In recent years, the power industry has experienced significant changes on the distribution power system primarily due to the implementation of smart-grid technology and the incremental implementation of distributed generation. Distributed Generation (DG) is simply defined as the decentralization of power plants by placing smaller generating units closer to the point of consumption, traditionally ten mega-watts or smaller. While DG is not a new concept, DG is gaining widespread interest primarily for the following reasons: increase in customer demand, advancements in technology, economics, deregulation, environmental and national security concerns. The distribution power system traditionally has been designed for radial power flow, but with the introduction of DG, the power flow becomes bidirectional. As a result, conventional power analysis tools and techniques are not able to properly assess the impact of DG on the electrical system. The presence of DG on the distribution system creates an array of potential problems related to safety, stability, reliability and security of the electrical system. Distributed generation on a power system affects the voltages, power flow, short circuit currents, losses and other power system analysis results. Whether the impact of the DG is positive or negative on the system will depend primarily on the location and size of the DG. The objective of this research is to develop indices and an effective technique to evaluate the impact of distributed generation on a distribution power system and to employ the particle swarm optimization technique to determine the optimal placement and size of the DG unit with an emphasis on improving system reliability while minimizing the following system parameters: power losses, voltage deviation and fault current contributions. This research utilizes the following programs to help solve the optimal DG placement problem: Distribution System Simulator (DSS) and MATLAB. The developed indices and PSO technique successfully solved the optimal DG sizing and placement problem for the I 13-Node, 34-Node and 123-Node Test Cases. The multi-objective index proved to be computational efficient and accurately evaluated the impact of distributed generation on the power system. The results provided valuable information about the system response to single and multiple DG units.

The increasing demand for sustainable energy is one of the driving forces behind the increasing use of wind power by means of wind-turbines in electric power systems. Modern wind turbines commonly employ variable-speed-generator technology associated with a power-electronic converter as part of the grid connection. A drawback of the use of power electronics is the presence of harmonic emissions. Consequently a systematic study on distortion from wind power installations is needed; this holds for individual wind turbines as well for complete installations.In the work, measurements and analysis of harmonic emissions were performed on a number of wind-turbines in several wind parks in Northern Sweden. The measurements on the individual wind turbines reveal that the harmonic emissions are different from each other, even for different turbines from the same manufacturer. However in general the characteristic harmonics dominate the harmonic emissions. Furthermore, a long-term measurement shows that the dominant frequencies in the emission change with time.The total emission from a wind park into the public grid is determined by the emission from individual turbines and by the properties of the wind park. To study the impact of the wind park on the propagation a ``transfer function'' method has been introduced, and applied by means of calculation and simulation. The method is based on a mathematical model that predicts the harmonic propagation from the wind turbines to the public grid in the frequency range up to 50 kHz. Applying the model to three example parks reveals that, the amplitudes at the resonance frequency are strongly dependent on the resistance of both underground cables and transformer, especially at high frequencies. In other words, the higher order harmonics are damped a lot.In conclusion, wind park harmonic emissions into the public grid are due to the combination of emission from individual wind turbines and the propagation through the collection grid.

In this report, the effect that a higher penetration ofrenewable energy sources has on electric power grid stability isevaluated. The report also compares different methods of stabilizingan unstable grid. The model used is a two-area four-machinesystem and the main objective is to stabilize the synchronousgenerators such that they revert back to synchronism afterbeing subjugated to a small signal disturbance. The stabilizationmethods consists of supplementary Power System Stabilizers(PSSs) complementing the exciter systems of the synchronousmachines, as well as two types of converter-based controllers inthe renewable energy source: Grid-Following (GFL) convertersand Grid-Forming (GFM) converters. The results show thata system with renewable energy sources is more sensitive todisturbances and has a larger rotor angle deviation from a steadystate when using only GFLs compared to the conventional gridwithout PSSs. It is also found that a conventional grid requiressupplementary PSSs to be stable. This is also the case for asystem with renewable energy controlled by GFL. The systemwith GFM controllers does however not need supplementary PSSto be stable. This leads to the conclusion that GFM is morepreferable than GFL to control a grid with a higher penetrationof renewable energy. / I denna rapport utvärderas hur en högre andel förnybara energikällor påverkar stabiliteten i elnät, och jämför också olika metoder för att stabilisera ett instabilt nät. Modellen som används var ett två-områdes-fyrmaskinsystem och huvudsyftet är att stabilisera synkrongeneratorerna så att de återgår till synkronism efter att ha utsatts för en liten småsignalsstörning. Stabiliseringsmetoderna består av kompletterande Power System Stabilizers (PSS:er) som kompletterade exciteringssystemen i synkronmaskinerna, samt två typer av omvandlarbaserade styrenheter i den förnybara energikällan: Grid-Following (GFL)-omvandlare och Grid-Forming (GFM)omvandlare. Resultaten visar att ett system med förnybara energikällor är mer känsligt för störningar och har en större rotorvinkelavvikelse från ett stationärt tillstånd när GFL-kontroller används jämfört med det konventionella nätet utan PSS:er. Det visar sig också att ett konventionellt nät kräver kompletterande PSS:er för att vara stabilt. Detta är också fallet för ett system med förnybar energi som enbart kontrolleras av GFL-omvandlare. Systemet med GFM-omvandlare behöver dock inte kompletterande PSS för att vara stabilt. Detta leder till slutsatsen att GFM är mer att föredra än GFL för att kontrollera ett nät med högre andel förnybar energi. / Kandidatexjobb i elektroteknik 2022, KTH, Stockholm

