Microgrids are local power systems of different sizes located inside the distribution systems. Each microgrid contains a group of interconnected loads and distributed energy resources that acts as a single controllable entity with respect to the grid. Their islanding operation capabilities during emergencies improve the resiliency and reliability of the electric energy supply. Due to its low kinetic energy storage capacity, maintaining microgrid stability is challenging under system contingencies and unpredictable power generation from renewable resources. This dissertation highlights the potential benefits of flexibly utilizing the battery energy storage systems to enhance the stability of microgrids. The main contribution of this research consists in the development of a storage converter controller with an additional stability margin that enables it to improve microgrid frequency and voltage regulation as well as its induction motor post-fault speed recovery. This new autonomous control technique is implemented by adaptively setting the converter controller parameters based on its estimated phase-locked loop frequency deviation and terminal voltage magnitude measurement. This work also assists in the microgrid design process by determining the normalized minimum storage converter sizing under a wide range of microgrid motor inertia, loading and fault clearing time with both symmetrical and asymmetrical fault types. This study evaluates the expandability of the proposed control methodologies under an unbalanced meshed microgrid with fault-induced feeder switching and multiple contingencies in addition to random power output from renewable generators. The favorable results demonstrate the robust storage converter controller performance under a dynamic changing microgrid environment.

The goal of this study was to determine the economic feasibility of battery energy storage system (BESS). Three major economic benefits derived from BESS using were studied: 1. Energy Purchase Shifting, 2. Distribution Feeder Deferral, 3. Outage Avoidance. The economic analysis was based on theoretical modeling of the BESS and distribution system. Three simulation models were developed to quantify the effects of different parameters, such as: BESS round-trip efficiency, life span, rated power, rated discharge time, marginal cost of electric energy, 24 h feeder load profile, annual load variation, feeder load growth rate and feeder length. An optimal battery charging/discharging method was presented to determine the differential cost of energy (DCE). The annual maximum DCE was calculated using stochastic probability analysis on seasonal load variation. The net present value was evaluated as the present value difference between two investments: first, the distribution feeder upgrade without BESS deferral, and second, with BESS deferral. Furthermore, the BESS’s contributions under different outage strategies were compared. It was determined that feeder length is the most significant parameter. The economics of the studied system becomes favorable when the feeder length exceeds a critical value.

A method has been developed, Physical Hybrid Model, to investigate the physical large scale electrical effects of a Battery Energy Storage System (BESS) on a distribution grid by scaling the response from a small size Research Development and Demonstration (RD&D) platform. In order to realize the model the control system of an existing RD&D platform was refurbished and stability of components ensured. The Physical Hybrid Model proceeds as follows: Data from a distribution grid are collected. A BESS cycle curve is produced based on analyzed measurements. Required BESS power and capacity in investigated grid is scaled down by factor k to that of the physical test installation of the RD&D platform. The scaled BESS cycle is sent as input to control of the battery cycling of the RD&D platform. The response from the RD&D platform is scaled – up, and used in simulation of the distribution grid to find the impact of a BESS. The model was successfully implemented on a regional distribution grid in southern Sweden.

This thesis presents the application of a series interconnection module for small distributed generation (DG) or renewable energy systems integration in the distribution network. The concept used one set of voltage source converter (VSC) with battery energy storage system to control the injected voltage magnitude and phase angle for power injection and voltage sag mitigation applications. Through an energy storage device and the VSC, the module allows storage of surplus energy during off peak period and release for use during daytime peak load period, therefore, exhibits a load leveling characteristic. Due to its series connection characteristic, it is convenient in preventing islanding operation and suitable for voltage sag mitigation. The concept is suitable for locations where the voltage phase shift is not a problem. Due to the use of only one set of VSC, it is economic for customer site distributed energy resource applications.

