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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 (Rₗ) and capacitance (Cₗ) of dc-link capacitor. The mathematical proof is presented that based on y = f, Rₗ and Cₗ are estimated separately by decoupling the cable parameter. Second, a novel degradation indicator D is proposed that combines increasing trends of both Rₗ and Cₗ^-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 Rₗ is 1. 18 dis/m. 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.
Lu et al. (Tue,) studied this question.