*5.2. Simulation Analysis with Fault in the Biomass Unit*

At the beginning of the simulation, the islanded forest microgrid operated steadily, with the wind power generation system, the hybrid energy storage system, and the biomass power unit providing stable power output to the DC side. At the 6th second of the simulation, the output of the biomass unit was reduced instantaneously due to a fault of the micro-gas turbine, which led to a large power gap in the DC microgrid. In Figure 13a, simulation results of dashed lines and solid lines are obtained when the basic method of sectional coordinated control and the improved method of hybrid complementary energy storage control based on the prediction model were adopted, respectively. It can be seen from the figures that when the basic method was utilized, the DC voltage dropped rapidly to about 0.93 pu, due to the sudden decline of the system power. According to the control principle of the converters in the DC microgrid, the system will switch to load-shedding mode and the load will be shed according to priority until the voltage restores. However, when the hybrid complementary energy storage control method based on converter prediction model was employed, once the system switched to emergency mode, the hybrid complementary energy storage unit started immediately. Under the fast predictive control of the converter, instantaneous power compensation was provided to relax the downtrend of DC voltage. At this time, the change of DC voltage did not exceed the mode switching threshold, which avoided load shedding and improved the capability of fault ride-through.

In order to compare the control speed of the predictive control method of converter with traditional PI control in the inner loop, the converter control with PI regulator in the inner loop of the hybrid energy storage was simulated and analyzed, as shown in the solid lines in Figure 13b. The dotted lines in the figure still indicate the simulation results with the basic coordinated control being used alone. At the instant of DC voltage drop, the output power of the hybrid energy storage system increased instantaneously, with adaptive modification of the adjustment coefficient in the outer loop of the control. However, the output power of the system could not meet the power requirement and presented unsatisfactory dynamic characteristics, due to the using of PI control which was essentially hysteretic in the inner loop. As a result, the DC voltage was still below the mode switching threshold, which made it difficult to achieve substantial improvement of power quality and reliability.

*Appl. Sci.* **2019**, *9*, 2523 15 of 19

**Figure 13.** Performance of each unit in the DC microgrid under fault condition in the biomass unit: (**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 13.** Performance of each unit in the DC microgrid under fault condition in the biomass unit: (**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.
