A Supplementary Control Design for Multi-Vendor Realization of Parallel VSCs: A Loop Shaping Approach

The formation of hybrid alternating current (AC)-(DC) direct current systems from converters built by different manufacturers has attracted considerable attention in recent years. In multi-vendor AC-DC systems, the converter stations and their controllers are designed independently due to confidentiality requirements. If the converters have a physical connection at their AC side, unforeseen interactions among adjacent converters may disrupt stability and alter the dynamic performance of the converters from that intended by their designers. This paper contributes to seamlessly integrating converters with independently designed controllers into a multi-VSC (voltage-sourced converter) system. An
$H_\infty$
control problem is defined to design two supplementary filters (SFs) per converter, one for the direct (d)-axis and one for the quadrature (q)-axis control loop, to simultaneously stabilize the multi-VSC system and minimize the perturbation of the dynamic response of the interconnected converters from the vendors' designed dynamic behavior. Adding the SFs to the control system of converters will not cause new disruptive interactions, because the coupling dynamics among the converters are considered in designing the SFs. It is also analytically shown that employing the proposed SFs increases the robust stability margin of the multi-VSC system. Various studies based on the nonlinear model of a 2-VSC system verify the effectiveness of the presented method in integrating independently designed converters into a multi-VSC system.