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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.

Ensuring a good voltage waveform for the modular multilevel converter (MMC) is always of great importance. This article reveals that the modulation harmonic features of MMC would be adversely affected when applying circulating current suppression control. This problem becomes more severe in medium-voltage MMCs which has small number of submodules. A straightforward decoupled modulation scheme is proposed in this article to overcome this problem. Effectiveness of the proposed scheme is verified by simulation and experimental results.

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.

Modular multilevel converters (MMC) have an inevitable double-frequency circulating current between phases because of the three-phase parallel structure. This leads to extra power loss of the converter. The traditional circulating current suppression strategy suppresses the double-frequency content by superimposing the circulating current suppression signal into the reference of upper and lower arms. Due to the addition of the suppression signal, the upper-and lower-arm modulation reference waves are distorted. That affects the quality of output waveform on AC voltage. This paper proposes a decoupled circulating current suppressing strategy which suppresses the circulating current without affecting the output waveform on the AC side. Simulation and experiment results show the effectiveness of the proposed strategy.