Grid power quality improvement and active power injection methods through solar interface active filters in presence of nonlinear loads

Abstract

This work proposes an efficient integration method of solar energy with industrial nonlinear loads, accounting for dynamic variations in solar power and load harmonic profiles. The involved dynamics necessitate that the solar interface front-end inverters be equipped to simultaneously handle both harmonic distortions and active power injection characteristics. In this work, an instantaneous load and source characteristic mapping is proposed for the solar-interfaced front-end inverter operated as a shunt active power filter cum active power injector in order to accommodate the dynamic variations of the source and load. As per requirement, to separate the load-demanded and switching harmonics, a new load network time constant-based passive filter design diverging from traditional passive filter design methods is demonstrated. Further, a comprehensive explanation of the current controller modeling methodology is elaborated, considering the essential bandwidth required for proficiently managing load-demanded harmonics in both grid injection and drawing modes of operation. Later, in grid injection mode, the influence of grid and system impedance interactions on the power quality is analyzed for weak grid scenarios to derive the requisite bandwidth conditions to ensure resilient power transfer. Subsequently, the experimental validation of the solar interfaced shunt active power filter (SISAPF) with active power injection capability is carried out in both grid power drawing and injection modes, demonstrating the effectiveness of the enhanced bandwidth in handling load-demanded harmonics and solar power dynamic variations. Under random solar and nonlinear load variations, this work demonstrates an adaptive controller bandwidth realization for a PV-interfaced front-end inverter (PIFI) delivering the power to the EV charging station. For an instantaneous adaptive bandwidth realization, a novel approach to model source intermittency and load nonlinearity in terms of angular frequencies is presented. In particular, the load nonlinearities are modeled in a synchronous reference frame, and it identified that the captured nonlinearity in the reactive component is sufficient to model the load dynamic variations. With the dynamic source and load modeling, a method of rolling gain adjustment for the controller is demonstrated for adaptive accommodation of source and load characteristics. Thus, with the adaptive controller rolling gain adjustment, the PIFI can handle multiple functionalities of harmonic compensation, maximum power point tracking, and simultaneous active power injection. The harmonic correction eliminates the need for a power factor correction circuit in the charging station. The simultaneous active power injection with adaptive bandwidth realization supports efficient solar integration with the load/grid network across all source/load power variations. The efficacy of the proposed sources and load dynamic modeling in realizing the adaptive controller gain for achieving multi-functionalities are demonstrated experimentally on the lab prototype. Apart from this, a new series solar inverter configuration is proposed to share the power in terms of voltage, unlike parallel inverter configurations. Elevated DC potential and circulating current due to common-mode voltage (CMV) would degrade the solar inverter’s life in a single-stage parallel inverter. The proposed topology eases the stress on the DC bus and protects the solar inverter from the issues associated with elevated DC potential (Potential-induced degradation effect, switch operating voltage stress, etc.). Therefore, this work proposes a solar-based series inverter configuration to demonstrate the superior power quality shaping with the active power injection capability in grid-interfaced nonlinear load applications. The proposed configuration realizes the AC network in series. It ensures the reduced AC filter size and DC voltage compared to the conventional inverter topologies. The series inverter configuration mimics the double switching frequency across the load, making the system’s overall bandwidth higher than the conventional inverter. The improved bandwidth ensures reduced attenuation towards lower-order load-demanded harmonics. In addition, as the filter size plays a role in harmonic processing capability, in this work, an optimum filter size is derived by analyzing the nonlinear load current rise time signatures at different load resistances and filter inductances. With the appropriate filter sizing, the dynamic solar power and harmonic injection are accomplished by developing a dedicated closed-loop control mechanism for series inverters. The developed control methodology instantaneously derives combined references to individual inverters through instantaneous AC load harmonic segregation and DC solar power computation. With the derived control mechanism, the superiority of the proposed series inverter over a parallel isolated inverter in terms of enhanced harmonic compensation and dynamic solar power injection is demonstrated through experimental results.