An Novel Online Monitoring Scheme of dc-link Capacitors in Multistring Photovoltaic dc/dc Converters Without Capacitor Voltage and Current

These dc-link capacitors of multistring photovoltaic (PV) dc/dc converters face a great threat in ageing failure. Neither voltage nor current across dc-link capacitor is available in practice, so it is challenging to realize its online monitoring based on equivalent electrical capacitance (C x ) estimation. First, a novel estimation method is proposed using the common dc voltage and the PV cable current. The estimated C x is proven independent of the output characteristics of PV module, the operating condition of grid load and the variation of PV cable parameter. Then, a high-efficiency online monitoring scheme is proposed to monitor each dc-link capacitor daily after the dc/dc converter is locked routinely. The ageing process can be traced and the residual lifetime can be predicted. Online experimental results indicate the estimated C x has the error within 5 %. In practical applications of multistring PV system, the proposed scheme is highlighted with cost-effective, accurate, nonintrusive and robust advantages.