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

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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y = f$ </tex-math></inline-formula> is optimized with the information of the series resistance (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{x}$ </tex-math></inline-formula>) and capacitance (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{x}$ </tex-math></inline-formula>) of dc-link capacitor. The mathematical proof is presented that based on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y = f$ </tex-math></inline-formula>, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{x}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{x}$ </tex-math></inline-formula> are estimated separately by decoupling the cable parameter. Second, a novel degradation indicator D is proposed that combines increasing trends of both <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{x}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{x}^{-1}$ </tex-math></inline-formula>. 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{x}$ </tex-math></inline-formula> is 1.18 dis/m<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula>. The breakthrough lies in the mitigation of uncertain cable effect using neither voltage <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nor</small> current across dc-link capacitors. Convenient implementation and robust performance are emphasized.