Showing papers by "Matti Lehtonen published in 2012"

Impact of MV Connected Microgrids on MV Distribution Planning

Abstract: The planning and development of distribution networks with a substantial penetration of microgrids connected to the medium voltage (MV) network form the main themes of this paper. The impact of microgrids is assessed in terms of their effect on optimal network topology, losses, reliability, reserve connections, network upgrade and expansion savings. The earning base of the distribution system operator also comes under scrutiny. A suburban MV cable network is planned using a network planning algorithm developed by the authors, first with optimal routing for demand-only nodes and then with a 33% penetration of randomly located microgrids. The network is then expanded to meet the requirements of a future planning horizon, in order to compare the expanded and upgraded optimum MV network topology with and without microgrids. Apart from visually depicting the topological differences, the savings such microgrids can give to the long term distribution network investment and running costs are quantified in terms of the investment costs, loss costs and interruption costs. When networks are planned with optimal rather than full backup, the introduction of microgrids is shown to have a considerable saving impact on all cost components except the cost per unit power transfer in the distribution network.

Impacts of Fault Diagnosis Schemes on Distribution System Reliability

Abstract: Design and development of fault diagnosis schemes (FDS) for electric power distribution systems are major steps in realizing the self-healing function of a smart distribution grid. The application of the FDS in the electric power distribution systems is mainly aimed at precise detecting and locating of the deteriorated components, thereby enhancing the quality and reliability of the electric power delivered to the customers. The impacts of two types of the FDS on distribution system reliability are compared and presented in this paper. The first type is a representative of the FDS which diagnoses the deteriorated components after their failing. However, the second type is a representative of the FDS which can diagnose the failing components prior to a complete breakdown and while still in the incipient failure condition. To provide quantitative measures of the reliability impacts of these FDS, the comparative and sensitivity case studies are conducted on a typical Finnish urban distribution network.

Transient algorithm based on earth capacitance estimation for earth-fault detection in medium-voltage networks

Abstract: This study presents a new earth-fault detection algorithm for both unearthed and compensated neutral medium-voltage networks. The proposed algorithm is based on the transient estimation of the earth capacitance of the background network during the first one-eighth of the power frequency period. The effects of feeder impedances and the Petersen coil (in compensated networks) are compensated using proposed compensation factors. The estimated capacitance accurately indicates if the feeder is faulted or not. The capacitance can be estimated with a maximum error of ±10 %. The algorithm is not reliant on the fault resistance and the faulted feeder parameters; it mainly depends on the background network. The network is simulated using the ATP/EMTP program. Different fault conditions are covered in the simulation process, including different fault inception angles and high fault resistances.

Multi-end correlation-based PD location technique for medium voltage covered-conductor lines

Abstract: In Finland, overhead covered conductors (CC) are commonly used in the medium voltage (MV) networks because the distribution loads in rural areas are not heavy and are widely distributed in the forested terrain. Such parts of the network are exposed to leaning trees and therefore partial discharging into the overhead CC frequently occurs. In this paper, a multi-end correlation-based technique for locating partial discharge (PD) sources on CC lines is presented. The theory of maximum correlation factor is used in order to find the time differences between signal arrivals at three synchronized-measuring points. This helps to solve the problem of varying parameters of the CC line that can change the propagation velocity and the problem of identifying the arrival times of the corresponding PD pulses at different measuring points. The algorithm is developed and tested using a simulated study and experimentally verified using an alternating current (ac) energized system. The Electromagnetic Transient Program-Alternative Transient Program (EMTP-ATP) is used to implement and analyze a PD monitoring system. The Discrete Wavelet Transform (DWT) is used to de-noise PD signals measured from a Rogowski coil utilized as a high frequency transducer. The noise model is added to the simulated-power network signals and then a DWT-based de-noising process is carried out. The proposed algorithm performance is evaluated by extracting the high frequency features using a windowed-standard deviation. The simulated and experimental evaluations prove that accurate PD source location can be achieved using the proposed algorithm technique.

Comparison of Air-Gap Thermal Models for MV Power Cables Inside Unfilled Conduit

Abstract: This paper studies the effects of natural convection on longitudinal heat transfer and on the air-gap thermal resistance of cables inside conduit installations. Oversimplification of the physical placement of cables inside unfilled conduits is the main shortcoming in currently available thermal models. The study closely investigates the share of each heat-transfer mechanism and the effect of the natural placement of trefoil cables inside the conduit. Measurements from various installation setups are investigated for their impact on heat transfer. The installation-dependent convection correlations adopted in this study have broader applications for the dynamic thermal rating of underground cables inside conduit, troughs, and tunnels. Laboratory measurements are compared with numerical solutions from the IEC 60287 standards, Electra 143 methods, and FEA simulations.

