*2.3. Coordination Control*

With no frequency and reactive power in the DC microgrid, the DC voltage fluctuation is mainly caused by the uncertainty of system input power [25]. As the DC bus voltage reflects the internal power state of the microgrid and is a sign of system stability, power–voltage regulation is the core of stability control, as well as energy management. This paper utilizes DC bus voltage as the basis for operation mode, switching of each unit to effectively simplify system control.

In this method, the various units of the system are divided into a slack terminal, which is responsible for adjusting power, and power terminals, with constant output in different working modes, as is shown in Figure 2. They are not static, and switch with the change of the operating state of the system. The corresponding switching diagram of control mode is shown in Figure 3. For example, under free mode and emergency mode, the primary power regulating units of the system are the

unit/Load.

biomass power generation unit and the energy storage unit, respectively. In power limiting mode, the corresponding power control unit is the wind power system. In addition, the supercapacitor functions under the condition of rapid power fluctuation in free mode and emergency mode, while kinetic energy regulation of the wind turbine acts under large power disturbances in power limiting mode as well as load shedding mode. power disturbances in power limiting mode as well as load shedding mode.

mode and emergency mode, while kinetic energy regulation of the wind turbine acts under large

mode, the corresponding power control unit is the wind power system.

*Appl. Sci.* **2019**, *9*, 2523 5 of 19

example, under free mode and emergency mode, the primary power regulating units of the system are the biomass power generation unit and the energy storage unit, respectively. In power limiting

*Appl. Sci.* **2019**, *9*, 2523 5 of 19

example, under free mode and emergency mode, the primary power regulating units of the system

**Figure 2.** Basic principle of operation mode-based sectional coordinated control strategy for DC microgrid in forest: (**a**) Biomass power generation unit; (**b**) Energy storage unit; (**c**) Wind power **Figure 2.** Basic principle of operation mode-based sectional coordinated control strategy for DC microgrid in forest: (**a**) Biomass power generation unit; (**b**) Energy storage unit; (**c**) Wind power unit/Load. microgrid in forest: (**a**) Biomass power generation unit; (**b**) Energy storage unit; (**c**) Wind power unit/Load.

**Figure 3.** State switching diagram of system control mode. *k*i (i = 1…8): the hysteresis bandwidth in different modes. **Figure 3.** State switching diagram of system control mode. *k*<sup>i</sup> (i = 1 . . . 8): the hysteresis bandwidth in different modes.

**Figure 3.** State switching diagram of system control mode. *k*i (i = 1…8): the hysteresis bandwidth in

different modes. In the DC microgrid, as the voltage value at the DC bus of each unit makes very little difference, In the DC microgrid, as the voltage value at the DC bus of each unit makes very little difference, the working mode of each unit can be determined only by detecting the DC voltage at the outlet of each converter. Consequently, there is no need to establish a complex communication system by In addition, the supercapacitor functions under the condition of rapid power fluctuation in free mode and emergency mode, while kinetic energy regulation of the wind turbine acts under large power disturbances in power limiting mode as well as load shedding mode.

the working mode of each unit can be determined only by detecting the DC voltage at the outlet of each converter. Consequently, there is no need to establish a complex communication system by utilizing an operation mode-based sectional coordinated control strategy. With the characteristic of plug and play, the coordinated control method has the advantage of simplicity and reliability. In addition, it boasts the feature of self-adaption besides coordination of different units. Due to the fact that the operation mode of the system is divided based on DC voltage, and under each mode power regulation is conducted by the most suitable controllable unit, the stability of the islanded system can be ensured. utilizing an operation mode-based sectional coordinated control strategy. With the characteristic of plug and play, the coordinated control method has the advantage of simplicity and reliability. In addition, it boasts the feature of self-adaption besides coordination of different units. Due to the fact that the operation mode of the system is divided based on DC voltage, and under each mode power regulation is conducted by the most suitable controllable unit, the stability of the islanded system can be ensured. In the DC microgrid, as the voltage value at the DC bus of each unit makes very little difference,the working mode of each unit can be determined only by detecting the DC voltage at the outlet of each converter. Consequently, there is no need to establish a complex communication system by utilizing an operation mode-based sectional coordinated control strategy. With the characteristic of plug and play,the coordinated control method has the advantage of simplicity and reliability. In addition, it boasts the feature of self-adaption besides coordination of different units. Due to the fact that the operation mode of the system is divided based on DC voltage, and under each mode power regulation is conducted by the most suitable controllable unit, the stability of the islanded system can be ensured.