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The enhancement of the reliability, security, and resiliency of electric power systems depends on the availability of fast, accurate, and robust dynamic state estimators. These estimators should be robust to gross errors on the measurements and the model parameter values while providing good state estimates even in the presence of large dynamical system model uncertainties and non-Gaussian thick-tailed process and observation noises. It turns out that the current Kalman filter-based dynamic state estimators given in the literature suffer from several important shortcomings, precluding them from being adopted by power utilities for practical applications. To be specific, they cannot handle (i) dynamic model uncertainty and parameter errors; (ii) non-Gaussian process and observation noise of the system nonlinear dynamic models; (iii) three types of outliers; and (iv) all types of cyber attacks. The three types of outliers, including observation, innovation, and structural outliers are caused by either an unreliable dynamical model or real-time synchrophasor measurements with data quality issues, which are commonly seen in the power system. To address these challenges, we have pioneered a general theoretical framework that advances both robust statistics and robust control theory for robust dynamic state and parameter estimation of a cyber-physical system. Specifically, the generalized maximum-likelihood-type (GM)-estimator, the unscented Kalman filter (UKF), and the H-infinity filter are integrated into a unified framework to yield various centralized and decentralized robust dynamic state estimators. These new estimators include the GM-iterated extended Kalman filter (GM-IEKF), the GM-UKF, the H-infinity UKF and the robust H-infinity UKF. The GM-IEKF is able to handle observation and innovation outliers but its statistical efficiency is low in the presence of non-Gaussian system process and measurement noise. The GM-UKF addresses this issue and achieves a high statistical efficiency under a broad range of non-Gaussian process and observation noise while maintaining the robustness to observation and innovation outliers. A reformulation of the GM-UKF with multiple hypothesis testing further enables it to handle structural outliers. However, the GM-UKF may yield biased state estimates in presence of large system uncertainties. To this end, the H-infinity UKF that relies on robust control theory is proposed. It is shown that H-infinity is able to bound the system uncertainties but lacks of robustness to outliers and non-Gaussian noise. Finally, the robust H-infinity filter framework is proposed that leverages the H-infinity criterion to bound system uncertainties while relying on the robustness of GM-estimator to filter out non-Gaussian noise and suppress outliers. Furthermore, these new robust estimators are applied for system bus frequency monitoring and control and synchronous generator model parameter calibration. Case studies of several different IEEE standard systems show the efficiency and robustness of the proposed estimators. / Ph. D.

Frequency monitoring network (FNET) is an Internet based GPS synchronized wide-area frequency monitoring network deployed at distribution level. At first part of this thesis, FNET structure and characteristics are introduced. After analysis and smoothing FDR signals, the algorithm of event trigger is present with Visual C++ DLL programming. Estimation of disturbance location method is discussed based on the time delay of arriving (TDOA) in the second part of this work. In this section, author shows the multiply method to calculate event time, which is important when deal with pre-disturbance frequency in TDOA part. Two event kinds are classified by the change of frequency and the linear relationship between change of frequency and imbalance of generation and load power is presented. Prove that Time Delay of Arrival (TDOA) is a good algorithm for estimation event location proved by real cases. At last, the interface of DLL module and the key word to import and export DLL variables and function is described. At last, PSS compensation optimization with a set of nonlinear differential algebraic equations (DAE) is introduced in detail. With combining the bifurcation theory of nonlinear system and the optimization theory, the optimal control of small-signal stability of power electric systems are solved. From the perspective of stability margin, global coordination of controller parameters is studied to ensure the stable operation of power grids. The main contents of this thesis include: ï¼ 1ï¼ Models of power systems and test power electric systems. Tht5e dynamic and static models of the elements of power systems, such as generatorbbs, AVRs, PSSs, loads and FACTS controllers are presented. Method of power system linearization modeling is introduced. Three test power systems, WSCC 9-bus system, 2-area system, New England 39-bus system, are used in thesis. ï¼ 2ï¼ Multi-objective optimizations based on bifurcation theory. The optimization models, damping control-Hopf bifurcation control, voltage control-damping control, are presented. Pareto combined with evolutionary strategy (ES) are used to solve multi-objective optimizations. Based on traditional PSS parameters optimizations, it can be formulated as a multi-objective problem, in which, two objectives should be taken into account. The minimum damping torque should be identified. / Master of Science

