Control of Dynamic Capacitor

Industrial plants are faced with stringent requirements by the utility to maintain a near unity power factor. These plants have traditionally utilized switched capacitor banks as a cost-effective means for power factor correction (PFC). However, if there is a significant level of harmonics present in the current that is disturbing the neighboring loads, then a separate unit for harmonic filtering is required as well. By the time a realistic system is configured, significant cost and complexity are encountered. The concept of a dynamic capacitor (D-CAP) was previously introduced as a means by which to achieve dynamic VAR injection as well as active harmonic filtering in one single integrated unit. The topology consists of taking an existing asset like a PFC capacitor, adding a pair of ac switches and small LC components per phase, and thereby realizing an asset with augmented functionalities. Although the power stage is simple, the underlying control is very challenging. This paper discusses the intricacies of controlling such an architecture and provides keen insights into how one would go about designing a controller.