Multilevel Converters for Battery Energy Storage: How Many Levels and Why?

This work explores the potential benefits of cascaded H-bridge multilevel converters in low-voltage applications, particularly grid-attached battery energy storage systems (BESS). While some benefits of these are discussed in literature, this work seeks to create practical, quantitative models for system performance in terms of a number of key performance parameters. These models are then used to find the trends in these performance parameters with an increasingly high order converter, starting to answer the question of how many levels are best. The system performance parameters modelled are power loss, thermal performance and reliability. Wherever practical models and assumptions are validated, be that experimentally or through comparison with existing methods – this work includes a number of experimental series. The resulting trends explored highlight a number of interesting trends, principally: total power loss can be much lower, particularly at high switching frequencies; system thermal performance can be much improved owing to more efficient heatsink utilisation; and due to these thermal benefits, the system reliability based on switching device failure does not suffer as one might expect, and can in fact be higher under some conditions. The investigation also considers the use of cutting-edge switching device technology, such as gallium nitride power transistors, which a multilevel converter enables the use of, and in turn can significantly reduce power dissipation and increase switching frequency. Overall, the work adds new arguments in favour of multilevel converters in such applications and lowers the barrier to practical implementation by answering a number of questions a designer would likely ask. The key novel contributions of this work are the results of the trends that were found in terms of converter power loss, system thermal performance and switching device reliability with respect to multilevel converter order – with the methodologies created for these being somewhat novel in their own right. Along the way, however, other novel work was conducted including: an experimental investigation in to the accuracy of voltage-capacitance curves provided by manufacturers; experimental derivation of relationships for predicting MOSFET body diode performance from readily available device parameters; analysis showing the potential impact of GaN devices on converter efficiency; an experimental validation of GaN device gate turn-on energy; creation and validation of empirical relationship for predicting how heatsink performance varies with more devices of a smaller size; as well as an exploration of whether the extreme small size of some modern power transistors could lead to unexpected thermal cycling issues

Active control of medium-voltage cascaded three-level neutral-point-clamped converters

Three-Level Neutral-Point-Clamped (3L-NPC) converters have been widely used in the high-power motor drives. In recent years, a novel cascaded 3L-NPC converter has been developed and adopted in the ANGLE-DC project − a 30 MVA MVDC link demonstration project in North Wales, UK. This cascaded configuration provides exceptional waveform quality, modular design and a cost-effective solution to MVDC applications. Although the control strategy for a single 3L-NPC converter has been well established, control of the cascaded 3L-NPC converter is still under-researched. The potential challenges to control strategy design arising from their cascaded connections need to be specifically explored. In particular, due to the series DC connection, the voltage imbalance across 3L-NPC submodules (SMs) may occur and influence the system stability. This issue may occur in converter stations where power is controlled in either point-to-point or multi-terminal systems. Beyond the electric characteristic, thermal characteristic is also vital to the performance of system. Thermal imbalance of 3L-NPC SMs may occur in a cascaded 3L-NPC converter even the voltage and power are equally shared, which poses great challenges to the system reliability. To address aforementioned challenges, this thesis developed suitable control schemes for the cascaded 3L-NPC converter system and demonstrated their operation using a 30 kVA MVDC testbed based on the real ANGLE-DC project. The DC voltage imbalance was analysed through a small-signal model-based approach. Two DC voltage balancing methods with and without communications were presented. The PI-based method can automatically switch to the droop-based method upon failures of communication. The DC voltage imbalance of the cascaded 3L-NPC converter is further investigated in a three-terminal MVDC network in consideration together with the interactions of control characteristics between different converter stations and the power control accuracy. Then suitable control scheme was proposed. Multiple crossovers due to the interactions are avoided while DC voltage balance and power control accuracy are achieved as well. To mitigate the thermal imbalance, a thermal sharing controller was superposed on the DC voltage balancing controller to regulate the active and reactive power of each SM according to their individual junction temperatures. The thermal stresses are hence equally shared in presence of mismatched component parameters and cooling system failures. The effectiveness of presented methods in the thesis has been verified in MATLAB/Simulink simulation and experimentally validated

Power Converter of Electric Machines, Renewable Energy Systems, and Transportation

Power converters and electric machines represent essential components in all fields of electrical engineering. In fact, we are heading towards a future where energy will be more and more electrical: electrical vehicles, electrical motors, renewables, storage systems are now widespread. The ongoing energy transition poses new challenges for interfacing and integrating different power systems. The constraints of space, weight, reliability, performance, and autonomy for the electric system have increased the attention of scientific research in order to find more and more appropriate technological solutions. In this context, power converters and electric machines assume a key role in enabling higher performance of electrical power conversion. Consequently, the design and control of power converters and electric machines shall be developed accordingly to the requirements of the specific application, thus leading to more specialized solutions, with the aim of enhancing the reliability, fault tolerance, and flexibility of the next generation power systems