електрична мережа

Distribution networks with a voltage of 35 kV are mostly used to supply electricity consumers located in rural areas. These networks, which mainly have overhead lines, normally function according to the radial principle, although in most cases they are closed or with two-way power. In recent years, the structure of 35 kV electric network has been changing, they are being transformed from aerial to aerial- cable due to the construction of 35 kV cable lines in rural areas, which in most cases are performed with cables with cross-linked polyethylene insulation.

One of the current issues in a complex electrical network with isolated (compensated) neutral is the selective identification of the damaged element during a single-phase-to-ground fault (SPGF). In order to achieve selective and reliable operation of protections against SPGFs in complex networks with isolated (compensated) neutral, besides information about the zero-sequence current and connection, information about phase voltages, zero-sequence voltage of the electrical network, and the operation of protection devices against adjacent SPGFs is necessary.

One of the problems in the electrical network with isolated (compensated) neutral is the selective determination of the damaged element in a single-phase earth fault. For selective and reliable operation of protections against single-phase earth faults in networks with isolated (compensated) neutral, in addition to the zero-sequence current, information on phase voltages and zero-sequence voltage of the electrical network is required. Many transformer substations do not have traditional electromagnetic voltage transformers.

The arc overvoltages during the single phase to earth faults in electrical distribution grids of 6-35 kV are the object of the research in this paper. The development of 35 kV distribution electrical grids is accompanied by the construction of new overhead and cable power lines. It causes a change of the capacitive earth fault current in the grids and also affects the multiplicity of overvoltages in electrical distribution grids during the single phase to earth faults.

The object of research is the choice of the optimal technical solution to prevent the development or the long existence of ferroresonance processes in electrical distribution grids of 6-35 kV, which are operated in isolated neutral mode. A lot of factors such as the grounding mode of a grid neutral, its capacitive current of a short circuit to the ground, the type and number of voltage transformers (VT) etc. influence on the probability of occurrence and duration of ferroresonance processes.

Medium voltage distribution power grids mostly operate with ungrounded neutral points of transformers. The philosophy of these electrical grids is based on the assumption that they are more reliable than grids with grounded neutral. However, transient currents during single phase or multiple faults to ground, depending mostly on the phase to ground capacitances of the used cables connected to power grid substation buses, frequently have intermittent nature due to arising arc.