Online Degradation Fault Prognosis for DC-Link Capacitors in Multistring-Connected Photovoltaic Boost Converters Subject to Cable Uncertainties

Abstract

In the application of multistring photovoltaic (PV) power generation, each dc string has a boost converter and the degradation fault of its dc-link capacitor can cause power outage. A comprehensive fault prognosis technology is proposed using the measurable busbar voltage and per-string cable current. The challenge is to avoid the effect of multistring cable uncertainties. First, a 3-D plane $y = f$ is optimized with the information of the series resistance ( $R_{x}$ ) and capacitance ( $C_{x}$ ) of dc-link capacitor. The mathematical proof is presented that based on $y = f$ , $R_{x}$ and $C_{x}$ are estimated separately by decoupling the cable parameter. Second, a novel degradation indicator D is proposed that combines increasing trends of both $R_{x}$ and $C_{x}^{-1}$ . The D-based evaluation sensitivity is improved compared with the single estimated parameter. Simulation verifies that the D-based performance is robust to multistring cable uncertainties and varying operating conditions of PV module and load. Online experiments upon the grid-connected platform clarify that the maximum error of parameter estimation is 1.6% and the D-based evaluation resolution relative to $R_{x}$ is 1.18 dis/m $\Omega $ . The breakthrough lies in the mitigation of uncertain cable effect using neither voltage nor current across dc-link capacitors. Convenient implementation and robust performance are emphasized.