Morgongåva is an urban centre in Sweden, with several challenges in the electrical power grid. In order to use the power grid more efficiently, this report investigates potentials of installing a battery energy storage system (BESS). Focus lies on finding economic and technical benefits of reducing power peaks, which occur during high demand hours when transmitting energy is more expensive. This method is referred to as peak shaving. Further, economic calculations if installing a BESS are based on electricity pricing data. Calculations regarding technical benefits are based on net power demand data. Further, the study shows that the usage of the grid, which was measured with the load factor, would increase and thus allow installation of more power sources and connecting more load to the grid. The load factor was estimated to increase by an average of 2.12 percent each month in 2019. In one year, the economic profit was estimated to be 91,000 kr. The conclusion is that there are economic profits for Sala-Heby Energi of installing a BESS, but more importantly a BESS has technical consequences in the power grid. Where technical benefits are important in order to reach the goals of Agenda 2030 but also to obtain a more reliable grid for the customers. A sensitivity analysis shows that the model is robust. Thus, the conclusion is that Sala-Heby Energi and the local electricity grid in Morgongåva would benefit from installing a BESS.

In deregulated electricity markets, profit driven electricity retailers compete to supply cheap reliable electricity to electricity consumers, and the electricity consumers have free will to switch between the electricity retailers. The need to maximize the profits of the electricity retailers while minimizing the electricity costs of the electricity consumers has therefore seen a drastic increase in the research of electricity markets. One of the factors that affect the profits of the electricity retailers and the energy cost of the consumers in electricity retail markets is the supply and demand. During high-supply and low-demand periods, the excess electricity if not managed, is wasted. During low-supply high-demand periods, the deficit supply can lead to electricity blackouts or costly electricity because of the volatile electricity wholesale spot market prices. Research studies have shown that electricity retailers can achieve significant profits and reduced electricity costs for their electricity consumers by minimizing the excess electricity and deficit electricity. Existing studies developed load forecasting models that aimed to match electricity supply and electricity demand. These models reached excellent accuracy levels, however due to the high volatility character of load demand and the rise of new electricity consumers, load forecasting alone is unable to mitigate excess and deficit electricity. In other studies, researchers proposed charging the electricity consumers’ batteries with excess electricity during high-supply low-demand periods and supplying their deficit electricity during low-supply high-demand periods. Electricity consumers’ incorporating batteries resulted in minimized excess and deficit electricity, in turn, maximizing the profits for the electricity retailers and minimizing the electricity costs for the electricity consumers. However, the batteries are consumer centric and only provide battery energy for the battery-owned consumer. Electricity consumers without battery energy during low-supply highdemand periods have electricity blackouts or require costly electricity from the electricity wholesale spot market. The peer-to-peer (P2P) energy sharing framework which allows electricity consumers to share their energy resources with one another is a viable solution to allow electricity consumers to share their battery energy. P2P energy sharing is a hot topic in research because of its potential to maximize the electricity retailers’ profits and minimize the electricity consumers’ electricity costs. Due to the increased profits for the electricity retailer and reduced electricity costs for the electricity consumers from implementing battery charging and P2P energy sharing, this dissertation proposes a day-ahead electricity retail market structure in which the electricity retailer supplies consumers’ batteries with excess electricity during high-supply low-demand periods, and during low-supply highdemand periods the electricity retailer discharges the consumers’ batteries to supply their deficit supply or supply their peers’ deficit supply. The electricity retailer aims to maximize its profits and minimize the electricity cost of the electricity consumers in its electricity retail market, by minimizing the excess and deficit electricity. The problem is formulated as a non-linear optimization model and solved using game theory. This dissertation compares the profits of the electricity retailer and electricity costs of the consumers that charge their batteries with excess electricity, discharge their batteries and purchase electricity from their peers to supply their deficit supply, with consumers that only charge their batteries with excess electricity but do not share their battery energy with their peers, consumers that only purchase electricity from their peers to supply their deficit supply but do not employ a battery, and consumers that neither employ a battery nor purchase electricity from their peers to supply their deficit supply. The results show that the consumers that charge their batteries with excess electricity, discharge their batteries and purchase electricity from their peers to supply their deficit supply achieved the lowest electricity cost and highest profits for the electricity retailer. / Dissertation (MEng)--University of Pretoria, 2020. / Electrical, Electronic and Computer Engineering / MEng / Unrestricted

