Showing papers by "Matti Lehtonen published in 2009"

Impacts of automatic control systems of loop restoration scheme on the distribution system reliability

Abstract: Loop restoration scheme (LRS) is a special feeder automation (FA) scheme, which is used by utilities to improve distribution system reliability. The LRS is controlled and managed by its automatic control system (ACS). The impacts on distribution system reliability indices of implementing LRS mainly depend on the type of its ACS. Two common types of ACS of LRS are presented and used in this study. Successful operation of ACS is dependent on the protection and automatic control functions of switching devices of LRS. Different failure modes of these switching devices can therefore affect the procedure of ACS in fault detecting, isolating and service restoration. The impacts of failure of protection and automatic control functions of switching devices and fuse of lateral distributors on reliability indices are illustrated. The worth of implementing LRS and its ACS type is represented by the reduction in expected customer interruption cost. A distribution test system is utilised to examine the impacts of two common types of ACS of LRS on the distribution system reliability. Selecting the type of ACS of LRS by utilities relies on the desired level of load-point and system reliability improvement. This study aims to quantitatively assess the impacts of two common types of ACS of LRS on the distribution system reliability.

Simplified probabilistic selectivity technique for earth fault detection in unearthed MV networks

Abstract: A novel selectivity technique to identify the faulty feeder in unearthed Medium Voltage (MV) networks is introduced. The proposed technique is based on a simplified probabilistic method applied to transient features extracted from the residual currents only using the discrete wavelet transform (DWT). DWT will enhance the accuracy of localising fault events with different fault resistances and the probability will give the protection decision (detection/selectivity). The standard deviation of a window of the DWT detail coefficient is used to detect the fault and an adaptive threshold is used as input to the probability function to estimate the faulty feeder. A faulty unearthed 20 kV network is simulated by ATP/EMTP imitating practical fault cases such as arcing and tree leaning faults. The time-domain test cases provide evidence of the efficacy of the proposed technique.

Probabilistic model for MV spark-gap characteristics with lightning induced overvoltage superimposed on AC voltage

Abstract: In this paper, the breakdown probability of 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 introduced 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 results show 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. Finally, 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 voltages using a statistical approach.

An Expert System Realization of Adaptive Autonomy in Electric Utility Management Automation

Abstract: Earlier we introduced a novel framework for implementation of Adaptive Autonomy (AA). This study presents an expert system realization of the AA framework, referred to as Adaptive Autonomy Expert System (AAES). The proposed AAES is based on the extracted rules from the Expert’s Judgment on proper Levels of Automation (LOA) for various environmental conditions, modeled as Performance Shaping Factors (PSFs). Decision fusion method is used as expert system inference engine, where eight decision fusion methods are developed as prospective ones. The AAES is realized in the practical case of electric power Utility Management Automation (UMA) for the Greater Tehran Electricity Distribution Company (GTEDC). The practical list of PSFs and the judgments of GTEDC’s experts are used as the expert system rule base in this research. The results of implementing the proposed AAES to GTEDC’s network are evaluated according to two criteria: average error and error margin. Five out of eight decision fusion methods are proven to be suitable inference engines, due to both criteria. Evaluation of the results shows that the proposed AAES can estimate proper LOAs for GTEDC’s UMA system, which change due to the changes in PSFs; thus providing a dynamic (adaptive) LOA scheme for UMA.

