Interlinking converters and their contribution to primary regulation: a review

Abstract

Classical power systems, which are typically structured in top-down topologies, are gradually evolving towards more decentralized systems comprised by clusters of smart subgrids to cope with the increasing penetration of distributed generation, energy storage systems and controllable loads. These clusters will increase the overall reliability, optimize resource usage and reduce investments in back-up systems. However, tying subgrids via passive devices (tie lines or power transformers) poses certain problems from the point of view of modularity and controllability. They also limit the connection capability of subgrids, as it is expected that systems with different voltage natures (ac and dc) will coexist in the future network. In this context, interlinking converters (IC) have emerged as a universal approach for the interconnection of such subgrids regardless of their characteristics. These power converters not only provide power flow control, but they also improve the power quality of networks through different ancillary services. Therefore, ICs are expected to be the energy routers of the future, smartly connecting and managing the interaction among grids. In the literature several topologies and control techniques have been proposed for this type of converters to transfer power between grids and provide support under contingencies. However, there are no classifications from the point of view of the participation of ICs in the primary regulation of the power system. The aim of this paper is to 1) identify the main characteristics of ICs compared to conventional interconnections based on passive devices, 2) review the most usual IC topologies depending on the nature of the grids they are interconnecting (ac-ac, ac-dc or dc-dc) and 3) analyse and compare the different control approaches for the primary regulation via ICs and propose a general classification based on this analysis regardless of the number of conversion power stages of the IC and the nature and characteristics of tied grids.