| Abstract |
A full-bridge inverter for interfacing the utility grid is developed for using in a Green Power Inverter application. The inverter is feed from an arbitrary green power source (fuel cell, photovoltaic, small wind turbine, etc.) through a rectifier into the dc-link. In order to maintain a sinusoidal grid current with low harmonic distortion and a high power factor, the inverter is controlled to emulate a negative resistance towards the grid. The size of the emulated resistor is determined by the dc-link voltage controller, which tries to maintain a constant dc-link voltage. This is however not possible, while the power into the dc-link is constant and the power out of the dc-link is a second powered sinusoidal with an amplitude of two times the average power. For that reason a small ripple is present. In order to lower the transmitted high frequency current ripple, due to the operation of the inverter, a LCL filter is inserted between the grid and the inverter. It is shown that the LCL filter may be regarded as an inductor for frequencies slightly below the filters resonant frequency; hence the control of the filter becomes easy. On the other side, the z-plane poles for the filter, even with parasitic resistances, lie closer to the border of the unit-circle. Adding a resistance in parallel with the outer inductor in the LCL filter shows to improve the stability on the cost of a little higher loss. A 1 kW Green Power Inverter was designed and implemented in the laboratory. The result shows that the LCL filter is stable when the damping-resistor is added. The total harmonic current distortion was measured below 4,0 the power factor is better than 0,99 for an input power above 300W. |
