Showing papers by "Matti Lehtonen published in 1997"

Characteristics of earth faults in power systems with a compensated or an unearthed neutral

Abstract: This paper presents an analysis and comparison of earth fault characteristics in the medium voltage distribution networks (20 kV) with high impedance earthing. The results presented are based on the evaluation of 341 real case data recordings, obtained from substations of distribution networks with an unearthed or a compensated neutral. The networks were mainly of overhead construction, with a smaller share of underground cables.

An integrated solution for protection and automation of power systems

Abstract: The paper presents an integrated solution for power system protection and medium voltage network and substation automation. For network automation, the remote control and monitoring of sectionalizing and tie switches is discussed. With regard to the substation automation, the integration of numeric protective relays into the substation telecontrol system (SCADA) is studied. A solution is presented, where the protective relays are connected to the station computer via a fibre optic based local area network. The station computer, in turn, works as a remote terminal unit of the telecontrol system. An advanced solution is presented for the integration of the telecontrol system and the distribution data management system (DMS). Here the data communication works two-ways. From the operator's point of view, the system appears as one entity. The telecontrol system is responsible for the control commands and on-line measurements. The role of DMS is to display the network topology and to analyse the network condition by various calculations. To illustrate the operation of the integrated automation system, the applications for fault location and supply restoration are presented in more detail. The fault location is based on two different solutions. By computational means it is possible to produce an estimate for the fault distance from the substation. This function is based on an analysis of fault currents recorded by the numerical relays at the substation. Since there usually are many line branches, the computational means also yield several possible fault locations. In order to find the actual fault location, the remote reading of the fault indicators, mounted at the disconnector stations, is used. Finally the function for automatic fault isolation and fault restoration is discussed. This application is based on the remote control of line switches by the telecontrol system, combined with the topology analysis and fault location analysis in the DMS system.