Experimental verification of distribution transformer model under lightning strokes

Abstract: In this paper, the distribution transformer model under lightning impulses introduced by Piantini at no load conditions is investigated This model is modified where this modification takes more than one resonance frequency into consideration during the model parameters calculation Therefore, the frequency response of the simulated transient voltage is improved comparing with Piantini model The 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 The comparison showed a good agreement between the simulation and experimental results

Urban distribution networks - some general planning observations

Abstract: Efficiency and performance targets are nowadays being tangibly addressed by the regulation authorities. Customer demands on distribution networks, both qualitative and quantitative, are increasing. Whilst most urban distribution network development occurs in the context of existing and ageing developed networks, it is legitimate to highlight some general planning targets showing the cost implications of various medium voltage planning philosophies, such as voltage levels, the use of parallel conductors in congested and heavily loaded regions, and full or economically optimal backup connections and switch placement. This paper uses a new automatic network routing algorithm to show the topographical impact of these considerations on a realistic network, serving to both demonstrate some aspects of the algorithm and highlight planning concerns going into the future. It is the authors’ wish to illustrate the new algorithm, because it directly addresses a comprehensive range of actual planning requirements in real distribution networks, both urban and rural, although it is the former that forms the focus of this paper.

An intelligent fuzzy expert system for realization of adaptive autonomy concept in utility management automation

Abstract: : Intelligent control and automation is associated with expert systems; especially, when it needs to human expertise. Earlier we introduced a framework for implementation of adaptive autonomy (AA) in human-automation interaction systems, followed by a data-fusion-equipped expert system to realize that. This paper uses fuzzy sets concept to realize the AA expert system, in a real automation application. The presented adaptive autonomy fuzzy expert system (AAFES) determines the Level of Automation (LOA), adapting it to the changing Performance Shaping Factors (PSF) of automation system. The paper includes design methodology and implementation results for AAFES, and discussion on results. Results show that AAFES yields proper LOAs, even in the new contingency situations. This is caused by AAFES’s higher intelligence than the crisp (binary) one. Moreover, since AAFES deals with fuzzy linguistic PSFs, it more realistically represents the experts’ opinion.

Impacts of Loop Restoration Strategy on distribution system reliability

Abstract: Implementation of Distribution Automation (DA) in electric power distribution systems can result in customer satisfaction with improved reliability, consistent quality and high-tech services. Loop Restoration Strategy (LRS) is a special DA method in the feeder level which is used by electric utilities to improve distribution system reliability. The effects of the implemented LRS on the reliability indices of a given distribution system mainly depend on its Automatic Control System (ACS). Selecting the type of ACS of LRS by electric utilities relies on the level of improvement required at load points and system oriented reliability indices. A step by step evaluation procedure is proposed in this paper to quantitatively assess the impacts of two common types of ACS of LRS on reliability indices of a distribution reliability test system.

Simulation performance of lightning discharges around medium voltage underground cables

Abstract: This paper studies the performance of an MV underground cable due to a nearby lightning discharge The simulation is made with Finite Element Method (FEM) The effects of lightning stroke location, resistivity of the soil and underground cable configurations are considered The results of the numerical computation show how electric field intensity and potential distribution vary with the factors mentioned above Estimations of the failure rate of the underground cable are made based on the real lightning statistics in Finland, considering homogeneous and inhomogeneous soils It is expected that the results will provide a good understanding of the need for lightning protection on MV underground cables

Modeling On-line Three-phase PD Monitoring System for MV Overhead Covered-Conductors

Abstract: In this paper, EMTP-ATP simulation environment is used to model on-line three-phase partial discharge (PD) measuring system using Rogowski coil for the monitoring of falling trees on the covered-conductor (CC) overhead distribution lines. An on-line single-phase PD measuring system has already been simulated and experimentally verified. The previous model has been extended for a three-phase system to analyze and resolve the real problems faced by the utilities. The present model is investigated under various conditions with trees leaning on more than one phases. The model can be used to estimate the length of the practical CC line at which PDs due to falling trees can be detected; thus, deciding the number and positioning of the sensors over a particular length of the CC overhead distribution line.

