Harmonic distortion and unbalance analysis in multi-inverter photovoltaic systems

The growing penetration of distributed generation connected to the grid by power electronics converters can adversely affect the power quality. The capability of the system to host large harmonic distortions and unbalance in three-phase systems may be an issue. In case of Photovoltaic (PV) systems, the harmonic injection and unbalance are due to the transistors commutated by Pulse Width Modulation (PWM) inside the converters. Thus, power converters, to comply with the distortion limits, are equipped with filters, and control methodologies are even required for the compensation of power unbalance. In this paper, the main goal is the quantification of the impact of harmonic distortion on the unbalance. After the presentation of a theoretical approach to deal with harmonic distortion and unbalance by appropriate indicators, a PV system with multi-inverter configuration is tested. The measurements of power quality of the worst and the best converters are compared to the overall measurements of power quality for all the inverters.

I. Introduction

In grid-connected or stand-alone Photovoltaic (PV) systems the power converters are key components. They permit to extract the maximum power from PV generators and to inject power into the grid. Their architecture differs as a function of the size and the number of the installed converters [1]. In the centralized configuration, inverters with high capacity, up to 500 kW, are connected to several (50–100) PV strings in parallel: each string is composed of 15–25 series-connected modules [2]. In this case, installation costs are reduced thanks to few converters. If each converter is equipped with a conventional Maximum Power Point Tracker (MPPT), it cannot properly work in case of partial shadings or module failures, because local MPPs arise besides the global MPP, and the PV system performance drops significantly [3]. An algorithm, able to trace the current-voltage (1-V) characteristic curves in a single sweep, can identify the global MPP [4]. The centralized configuration can be the best solution in case of multi-megawatt ground-mounted PV systems without shading problems. In other cases, it is diffuse the usage of centralized converters equipped with many MPPTs [5]. In [6] a 330 kVA inverter is equipped with six MPPTs, each of them connected with a portion of the PV field affected by shadows at different hours of the day. On the other hand, smaller converters have relatively lower efficiency and higher installation costs. In the micro-inverter configuration, each PV module is equipped with a single inverter installed on its rear side: an individual MPPT is present and shading losses are minimized [7]. Although the impact of the fault of a single module is minimum for the PV system performance, the higher number of converters leads to a higher installation cost. In addition, these converters may have a reduced life, because they are installed outdoor and are subject to a continuous thermal stress [8]. In small PV applications, such as residential plants, the impact of inverter price on the system cost is lower, and micro-inverters can be the optimal solution.