Modelling the Connection of Alternative Energy Sources to the Three-phase Utility Grid for Circuit Simulation

The paper proposes a model for the practical circuit needed to interface a VSI to the three-phase utility grid for connection of alternative energy sources. The model is deduced by considering both the inverter and grid voltages as being constituted by a superposition of modal (differential and omopolar) components. This approach allows identifying a transfer function of the interface, whose Bode diagrams can aid the design and help to check the compliance with the general requirements. As an application example, the problem of interfacing a battery of fuel cells to the grid is considered. Here, besides dimensioning the passive circuit, it is successfully explored by means of PSpice simulations a closed loop solution aimed at controlling the output power. This is done by feeding back the grid currents transformed in the two-phase frame. In the simulated control system, the errors signals are minimized by adding compensating signals to cancel the contributions due to both grid voltage and cross coupling between the inverter differential modes