Mid Sweden University

This thesis aims to contribute with knowledge on the subject of integrating electricalenergy storage in power systems with emphasis on energy storage coordinated withhydropower production units for the primary purpose of frequency regulation.Firstly, the thesis describes the theoretical background underlying the performanceand 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 hydropowerunits follow. A review of hydropower combined with energy storage systems hasbeen 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 mediumvoltage levels. The primary application of the system is active power injection andfrequency regulation as an auxiliary unit complementing hydropower plants. Aninitial emphasis is placed on energy storage technologies with high power densityand low energy density.For high power applications, the converter becomes a larger share of the investmentin energy storage systems. An outline of the design of a multilevel inverter withintegration of supercapacitors is presented. One of the main objectives of the designprocess has been on integrating supercapacitors without a DC/DC interface. Therisks and benefits are discussed compared to the alternatives. Risks are associatedwith 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 forthe 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 harmoniccontent 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 ispresented. These results comprise the foundation for Paper (I).The variable voltage output from supercapacitors relating to the state of charge ofsupercapacitors is apparent in the design. A control scheme for the inverter addressing the variable voltage output from the supercapacitors in the submodules issuggested 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 dimensionedare able to meet power provision of the Fast Frequency Response ancillary service.