Leakage Current Mitigation in Photovoltaic String Inverter Using Predictive Control with Fixed Avera

Abstract

This work proposes and validates model predictive control as an alternative control strategy for H-NPC converters for single-phase grid-tied string photovoltaic systems. The presented control scheme achieves the good quality current waveforms with unity power factor, dc-link voltage control, neutral-point voltage minimization. Furthermore, the predictive controller has been further enhanced by including an average device switching frequency restriction and a dv/dt mitigation. The main contribution of this paper is the avoidance of the potential leakage current due to parasitic capacitance of the photovoltaic modules by using a predictive model based control technique instead of modulated-schemes and eliminating high-frequency common-mode voltage components.

Existing System

Parasitic Capacitor Model, Modulation Technique

Proposed System

This paper shows the implementation of FCS-MPC on a single-phase photovoltaic grid-connected system based on the commercial H-NPC PV inverter. The programmed control scheme provides current reference tracking, dc-link capacitor voltage balance through the neutral-point voltage minimization and average semiconductor switching frequency limitation. The contributions of this paper are three. The first one is the use of a moving-window to compute the average device switching frequency with negligible computational cost, which allows one to control and almost fix the average switching frequency, that usually is a challenge in predictive control. The second one is the minimization of the CMV to avoid potential leakage currents. In addition, a very simple normalization method to select the weighting factors is presented to reduce the converter commissioning. We present a full description that validates the grid current control with unity power factor, neutral-point voltage (NPV) minimization and non-switched CMV to avoid leakage currents.

Architecture

Proposed control scheme for the H-NPC PV system