Comparative study of on-line three phase PD monitoring systems for overhead covered conductor distribution lines

Abstract: In this paper, ATP-EMTP simulation environment is used to compare the performance of on-line three-phase partial discharge (PD) monitoring system for overhead covered-conductor (CC) distribution lines. The PD measuring system has been developed using two different types of CC line models, i.e. Clarke's and JMarti models. Rogowski coil is used as the sensor to detect and localize PDs caused by falling trees on the CC lines. The various scenarios of falling trees on CC lines are investigated. The model can be used to estimate the magnitude and location of PDs on practical CC lines due to falling trees, thus deciding the number and positioning of sensors over a particular length of the CC overhead distribution lines.

DWT and Bayesian technique for enhancing earth fault protection in MV networks

Abstract: In this paper, a Bayesian selectivity technique is introduced to identify the faulty feeder in compensated medium voltage (MV) networks. The proposed technique is based on a conditional probabilistic method applied on features extracted from the residual currents only using the Discrete Wavelet Transform (DWT). DWT enhances to localize initial transients generated in the network due to the fault event. The absolute sum of a window of the DWT detail coefficient is used to detect the fault. The conditional probability provides the selectivity decision. The fault cases occurring at different locations in a compensated 20 kV network are simulated by ATP/EMTP concerning practical fault case such as arcing faults. Test results corroborate the efficacy of proposed technique.

Evaluation of probabilistic-based selectivity technique for earth fault protection in MV networks

Abstract: In [1], a Bayesian selectivity technique has been introduced to identify the faulty feeder in compensated medium voltage (MV) networks. The proposed technique has been based on a conditional probabilistic method applied on transient features extracted from the residual currents only using the Discrete Wavelet Transform (DWT). In this paper, the performance of this selectivity technique is evaluated when the current transformers (CTs) impacts are considered. The CTs are modeled considering their frequency characteristics. Furthermore, network noises are added to the simulated signals. Therefore, the algorithm can be tested at different practical conditions, such as nonlinear characteristics of the measuring devices and the impact of noise as well. The fault cases occurring at different locations in a compensated 20 kV network are simulated by ATP/EMTP. Results show a reduction in the algorithm sensitivity with considering CT and noise effectiveness.

AAFES: An Intelligent Fuzzy Expert System for Realization of Adaptive Autonomy Concept

Abstract: Intelligent control and automation is associated with expert systems; especially, when it needs to human expertise. Earlier we introduced a framework for implementation of adaptive autonomy (AA) in human-automation interaction systems, followed by a data-fusion-equipped expert system to realize that. This paper uses fuzzy sets concept to realize the AA expert system, in a real automation application. The presented adaptive autonomy fuzzy expert system (AAFES) determines the Level of Automation (LOA), adapting it to the changing Performance Shaping Factors (PSF) of automation system. The paper includes design methodology and implementation results for AAFES, and discussion on results. Results show that AAFES yields proper LOAs, even in the new contingency situations. This is caused by AAFES’s higher intelligence than the crisp (binary) one. Moreover, since AAFES deals with fuzzy linguistic PSFs, it more realistically represents the experts’ opinion.

Performance analysis of lightning overvoltage on medium voltage distribution lines equipped with covered conductors

Abstract: This paper gives the performance of lightning overvoltage due real lightning discharges that have occurred in distribution power lines equipped with covered conductors Evaluation and simulations are based on real lightning data collected from the Finnish Meteorological Institute (FMI) from 1998 to 2008 The presented feeder is an overhead distribution line with a covered conductor subjected to indirect lightning strokes to ground and direct lightning strokes to a line support Evaluation of induced overvoltage from indirect stroke to the line is analyzed with MATLAB With the Electromagnetic Transient Program (EMTP), simulations of lightning strokes are performed with different lightning current characteristics All cases are considered with the modelling guidelines for fast transients as specified in some lightning literature Simulations are made to compare the resulting overvoltages and energy absorptions of surge arresters for all the analyzed cases

Current differential relay with a power-current spectrum blocking for transformer protection

Abstract: In this paper, a combined power-current spectrum blocking for current differential protection of power transformer is proposed. The proposed algorithm can exclusively classify the internal faults associated with transformer inrush currents. This combination is assessed based on the individual analysis of differential power and power spectrum releasing functions and second harmonic current blocking considering modern design of power transformer. The investigation involves both theoretical and experimental studies in order to validate the new algorithm. Also, the scheme stability and sensitivity boundaries are obtained covering broad band of operational and fault conditions.

