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Due to the increasing integration of renewable energies into the distribution grid, a deterioration of the grid power quality is expected. Consisting of a low-frequency transformer and a fractionally rated power electronic converter, hybrid transformers (HTs) can be applied to ensure a high power quality by controlling voltage, active and reactive power dynamically. For the application in conventional grids, HTs have to withstand considerable overvoltages and overcurrents during voltage surges or short circuits. Although HTs recently gained increasing research interest, protection requirements and protection concepts have not been discussed yet. These aspects are, however, essential to evaluate the practical potential of HTs. In this article, protection requirements are derived and a protection concept is developed and verified by simulations and experiments for a 100-kVA hybrid distribution transformer. Although the protection concept has a considerable impact on the design and performance of the converter, it increases the total losses and volume of the HT only insignificantly. An efficiency of 98.48% is achievable for an HT including protection, which is only slightly lower than the efficiency of 98.64% for a comparable conventional transformer. This renders the HT a promising solution for today's and future ac grids. [ABSTRACT FROM AUTHOR]

Modern wide-bandgap devices, such as SiC- or GaN-based devices, feature significantly reduced switching losses, and the question arises if soft-switching operating modes are still beneficial. For most of the semiconductor devices, only limited information is available to estimate the switching losses. Especially, if a wide operating range is desired, excessive measurements have to be performed to determine the switching losses for arbitrary operating points. Therefore, in this paper, a fast calculation method to determine the switching losses based on the charge equivalent approximation of the mosfet capacitances, relying only on datasheet parameters, is presented. In addition, the turn-off losses at high switching currents are investigated, and an analytical expression to estimate the maximum current range for which the mosfet can be turned off with negligible switching losses is proposed. [ABSTRACT FROM AUTHOR]

This paper presents a combined phase-shift and frequency modulation scheme of a dual–active-bridge (DAB) ac–dc converter with power factor correction (PFC) to achieve zero voltage switching (ZVS) over the full range of the ac mains voltage. The DAB consists of a half bridge with bidirectional switches on the ac side and a full bridge on the dc side of the isolation transformer to accomplish single-stage power conversion. The modulation scheme is described by means of analytical formulas, which are used in an optimization procedure to determine the optimal control variables for minimum switch commutation currents. Furthermore, an ac current controller suitable for the proposed modulation scheme is described. A loss model and measurements on a 3.3-kW electric vehicle battery charger to connect to the 230 \textV_\textrms  / 50-Hz mains considering a battery voltage range of 280–420 V validate the theoretical analysis. [ABSTRACT FROM AUTHOR]

Hardware-in-the-Loop tests of components for high-voltage dc transmission grids, for example circuit breakers, require converter systems capable of generating a high output current with fast transients of the output signal at different output voltages. In this paper, a three-level buck converter (N3L) employing three different supply voltages to enable a wide voltage operation range is presented. The modulation of the converter is investigated in detail with special focus on ripple cancellation, the startup of the interleaved converter and the level shifting between different converter output voltages. A prototype system is designed and constructed, optimized for pulsed power operation and compared to a two level and to a neutral point clamped converter design. The system has been simulated numerically and the design boundaries are validated by measurements on the prototype system. [ABSTRACT FROM AUTHOR]

Standard pulsewidth modulation (PWM) schemes for modular multilevel converters (MMC) have neglected deviations of the module voltages from the mean module voltage of an MMC arm as well as the dynamic change in the module voltages since their last measurement and during the switching period. This induces an output voltage error. Large output voltage errors disturb the higher level closed-loop control of the MMC, such that control targets cannot be met anymore. The output voltage error can be reduced using advanced PWM methods that predict and include the dynamic change of the individual module voltages during a switching period in the calculations for the duty cycle. This paper shows measurement results obtained from an experimental medium-voltage prototype, proving that advanced methods lead to much lower output voltage errors than the standard methods, but cannot fulfill the expectations raised by the simulations documented in the literature. In order to overcome the limitations of the old method, a new method that includes the semiconductor voltage drop and parasitic resistances is introduced and evaluated with measurements in this paper. Furthermore, simulation results showing the influence of the considered modulation methods on the closed-loop current control performance are presented. [ABSTRACT FROM AUTHOR]