Analysis and performance evaluation of a three-phase sparse neutral point clamped converter for industrial variable speed drives

This paper analyzes the operation and characterizes the performance of a three-phase three-level (3-L) Sparse Neutral Point Clamped converter (SNPCC) for industrial variable speed drives (VSDs). The operating principle of the SNPCC, which advantageously employs a lower number of power transistors than a conventional 3-L inverter, is described in detail, focusing on the AC-side differential-mode and common-mode voltage formation and on the DC-side mid-point current generation processes. The degrees of freedom in the SNPCC modulation scheme are defined and several switching sequences are investigated. Afterwards, the stresses on the active and passive components (e.g. semiconductor losses, machine phase current ripple, DC-link capacitor RMS current, etc.) are calculated by analytical and/or numerical means, enabling a straightforward performance comparison among the identified switching sequences. The most suited modulation strategy for VSD applications is then selected and a chip area sizing procedure, aimed at minimizing the total semiconductor chip size, is applied to a 800V 7.5kW three-phase system. The performance limits of the designed SNPCC are evaluated and finally compared to the ones of conventional 2-L and 3-L solutions, highlighting the promising cost/performance trade-off of the analyzed topology.

Architecture and control of an electric vehicle charging station using a bipolar DC bus

DURING the last decade, Electric Vehicles (EVs) have become a reality, and several products that offer a cleaner alternative for transportation have become available on the market. However, despite the numerous advantages of EVs, drivers are still more inclined to use conventional vehicles because they do not see them as a real alternative to transportation. The main reasons for this are the long refueling process using conventional overnight charging and their limited mileage capacity. Several options have been explored in order to address this reticent behaviour toward EVs. Among these alternatives, the high-power fast charging process of the battery packs holds the potential to facilitate large-scale adoption of EVs. However, to reduce the charging times and also meet all the challenges and requirements of this growing application, new high-performance architectures must be conceived and developed. Framed by this context, the main goal of this thesis is to contribute the development of fast-charging stations (FCS) configurations, control schemes and coordination methods to facilitate its grid integration. The increased power levels and the amount of energy involved in transportation, make multilevel power converters as the most suitable topologies for enabling the station. Aiming in this direction, a novel architecture for FCS is proposed, based on the use of a bipolar dc bus enabled by a central Neutral Point Clamped Converter. Given the selected dc configuration, the balancing of the dc voltages becomes more complex. This is related with the stochastic nature of the EV charging load, leading to unbalanced dc loads. To overcome this issue two balancing methods are proposed based on the use of a balancing circuit that enhances the central converter capabilities. Moreover, the architecture enables the inclusion of energy storage and generation stages, allowing to extend its functionality. To fully explore the potential benefits of FCS, a third balancing mechanism is developed based on the use of an energy buffer. Without altering its main function, its power consumption can be managed toward aiding the balancing tasks. Additionally, the inclusion of these optional stages requires a proper management of the energy available in the system. A novel generalized energy management strategy is proposed, that allows to evaluate the economical benefits of the different configurations.

Architecture and control of an electric vehicle charging station using a bipolar DC bus

DURING the last decade, Electric Vehicles (EVs) have become a reality, and several products that offer a cleaner alternative for transportation have become available on the market. However, despite the numerous advantages of EVs, drivers are still more inclined to use conventional vehicles because they do not see them as a real alternative to transportation. The main reasons for this are the long refueling process using conventional overnight charging and their limited mileage capacity. Several options have been explored in order to address this reticent behaviour toward EVs. Among these alternatives, the high-power fast charging process of the battery packs holds the potential to facilitate large-scale adoption of EVs. However, to reduce the charging times and also meet all the challenges and requirements of this growing application, new high-performance architectures must be conceived and developed. Framed by this context, the main goal of this thesis is to contribute the development of fast-charging stations (FCS) configurations, control schemes and coordination methods to facilitate its grid integration. The increased power levels and the amount of energy involved in transportation, make multilevel power converters as the most suitable topologies for enabling the station. Aiming in this direction, a novel architecture for FCS is proposed, based on the use of a bipolar dc bus enabled by a central Neutral Point Clamped Converter. Given the selected dc configuration, the balancing of the dc voltages becomes more complex. This is related with the stochastic nature of the EV charging load, leading to unbalanced dc loads. To overcome this issue two balancing methods are proposed based on the use of a balancing circuit that enhances the central converter capabilities. Moreover, the architecture enables the inclusion of energy storage and generation stages, allowing to extend its functionality. To fully explore the potential benefits of FCS, a third balancing mechanism is developed based on the use of an energy buffer. Without altering its main function, its power consumption can be managed toward aiding the balancing tasks. Additionally, the inclusion of these optional stages requires a proper management of the energy available in the system. A novel generalized energy management strategy is proposed, that allows to evaluate the economical benefits of the different configurations.