Earth faults are the most common disturbances in medium voltage networks. They cause threats to equipment and people, and reduce the quality and reliability of electricity supply. Detection and localization of earth faults is a key challenge to network operators, therefore it is important to constantly develop criteria that allow for an unequivocal assessment of the current network state. This article focuses analysis of effectiveness of classic zero-sequence overcurrent criterion and on modification of the zero-sequence overcurrent criterion applied in fault current passage indicators, which are the devices used to effectively detect and locate earth faults. Two new adaptation criteria for networks with resistor grounded neutral point have been proposed, which enable significant improvement of the effectiveness of earth faults detection, in particular high impedance ones. The effectiveness of the criteria was verified computationally and in simulations. In certain situations, the proposed solutions may be over 40% more effective than the classic criterion. The value of detected transition resistance R _F also increases significantly. All the above-mentioned advantages make their potential use in practice very beneficial.

This paper presents the final development of an expert system utilizing a measurement of cable screen earthing transient current. The developed system allows for identification and location of earth fault in underground cable and mixed lines (underground cable and overhead line) and monitoring of an earthing system and cable screen connections. The unique feature of the developed earth fault locating system is the possibility of identification of line type and branch of the MV underground cable or mixed feeder under earth fault conditions. As a result, the time to remove failure can be greatly reduced and the number of earth fault indicators installed in the distribution network can also be reduced. Unfortunately, in order to operate properly, the previously developed system requires a fundamental—50 Hz component of the measured zero-sequence cable core current and cable screen earthing current; therefore, short transient earth faults without steady-state earth fault currents cannot be localized and categorized even though the transient earth faults have a negative impact on the power system. According to measurements performed by the authors, transient earth faults are relatively frequent, which causes stress to insulation. The number of transient earth faults may be reduced by ensuring proper maintenance of the distribution system. Unfortunately, because of the very large area of the distribution feeder, often in the range of tens of kilometers or even around a hundred kilometers, and many potential causes of earth faults, it is very difficult to localize the transient earth fault and determine the cause of the earth fault. Herein, we present the possible causes of transient earth faults and methods developed for the analysis of transient earth faults. Moreover, the novel algorithm for transient earth fault detection and location is proposed. The proposed algorithm has a self-learning capability and can identify branches of the distribution feeder under transient earth fault conditions. The effectiveness of the proposed algorithm is confirmed thanks to the performed network experiment.

This paper presents current flow in cable screens and a cable screen earthing system. Moreover, the paper presents the methodology for the detection of problems in cable screens, such as open phase in a cable screen and high contact resistance of a cable screen in a cable joint. The is based on cable screen current measurements and allows for localization of the erroneous connection–high contact resistance or open phase. The phenomenon is simulated in PowerFactory Software. Moreover, exemplary cable screen current measurements are presented.

The article presents the results of research in the field of limiting the effects of overvoltages and improving the conditions for the self-extinguishing of transient faults in medium voltage networks with earth fault current compensation. The aim of the research was to estimate the level of overvoltages generated by an earth fault during re-ignition of the electric arc and to assess the possibility of reducing them by increasing the attenuation of the earth fault circuit. The results of the conducted tests show that to increase the attenuation, which ensures a significant effect of limiting such overvoltages, it is enough to change the way of operation of the devices forcing an additional active component of the earth fault current (AWSCz/ACFA). In Poland, such devices are commonly used to improve the effectiveness of earth fault protection. It was also found that the proposed solution enables accurate tuning of the Petersen coil in networks with natural asymmetry of earth capacitances. Therefore, changes in the operation of AWSCz/ACFA devices may have a beneficial effect on the limiting the effects of repeated ignitions during earth faults and, at the same time, enable the accurate tuning of the Petersen coil and increase its ability to extinguish arc-fed faults. Research and theoretical analysis of the issue were carried out on the basis of data characterizing the parameters of the earth fault circuits of the real 15 kV network.

The detection and isolation of short circuits within the MV networks can be improved by the installation of disconnectors together with short-circuit flow passage indicators (FPI) inside the network. The proposed further improvements of FPI operation based on the change of operating criterion to the admittance one and the installation of zero-sequence resistive current forcing system (AWSCz). The operation of such system is analyzed considering the influence of high transition resistances at the place of fault, errors generated within the zero-sequence current measurement systems as well as the poor setting value divisibility of FPI. The proposed decision making system, placed in HV/MV substation, analyses the information obtained from substation measuring systems and from distributed earth fault FPI sensors to issue the proper decision which sectionalizing disconnectors should be open to isolate the faulted line section.

The article presents a method for detecting earth faults in a compensated medium-voltage network, which can be used especially in fault current passage indicators, but also in standard protection devices. The method is based on the adaptation process of the zero-sequence current protection setting, with the adaptation factor depending on the current value of the zero-sequence voltage. In this article, a comparative analysis of the classical and adaptive zero-sequence current criterion is conducted, taking into account the errors of the measuring system. The results of simulations performed in the PSCad environment are also presented. For a representative set of example short circuits, it was shown that the effectiveness of the presented criterion can be more than 50% higher than that of the standard criteria, taking into account the range of detected transition resistances. A comparison of the proposed method with admittance criteria was also drawn, and it was shown that it is suitable for the detection of high-resistance earth faults.