Medium-voltage power conditioning systems (MV-PCS) are used to interface battery energy storage systems (BESS) to medium-voltage ac distribution systems. MV-PCS usually include an ac-dc stage, also referred to as the front-end converter (FEC). In this paper, several FEC topologies are categorized according to the type of BESS connection and evaluated. The specific connections are centralized, distributed, and hybrid. The evaluation is performed in terms of power density, FEC efficiency, and construction complexity. Finally, this evaluation is used to provide design guidelines and highlight future research aspects.

A Review of Medium-Voltage Front-End Converters for Grid-Connected Battery Energy Storage Systems

The innovation pace of modern and future power electronic systems such as energy storage systems and D-FACTs is driving the utilization of H.V. SiC MOSFETs. This paper presents the static and dynamic characterization of two configurations of a 3.3-kV SiC MOSFET half-bridge power module. The freewheeling diode in the first configuration is the body diode of the MOSFET. In contrast, the second configuration consists of both the body diode and an external anti-parallel SiC junction barrier Schottky (JBS) diode per switching position. The static and dynamic characteristics of these two configurations are investigated and compared at different junction temperatures. Unlike other evaluations, the comparison shows that the second configuration increased switching losses, switching times, and drain current overshoots. However, it decreased drain-to-source voltage overshoot and slew rates compared with the first configuration.

Static and Dynamic Characterization of 3.3-kV SiC MOSFET Modules With and Without External Anti- Parallel SiC JBS Diode

Availability of high-voltage power modules based on silicon carbide (SiC) is enabling the realization of multilevel topologies to solve a variety of issues in power distribution systems. However, the relatively low voltage ratings of currently-available semiconductor devices increase the complexity of the required solutions. Stacking lower-cost and lower-voltage devices to derive a higher-voltage switch can help reduce system complexity at the expense of more a complex switching position. Thus, this stacking solution comes with its own set of challenges. This paper presents the design and experimental results for an 11-level 13.8 kV neutral-point-clamped flying-capacitor converter that uses stackable switching positions for three-phase current compensation in a distribution system. The converter is successfully used for direct grid-connection and upstream current compensation with a 4.16 kV LL grid.

Construction and Testing of a 13.8 kV, 750 kVA 3-Phase Current Compensator Using Modular Switching Positions

Grid-connected medium-voltage (MV) distribution flexible ac transmission systems (D-FACTS) can be used to perform many functions such as improving the quality of electric power in distribution systems. This paper provides a design methodology for a 2-MVA MV D-FACTS based on a flying-capacitor converter (FCC) topology for three-phase four-wire 25-kV distribution systems. By considering the breakdown and utilization voltages of the available power modules as the main design factor, 10-kV silicon-carbide (SiC) MOSFET half-bridge power modules was chosen with 73% utilization factor. Then, sizing of the flying capacitor cells, dc-bus capacitance, and output LCL filter was performed. Thereafter, the required parts for these components were selected to show the feasibility of fabrication. The power losses of switching modules were investigated for one phase of the converter. Finally, the envisioned MV-FCC functions of compensating for three-phase unbalanced currents and injecting reactive power complementing switched capacitor banks are illustrated through simulations.

Ahmed Rahouma

Papers

Construction and Testing of a 13.8 kV, 750 kVA 3-Phase Current Compensator Using Modular Switching Positions

A Medium-Voltage SiC Flying Capacitor Converter Design for 25-kV Distribution Systems