A New THD Measurement Method with Small Computational Burden using a SOGI-FLL Grid Monitoring System

This article proposes a new method for obtaining the total harmonic distortion (THD) using few math operations, a low-pass filter (LPF), and a grid monitoring system able to provide the fundamental and harmonics components of the grid voltage. The method is particularly developed by using the second-order generalized integrator (SOGI) error signal notch filter transfer function characteristics of a standard SOGI-FLL monitoring system. The method is accurate and has a small computational burden, so it is suitable for the online assessment of the grid voltage or current THD and can easily be implemented into a digital signal processor. The accuracy and transient response of the system have been analyzed, showing that they can easily be determined by the tuning of the SOGI filter and the LPF. The method also shows to be robust to grid perturbations, such us voltage sags, or swells, and frequency step changes. Simulations and experimental results are provided to validate the proposed THD method. Moreover, comparison with a fast Fourier transform (FFT)-based THD is also presented, which shows that the proposed THD method results to be faster, more accurate, and simpler than the FFT-based one.