Browsing by Author "Lehtonen, Matti, Prof."

Now showing 1 - 6 of 6

Enhancing the performance of flexible AC transmission systems (FACTS) by computational intelligence
(2011) Othman, Ahmed Mohamed
Sähkötekniikan korkeakoulu | Doctoral dissertation (monograph)
The thesis studies and analyzes UPFC technology concerns the management of active and reactive power in the power networks to improve the performance aiming to reach the best operation criteria. The contributions of the thesis start with formatting, deriving, coding and programming the network equations required to link UPFC steady-state and dynamic models to the power systems. The thesis derives GA applications on UPFC to achieve real criteria on a real world sub-transmission network. An enhanced GA technique is proposed by enhancing and updating the working phases of the GA including the objective function formulation and computing the fitness using the diversity in the population and selection probability. The simulations and results show the advantages of using the proposed technique. Integrating the results by linking the case studies of the steady-state and the dynamic analysis is achieved. In the dynamic analysis section, a new idea for integrating the GA with ANFIS to be applied on the control action procedure is presented. The main subject of the thesis deals with enhancing the steady-state and dynamics performance of the power grids by Flexible AC Transmission System (FACTS) based on computational intelligence. Control of the electric power system can be achieved by designing the FACTS controller, where the new trends as Artificial Intelligence can be applied to this subject to enhance the characteristics of controller performance. The proposed technique will be applied to solve real problems in a Finnish power grid. The thesis seeks to deal, solve, and enhance performances until the year 2020, where the data used is until the conditions of year 2020. The FACTS device, which will be used in the thesis, is the most promising one, which known as the Unified Power Flow Controller (UPFC). The thesis achieves the optimization of the type, the location and the size of the power and control elements for UPFC to optimize the system performance. The thesis derives the criteria to install the UPFC in an optimal location with optimal parameters and then designs an AI based damping controller for enhancing power system dynamic performance. In this thesis, for every operating point GA is used to search for controllers' parameters, parameters found at certain operating point are different from those found at others. ANFISs are required in this case to recognize the appropriate parameters for each operating point.
Influence of national and company interests on European electricity transmission investments
(2011) Supponen, Matti
Sähkötekniikan korkeakoulu | Doctoral dissertation (monograph)
The objective of this study is to analyse to what extent national and company interests prevent electricity transmission investments which would be beneficial for Europe. The study has provided strong evidence that national and company interests have influence on cross-border transmission investments. It indicates that national and company interests have contributed to serious underinvestment in the European transmission network from the overall welfare point of view. Vertically integrated TSOs prioritise investments for increasing exports which increase income for their owner, while profitable interconnectors in import direction are often not developed at all. The behavioural pattern is less clear with the majority of the TSOs which are publicly owned and can be considered as semi-independent. The study has demonstrated that there is an important dimension of welfare distribution between the countries connected but also within the countries due to the change in the market outcome when an interconnector is built. There are a number of factors that potentially influence the motivation of a TSO for investing to increase cross-border capacity. The study shows that it is possible to develop objective criteria for interconnector investments. Social welfare benefits from price arbitrage should be one criterion but several other criteria should be used as well including price convergence, security of supply and competition benefits. Flaws in market design, capacity calculation and capacity allocation need to be addressed to provide efficient signals for interconnector investments. This should include designing of price zones, on which a proposal is made in this study, and working on the loop flow problem which is already acute for some countries in Europe. The study proposes an approach for addressing the cost-allocation of future transmission investments in order to take into account European wide the costs and benefits of those investments.
Lightning-induced overvoltages in medium voltage distribution systems and customer experienced voltage spikes
(2010) Sabiha, Nehmdoh A
Aalto-yliopiston teknillinen korkeakoulu | Doctoral dissertation (monograph)
In Finland, distribution transformers are frequently subjected to lightning strokes for which they are continuously protected by spark-gaps. So, the breakdown probability of medium voltage (MV) spark-gaps is modeled using the Gaussian distribution function under an impulse voltage test in accordance with the IEC 60060-1 standard. The model is presented in the form of the well-known Gaussian tail probability. Accordingly, a modified probabilistic model is proposed to study the effect of impulse voltage superimposed on the ac voltage on the breakdown probability of MV spark-gaps. The modified model is verified using experimental data, where the experimental setup is arranged to generate a range of impulse voltages superimposed on the ac voltages. The experimental verification shows evidence of the efficacy of the proposed probabilistic model. Furthermore, the proposed model is used to evaluate single-phase, two-phase and three-phase spark-gap breakdown probabilities in the case of lightning induced overvoltages. These breakdown probabilities are used along with the simplified Rusck expression to evaluate the performance of MV overhead lines above a perfectly conducting ground under lightning-induced overvoltages using a statistical approach. In order to study the overvoltages propagating through the transformer to its low voltage side, the high frequency model of the transformer is investigated. First, the investigation is carried out using model introduced by Piantini at no-load condition. This model is modified to take more than one resonance frequency into consideration. Therefore, the frequency response of the simulated transient voltage is improved. A verification of the modified model is carried out through the comparison between the experimental and simulation results, in which the time domain simulation is carried out using ATP/EMTP while MATLAB is used to identify the model parameters. As this model is found suitable only for unloaded transformer, an accurate and simplified model is proposed concerning unloaded and loaded conditions as well. The proposed high frequency transformer model is experimentally verified under different balanced load conditions considering two different practical distribution transformers. Then the impact of low voltage (LV) network feeder numbers, lengths, types and loads on the lightning-induced overvoltage reached at the service entrance point is investigated with and without MV spark-gap operation. The high frequency model representation of the distribution transformer and low voltage network are combined in a single arrangement in the environment of ATP/EMTP. A simplified low voltage surge arrester model is represented and verified. Finally, a study is carried out to mitigate the overvoltages by allocating the surge arrester at secondary side of the distribution transformer with concerning MV spark-gap operation.
