Driven by the large-scale integration of distributed power resources, grid-connected voltage-source converters (VSCs) are increasingly required to operate as grid-forming units to regulate the system voltage/frequency and emulate the inertia. While various grid-forming control schemes have been reported, their transient behaviors under large-signal disturbances are still not fully explored. This paper addresses this issue by presenting a design-oriented transient stability analysis of the grid-forming VSCs. First, four typical grid-forming control schemes, namely the power-synchronization control (PSC), the basic droop control, the droop control with low-pass filters (LPFs), and the virtual synchronous generator (VSG) control, are systematically reviewed, whose dynamics are characterized by a general largesignal model. Based on this model, a comparative analysis on the transient stabilities of different control schemes is then carried out. It reveals that the PSC and the basic droop control can retain a stable operation as long as there are equilibrium points, due to their non-inertial transient responses; while the droop control with LPFs and the VSG control can be destabilized even if the equilibrium points exist, due to the lack of damping on their inertial transient responses. With the phase portrait, the underlying stability mechanism is explicitly elaborated, and the quantitative impacts of the controller gains and the virtual inertia are clearly identified. Subsequently, controller design guidelines are proposed to enhance the system damping as well as the transient stability. Finally, experimental results are provided to verify the theoretical analysis.

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In this study, a new non-isolated high voltage gains dc/dc converter using coupled inductor and voltage multiplier techniques (diode/capacitor) is presented. The voltage gain will be increased by increasing the turns ratio (N) and the number of stages of the VM units. The proposed converter capable to more increase the output voltage gains with transfer energy which is stored in coupled inductance. Also, the voltage multiplier unit causes to further increase in the output voltage level of the proposed converter. Besides, the nominal value of the semiconductors is low due to these are clamped to the capacitors available on the voltage multiplier units. The normalized voltage stress across the semiconductors is low which this case is compared in the comparison section. Therefore, the power loss of switch can be reduced by using a switch with a lower rating (lower RDS(on)) and power diodes with the low nominal rating. As a result, the overall efficiency of the proposed converter will be high. To confirm the benefits of working in this paper, comparison results for different items with other works are provided in section 4. The principle of operation, the theoretical analysis and the experimental results of a laboratory prototype for N(N2/N1)=2 and n=2 stage in about 260W with operating at 40kHz are provided.

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