Multilevel Multiplexed Inverters for Applications up to 1500 Volts and 100 kW. by Kepa Odriozola Sagasta

Multilevel Multiplexed Inverters for Applications up to 1500 Volts and 100 kW. by Kepa Odriozola Sagasta

SUMMARY

 Renewable energy, including solar and wind, is at the heart of the transition to a less carbon intensive and more sustainable energy system. Over the past two decades, there has been a significant increase in photovoltaic (PV) power installed in new solar power plants. This increase in power has also led to an increase in DC bus voltage up to 1500V (the limit between LV and MV in DC). It is worth noting that grid integration of distributed energy sources such as PV systems with battery based energy storage systems (BESS), both utility-scale and residential and industrial, is gaining importance. At the same time, the global data sphere is projected to grow from 33 zettabytes (ZB, 1021 bytes, one trillion gigabytes) in 2018 to 175 ZB in 2025. Due to the data tsunami, digitization, high-speed wireless networks, new data-intensive technologies, and the growing demand for cloud-computing have led to the development of data centers, which have become an electricity intensive industry. For example, several studies have estimated that between 2025 and 2050, the ICT industry could account for up to 20% of global electricity consumption. The increase in installed power of data centers and solar power plants implies high power converters (MW or more) connected in parallel. The increase in voltage for PV and energy storage is a strong challenge to reduce the level of current in the devices in order to obtain more compact systems, because today, with the current solutions, the objectives of efficiency, cost and power density are not met. Therefore, new high-efficiency power electronic systems and associated control strategies are needed to make this transition possible. This thesis is part of the study and design of a new family of high efficiency three phase multilevel converter topologies, called Multiplexed, intended for UPS, Energy Storage and Solar Inverter applications. The case studied, foresees a converter with a high voltage DC bus with a voltage variation range from 900VDC to 1500VDC, connected to a grid or a three-phase load of American (480VAC/60Hz) or European (400VAC/50Hz) voltage, and a bidirectional power ranging from a few tens of kW to 100kW. First, we will explain the general structure of multiplexed topologies. Basically, the structure is composed of a chopper stage (consisting of two symmetrical DC-DC converters) connected to a three phase DC-AC inverter stage. There are many variants depending on the topologies chosen for each conversion stage. In a second part, we will present the selected variants as a solution to the studied applications. We will also specify, the approach adopted for the dimensioning of these solutions, by considering the active elements (semiconductors) and the passive elements. We will present the performance results obtained via simulation. We will specify the control principle associated with this new range of topologies. Indeed,one of the main features of these structures is that the two stages, chopper and inverter, are connected without intermediate filter which implies that the inverter must re-switch the voltage that has already been switched by the chopper stage. The PWM modulation strategy that has been developed for this purpose will then be presented and detailed. Finally, based on the models and results obtained in the previous sections, we will show the realization of the 100kW prototypes built in order to validate the architecture and the control strategy proposed around the 'Multiplexed' concept.

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