Measuring picosecond flashover in pressurized sulfur hexafluoride (SF6)
(2011) Klüss, Joni Viljami
Sähkötekniikan korkeakoulu | Doctoral dissertation (monograph)
The exceedingly fast speed of breakdown in gas presents numerous challenges for measurement systems. Data integrity and analysis of recorded results are considerably affected by the measurement setup. In efforts to remove the effect of the measuring system and verify observations pertaining to the physical phenomenon of breakdown, two systems are compared. In addition, issues concerning the design, manufacturing and operation of high voltage pressurized spark gaps are discussed. Risetime of voltage collapse is measured in a pressurized SF6 spark gap using D-dot sensors and oscilloscopes with analogue bandwidth ranging from 6 to 30 GHz. Results from a 50 GHz series sampling oscilloscope are also presented. Inter-electrode distance is varied between 0.1 and 0.9 mm and pressure is increased from 1 to 19 bar. Breakdown voltages are measured ranging from c. 10 to 180 kV. Risetime values ranging from 50 to 800 ps and distinct variations in the voltage collapse waveform are observed. Measurements are documented and significant variables affecting the breakdown process and risetime analysis are identified. Risetime dependency to pressure, gap distance and electric field strength are presented using several analysis methods and compared to known theory. Methods to overcome or compensate the identified variables are discussed. Finally, optimal solutions for measurement systems and future research are presented.
Modeling and experimental investigation of lightning arcs and overvoltages for medium voltage distribution lines
(2011) Omidiora, Michael Adebayo
Sähkötekniikan korkeakoulu | Doctoral dissertation (article-based)
In this dissertation, lightning overvoltages in Medium Voltage (MV) lines are thoroughly investigated. The other goal is to propose new protection schemes for the designs. The lines consist of overhead lines, underground cables and covered conductors. These overvoltage problems range from direct and indirect strokes to lightning arcs. All the models and simulations are developed using the Electromagnetic Transient Program (EMTP) and Finite Element Method (FEM), while MATLAB is used for post-processing the results and identification of the model parameters. Improvement in the surge protection of MV overhead lines is demonstrated with a combination of surge arresters and a shield wire. Using the IEEE 34-node feeder injected with multiple lightning strokes, the feeder is simulated using EMTP. The response of the line is modeled both with and without the surge protection devices. The simulation study extends to the performance of a MV underground cable due to a nearby lightning discharge using FEM. The use of shield wire for limiting the overvoltage stress in the cables is proposed. A numerical analysis and simulations are performed to determine the outage rate of MV covered conductors due to lightning strokes of different characteristics. The optimum distance for surge protective devices on the conductors is also assessed. An enhancement in the surge analysis of distribution lines with the shielding effect of trees is proposed. An experimental study shows that a tree can intercept a lightning stroke in the vicinity of a distribution line. This study also analyzes experimental results of the shielding effectiveness of a tree and the induced voltages existing between the tree and the distribution line. The study is extended to evaluate the induced voltage on a distribution line for larger clearances using a Rusck model. This work investigates the lightning arc between an overhead line and a nearby tree under artificial rainfall. A full-scale laboratory experiment confirms that a direct stroke to a tree can cause severe damage to nearby power lines by initiating an arc channel through air to the conductors. A complete model of this phenomenon is developed by combining the existing static and dynamic arc equations. The model is accomplished by the bilateral interaction between the EMTP and Transient Analysis Control System (TACS) field. The experimental results have been reproduced by the computer simulations. The performance of the arc phenomenon is examined using a typical Finnish distribution network design. Using the modified arc model, the lightning arc performance of the MV/ LV network under the influence of nearby trees and the network characteristics is evaluated.
Reliability evaluation of smart distribution grids
(2011) Kazemi, Shahram
Sähkötekniikan korkeakoulu | Doctoral dissertation (monograph)
The term "Smart Grid" generally refers to a power grid equipped with the advanced technologies dedicated for purposes such as reliability improvement, ease of control and management, integrating of distributed energy resources and electricity market operations. Improving the reliability of electric power delivered to the end users is one of the main targets of employing smart grid technologies. The smart grid investments targeted for reliability improvement can be directed toward the generation, transmission or distribution system level. However, radial operating status, aging infrastructures, poor design and operation practices and high exposure to environmental conditions have caused the electric power distribution systems to be addressed as the main contributor to the customer reliability problems. Therefore, developing a smart distribution grid can be an attractive reliability enhancement solution for the electric utilities. Whenever the targeted reliability enhancement solutions are limited to the simple conventional solutions, the available reliability assessment techniques can be easily used for purposes of the value-based reliability planning. However, the electric utilities face a challenge when the reliability enhancement solutions include sophisticated measures such as those of the smart grid technologies. Generally, the available reliability assessment approaches cannot be employed directly for such purposes. In this situation, it is necessary to develop a reliability evaluation approach for predicting the reliability performance of the electric power distribution systems when employing such sophisticated solutions. A novel approach is proposed and demonstrated in this thesis for reliability assessment of an electric power distribution system when employing the advanced reliability enhancement technologies. In the proposed reliability evaluation approach, the overall impacts of the targeted reliability enhancement solutions on the sustained interruptions, momentary interruptions and voltage sags experienced by the customers are taken into account. The results of various reliability case studies directed in this thesis show that employing a suitable set of the smart grid technologies in the functional zone of an electric power distribution system can virtually mitigate all the reliability indices. It is also possible to reduce the range of variation of the reliability indices among different customers. In addition, there is a possibility to reduce the burden on the utility repair crews.