Online Capacitor Condition Monitoring for Cascaded-H-Bridge Type Converters

The cascaded-H-bridge (CHB) type converter is widely used in grid-tied applications, such as static var generator (SVG), static synchronous compensator (STATCOM), and photovoltaic system. In these situations, the dc-link capacitor is a key component, which serves as energy buffer and determines the quality of output waveform. Thus, acquiring the health status of these capacitors is critical for the reliable operation of the whole system. In this article, a general capacitor condition monitoring approach based on improved recursive least square (RLS) for CHB-type converters is proposed. The key equations are deduced from the basic control strategies and no extra sensors are introduced. The only signal required is the dc-link voltage, which is already collected for control purpose. The RLS algorithm is modified by adding self-adapted forgetting factor, to achieve a good performance on both identification accuracy and tracking speed. The capacitance estimation error is below 1%, and shows good feasibility in various working conditions and applications. The principle, main steps, case study, and proof-of-concept of the proposed method are presented.