A novel control strategy to enhance the current quality in grid tied solar inverter

Abstract

In recent times, along with the growth of the solar energy extractions, the grid connected solar inverters are gaining more popularity. The huge deployment of solar inverters may reduce the power quality of utility grid, if they deployed without proper power quality measures. As per the IEEE 1547 standards, the current total harmonic distortion (THD) should not be more that 3 % at the point of common coupling. The current THD can be controlled via inverter switching modulation derived based on the closed loop control of the solar inverter. In closed loop control to generate modulation index for switching, many solar inverter manufacturers uses grid voltage as a reference to achieve anti-islanding mode. But this brings close coupling between grid voltage and the inverter output current THD. In general power inverters are intended for industrial applications and vulnerable to more distorted grid. Therefore the coupling between grid voltage and the inverter current would shoot up the inverter current THD more than 3%. In order to comply the standard, inverter manufacturer need to achieve the decoupling between the grid voltage and the current THD to bring current THD less than 3% without loosing the feature of anti-islanding operation. To achieve the decoupling, in this work, a new rotatory reference frame based method is devised to derive the modulation index for the inverter switching from distorted grid voltage. The derived modulation by proposed method is used not only generate the switching pulses to achieve the current THD less than 3% irrespective of grid condition but also ensures the inverter operate in anti-islanding mode. The efficacy of the proposed method substantiated using MATLAB, PSIM simulations and verified with the hardware results.