Search

Modular multilevel converters are well known in the energy sector. Generally, their stable operation is at the expense of numerous sensors, communication burden, and computationally expensive balancing strategies that challenge their expansion to cost-driven applications. Hence, introducing a sensorless voltage-balancing strategy with a simple controller is an attractive objective. Diode-clamped modular multilevel converters (MMCs) offer a simple and effective solution by providing a unidirectional balancing path between two modules through a diode. Ideally, the modulation technique should compensate for the lack of bidirectional energy transfer; hence open-loop operation is possible. Although the sensorless operation is desirable to reduce costs, good knowledge of the modules’ voltages for system monitoring, and protection functions still improves operation in some applications or is mandatory in others. However, information should not be at the cost of additional sensors and communication bandwidth. This article develops a compensated state-space model for diode-clamped MMCs to estimate module voltages using an optimal estimator without any direct measurement at module levels. The model considers the effect of the diode-clamped branches and their balancing effect, resulting in 30%–50% reduction in estimation error compared to the conventional models using similar estimators. Simulations and experiments further confirm the provided analysis, where the estimator achieves >97.5% accuracy.

Advances in the fields of renewable generation, electric vehicles, and energy storage systems push forward the research on ac–dc and dc–ac grid-tied power converters. However, the variabilities of power converters create new challenges in modeling and control. Existing state-space models fail to accurately describe various types of grid-tied converters (GTCs), particularly those with grid-supportive services, which are increasingly required by upcoming grid codes. As such, this article first proposes to classify GTCs into four basic types according to their ac and dc characteristics. Subsequently, corresponding detailed state-space models of GTCs are introduced, which serve as a useful tool for stability analysis. On top of that, this article further proposes control implementations of grid-supportive services related to active and reactive power control, including droop, inertia, and oscillation damping. Finally, simulation and experimental results demonstrate the effectiveness of the proposed models and grid-supportive services.

Larger battery packs, an efficient battery management system, highly efficient electromechanical conversion, and fast battery charging are the key factors for extending the drive range of electric vehicles and the expansion of electromobility. We present a novel highly stable modular multilevel powertrain design with a variable dc-bus voltage, active inner battery energy flow control, and reduced filter size. The underlying powertrain design replaces conventional hard-wired batteries by a modular multilevel architecture, thus modularizing the battery pack and increasing the energy efficiency, reducing the output filter size, and providing full control over the output voltage and energy flow of the battery cells. We analyse the large-signal stability of the proposed system and compare it to dc/dc-dc/ac conventional multiconverter powertrains, demonstrating better stability and lower filter requirements. Further, we experimentally evaluate the proposed system structure. In the experimental setup, we achieve stable converter operation with the output filter capacitor reduced by a factor of 24 and the filter inductor reduced by a factor of 17 compared to dc-dc-dc-ac reference system, thereby leading to a significant reduction of system cost, size, and weight.

Modular multilevel converters and cascaded H-bridge converters are established concepts in ultrahigh-voltage systems. In combination with batteries, these circuits allow dynamically changing the series/parallel configuration of the battery modules, also known as reconfigurable batteries. A large body of research discusses applications of such systems in grid-storage and electromobility with a single load, such as an electric drive. However, most studies neglect the presence of auxiliary loads, which require additional voltage levels and isolation from the high-voltage battery. This article proposes a decoupling method that can fork a second (galvanically isolated) output off a reconfigurable dc battery without negatively affecting the main output by exploiting the many control degrees of freedom in the modular circuit. The proposed system provides a nonisolated port for the dc link of the motor-drive inverter(s), while generating an additional isolated output at another voltage for auxiliaries. The auxiliary output benefits from the interleaving effect, which increases the operating frequency of the transformer and reduces the filter size. The solution does not require additional active switches for the auxiliary port and can operate with a wide range of voltages. The integrated auxiliary circuit can also balance the modules during idling and charge/discharge cycles. Both simulation and experiments validate the analysis.

The modular multilevel converter (MMC) is a popular solution in high-voltage applications with significant potential in others. However, its stable operation is at the expense of numerous sensors, high communication burden, and complicated balancing strategies, which reduce its cost-effectiveness. Hence, the introduction of a sensorless voltage-balancing strategy with a simple controller is an attractive objective. The diode-clamped MMC offers the simplest and yet a highly effective solution for creating a balancing path between two modules through a diode with a fraction of the rated current. To compensate the lack of bidirectional energy transfer, a modified modulation technique is necessary. Level-adjusted phase-shifted-carrier (LA-PSC) modulation introduces a small circulating current that ensures the correct balancing direction. Although the open-loop operation is possible, monitoring and protection functions may still necessitate the independent tracking of the modules’ voltages. This article proposes a simple voltage-estimation method based on back-propagation without additional measurement. The algorithm further considers the balancing effect of the diode-clamped and the introduced circulating currents that reduces steady-state errors as well as fluctuations. Simulations and experiments verify the provided analysis and confirm that the proposed estimator can track the modules’ voltages with <1% and <3% errors in balanced and imbalanced conditions, respectively.