Mid Sweden University

Increased penetration of the combined operation of hydraulic power plants with electrical energy storage systems has pushed the state of the art of hybrid operation of hydropower. As the body of work is growing this review article aims to provide a thematic review on the objectives of hybrid operation of hydropower and the control strategies and dimensioning methodologies of auxiliary energy storage systems to achieve these objectives. The works reviewed are categorized thematically according to whether the contribution is a commercial objective or rather contributing to increased security of supply. Increased revenue from being able to qualify for ancillary services such as frequency regulation and reduced operational costs through reducing wear are the primary commercial objectives. Improved black-start capability, stability regions and island operation are objectives relating to security of supply. State-of-the-art methodologies on control and dimensioning of hybrid systems for the proposed objectives are presented. A quantitative survey of commercial hybridization projects focusing on the Nordic countries has been performed, highlighting regional disparities in technology adoption. The review highlights gaps in the literature concerning the translation of wear reduction metrics into operational cost savings. 

This work presents a method for designing an energy storage system (ESS) operating in an islanded power system formed by a single synchronous machine. The method utilizes stability and performance criteria to define the island operation. The Nyquist stability criterion is used to ensure a stable response and a performance criterion is used to evaluate the island system s ability to contain the frequency within specified limits during island loading steps. These criteria are derived using the same framework as Frequency Containment Reserve (FCR) criteria in the Nordic Synchronous Area. A closed-loop model of the island system, including a standard hydropower unit model and an ESS model, is used in MATLAB/Simulink. The models response is optimized together with the defined criteria to obtain the size and control parameters for the ESS. A case study with a Swedish hydropower unit with known power response dynamics operating in island mode is presented. Turbine test data is used to parametrize the linear hydropower model and an ESS is designed separately for FCR provision and island operation. The results of the case study show that 86 % more power from the storage system is needed for island operation than FCR provision according to the two defined sets of criteria. It is also shown that while island operation is the expected dimensioning mode for the ESS, the resulting EES size is very sensitive to the criteria and dynamics defined for the island system, with the load step size having the strongest influence on the ESS size. 

This study investigates a Cascaded H-Bridge converter with Electric Double-Layer Capacitors as integrated energy storage components. As the DC-link voltages are variable, the modulation index and number of submodules contributing to the active power delivery vary according to state of charge. The nearest level control algorithm for this application is studied, and expressions for the duty cycle of conventional Nearest Level Modulation are derived. A modification of the sort and select algorithm to determine which submodule is to be inserted and bypassed when using the Nearest Level Control algorithm is proposed to distribute the activation time and the experienced RMS current of the submodules. Expressions for the duty cycle of each inserted submodule for the proposed algorithm is presented and compared to the conventional. Simulation experiments of current sharing between submodules under active power delivery for the conventional and proposed Nearest Level Control is conducted for an 11- level, 41-level and 61-level converter. Simulation experiments show a reduction in RMS current for the submodule experiencing the highest thermal stress. Over the course of power delivery and increasing modulation index, the peak RMS current increase for the conventional nearest level modulation while it is kept constant for the proposed modulation scheme. 

An intrinsic feature of EDLCs is their voltage dependency on the state of charge. This voltage variation is typically addressed by introducing a DC-DC interface between the energy storage components and the switching bridge for the integration in power electronic inverters. While this additional converter interface ensures a nominal voltage output, it introduces additional losses, cost and may reduce system reliability. This paper proposes a solution to mitigate the need for an additional inverter interface by utilizing redundant submodules in the Cascaded H-Bridge converter and scaling the initial modulation index to produce the desired output at varying voltage levels of the EDLCs. A reduced state model predictive control scheme combined with a sort-and-select algorithm is employed to implement the control and balance. A simulation study of a 41-level converter injecting 0.5 MW of active power into the grid is presented. A proof-of-concept prototype converter with direct integrated EDLCs is presented.

This paper presents an experimental investigation on switching transients in a submodule of a cascaded H-bridge converter with the aim of investigating effects of direct integration of supercapacitor modules in switching converters. Departing from simulation studies focusing on overvoltage transients and effects from the Equivalent Series Inductance of the supercapacitor module. A passive absorption circuit is introduced to mitigate the transients on supercapacitor and switching components. The simulations are compared to experimental results on a submodule prototype. An operating point of the submodule corresponding to an output power of 6.2kW is investigated in a double pulse test. The experimental results show that effects of the series inductance across the switches are mitigated. The relationship between the equivalent series inductance of the supercapacitor and the rate of change of current is identified as causing overvoltages across the supercapacitor. Methods to handle the rate of change of current and high frequency voltage ripple across switches are suggested.