Determining wave propagation characteristics of MV XLPE power cable using time domain reflectometry technique

Abstract: In this paper, the wave propagation characteristics of single-phase medium voltage (MV) cross-linked polyethylene (XLPE) power cable are determined using Time Domain Reflectometry (TDR) measurement technique. TDR delivers the complex propagation constant (attenuation and phase constant) of lossy cable transmission line as a function of frequency. The frequency-dependent propagation velocity is also determined from the TDR measurements through the parameters extraction procedure. The measurement results can be used to localize the discontinuities as well as the design of communication through distribution power cables.

Windowed standard deviation-based transient feature extraction for earth fault protection in MV networks

Abstract: In this paper, the windowed standard deviation (WSTD) is used to extract the initial transients created in residual current waveforms generated due to earth faults in Medium Voltage (MV) compensated networks. The sliding window width used for computing the standard deviation is evaluated from the frequency filter point of view. The fault cases occurring in a compensated 20 kV network are simulated by ATP/EMTP concerning practical fault case such as arcing faults. Results provide evidence that the windowed standard deviation can be used to extract initial transients although they are non-stationary sinusoidal signals.

EMTP simulation of lightning overvoltage discharge to medium voltage overhead lines with covered conductors

Abstract: This paper presents the evaluation and simulation of real lightning discharges that have occurred in medium voltage overhead lines equipped with covered conductors (CC). Lightning stroke current selections are based on real lightning data collected from the Finnish Meteorological Institute (FMI) from 1998 to 2008. The presented feeder is an overhead distribution line with a covered conductor subjected to direct lightning strokes to a line support. With the ATP/EMTP (Alternative Transient Program/ElectroMagnetic Transient Program), simulations of lightning strokes are conducted with different lightning current characteristics. All cases are considered with the modelling guidelines for fast transients, as specified in some lightning literature. Simulations are made to compare the resulting overvoltages and energy absorptions of surge arresters for all the analyzed cases.

A new transient current based centralized selectivity algorithm for earth faults in isolated and compensated neutral MV networks

Abstract: This paper presents a new centralized selectivity algorithm for earth-faults in isolated and compensated neutral medium voltage (MV) networks. The algorithm proposes to investigate and utilize the relation between the transient phase current changes in the substation transformer and the transient residual currents for each feeder and section of the network. The transient RMS values of the residual currents for each feeder and section and the transformer phase current changes are calculated, in a constant sliding window of 2.5 ms, using a sampling frequency of 25 kHz. The proposed fault indication levels for each feeder and section will be calculated. The faulted feeder and section can be selected precisely from the higher fault indication levels, without any voltage measurements. The proposed algorithm is valid for isolated and compensated neutral networks. A 20 kV, 251 km medium voltage distribution network is simulated by ATP/EMTP program. Extensive simulations are performed to validate the proposed algorithm.

Faulty Phase Identification Using Probability Tail Function for EHV Transmission Systems

Abstract: -In this paper, a novel simplified probabilistic technique is investigated to identify the faulty phase. Estimating the faulty phase enhances the operation of single-pole autoreclosure which is applied in Europe countries. The proposed technique is based on the probability tail functions applied on the standard deviations of Discrete Wavelet Transforms (DWT) of the phase currents. An adaptive setting is proposed to be suitable for different faulty phase. The setting signal is selected as the maximum standard deviation of the DWT of phase currents. The earth faults and phase faults are correctly discriminated. Validation of the proposed algorithm is verified via ATP/EMTP simulation. Test results corroborate evidence of the efficacy of proposed technique.