Minimum DC Bus Capacitance Limit for STATCOM

Three-phase STATCOMs are popularly installed in distribution systems for maintaining voltage stability. With a view to enhancing the reliability and life-time of converters installed in such a system, constant effort is being made to minimize the use of failure-prone electrolytic capacitors. This necessitates the choice of optimal capacitance for the DC bus, leading to a tight design with reduced operating margin. However, the scope of reduction of the DC bus capacitance needs to be looked into without compromising the control, stability, and performance of the converter. Modeling of the converter yields operational limits associated with stability for a given DC bus capacitor design. This paper presents a model of a STATCOM, taking the impact of small DC bus capacitor into account. The minimum capacitance limit to ensure independent control of multiple control loops is analyzed and validated in this paper.