Abstract
Multilevel Voltage-Source Converters (VSC) based on modular structures are envisioned as a prominent alternative for grid and industry applications. Foremost among these are the Cascaded H-Bridge (CHB) and the Modular Multilevel Converter (MMC). In this context, depending on the application and the power conversion structure, unbalanced operating conditions can be asked to the converter. Previous investigations regarding the operation and the solutions for modular structures under unbalanced conditions have already addressed this topic, but information is dispersed over a wide number of sources. This paper identifies, classifies, and analyzes the intercluster active power balancing strategies for the adequate operation of the most commonly used modular structures in some typical unbalanced operating scenarios: the Static Synchronous Compensator (STATCOM) under unbalanced voltage and/or current conditions, the unequal power generation in large-scale photovoltaic (PV) power plants, and the uneven power distribution in a battery energy storage system (BESS). Each of the applications has been independently studied so as to provide a comprehensive analysis of the alternative techniques found in the specialized literature, clearly explaining their respective strengths and drawbacks. Several future challenges have been identified during the study, which will involve greater research effort in this key research topic.
Highlights
T HE dependency on fossil fuel-based conventional generation or the poor energy quality supplied by an aged power grid, among others, have led to an unsustainable energy system
Multilevel Voltage-Source Converter (VSC) based on modular structures are envisioned as essential components of the future power grid
Unbalanced condition is a mandatory scenario that these structures must operate in different applications such as STATCOM, large-scale PV power plants, or battery energy storage system (BESS)
Summary
T HE dependency on fossil fuel-based conventional generation or the poor energy quality supplied by an aged power grid, among others, have led to an unsustainable energy system. The unbalance may ask the converter to give negative-sequence current and/or voltage to balance the current and/or the voltage at the point of common coupling (PCC) Despite their advantages, multilevel VSCs based on modular structures present some drawbacks under unbalanced conditions. The aim of this paper is to perform a comprehensive review of the problems that the unbalanced operation suppose for multilevel VSCs based on modular structures, to classify the proposed solutions, and to identify future challenges. The classification and analysis of the most important modular converter structures, as well as their operation under unbalanced conditions will support researchers and developers to adopt the most appropriate solution This will contribute to achieving a more robust future power grid.
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