Improved harmonics estimation schemes-based shunt active power filter for quality enhancement under high distortions

Harmonics current estimation is important in a shunt active power filter (SAPF) to ensure proper nonlinear load harmonics compensation. Among the techniques that stand out as effective solutions for harmonics estimation are the second-order generalized integrator (SOGI) method and its derivatives-based schemes due to their good transient response and high filtering capability. However, these methods may perform poorly when DC disturbance is inherent in their input current. Therefore, advanced approaches have been proposed in the literature to reject the DC disturbance effect and improve the SAPF harmonics compensation performance. Nevertheless, in such methods, the harmonics estimation is achieved by subtracting the estimated DC-free fundamental currents from the actual ones, which contain the DC component, thus, the DC component will appear in the estimated harmonics. This leads to compromised harmonics estimation accuracy and erroneous compensating current reference generation, therefore, the quality of the injected grid current is questionable. To address this issue, three innovative harmonics estimation schemes shaped based on an enhanced SOGI (ESOGI) suitable for DC component estimation/rejection are proposed in this paper for a three-phase SAPF system. These schemes are the double ESOGI (DESOGI), double-multiple ESOGI (DMESOGI), and modified DSOGI with prefilter (mDSOGI-WPF), which allow estimation of the DC component and rejecting it from the estimated harmonics, thereby ensuring accurate DC-free harmonics estimation. Such methods-based harmonics estimation can contribute to effectively compensating for the nonlinear load harmonics, improving the injected grid currents quality, and maintaining ripple-free DC bus voltage control, even under highly distorted sensed currents. A comparative study through real-time hardware-in-the-loop (HIL) testing based on OPAL-RT is conducted between the proposed control schemes and a DSOGI-WPF method-based control scheme under distorted conditions. The HIL testing results confirm the superiority of the proposed methods regarding the injection of pure sinusoidal currents with low THD into the grid, total rejection of the DC disturbance from the injected currents, and proper control of the DC bus voltage. In addition, they highlight that the DMESOGI-based control scheme is found to be slightly superior to others.