Showing papers by "Matti Lehtonen published in 2004"

Dynamic thermal modelling of power transformers

Abstract: Summary form only given. The aim of This work is to introduce hot-spot and top-oil temperature thermal models for more accurate temperature calculations during transient states, based on data received in a normal heat run test (i.e. the top oil in the tank of transformer and the average winding-to-average oil gradient). Oil viscosity changes and loss variation with temperature are taken into account. The new thermal models will be validated using experimental (fiber optic test) results obtained at varying load current on a 250 MVA-ONAF-cooled unit, a 400 MVA-ONAF-cooled unit and a 605 MVA-OFAF-cooled unit. The results are also compared with the IEEE-loading guide (1995) Annex G method.

A wireless sensor concept for managing electrical distribution networks

Abstract: In this paper, methods and analysis of a simple wireless sensor concept for detecting and locating faults as well as for load monitoring are presented. The concept is based on distributed wireless sensors that are attached to the incoming and outgoing power lines of secondary substations. A sensor measures only phase current characteristics of the wire it is attached to, is not synchronized to other sensors and does not need configuration of triggering levels. The main novelty of the concept is in detecting and locating faults by combining power distribution network characteristics on system level with low power sampling methods for individual sensors. This concept enables the sensor design to be simple, energy efficient and thus applicable in new installations and for retrofit purposes in both overhead and underground electrical distribution systems.

Travelling waves based earth fault location in 400 kV transmission network using single end measurement

Abstract: This paper investigates the problem of fault location based on transient signals obtained at single-end of the transmission line. Current travelling waves are utilized to locate faults on transmission system. In this paper a review of different travelling wave methods in fault location is presented. The wavelet transforms is used for analyzing power system fault transients to determine the fault location and to provide time resolution for the high frequency components of the fault transients. These signals are decomposed into their modal components. The aerial and the ground mode. Wavelet transform is found to be an excellent tool for identifying the travelling wave reflections from the fault. Validation of fault location is carried out by ATP/EMTP simulations for typical 400 kV power system faults.

Introducing prob-a-sag - a probabilistic method for voltage sag management

Abstract: Comprehensive voltage sag management in a power distribution system includes the technical and economic impact of sags, annual frequency of sags, as well as the effect of possible mitigative means. A probabilistic approach is required for performing the task in complicated industrial processes. Prob-a-sag is a novel method combining all these features. Two-dimensional arrays are used for expressing the quantities and carrying out the analysis or optimization. Equipment sag sensitivity is expressed as tripping probability, which enables the probabilistic sensitivity assessment of a large process. The method is very flexible; increasing the array resolution improves the result precision. If sag quantities other than depth and duration are preferred, we may increase the number of array dimensions accordingly. Prob-a-sag is compatible with any spreadsheet application, or may be implemented in sophisticated network analysis software.

Overvoltage protection, faults and voltage sags

Abstract: Distribution transformers in rural networks have to cope with frequent lightning induced overvoltages and thus must be protected using spark gaps or surge arresters The overvoltage protection type has an influence on the system behaviour and on the interruptions and voltage sags experienced by power system customers In this paper, measurement results showing the effect of overvoltage protection type on established faults in an unearthed medium voltage overhead line network are presented Detailed results of fault frequencies and fault types are shown Further, a probability function is established to describe the influence of overvoltage protection type on voltage sags experienced in the network This latter aspect is important when limiting the effects of interruptions and voltage sags for sensitive customers

Energy meter behaviour under non-sinusoidal conditions

Abstract: Currently, there are two energy meters types installed: electromechanicals and electronics. The first ones are still the most frequent. However, new facilities tend to use electronics energy meters based on solid-state technology. Electromechanicals devices present characteristics already well known that allows us to measure only energy consumption. However, now the rules that manage the energy supply are totally new, there is a competitive electricity market. This way, detailed information on the energy consumption of each client is needed. In this point, electronic meters seem to be the best option to face the new challenges. This study will review the operating principles of solid-state energy meters. The behavior of some three phases models under nonsinusoidal conditions is studied. Experimental setup and test methodology are discussed as well.

Voltage sag distributions caused by power system faults

Abstract: Summary form only given. Voltage sag distributions caused by faults at different voltage levels and experienced by low voltage customers were established for four different power system areas. The shares of different fault types at each voltage level and the sag propagation throughout the power system were taken into account. The results show that the origin of sags in urban and rural areas tends to be different. This data is needed when, for example, planning measures for sag mitigation in different parts of the power system.

TCP/IP Communication Aspects in monitoring of a Remote Wind Turbine

Abstract: Today there is a growing interest in utilizing different distributed energy resources (DER) in the power system. A problem that the system operators, owners and power utilities have to consider is thus how this technology can be integrated within the existing information systems. Another emerging problem is what type of telecommunication architecture should be used for remote monitoring and control of these units that normally are distributed over a large geographical area. One possible solution is to use the commercially available TCP/IP based telecommunication infrastructure and extend it with different technologies when needed to reach a distant DER unit. The commercial TCP/IP telecommunication network poses, however, concerns regarding the reliability, response time and level of security that can be achieved. This paper addresses different protocol aspects and presents a pilot study where a wide area internet and a local packet switched radio network were used to monitor parameters of a wind turbine located approximately 1000 km away from the control center. In the pilot system, the applicability of the protocol based on the IEC 61400-25 draft standard as well as a protocol based on a proprietary solution were assessed. Main emphasize was put to the evaluation of the system architecture as well as the reliability and response characteristics of the tested communication solution in the pilot installation. Based on the test results, the general usability of a commercial wide area internet and packet switched radios as a communication means in monitoring of distant wind power plants is discussed.

Measured sagged voltages caused by identified power system faults

Abstract: Results of voltage sag measurements from five identified fault cases are presented in this paper. All the measurements were performed at one 110/20 kV substation on the 20 kV side. One fault was caused by a fallen 110 kV tower and two cases represent fallen trees on a 20 kV medium voltage feeder supplied by the substation in question. Another case concerns a fault, caused by a small animal, that occurred in a neighbouring MV network. The fifth fault case is an example of the most typical fault type, an earth fault in the 20 kV network where the fault itself causes no sagged voltages at low voltage customer locations but the circuit operations pertaining to the fault clearing procedure do. The measurement arrangements are explained and the measurement results of these particular faults are presented. By analyzing identified faults in this way, understanding of the system performance can be deepened.