A High-Efficiency Online Monitoring Strategy for DC-Link Capacitors of Multistring DC/DC Converters in Photovoltaic Applications

The multistring topology is widely used in photovoltaic (PV) power generation applications. These dc-link capacitors of multistring dc/dc converters suffer from different ageing severities, so realizing their independent online monitoring is critical. The equivalent capacitance (
C_x
) is a key ageing indicator. First, a novel method for
C_x
estimation is proposed independent of both voltage and current across dc-link capacitor. The voltage of common dc bus and the output current of dc string are used. Second, a high-efficiency online monitoring strategy is proposed. The procedure is carried out daily after the conventional exits of dc strings. A set of data are collected for the linear fitting. It is proven the fitting curve has the intersection with
y
-axis that turns out to be the reciprocal of
C_x
. Simulation results clarify that the
C_x
estimation has high accuracy and the monitoring performance shows robustness to environment changes and load variations. Online experiments show that the error of
C_x
estimation can be limited within 3.6%. The novelty of proposed strategy lies in its high-efficiency characteristics that the independent online monitoring of multistring dc-link capacitors in PV applications can be achieved without extra hardware and intrusive operation.