Licentiate Seminar in electronics with Viktor Döhlen

Licentiate Thesis: Enhancing hydropower with energy storage systems - Merging perspectives on inverter design and industrial application

Date: December 19th, 2024 at 13:00

Room: O102 campus Sundsvall and Zoom 

Main supervisor: Professor Kent Bertilsson, Mid Sweden University

Assistant supervisor: Doctor Sobhi Barg, Mid Sweden University

Opponent/External reviewer: Professor Massimo Bongiorno, Chalmers University of Technology

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Abstract

This thesis aims to contribute with knowledge on the subject of integrating electrical energy storage in power systems with emphasis on energy storage coordinated with hydropower production units for the primary purpose of frequency regulation.

Firstly, the thesis describes the theoretical background underlying the performance and stability criteria placed on entities delivering frequency regulation and how hydropower units fail to meet new criteria. A discussion on how energy storage design can ensure continued delivery of frequency regulation services for hydropower units follow. A review of hydropower combined with energy storage systems has been performed to describe the state of the art of combined control and technical implementation of hydropower enhanced with energy storage systems. Results from the review are quantitatively compared and discussed. These results becomes input to a converter design process for integrating energy storage systems to medium voltage levels. The primary application of the system is active power injection and frequency regulation as an auxiliary unit complementing hydropower plants. An initial emphasis is placed on energy storage technologies with high power density and low energy density. For high power applications, the converter becomes a larger share of the investment in energy storage systems. An outline of the design of a multilevel inverter with integration of supercapacitors is presented. One of the main objectives of the design process has been on integrating supercapacitors without a DC/DC interface. The risks and benefits are discussed compared to the alternatives. Risks are associated with unfavourable operating conditions for the energy storage and switching components due to current and voltage ripple. An investigation on reducing voltage ripple for direct integration has been conducted, presenting a reduction in overvoltage transients to safe operating areas for the energy storage and switching components. The results are verified from lab experiments, published in Paper (II) and further reduction of switching ripple is presented in the results section of the thesis. The converter prototype design aims to deliver high power capability, low harmonic content and low investment costs. The approach taken is using a modular and scalable multilevel inverter topology while minimizing overhead costs per submodule. Simulation studies evaluating a proposed converter design, cost and efficiency is presented. These results comprise the foundation for Paper (I). The variable voltage output from supercapacitors relating to the state of charge of supercapacitors is apparent in the design. A control scheme for the inverter addressing the variable voltage output from the supercapacitors in the submodules is suggested based on reduced state Model Predictive Control theory. Simulation studies on the control of the submodule are presented in the Results section of the thesis. The supercapacitors experience even discharge levels and if correctly dimensioned are able to meet power provision of the Fast Frequency Response ancillary service.

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