Modeling and analysis of current harmonic distortion from grid connected PV inverters under different operating conditions

• Causes of the current harmonics are summarized.
• The harmonic amplitude increases as PV output power level decreases.
• Field measurement for current harmonics is carried out at a microgrid.
• DC-link voltage ripple cause a series of odd harmonics in the output current.
• A time-varying model is proposed to reveal the characteristics of the harmonics.

Due to the fast growth of photovoltaic (PV) installations, concerns are rising about the harmonic distortion generated from PV inverters. High current total harmonic distortion (THD) occurs when PV inverters operate under light load conditions due to low solar insolation. A general model modified from the conventional control structure diagram is introduced to analyze the harmonic formation process. Causes of the current harmonics are summarized and its relationship with output power levels is analyzed. The field measurement for current harmonics is carried out at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) microgrid to validate the analysis results. In particular for two-stage inverter, unlike previous papers that assume the DC-link voltage is constant, the DC-link voltage ripple is identified as the source of a series of odd harmonics. A mew periodic time-varying model is proposed by including the DC-link voltage ripple into the conventional current control loop model. This model is able to simulate the characteristics of the harmonic components and show their dependence on the DC-link voltage ripple.