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This article presents two simple strategies for output dv/dt reduction in modular multilevel converters through coordination of the switching transients of the submodules. The first method is achieved by adding a delay time between the switching transients of two submodules, which would be switched simultaneously in regular switching strategies of modular multilevel converters. The second method is to inject a reverse dv/dt to the converter's output by switching redundant submodules. Both methods do not affect the switching speeds of the semiconductor devices and require no additional hardware components. They can be applied with different modular multilevel converters modulation strategies. Circuit analysis of the proposed methods and their effects on the converter operation are presented. Simulation and test results of a 1.7-kV SiC-device-based modular multilevel converter are provided to validate the circuit analysis. It is shown that the proposed methods can reduce the converter's output dv/dt to half of a single switching device's dv/dt. [ABSTRACT FROM AUTHOR]

A new series-type hybrid circuit breaker (S-HCB) concept for dc fault protection is reported that is fundamentally different from other prior-art hybrid circuit breakers (HCB) or solid-state circuit breakers (SSCB) in the literature. The S-HCB conducts its load current through metal wires instead of power semiconductor switches and curtails its fault current to near zero throughout the entire opening process of a series mechanical switch. It offers the low on-resistance of the conventional mechanical contacts for normal operation and µs-scale fault response even faster than the fast-acting SSCBs. A proof-of-concept S-HCB prototype experimentally demonstrates the interruption of a fault current of 30 A within 10 μs at a dc voltage of 600 V. The operating principle, simulation, and experimental results are discussed in this letter. [ABSTRACT FROM AUTHOR]

The unprecedented high-speed switching of wide bandgap power devices may change the partial discharge (PD) behaviors of power modules, busbars, and loads. However, very little research has been published on PD behaviors under square-wave pulses with ultrafast dv/dt. To address this problem, this article studies the PD behaviors of direct bonded copper samples with different trench distances and chamfer radii under single and repetitive square pulses with ultrafast dv/dt. The results show that for the single-pulse excitation partial discharge inception voltage (PDIV) decreases with increasing rise time if the pulsewidth is shorter than 300 μs, and PDIV decreases with increasing pulsewidth. For repetitive pulse excitation, PDIV increases linearly with trench distance if the chamfer radius is larger than 0.5 mm, while it follows a concave curve if the chamfer radius is less than 0.5 mm. On the other hand, for a fixed trench distance, the PDIV changes quadratically with the chamfer radius increasing from 0 (right angle) to 0.7 mm. Based on the experimental results, a PD model with bubble expansion theory is to explain the PD mechanism under ultrafast dv/dt excitation, and an empirical equation for PDIV prediction is also derived. [ABSTRACT FROM AUTHOR]

This article presents the design and development of a 7-kV dc bus, 1-MVA, 0–1000 Hz modular multilevel converter (MMC) prototype for high speed medium-voltage variable frequency drives. The system is designed based on 1.7-kV silicon carbide (SiC) mosfets. First, the full in-depth design of the prototype is provided, including the submodules, arm inductors, and system integration. Then, the unique control scheme and the control hardware design are presented to achieve the full-frequency range operation. The power loss based on Si and SiC devices is analyzed. Finally, experimental results are presented to demonstrate the performance of both the SiC-based submodule and the full-scale MMC system. Compared with the traditional Si-based solution, the established SiC MMC can reduce the drive system volume by 75% and achieve 325% power density, and the peak energy efficiency reaches around 99.35%. [ABSTRACT FROM AUTHOR]

Errors in the injected high-frequency circulating currents are originally investigated for modular multilevel converters (MMCs) in low-speed drive operation. The mathematical analysis reveals that the magnitude and phase errors in the injected circulating currents cause serious energy imbalance in the MMC arms, resulting in large voltage fluctuations in the submodules. Furthermore, these fluctuations get larger when the circulating currents are injected at higher frequencies, which reduces the drive system stability and damages the submodule capacitors. A remedy approach is proposed to minimise the error effects by adjusting the circulating current command. A full-scale system comprising a 1 MVA MMC and a 1 MW induction machine is simulated to verify the theoretical analysis. The experimental investigation of a down-scale MMC feeding an inductor is also conducted to validate the theoretical results. [ABSTRACT FROM AUTHOR]

An optimized model predictive control (MPC) method is proposed based on the selected switching states to control the four-level hybrid-clamped converter. The proposed method utilizes 64, instead of 512, switching states, compared with a conventional MPC, which greatly eases the calculation burden while achieving the high-quality regulation of output currents and the balance of capacitor voltages. The steady-state performance at both high and low frequencies is investigated, and results are compared with those by the traditional carrier phase-shifted pulsewidth modulation method. The dynamic response to the current or frequency step change is also evaluated. Both simulation and experimental results validate the feasibility and effectiveness of the optimized MPC method. [ABSTRACT FROM AUTHOR]

Without active control, the voltage fluctuations of the dc-link and flying capacitors in a 4-level hybrid-clamped converter (4L-HCC) become excessively large in low-frequency operation. This paper studies the switching state redundancy inherent in a 4L-HCC and proposes a new capacitor voltage balancing and stabilizing method to minimize the dc-link and flying capacitor voltage fluctuations. A straightforward algorithm is developed based on a logic table to select the suitable switching states. Verified by simulation and experimental testing results, the new method and algorithm virtually eliminate all the big voltage fluctuations of capacitors at low and zero frequencies, greatly enhancing the robustness and reliability of a 4L-HCC. The capacitor size can be reduced by more than 80% compared with the conventional method. [ABSTRACT FROM AUTHOR]

A novel submodule capacitor discharging control strategy is proposed and implemented for modular multilevel converters (MMCs). By sequentially controlling the arm currents of an MMC, all submodule capacitors are discharged rapidly through internal arm resistors without additional devices and circuits, resulting in improved performance and reduced cost. A closed-loop control method is investigated for the effectiveness of the new control strategy. A comparison study is evaluated between the proposed method and the traditional methods using bleeding resistors, auxiliary resistors, and load resistors. Validated by the simulation and experiment, the proposed method has the maximum benefits including zero hardware cost, fast discharging time, and negligible load current impact. [ABSTRACT FROM AUTHOR]