Enhanced Current Capability for Modular Multilevel Converters by a Combined Sorting Algorithm for Capacitor Voltages and Semiconductor Losses

Abstract

The modular multilevel converter (MMC) has become very attractive for high-and medium-voltage applications, generating excellent waveforms at very high efficiencies. One of the main challenges is the appropriate selection of inserted submodules (SMs), commonly done by capacitor voltage balancing algorithms. However, the semiconductor stress can only be balanced up to a certain degree by conventional algorithms, since the stress is not directly monitored. An uneven stress distribution between the SMs does not only result in different lifetime expectations, but also in increased maximum temperatures, for which each SM needs to be designed. With the goal of more effective utilization of chip area, a new balancing approach is introduced for monitoring and balancing not only the capacitor voltages but also the average power losses in each SM. In this way, the MMC current capability is significantly increased only by software without deteriorating the system performance and efficiency.

Reinforcement Learning Based Modulation for Balancing Capacitor Voltage and Thermal Stress to Enhance Current Capability of MMCs

“…In addition to the SM voltage balancing control, the thermal stress balancing of semiconductors between SMs is also important to enhance the current capability and to increase the lifetime of the MMC system [7,8]. Junction temperature of the semiconductor (Tj) is affected by loss caused in semiconductor (P) and parameters of thermal equivalent circuits, such as a thermal resistance (2 -.(/0!)…”

“…The average conduction losses caused in the SM can be expressed with the insertion state of the SM and the arm current in ( 7)- (10). [7] The switching losses are ignored due to low switching state changes.…”

“…In addition to the SM voltage balancing control, the importance of equalizing thermal stress between semiconductors has emerged recently in several studies [6][7][8]. The imbalance of the thermal stress not only reduces the current capability of converters [7] but also increases design costs due to additional margin for the safe operation. Furthermore, different lifetime expectancy of semiconductors deteriorates reliability and safety of the converter system.…”

“…Furthermore, different lifetime expectancy of semiconductors deteriorates reliability and safety of the converter system. To distribute the thermal stress evenly, cost functions considering semiconductor losses and junction temperature are introduced in [7] and [8].…”

Investigation of Modular Multilevel Converters for E-STATCOM Applications

“…The power module FF450R17IE4 from Infineon is consid- ered. The power losses of each semiconductor are calculated for both operation points according to [16]. The thermal model is applied according to [15].…”