method proposed in this paper was adopted, under fast predictive control of converter, the wind turbine unit and the energy storage systems provided corresponding power support for the system according to the change rate of DC voltage, with additional power output suppressing the random fluctuation. The proposed method ensured the stability of system power and improved the quality of the DC voltage. *Appl. Sci.* **2019**, *9*, 2523 17 of 19 fast predictive control of converter, the wind turbine unit and the energy storage systems provided corresponding power support for the system according to the change rate of DC voltage, with additional power output suppressing the random fluctuation. The proposed method ensured the stability of system power and improved the quality of the DC voltage.

**Figure 15.** Performance of each unit in DC microgrid with simulated natural wind speed. **Figure 15.** Performance of each unit in DC microgrid with simulated natural wind speed.
