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Bipolar LVdc distribution is a promising solution for better integration of renewables and enhancement of system efficiency and reliability, but it also faces the challenge of pole voltage imbalance. By simple modifications of the bidirectional CLLC dc/dc converter, this article presents a dc/dc converter with integrated pole voltage balance capability. The converter presents less number of semiconductors, smaller overall footprint, and lower power loss compared to the employment of additional pole voltage balancers. Operating principle of this dc/dc converter and its inherent pole voltage balancing capability are explained, while further control scheme is also developed to compensate the small pole voltage difference caused by the dead time effect. Simulation results with 150 kW/±375 V bipolar output demonstrate the effectiveness of the converter, and the experimental results from a 500 W/±50 V downscaled prototype further prove the proposed concept.

This article presents a dc solid state transformer based on three-level power module (PM) (TL-DCSST) for interconnection of medium-voltage dc (MVdc) and low-voltage dc distribution system. The proposed TL-DCSST adopts the low-voltage insulated-gate bipolar transistor (IGBTs) with excellent switching characteristics to build medium voltage PM and reduces the number of medium frequency transformer (MFT). This topology features inherent antibias mechanism for MFTs on the primary side, which saves the resonant capacitor at primary side. Meanwhile, the active front end is integrated to realize voltage and power regulation, low voltage ride-through and DC fault handling for MVdc side. In this article, the topology, operation principle, interleaving modulation, circuit parameter design, and control schemes are presented and analyzed comprehensively. At last, a prototype rated at 7 kW has been built and tested, and the experimental results verify effectiveness of the proposed TL-DCSST.

Recently, remarkable progress has been made in the voltage-source-converter-based high-voltage direct current (HVdc) transmissions. For the upcoming need to interconnect multiple dc transmission lines with different voltage levels, high-voltage and large power dc/dc converters are necessary. This article investigates a nonisolated bidirectional dc/dc converter that is composed by cascaded half-bridge submodules, series-connected diodes, and mechanical disconnectors. This hybrid topology presents very attractive features, such as low cost, high efficiency, and light weight. Operation principle, parameter design, and dedicated control strategies are developed. Simulation of a 150-MW, 200 kV/300 kV dc/dc converter has been performed to verify its performance and evaluate the efficiency. Moreover, a downscaled three-phase prototype rated at 500 V/300 V 4.5 kW has been built and tested. The experimental results further confirm the effectiveness of the proposed dc/dc converter.

In the solid state transformers application, the three-level series resonant converter (TL-SRC) based on neural point clamped (NPC) half bridge has become popular in recently. Compared with the full bridge (FB), NPC half bridge can help adopt the low voltage IGBTs with excellent switching characteristics to build high voltage power module (PM) and reduce the number of medium frequency transformers (MFT). However, when TL-SRC operates in backward, there existed voltage imbalanced in the NPC half bridge between inner and outer switches, which increased a risk of overvoltage of the switches in the NPC half bridge. Therefore, this paper proposed two voltage balancing methods for inner and outer switches, and the experimental results verified the validity of the proposed voltage balancing methods through a 150kW back-to-back prototype.

In order to reduce footprint and improve efficiency, solid-state transformer (SST) is an attractive technology for traction and smart grid applications. As the key components of SST, the conventional isolated bidirectional DC/DC converter (IBDC) is hard to guarantee high efficiency and high power density at the same time, due to the limitations of switching characteristics of semiconductor devices and high insulation requirements of medium frequency transformer (MFT). In this paper, a new IBDC topology is proposed, which is characterized by utilizing low-voltage IGBTs with excellent switching characteristics to build high voltage configuration. This helps to reduce the number of the MFTs. Meanwhile, the zero-current switching (ZCS) conditions for all the IGBTs can be guaranteed. This significantly reduce switching losses. The operation principles of the proposed IBDC are analyzed, and the feasibility is verified by simulation results.

The present landscape for power transmission network is undergoing remarkable change with the significant progress made in HVDC transmission, especially the modular multilevel converter (MMC) which shows many operational advantages for high-power application. Similar to the role of AC-transformer in AC systems, there urgently needs DC/DC converter to interconnect multiple dc transmission lines with different voltage levels. This paper investigates a transformerless high-voltage DC/DC converter termed as hybrid-flying-cascaded DC/DC converter (HFC) is suitable for interconnecting high-voltage. This topology presents the merits of less amount of semiconductors, very attractive capital costs, light converter weight and high system efficiency. In this paper, operating principles of the HFC are analyzed, while the modulation and control schemes are also elaborated. Feasibility of the proposed topology is verified by simulation results based on a 300MW, 300kV/500kV HFC.