Neutral point treatment and earth fault suppression

Abstract: Earth fault management in medium voltage networks involves assessing of customer interruptions, challenges in fault clearing processes and safety issues. The major advantage by allowing sustained earth faults is the avoidance of customer interruptions. However, the network operation with sustained earth faults does not only include welfare. The paper will generally discuss the advantages and disadvantages of operating the network with sustained earth faults. In addition, in the paper, the Helsinki medium voltage network and the developments in neutral point treatment and network operation during earth faults will be outlined. The paper presents the earth fault analysis in the Helsinki 10 kV network and compares the results to a reference analysis. Based on the analysis, the paper presents the possibilities and limitations of a sustained operation with earth faults. The paper also presents the strategy steps of managing sustained earth faults as well in the 10 kV and 20 kV areas and the required long term development steps.

Switch and reserve connection placement in a distribution network planning algorithm

Abstract: Network planning algorithms should, but do not always, inherently consider outage mitigation in terms of switch and reserve connection placement This paper outlines our approach, which is close to optimum but not exhaustive, in that the main aim is to accurately enough imbed outage mitigation in the planning algorithm so that the lines are in the right place The methodology involves placing backup connections assuming full default switch placement, and then upgrading and or removing switches so as to maximize their cost benefit Three levels of switching are supported, two of which, network circuit breakers and automatic load making disconnectors, fulfill the smart paradigm, so that we can assess the impact of such switching on network topology and the rest of the switching, which is manual

Guest Editorial Special Section on Microgrids

Abstract: Despite the significant research efforts devoted to the microgrid and smart grid areas, numerous problems related to real world implementations still remain unsolved. The present special issue was announced with the objective of addressing and disseminating state-of-the-art R&D results on microgrids to bring together researchers from both academia and industry with the goal of fostering interactions among stakeholders. In response, 190 two-page extended abstracts were received and considered for the first round of reviews. Authors of about 60 selected abstracts were then invited to submit the full papers in the second round and out of them 27 high-quality manuscripts were ultimately approved and included in this special issue. These papers are organized according to the following four topics: 1) Microgrid dynamic performance, control/operational strategies, and voltage/frequency regulation. Here, 14 papers deal with the dynamic modeling and performance ofmicrogrid or its building blocks and propose appropriate control/operational strategies or voltage/frequency regulation approaches. 2) Reliability and power quality aspects of microgrids, where seven papers about the microgrid reliability assessment techniques, impact of distributed energy resources on microgrid reliability indices, and the microgrid power quality analysis are presented. 3) Fault modeling and protection schemes. This section consists of four papers focusing on series fault modeling in dc microgrids, overvoltage protection of photovoltaic generators, and protection schemes tailored for the microgrid operational characteristics. 4) Others: Two practical papers about the impact of microgrids on the planning of the medium-voltage distribution networks and characterization of the Vanadium Redox battery are presented.

A novel approach for assessing the impacts of voltage sag events on customer operations

Abstract: The business activities of the majority of industrial and commercial customers are dependent on some critical equipments that are susceptible to voltage sag events. This paper aims to propose a novel approach to estimate the financial impacts of voltage sag events on customer operations. The proposed technique is based on the event tree method. Using this approach, it is possible to consider the impacts of operational failure of various sensitive equipments, involved in the customer operations, on the financial losses expected from voltage sag events. A quantitative case study is conducted in the paper to illustrate the applicability of the proposed approach.

Optimal selection of voltage sag mitigation solution based on event tree method

Abstract: Modern commercial operations consist of critical equipments that are more susceptible to voltage sag events. The consistent and smooth running of these operations is often disrupted by voltage sags. Diverse range of solutions exist which can enhance the ride through capability of sensitive devices. This paper presents various voltage dip mitigation solutions and a methodology for analyzing the effectiveness and practical viability of these solutions. The proposed approach is based on event tree method which gives an indication about the extent of process interruption in the event of voltage sags. A case study is conducted in the paper with different combinations of practical mitigation solutions against voltage sags, in order to illustrate the applicability of the proposed approach.

A New Transient Impedance-Based Algorithm for Earth Fault Detection in Medium Voltage Networks

Abstract: This paper presents a new earth-fault detection algorithm for unearthed (isolated) and compensated neutral medium voltage (MV) networks. The proposed algorithm is based on capacitance calculation from transient impedance and dominant transient frequency. The Discrete Fourier Transform (DFT) method is used to determine the dominant transient frequency. The values of voltage and current earth modes are calculated in the period of the dominant transient frequency, then the transient impedance can be determined, from which we can calculate the earth capacitance. The calculated capacitance gives an indication about if the feeder is faulted or not. The algorithm is less dependent on the fault resistance and the faulted feeder parameters; it mainly depends on the background network. The network is simulated by ATP/EMTP program. Several different fault conditions are covered in the simulation process, different fault inception angles, fault locations and fault resistances.

