Operation of Unified Constant-frequency Integration Controlled Three-phase Grid-connected Inverter in Unbalanced Systems

T. Jin and K.M. Smedley (USA)


Grid-connected inverter, One-cycle control, power quality, unbalanced power systems, asymmetric power systems


In recent years, alternative energy sources such as fuel cells, photovoltaic (PV), wind power, etc. have become attractive due to their renewable and environmental friendly features. The power produced from them is in the form of dc in the cases of fuel cells and PV or variable ac in the case of windmill, therefore, in order to transmit those renewable energies to utility, inverters or rectifier/inverter combos are required. Furthermore, in order to eliminate harmonic contamination in the power lines and improve the transmission efficiency, the inverter should inject sinusoidal current to the grid with unity power factor and low harmonics. Many control methods for the grid-connected inverters were proposed in recent years. Theory and experiments have demonstrated that Unified Constant-frequency Integration (UCI) controller features excellent performance, simple circuitry, and low cost for a three-phase grid-connected inverter under three phase balanced conditions. In this paper, operation of this inverter in an unbalanced system is studied. Analysis and experiments show that with UCI control sinusoidal grid current injection can be realized, whether the grid voltages are balanced or not.

Important Links:

Go Back