Mitigation of harmonic distortion in DC-AC conversion for grid connected PV systems utilizing multi-level inverters

Abstract

With a large PV power station integrated into the grid, some problems including technical losses, voltage drop and harmonic distortion arise. This quantitative study intended to identify the inverter configurations, their impact on system performance and mitigation of harmonic distortion for improved quality of supply. In this study, the PV-grid connected system was modeled in DIgSILENT and MATLAB. First, the model for the PV-grid connected system was established consisting of transformers, transmission line, distribution lines, substations and PV grid. Second, mitigation measures including a multilevel inverter scheme was modeled. Finally, simulations were carried out to determine the impact of evacuation of the generated power on the total harmonic distortion (THD) and its mitigation. Results showed that inverters in use were three phase and centralized inverters with multistring and bidirectional battery capabilities respectively. The highest THD of 38.93% was observed at the point of common coupling (PCC). THD was higher at the nodes closer to the solar power plant and reduced further away from the PCC. Simulation of a conventional three phase inverter resulted into a THD of 31.06%. As a result of applying higher voltage levels, multilevel inverters were introduced. The output voltage from inverter has reduced harmonic distortion and a good quality of waveform. By using MATLAB, different multilevel inverter models were simulated and THD obtained. Application of a seven level cascaded H-Bridge inverter resulted into a THD of 21.60% or reduced by 30.50%. The THD performance further improved by application of a modified seven level cascaded HBridge inverter to 4.60%. An efficiency improvement of 85.20% was therefore achieved as a result of utilizing a modified seven level cascaded H-Bridge inverter. The THD of 4.60% lies within the acceptable harmonic limit of maximum 5% as required of IEEE 519-2014 Standard, hence its mitigation and improved network performance. This study recommends that all new solar power plants include multilevel inverters in their design as opposed to conventional three phase inverters in order to minimise the THD content introduced onto the grid.