Frequency synchronization of a single-phase grid-connected dc/ac inverter using a double integration method

Abstract

Many traditional grids have started to integrate the DC/AC inverters in their networks. This requires a good control to enhance the power quality. The grid frequency behaviour is one of the principal indicators that reveal the power quality. Therefore, it must be strongly controlled. Up to now, the most popular frequency synchronization method is based on phase-locked loop (PLL) control. It is known that setting a current proportional to voltage does not work well and it creates a negative resistor in a wide frequency range because the circuit is not passive. This paper presents an alternative idea of synchronization of a single-phase grid-connected DC/AC inverter by using a double integration method. It is simpler than PLL since it needs neither a phase comparator, nor a local oscillator. It provides a signal in phase with the current. The art of the method is how to remove the integrating constants without degrading the signal.