Voltage Estimation in Combination With Level-Adjusted Phase-Shifted-Carrier Modulation (LA-PSC) for Sensorless Balancing of Diode-Clamped Modular Multilevel Converters (MMCs)
Nima Tashakor, Davood Keshavarzi, Farzad Iraji, Shady Banana, Stefan M. Goetz
Abstract
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.