The compensated networks are affected by the phase capacity asymmetry of wiring, which causes the unbalancing of the phase voltages in the tuned condition of the arc-suppression coil. In the especially unfavourable cases this effect reduces the possibility of network operation with the tuned arc-suppression coil. The operation with the arc-suppression coil out of tune is accompanied by higher values of the fault currents during the ground faults and the natural frequency of the course of recovery voltage in the network after switching off or interrupting the ground fault. These disadvantages can only be removed by efficient elimination of the phase asymmetry. The new methods of eliminating the phase capacity asymmetry allow for controlling the phase asymmetry size (its reduction as well as its possible increase), they keep it within a specified tolerance band (Uo value in the tuned condition of the arc-suppression coil is typically maintained at levels from 1% to 4% of the network voltage phase value). The new methods of elimination of the phase capacity asymmetry have favourable effects on the network operation during the ground fault and do not reduce the protecting elements sensitivity to high-ohm ground faults. By improving the accuracy of the arc-suppression coil tuning the safety of the compensated networks operation can be enhanced. The other properties of the whole system of phase capacity asymmetry elimination designed to increase the compensated networks operation safety, to improve the reliability of groundfault protections and to locate the fault areas, can be fully utilized

Protection of transformers, as one of the most expensive equipment in the power system, against lightning overvoltage impulses is a vital task. This paper, for the first time so far, investigates the effects of a filtered spark gap on the protection level of transformers against lightning overvoltage impulses. The filter is an inductor that is placed in series with the transformer and before the spark gap aiming to reduce the voltage at the connection point of the spark gap, and hence, enhancing the protection level of the transformer under lightning overvoltages. The experimental laboratory tests are accomplished on a 400 kVA, 22/0.4 kV, Delta-Star ( Δ − Y ) connection type transformer under 110 kV, and 125 kV overvoltage impulses, whereas the size of the spark gap is set to 80 mm and two inductors of 35 μ H and 119 μ H are considered. In order to perform a more in-depth analysis, a model that works reasonably close to the empirical case is developed in the EMTP-RV software. An optimization algorithm is used to determine the sensitive parameters of the double-exponential function, which is used to reproduce the applied laboratory lightning impulse voltages in the EMTP-RV environment. Moreover, the transformer is modeled according to the Cigre Guidelines (Working Group 02 of Study Committee 33). The behavior of the spark gap is simulated as close as the practical situation using the disruptive effect method. The preciseness of the simulated filtered spark gap model is verified by comparing the results of the simulated model in the EMTP-RV with the results of experimental tests. After verifying the model, different sizes of inductors are studied in the EMTP-RV environment to investigate whether larger or smaller inductors provide better protection for the transformer under lightning conditions. A comparison is performed among the conventional spark gap, surge arrester, and the filtered spark gap to provide a better analysis of the potential of the proposed device. The results indicate that proper sizing of the inductor, within an effective range, slightly enhances the protection level of the transformer.

The compensated networks are affected by the phase capacity asymmetry of wiring, which causes the unbalancing of the phase voltages in the tuned condition of the arc-suppression coil. In the especially unfavourable cases this effect reduces the possibility of network operation with the tuned arc-suppression coil. The operation with the arc-suppression coil out of tune is accompanied by higher values of the fault currents during the ground faults and the natural frequency of the course of recovery voltage in the network after switching off or interrupting the ground fault. These disadvantages can only be removed by efficient elimination of the phase asymmetry. The new methods of eliminating the phase capacity asymmetry allow for controlling the phase asymmetry size (its reduction as well as its possible increase), they keep it within a specified tolerance band (Uo value in the tuned condition of the arc-suppression coil is typically maintained at levels from 1% to 4% of the network voltage phase value). The new methods of elimination of the phase capacity asymmetry have favourable effects on the network operation during the ground fault and do not reduce the protecting elements sensitivity to high-ohm ground faults. By improving the accuracy of the arc-suppression coil tuning the safety of the compensated networks operation can be enhanced. The other properties of the whole system of phase capacity asymmetry elimination designed to increase the compensated networks operation safety, to improve the reliability of groundfault protections and to locate the fault areas, can be fully utilized.

No abstract

The paper deals with the description and analysis of the method of the additional earthing of the affected phase used for the elimination of the fault current during an earth fault in a compensated network. The efficiency of this method is strongly influenced by the fault location and resistance and, in some cases, its application can lead to deteriorating the situation at the point of the fault and thus to increasing the risk of dangerous touch voltage. The paper analyses the results of the simulations of selected fault cases where the negative influence of the additional earthing of the affected phase on the fault current magnitude (and therefore on the occurrence of the dangerous touch voltage at the point of the fault and in the neighboring LV network) can be expected. All the theoretical conclusions and assumptions were consequently verified by experiments in a real distribution network. Their results were then used in the final part as a basis for proposing the measures to apply the method in MV distribution networks.