In this study, an examination regarding what benefits an aggregatedpopulation of Battery Energy Storage Systems (BESSs) could result incompared to when the individual units in the population are being usedseparately has been executed. The increased flexibility and reducedsafety margins as results of the aggregation was also examined. Thestudy was executed on behalf of the smart energy service companyCheckWatt AB and the study furthermore rests upon results of earlierperformed master theses on behalf of the company. By investigating previous work and studies through a literature study,the enabling of anumerical study was done. The numerical study wasbased on a simple model of a Virtual Power Plant (VPP) where severalBESSs are smartly controlled in order to be used for both local peakshaving and as common providers of the frequency reserve FrequencyContainment Reserve - Normal (FCR-N). The study involved the formation of a numerical model which simulated cases of both aggregated and non-aggregated populations of up to 45 load profile units, this in order for advantages and differences to be distinguished. The data used inthe simulations was received mainly from the CheckWatt AB andconsisted of photovoltaic (PV) electricity production and load data of 45 customers of the company. A sensibility analysis of the numericalstudy was also performed, which showed that the studied model andsystem were quite stable. The results of the simulations of the case of the study proved thatthere are some advantages connected to aggregation of several BESSs,and that the aggregation enabled an added value and a higher level offlexibility within the system. The safety margins connected todelivery of FCR-N could be reduced when aggregating several BESS,while a more extensive study is requested regarding safety marginsconnected to peak shaving. The study’s results further showed that anaggregator can be used as a sustainable and flexible solution forbalancing the electrical grid in the transition to a sustainableenergy system allowing a higher penetration of intermittentenergy sources.

The rise of Renewable Energy Sources (RES) such as wind and solar power, creates new challenges for electric power systems. One of these challenges occur in Frequency Containment Reserves (FCR) on power system because of decreasing system inertia from RES. The purpose of FCR is to regulate the system frequency after a disturbance that gives rise to a Rate of Change of Frequency (RoCoF) and an Instantaneous Frequency Deviation (IFD). Conventional electricity production such as hydro and nuclear power have a contribution for the amount of inertia in the system, while RES lack this contribution of inertia.This paper studies different cases of amount of inertia to understand the impact of lower amount of inertia caused by RES on power system. A power system was simulated and the IFD and SteadyState Frequency Deviation (SSFD) of the system were examined as the nuclear powers were substituted by wind powers. The results showed that a large amount of inertia implies a small IFD and vice versa.Furthermore, this paper also studies Battery Energy Storage System (BESS) as a power support for FCR when using RES. The conclusion for the impact of the battery was to use high injected power and triggering frequency level (TLF) and vice versa to get an acceptable IFD. In other words, this means that it is possible to keep the IFD within predefined limits by using batteries and identify the appropriate range of battery control settings.

Renewable energy resources are projected to claim a larger part of the Swedish power mix in coming years. This could potentially increase frequency fluctuations in the power grid due to the intermittency of renewable power generating resources. These fluctuations can in turn cause issues in the power grid if left unchecked. In order to resolve these issues, countermeasures are employed. One such countermeasure is for private actors to regulate power; in exchange they are financially compensated through reserve markets. The reserve market studied in this thesis is called Frequency Containment Reserve – Normal (FCR-N). Currently hydroelectric power provides almost all regulated power within this market. As the need for power regulation is expected to increase in the coming years, there exists a need to study other technologies capable of power regulation. This thesis focuses on one such technology called, virtual power plants. While virtual power plants are operating in other parts of the world, there were no virtual power plants operating in Sweden. As a result, the nature of an optimized virtual power plant and the economic benefits of optimization had not been previously investigated. To answer such questions, this thesis modelled and optimized the revenue of a virtual power plant. The examined virtual power plant consisted of cooling chillers, lighting, ventilation fans and a battery energy storage system. Where varying their total power demand allowed for them to provide power regulation. With the virtual power plant market in Sweden being in its infancy, this thesis serves as a first look into how an optimized virtual power plant using these components could function. To put the economic results of the optimization into context, a comparative model was constructed. The comparative model was based on a semi-static linear model. This is what the thesis’s industry partner Siemens currently uses. For the simulated scenarios, the optimized model generated at least 85% higher net revenues than the semi-static linear model. The increase in revenue holds potential to increase the uptake of virtual power plants on the Swedish market, thus increasing stability in the power grid and easing the transition to renewable energy. / Då förnyelsebara energiresurser antas omfatta en större roll av den svenska elproduktionen inom kommande år, så kan detta leda till att frekvensfluktueringar i elnätet ökar. Detta sker på grund av att den oregelbundna elproduktionen från förnyelsebara energiresurser inte matchas med konsumtion. Om dessa fluktueringar inte hanteras kan det i sin tur leda till skadliga störningar inom elnätet. För att motverka detta och således stabilisera elnätet används diverse lösningar. Ett sätt att åstadkomma ökad stabilisering i elnätet är att låta privata aktörer kraftreglera. De privata aktörerna som står för kraftregleringen gör detta i utbyte mot ekonomisk kompensation, genom att delta i reservmarknader. Den reservmarknad som studerades inom detta examensarbete kallas Frequency Containment Reserve – Normal (FCR-N). I nuläget står vattenkraft för nästan all reglerad kraft inom den här marknaden. Men då behovet av kraftreglering antas öka inom kommande år så behövs nya teknologier studeras som kan bistå med kraftregleringen. Den studerade teknologin inom detta examensarbete var ett virtuellt kraftverk. Då inga virtuella kraftverk var i bruk i Sverige då denna uppsats skrevs fanns det osäkerheter kring hur man optimalt styr ett virtuellt kraftverk och de ekonomiska fördelarna som detta skulle kunna leda till. Detta examensarbete modellerade och optimerade ett virtuellt kraftverk ur ett vinstperspektiv. Det virtuella kraftverket var uppbyggt utav kylmaskiner, ljus, ventilationsfläktar och ett batterisystem. Deras kraftkonsumtion styrdes på ett sådant sätt som lätt de bidra till kraftreglering på reservmarknaden. För att kunna analysera de ekonomiska resultaten från det optimerade virtuella kraftverket, så byggdes en jämförelsemodell. Denna jämförelsemodell är baserad på en semistatisk linjär modell, vilket är det som examensarbetets industripartner Siemens använder. Den ekonomiska jämförelsens resultat påvisade att inkomsten från den optimerade modellen var minst 85% högre än den semistatiskt linjära modellen, inom de studerade scenarierna. Denna inkomstökning skulle potentiellt kunna öka användningen av virtuella kraftverk på den svenska reservmarknaden vilket i sin tur skulle medföra högre stabilitet på elnätet. Genom att öka stabiliteten på elnätet kan således förnyelsebara energiresurser i sin tur lättare implementeras.

