At present, the distributed cooperative control method has been widely adopted in the secondary control of the conventional droop-based microgrid (MG). However, its application in the secondary control of Virtual Synchronous Generator (VSG) is relatively lagging. Based on the conventional virtual governor, this paper designs the frequency distributed secondary control strategy of VSG based on the theory of nonlinear heterogeneous multi-agents and presents the distributed active power sharing control strategy. The frequency will converge to the frequency reference value through the consensus protocol, instead of relying on the virtual frequency modulation coefficient. To reduce the communication burden, a control strategy based on event-triggered is adopted in the distributed secondary control. The distributed control is used in frequency strategy, compared with the VSG secondary control proposed previously, the distributed generations (DGs) can collaboratively work. Compared with the existing conventional droop-based distributed secondary control, the proposed scheme retains the characteristics of VSG and can provide virtual inertia to MG. An islanded microgrid test system was established to verify the proposed control strategy. The results show that the proposed control strategy can restore the frequency to the reference value and accurately share the active power, while greatly reducing the communication burden.

There are some existing power management methods considering economic factors, but these methods are limited by fixed structure of microgrid (MG) and mathematical model, which will be out of effect once the structure changes. In order to obtain the optimal power allocation scheme, this paper proposed the grey wolf optimization (GWO) for the distributed hierarchical control structure of MG, considering the economic dispatching problem. Firstly, the distributed generators (DGs) of MG are considered as multi-agent system, and the secondary controller is constructed. Secondly, the objective function and constraints of GWO are established. Thirdly, the calculation results of GWO are taken as the input of secondary controller, which are called virtual rated power (VRP). Finally, by adjusting VRP dynamically, the on-line real-time optimal power distribution of MG is achieved, and the system stability is proved by multi-agent consensus theory. This power allocation scheme is more flexible to realize the plug-and-play capability. The simulation model is established in Matlab/Simulink environment, and the simulation results show that the method is effective.

Pulse-width pulse-frequency (PWPF) modulators are widely used in spacecraft thruster control. Their dynamic characteristic is still lack of effective analysis tools. This paper presents a fractional describing function method to describe the frequency characteristics of PWPF. A frequency-dependent gain and phase shift are clearly described by fractional-order expression, and the fractional-order behaviors depict the nonlinear properties of PWPF modulators. This fractional describing function method can also be applied to other kinds of modulators.