Master of Science / Department of Electrical and Computer Engineering / Sanjoy Das / Noel Schulz / Electrical power, although convenient form of energy to distribute and use, cannot easily be stored in large quantities economically. Most electrical power generated by utility plants is consumed simultaneously in real time. However, in some cases, energy storage systems become crucial when power generated from sources does not fulfill peak power load demand in a power system or energy storage systems are needed as backup. Due to these reasons, various technologies such as batteries, ultracapacitors (UC), superconducting magnetic energy storage (SEMS) and flywheels are beneficial options for energy storage systems. Shipboard power systems must use one or more energy storage systems in order to backup the existing power system if locally generated power is unavailable. This will lessen the effect of voltage sags on power quality, and improve system reliability. This report mainly focuses on the design of a Boost DC-DC converter and the integration of that converter with a previously designed battery storage model, as well as the effect of varying loads at the end of the converter.

Although power system load modelling is a mature research area, there is a renewed interest in updating available load models and formulating improved load modelling methodologies. The main drivers of this interest are the introduction of new types of non-conventional (e.g. power electronic interfaced) loads, the requirement to operate power supply systems with increasing levels of renewable distributed generation and the implementation of various load control functionalities (e.g. demand side management). As the majority of existing load models do not allow for a full and precise analysis of these new operating conditions, it is essential to develop new load models and update load modelling techniques. This thesis presents a detailed study of modern loads, focussing on the requirements for their correct representation in power system analysis. The developed models of the individual loads are then combined using a new load aggregation methodology for developing aggregate load models, suitable for the analysis of both existing and future power supply systems (so called ’smart grids’). The methodology uses a circuit-based load modelling approach, as this allows reproduction of the instantaneous current waveforms of the modelled load for any given supply voltage. This approach retains all electrical characteristics of the loads and provides a more realistic representation of some important phenomena (e.g. harmonic cancellation and attenuation due to load and supply system interactions) which are often neglected in traditional load modelling procedures. Case studies of the UK residential and commercial load sectors are presented as illustrations of the load aggregation methodology. The results show significant short-term and long-term temporal variations in the load characteristics, which are not available or reported in the existing literature. This information allows for a more comprehensive assessment of demand-side management functionalities and correlation with locally connected distributed generation. Both of these effects are investigated in the thesis by quantifying the possible extent and range of changes in power system performance for some expected near future changes in load configurations and network operating conditions.

Thesis (MScIng)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: This thesis covers the development of a transformerless series dip compensator. Of all known power quality problems, voltage dips are the greatest reason for concern. Series injection dip compensators offer the advantage of only having to compensate for the decrease in supply voltage during a dip. This results in significant reduction in converter ratings and energy storage requirements. The aim of this thesis was to take up previous developments and combine them with new technologies to maximize their functionality. The new design was implemented with ultracapacitors to offer a maintenance-free device lifespan of 20 years. As they are very expensive, a new topology was introduced in this thesis to maximize their use so that they become viable for industry. Furthermore, a new method of daisy chaining switches was introduced to minimize costs involved in controlling them. A single-phase compensator, with this new topology and the new way of controlling switches, was designed and built according to specifications stated by Eskom. This ultracapacitor-based dip compensator was tested with a dip generator, developed by the University of Stellenbosch for different load currents. The experimental results confirmed simulations made with identical parameters. This thesis presents a reliable and cost effective solution for dip compensation. / AFRIKAANSE OPSOMMING: Hierdie tesis dek die ontwikkeling van ’n transformatorlose duik kompenseerder. Van al die bekende toevoerkwaliteit probleme wek duike in die spanning die meeste kommer. Serie-injeksie kompenseerders het die voordeel dat hulle net kompenseer vir die verlies in die toevoerspanning tydens die duik. Dit het ’n beduidende vermindering in die omsetterkenwaardes en energiestoorvereistes tot gevolg. Hierdie tesis mik om vorige soortgelyke ontwikkelings op te volg en te verbeter met nuwe tegnologie om die funktionliteit te maksimeer. Die ontwerp is geimplementeer met ultrakapasitore wat die onderhoudsvrye toestel ‘n leeftyd van 20 jaar toelaat. Omdat ultrakapasitore so duur is moes ‘n nuwe topologie onwikkel word om die gebruik van ultrakapasitore meer ekonomies aantreklik te maak. Daar is ook ‘n nuwe manier van skakelaar beheer ontwikkel wat toelaat dat baie skakelaars oor een optise veesel beheer kan word. ‘n Enkel fase dip kompensaeerder is toe onwikkel en gebou volgens Eskom se spesifikasies. Die ultrakapasitor gebaseerde omsetter is getoets met ‘n dip generator wat deur die Universiteit van Stellenbosch ontwikkel is. Die praktiese resultate bevestig die simulasies wat gedoen is met dieselfe parameters. Hierdie tesis lei tot ‘n betroubaare en ekonomiese oplossing vir duik kompensasie.

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy