In recent years, the modular multilevel cascaded converters (MMCCs) have attracted interest for potential applications in utility-scale photovoltaic and battery energy storage systems. However, the large numbers of cascaded modules increase the difficulty of implementation in a centralized control architecture. This paper presents a distributed control technique for three-phase MMCCs, which can significantly reduce complexity and increases the flexibility to expand the system. In addition, reference voltage analysis is performed to better understand the operation of the proposed method. Experimental results carried out on a seven-level star-connected cascaded converter are included to validate the proposed approach.

This article develops a solid-state transformer (SST), which integrates PV panels and EV chargers together through the open-dc modular multilevel converter (MMC). Different from the well-known MMC, which has a common dc link, the discussed MMC is open-circuited at the dc link so that it should be analyzed from the three-phase viewpoint. The developed SST performs like two parallel-connected cascaded converters, and they are operated with opposite power flow directions. Compared with the conventional SST solution using two separate cascaded H-bridge (CHB) converters, the developed SST has relatively low component count so that it is expected to be of superiority in cost, footprint, and even power density. In addition, the usage of zero sequence voltage in the CHB converter can be replaced by circulating currents in the open-dc MMC so as to greatly reduce the risk of overmodulation. After the thorough power flow analysis on the open-dc MMC and the corresponding controller design, the developed SST is capable of freely working at any operating points with PV panels and EV chargers. Moreover, the developed SST takes the unbalanced grid conditions into account, which is another key contribution of this article. Finally, the balanced and unbalanced operations of the developed SST are both confirmed by the laboratory test results, and furthermore, the dynamic performance of the developed SST considering the time-variant characteristics of PV panels and EV chargers is also demonstrated through the PLECS simulation platform.

This article develops a static synchronous compensator (STATCOM) based on the hybrid cascaded converter which has the advantage of reduced number of power transistors and floating capacitors compared to the cascaded H-bridge converter. A hybrid modulation scheme is employed in the hybrid cascaded converter to reduce the switching losses while maintaining the output performance, and the corresponding dc capacitor voltage balancing control scheme is proposed. The laboratory test results are provided to verify the feasibility of the developed STATCOM and the effectiveness of the proposed scheme. Furthermore, the proposed scheme is compared with the conventional scheme to reveal the improvement in output quality, power losses, and even fault ride through capability.

Compared with the well-known modular multilevel cascaded converter with single-star bridge-cells (MMCC-SSBC), the hybrid cascaded converter (HCC) has the advantage of the lower component count of power transistors and DC capacitors. Thereby, this paper explores the HCC in grid applications and proposes a hierarchical control scheme which allows the HCC to operate under unbalanced grid conditions and power imbalances among all cells. The HCC is further compared with the MMCC-SSBC under these unbalanced conditions to evaluate its performance. The laboratory test results are provided to verify the proposed control scheme and the comparison. The operational flexibility and reliability of the HCC are also discussed.

The penetration of renewable energy resources (RES) is decreasing the inertia of current power grid due to their intermittent nature. Thus, the introduction of energy storage systems (ESS) can cancel this influence out, thereby increasing the power grid resilience. This paper proposes a novel isolated dc-dc converter designed for ESS in low voltage dc (LVDC) power grids. This converter aims to facilitate the development, installation, and efficient operation of ESS. The proposed converter is based on the conventional three-phase dual-active-bridge (TP-DAB) converter and incorporates a three-phase Yn-Y transformer with a three-limb core, as well as auxiliary converters using multiple cascaded bidirectional choppers connected to the primary side of the transformer. Unlike the traditional TP-DAB converter, which suffers from large converter currents due to reactive current, the proposed converter features the fundamental-frequency unity-power-factor operation, effectively reducing the root-mean-square (RMS) and peak currents under a wide range of operating conditions, leading to higher efficiency compared with the TP-DAB converter. The paper presents the operating principles and control methods of the proposed converter and compares its performance with the conventional TP-DAB converter in terms of RMS and peak currents and power losses. The feasibility and practical applicability of the proposed converter are verified through a 150 V, 2.5 kW downscaled prototype.