Modelling of the electrical safety system in case of a phase failure

Faulty wires of the overhead power lines at the voltage 10 kV result in reduced quality of the electric power supplied to consumers, fire hazards in the area of the damaged overhead line, and increased risks of electrocution for people which, in most cases, is fatal. At present, the operating electricity networks have no devices of prompt blackouts when there are downed 10 kV power-lines. To create a protection providing electrical safety when wire is faulty, it is negative and zero sequence voltages that are used as information parameters, whereas the device itself is installed either at the end of the overhead 10 kV power lines or on the side of the low voltage of the 10/0.4 kV step-down transformer. To select the information parameter, to obtain its quantitative characteristics, and to determine the location of the protection device, laboratory studies were undertaken on voltages with the use of the computer simulation in the MATLAB software environment and physical modelling. The paper presents the results of the studies of negative and zero sequence voltages at various points in the 10/0.38 kV electric network and a transformer with the winding connection ‘delta-star with zero’ when the 10 kV overhead line is downed. It has been demonstrated that the negative sequence voltage more fully characterises the phase wire breakage of the overhead 10 kV voltage line than the zero-sequence voltage; the information about the negative sequence voltage can be obtained both at the side of a high and a low voltage of the step-down transformer with 10/0.4 kV. The installation of the protection device on the low voltage side of the 10/0.4 kV step-down transformer has been substantiated. Underlined is the importance of the installation of the negative sequence voltage protection device. Compared are the results of the research with those obtained earlier when investigating the voltages in a 10/0.38 kV electric network and the transformer with winding connection ‘star-star with zero’.