Xiao Li

This paper presents a review and comparison of high voltage-step-down ratio dc/dc converters based on the modular multilevel converter (MMC) or quasi-MMC, specifically designed for medium-voltage direct current (MVDC) grid-tied energy storage systems (ESS). The paper discusses various topology configurations and their operational features. The surveyed topologies are compared in terms of switching device cost, switching device power losses, passive component volumes, voltage-step-down ratios, and frequency multiplication ratios. Case studies are performed on selected topologies using specifications of 7-kV input voltage, 250-V-to-450-V output voltage, and 250-kW transferred power. This review highlights the potentials of MMC-based dc/dc converters and provides an analysis of the pros and cons of different topologies.

When ac and dc microgrids get exposed to load power changes or voltage transients, instability could occur owing to constant power loads’ (CPLs) behaviour. CPL has the negative impedance behaviour which is the reason behind such instability. A flexible alternating current transmission system (FACTS) devices are used in ac grid to help improve the power quality under such circumstances. Contrarily, dc networks doesn’t have such devices and suffers under similar situations. In this paper, the T-Type Modular Dc Circuit Breaker (T-Breaker) is proposed to act as a compensator device for dc networks when energy storage is integrated. Its compensation functions in addition to breaking and limiting a system’s current can be achieved using this single device. By implementing the T-Breaker’s shunt (current) and series (voltage) compensations, ride-through of grid transients can be achieved. T-Breaker’s series compensation function and how it can be integrated with the shunt compensation to improve the stability of a dc microgrid is the focus in this paper. In addition to modeling and simulating the system, both small signal and large signal stability are analyzed. Furthermore, a scaled down 270 V 5 kW system is utilized to validate T-Breake’s compensation functions and their influence on a microgrid’s stability.

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 .

The modular multilevel converter (MMC) is highly attractive to develop high-speed medium-voltage variable frequency drives (MV VFDs) due to its cascaded design, high-quality output, and strong reliability. However, high-speed MV VFDs have demanding requirements in capacitor voltage stabilization, current regulation, dynamic response, etc. The instability and distorted voltage/current could easily occur if MMCs are used as high-speed MV VFDs. This paper presents a high-speed VFD with SiC-based MMC feeding a permanent magnet synchronous machine. A new integrated control method is specifically proposed to address the aforementioned issues covering the low-/middle-/high-speed range. Performance analysis is conducted in terms of efficiency, power density, output current quality, and speed range. Steady-state and dynamic conditions are evaluated by a full-scale simulation model and a high-power SiC-based MMC testbed. Results show that the proposed method effectively removes the large capacitor voltage fluctuations, resonant circulating current, and saturated output voltage from zero to the highest speed. Compared with existing MMC VFDs of limited speed (< 4000 rpm), the SiC-based MMC VFD has three times higher speed (15000 rpm), improved efficiency (99.43%), high power density, and low current harmonic distortions.

Modular multilevel converters (MMC) are popular for high-voltage, high-power electric energy conversion. To improve the reliability of MMCs, fault diagnosis and tolerant control have been studied by many researchers. Fault reconfiguration, one important step of the fault tolerant control, has become a challenge, especially under high-voltage operation. Arm current overshoot occurs during the reconfiguration if the MMC does not provide redundant submodules (SMs). Existing publications focus on the fault detection and localization. This work investigates the reconfiguration and proposes a suppression control strategy for the overcurrent. The strategy does not require redundant SMs. The proposed method is simulated in Matlab/Simulink and validated by experiments based on a three-phase MMC using an RL load.