Experimental Evaluation of Rogowski Coil Performance for Locating PD in Energized Overhead Covered-Conductor Feeder

Abstract: An experimental analysis is performed to evaluate Rogowski coil performance for PD (partial discharge) location on overhead CC (covered-conductor) distribution lines. The measuring set-up is arranged in high voltage laboratory. A multi-end measuring method is chosen as a technique to locate PD source point on the line. A power transformer is used to energize one end of the CC line by the AC voltage source. The performance of Rogowski-coil is tested in noisy environment. The tests are carried out concerning different measurement conditions such as off-line and on-line PD measuring systems. The results obtained from the laboratory measurements confirm the capability of the Rogowski-coil in order to measure and locate the high frequency PD source on the CC line in consistent with energizing AC source. Chirp detector is used as a signal-processing tool in order to extract the PD signals and then apply the locator algorithm.

Air-core inductive current sensor for fast transients measurements in distribution networks

Abstract: Observation of fast transients in power networks can provide some of the most valuable information. The transients from partial discharge can provide the information about the power line insulation state; fault-produced fast transients can also provide information about the fault or problematic section location. For measurement of fast transients, there would also need to be a very fast sensor, as the pulsed measurements could require the bandwidth range reaching in the tens of MHz range. In this paper, a sensor for fast transients measurements is presented and tested. While very simple, this sensor can offer very good performance while being inexpensive and robust. The results of performance verification measurements show very good results when testing with high-frequency pulse source, presenting the potential of such sensor.

Modelling and analysis of lightning overvoltage protection of MV cable laterals connected with overhead lines

Abstract: This paper describes the lightning overvoltage protection schemes of cable laterals connected with MV overhead line. The various protection schemes are modelled and investigated are carried out on the simulation results. The simulations are performed using EMTP-ATP simulation environment, which is a strong tool to analysis transient behaviour of MV network components. The simulated lightning surge is struck on one of the overhead line phases and overvoltage effects are investigated down the cable connected with overhead line. The cable laterals are consisted of sub-laterals which are also simulated in this study. Generally, the surge arresters are installed at the junction of overhead line and cable and at far end of the cable. The various mitigation methods and protection schemes for protecting MV cable laterals are proposed on the basis of surge arresters locations, sub-laterals connections, and cable length. DOI: http://dx.doi.org/10.5755/j01.eee.123.7.1325

Modelling and evaluation of the lightning arc between a power line and a nearby tree

Abstract: SUMMARY This paper addresses the modification of the existing dynamic arc model for lightning arc simulation application. The modified dynamic arc model combines the static and the dynamic arc equations that are already established in the literature to reproduce an arc fault involving a lightning-struck tree and a nearby power distribution line during rainfall. The results of experimental studies regarding arc creation between a tree and a nearby power line with artificial rainfall were obtained. A complete model of this phenomenon is reproduced using the electromagnetic transient program (ATP/EMTP) and the existing static and dynamic arc equations. The essential reason for modifying this model is graphically shown with a comparison of the dynamic arc and the modified dynamic arc models. The performance of the arc phenomenon was examined using a typical Finnish distribution network design. By means of simulation using the modified dynamic arc, developed within the ATP/EMTP software with its Transient Analysis Control System features, the lightning arc performance of the MV/LV network under the influence of nearby trees and the network characteristics is analyzed. It is expected that such a model can be included in the existing ATP/EMTP software packages for simulating power system transients. Copyright © 2011 John Wiley & Sons, Ltd.

A method for ground fault distance computation in unearthed medium voltage distribution network

Abstract: In networks with unearthed or compensated neutral, earth fault signals consist of different frequency components, which result from charging and discharging of the network capacitances. The charge transient which usually is of higher amplitude and lower frequency than the discharge transient is most suitable for fault location. This paper discusses a method for estimating a single phase to ground fault location in radially operated unearthed distribution network. The proposed method is based on correlation of undamped charge transient frequency with the fault distance. The undamped complex frequency is defined from the development of single phase to ground fault bus impedance matrix of the circuit network in complex frequency domain. The accuracy of the method was tested using a simulation model and the network example with overhead lines. The analysis was performed using the EMTP-ATP software.

