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By combining partially rated converters with electromagnetic transformers, hybrid transformers can manage voltage and power flow in ac grids while enabling a variety of application expansions. For instance, dc bias in ac grids can lead to several detrimental impacts, and the existing suppression methods typically require supplementary equipment. However, hybrid transformers can achieve dc bias suppression by modifying their control strategies without any extra demands. The direct dc output control signal will lead to unbalanced arm power accumulation in hybrid transformers employing modular multilevel converters with half-bridge submodules, eventually causing the converter blocked. This letter proposes a novel asymmetric operation mode of hybrid transformers for dc bias suppression, being able to output a continuous and stable dc component without incurring any additional cost. It also avoids impacting the basic functions of hybrid transformers for voltage management and dc integration. Finally, the proposed operation method has been validated through simulations and experiments. The proposed operation approach is not solely aimed at achieving converter optimization, but rather to serve the specific needs of dc bias in ac grids, expanding the functionality of the hybrid transformers further.

The dc chopper (DCC) is the key equipment to realize the fault ride through (FRT) of the offshore wind voltage source converter based high voltage direct current (VSC-HVdc) system. In the existing projects, the solid-state switch is directly connected in parallel with the metal oxide varistor. However, the FRT process lasts for several seconds and the solid-state switch operates at high frequency. The hard-switching operation results in an increase in the number of switches and the limited usage capacity, respectively. In this letter, an active snubber circuit for high-capacity DCC is proposed to achieve soft-switching operation. By actively controlling the capacitor discharge by thyristor, the turn-off voltage and the loss under the multipulse condition are reduced. Furthermore, the working process of the active snubber circuit is analyzed and the parameter selection method is proposed. Finally, the 2 kV/600 A equivalent experimental platform is built, and the experimental results show that the turn-off overvoltage is eliminated, and the turn-on loss and turn-off loss of insulated gate bipolar transistor (IGBT) are reduced by 78%, which verifies the effectiveness of the proposed method.

The dc chopper (DCC) is the key equipment to realize the ac fault ride through (FRT) of the offshore wind voltage source converter (VSC)-HVdc system. The existing DCC is arranged independently, and the full control device is used to control the absorbed surplus power, which is costly and limits its further application. In this article, an modular multilevel converter (MMC) with integrated energy dissipation function using thyristor-based chopper modules (TCM MMC) is proposed. It distributes the chopper module that dissipates the surplus power of the system into the submodules of MMC, shares the energy supply and water-cooling system with the MMC, and uses the semicontrol device, namely thyristor, to cooperate with the LC circuit to realize the control of the energy dissipation function, thus providing a low-cost solution to the FRT problem. Furthermore, the control strategy and parameter matching method are proposed. The 0.75 kV/0.37 MW TCM MMC prototype is built, and the experimental results verify the effectiveness of the proposed scheme. Finally, the performance of TCM MMC in the different FRT process of the ±150 kV/700 MW VSC-HVdc system is analyzed, and the proposed scheme is compared with existing schemes in various aspects.

The dc circuit breaker (DCCB) is the core equipment for ensuring the safe and stable operation of the dc transmission system. It often requires high response times. In recent years, a new type of DCCB topology called forced resonant DCCB (FR-DCCB) has attracted widespread attention. It requires only a small amount of power electronic devices to provide mechanical switch zero crossing with LC circuit. And this topology offers advantages, such as multiple zero-crossing points, low cost, and the potential for faster response time. However, compared with the traditional DCCB solutions, the breaking process of FR-DCCB is extremely complex, which leads to the lack of mature control methods and parameter design theories. Therefore, to further improve the performance of FR-DCCB, this article proposes an efficient resonance control method based on the real-time feedback correction and a fast reclosing control method based on closing surge energy recovery, respectively, and also forms a complete optimization parameters design method. In addition, the parameters design method is presented to significantly reduce the oscillation time of FR-DCCB, thereby improving its response speed to meet engineering requirements. A prototype of FR-DCCB with the largest current breaking capacity has been developed and the effectiveness of the proposed methods has been verified.

Desaturation has always been one of the most severe threats to IGBT safety operations. The current mainstream IGBT drivers used in dc breakers primarily choose on-state voltage VCEsat as the monitor parameter of IGBT desaturation. However, this detection method requires the device first to enter the high-loss desaturation region, which may eventually lead to the thermal breakdown of the IGBT device. This letter reveals a new phenomenon (self-surge current mode) in which the gate voltage VGE rises prior to IGBT desaturation. This letter proposes a novel detection method that uses VGE as a desaturation precursor based on comprehensive analyses of this phenomenon. The proposed method enables highly reliable and sensitive detection while having little impact on the IGBT devices’ maximum current performance, preventing the IGBT device from entering the extremely high loss desaturation region. The experimental results in this letter show that compared to traditional VCE detection, proposed VGE detection can reduce detection time by more than 80% and IGBT junction temperature rise by more than 75%.