Lithium-ion based battery energy storage system has become one of the most popular forms of energy storage system for its high charge and discharge efficiency and high energy density. This dissertation proposes a high-efficiency grid-tie lithium-ion battery based energy storage system, which consists of a LiFePO4 battery based energy storage and associated battery management system (BMS), a high-efficiency bidirectional ac-dc converter and the central control unit which controls the operation mode and grid interface of the energy storage system. The BMS estimates the state of charge (SOC) and state of health (SOH) of each battery cell in the pack and applies active charge equalization to balance the charge of all the cells in the pack. The bidirectional ac-dc converter works as the interface between the battery pack and the ac grid, which needs to meet the requirements of bidirectional power flow capability and to ensure high power factor and low THD as well as to regulate the dc side power regulation. A highly efficient dual-buck converter based bidirectional ac-dc converter is proposed. The implemented converter efficiency peaks at 97.8% at 50-kHz switching frequency for both rectifier and inverter modes. To better utilize the dc bus voltage and eliminate the two dc bus bulk capacitors in the conventional dual-buck converter, a novel bidirectional ac-dc converter is proposed by replacing the capacitor leg of the dual-buck converter based single-phase bidirectional ac-dc converter with a half-bridge switch leg. Based on the single-phase bidirectional ac-dc converter topology, three novel three-phase bidirectional ac-dc converter topologies are proposed. In order to control the bidirectional power flow and at the same time stabilize the system in mode transition, an admittance compensator along with a quasi-proportional-resonant (QPR) controller is adopted to allow smooth startup and elimination of the steady-state error over the entire load range. The proposed QPR controller is designed and implemented with a digital controller. The entire system has been simulated in both PSIM and Simulink and verified with hardware experiments. Small transient currents are observed with the power transferred from rectifier mode to inverter mode at peak current point and also from inverter mode to rectifier mode at peak current point. The designed BMS monitors and reports all battery cells parameters in the pack and estimates the SOC of each battery cell by using the Coulomb counting plus an accurate open-circuit voltage model. The SOC information is then used to control the isolated bidirectional dc-dc converter based active cell balancing circuits to mitigate the mismatch among the series connected cells. Using the proposed SOC balancing technique, the entire battery storage system has demonstrated more capacity than the system without SOC balancing. / Ph. D.