Fault Location in MV Unearthed Distribution Network Using the Undamped Frequency of the Transient Signal

Abstract: This paper present a method to locate a fault location for single phase to earth fault in medium voltage (MV) unearthed network using undamped frequency of the transient signal. During the earth fault, a transient component in the fault signal provides valuable information for fault location purposes. The fault distance is formulated using the lumped parameter model. In this paper, the properties of Hilbert transformation are used to estimate the damping attenuation of transient signal. The networks considered are assumed to be radially operated and modelled using ATPDraw program. Furthermore, the accuracy of technique was tested on parameters such as a fault distance, fault resistance and the voltage inception angle. Some sources of error affecting the method accuracy, are explained. Finally, the possible correction technique is explained to compensate the computation error. Key-Words: Fault location, earth fault, isolated network, charge transient, discharge transient, symmetrical component, damping attenuation

Rogowski coil evaluation performance with different fault conditions in medium voltage distribution networks

Abstract: In this paper, Electromagnetic Transient Program-Alternative Transient Program (EMTP-ATP) test bench of 20 kV network is developed considering high frequency models of Rogowski coil and the distribution network. The bench test is used to evaluate the Rogowski performance with different fault scenarios such as different fault types, different fault resistances and inception angles. Furthermore, the tree leaning fault and arcing fault with characteristics of intermittent faults are considered in the study. Results show the possibility of using Rogowski coil for locating faults in

Intelligent fault diagnosis for on-line condition monitoring in smart distribution networks

Abstract: Partial discharge (PD) measurements can be regarded as an effective and reliable tool for on-line condition monitoring and asset management of high voltage (HV) apparatus. Recently, a novel application is observed in the monitoring of falling trees on covered-conductor (CC) overhead distribution lines. In this paper, Rogowski and Pearson coils are used as sensors to detect PDs for this specific application. These sensors are non-intrusive and superior to the conventional PD detecting methods. In the next stage of future developments, the wired sensor will be converted into a wireless one. The challenges faced while implementing future wireless technology are also described here. In future, the wireless sensors will be integrated into distribution management system (DMS) to detect and localize the falling trees. The proposed intelligent fault diagnosis system will improve the safety of CC lines and make them more attractive to utilities due to reduced maintenance costs and visual inspection work. In addition, the reliability of the distribution system will improve which is one of the significant characteristics of the future smart distribution networks.

A novel transient current-based differential algorithm for earth fault detection in medium voltage distribution networks

Abstract: This paper presents a novel transient current differential algorithm for earth fault detection in unearthed (isolated) and compensated neutral medium voltage (MV) networks. The proposed algorithm uses the transient residual currents, which are very sensitive for earth faults. The transient values of residual currents are calculated at each feeder in the network and used as a fault indicator. The flow of residual current is investigated. It is found that the residual current for the faulted feeder is equal to the summation of all residual currents for all other healthy feeders. Based on this investigation, a differential technique is proposed. A percentage restrain performance is proposed to ensure the selectivity and security of the algorithm. The transient analysis of the algorithm is very sensitive for fault incidence rather than steady state analysis. From practical point of view, the residual currents can be measured easily by one sensor for each feeder and with no need for voltage signals to apply this algorithm. The proposed algorithm is less dependent on the fault resistance and the faulted feeder parameters. The network is simulated by ATP/EMTP program. Different fault conditions are covered in the simulation process; different fault inception angles, fault locations and fault resistances.

Influence of rainfall on lightning flashovers from trees to medium voltage bare and covered conductors

Abstract: In this paper, a laboratory experiment was conducted to compare the performance of lightning discharge characteristics of bare conductor (BC) and covered conductor (CC), resulting from very-close lightning strokes to trees. Special attention is paid to the influence of rainfall on the lightning arc characteristics. All the experiments are performed using full-scale configurations. Other factor investigated is the effects of tree-to-line clearance. The studies reveal a substantial amount of new information which is required to fully understand the true behavior of very-close lightning interaction with power conductors in rainfall, which is often the true lightning situation.

The effect of fault resistance on the earth fault characteristics in unearthed and compensated neutral medium voltage networks

Abstract: In unearthed neutral medium voltage network the maximum earth fault current, at zero fault resistance, is limited by the total earth capacitance however for compensated neutral network the maximum earth fault current is limited by the total leakage conductance and the compensation branch. The effect of the fault resistance on the earth fault characteristics, in both unearthed and compensated neutral networks, is investigated in this paper. The investigations include the effect of the fault resistance on the fault current, fault voltage, the transient recovery voltage (TRV) and the rate of rise of the transient recovery voltage (RRTRV). A realistic 20 kV medium voltage network model is used for the required investigations. The network model has been implemented using EMTP-ATP program. Based on the simulation results, it is found that the fault resistance has a significant effect on the earth fault characteristics which is found different in compensated from unearthed neutral networks.