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Single phase-to-ground faults are the most common type of faults in 6–10 kV overhead distribution networks. Arc intermittent single phase-to-ground fault (PSP) are the most dangerous for the network and the damaged element. They are followed by intense transient processes and, as a result, dangerous overvoltage rate and significant transient current surges at the point of insulation damage. PSP transients also have a significant effect on the selectivity and operation stability of protection devices against this type of damage. Therefore, the development of the methods and means to improve the operation efficiency of 6–10 kV overhead networks in case of PSP and technical improvement of protection devices in many cases is due to the need to calculate the transient processes that occur during insulation breakdowns of the network phase to earth. For the systems under consideration, the reliability of transient processes calculations in case of PSP is determined mainly by the accuracy of estimation of the parameters of 6–10 kV overhead lines, first of all, of inductance, which generally depends on the frequency of the transient current components. In the scientific papers devoted to the study of transient processes in case of PSP in medium voltage electrical networks, including 6–10 kV overhead networks, constant (frequency independent) values of inductance are used as a rule in the equivalent circuits and in the models of transmission lines. An urgent task is to estimate errors caused by the application of this approach to determine the parameters of 6–10 kV overhead lines during the calculations and modeling of transient processes during PSP, and cases of its application. Advanced methods of modeling of electric power systems and their elements have been applied with the use of COMSOL Multiphysics and PSCAD software to obtain the frequency dependences of the inductances of a 6–10 kV three-phase overhead line and study of their influence on the calculation accuracy of transient currents and voltages in case of PSP. The parameters of 6–10 kV overhead line models developed in the indicated software packages at a frequency of 50 Hz are set in accordance with the reference data. The authors obtain the errors estimation to determine the parameters of transient currents and voltages during PSP in 6–10 kV overhead networks when using transmission line models. The frequency dependences of inductance, which are up to 40–50 % in amplitude are not considered. The results show that application of frequency-independent models is permissible only in the cases when parameters of the calculated equivalent circuit of the network and position of PSP point remain practically constant, when solving problems that require high accuracy to determine the parameters of transient currents and voltages, for example, to determine remotely the location of a ground fault, it is necessary to use frequency-dependent models of 6–10 kV overhead lines. Introduction of the developed recommendations to determine 6–10 kV three-phase overhead lines parameters allow us to increase the reliability of calculations and to avoid raw errors when solving the problems which are related to the study of transient processes in case of earth faults in the networks of the given voltage class.

As a rule, researchers do not consider the dependence of the inductance of cable lines on frequency in their scientific papers devoted to the calculation of transient processes during single phase-to-ground fault in 6–10 cable networks. In some cases, it can lead to significant errors in evaluation of current and voltage transient components parameters. Therefore, it is an urgent task to estimate defined errors and the scope of application of frequency-independent equivalent circuits and models of 6–10 kV cable lines during calculation and simulation of transient processes in case of single phase-to-ground fault. The authors applied PSCAD / EMTDC software to study the effect of the frequency dependence of the inductances of 6–10 kV cable lines on the calculation accuracy of transient processes during single phase-to-ground fault. It allows to simulate electric power systems models with the usage of both frequency-dependent and frequency-independent cable line models with round conductors only. To check the adequacy of the frequency-dependent three phase cable model developed in PSCAD software, the authors have used a frequency-dependent model of 6–10 kV three-phase cable with sector-shaped conductors designed in COMSOL Multiphysics software. The authors have developed an approach to develop of 610 kV cable lines models with frequency-dependent and frequency-independent parameters. The authors have obtained error estimation in transient current and voltage parameters during single phase-to-ground fault in cable networks models that do not consider the frequency dependence on inductance (for discharge components the error is 1520 %, for charging components the error is equal to 510 %). It is shown that models with cable line parameters defined according to spreading speed of electromagnetic wave, can be used for approximate calculation of transient current and voltage to solve most of tasks of investigation of transient processes during single phase-to-ground faults. Application of the developed recommendations to determine three phase medium voltage cable lines parameters will increase the calculation accuracy of transient processes during single phase-to-ground faults in 6–10 kV cable networks. Only application of frequency-dependent models of cable lines allows us to provide required accuracy to develop methods of distant earth fault localization in 6–10 kV networks.

A method and principles are discussed for universal multifrequency protection from single-phase short circuits to ground in compensated and uncompensated 6 – 10 kV cable networks based on monitoring the magnitude and sign of the zero-sequence capacitance using the components at the fundamental frequency and higher harmonics of the current and derivative of the zero sequence voltage.

When operating power autotransformers that have become widely used in networks of 110 kV and higher, it is often necessary to deal with open-phase modes caused by disconnections of one or two phases in the case of short circuits or in the case of single-phase repairs. This leads to asymmetry of the voltage of autotransformers, which affects the quality of power supply to consumers. For the autotransformer itself, an open-phase operation can be dangerous with respect to overloading of individual windings. The presence of an electrical connection between high and medium voltage causes the specifics in the analysis of their operating modes. In addition, while the theoretical analysis and the general approach to the calculation of asymmetrical modes of two-winding transformers are currently considered in sufficient detail, the task of analyzing the asymmetrical operating modes of three-winding autotransformers remains important. An approach to the calculation of autotransformer and combined asymmetrical operating modes of power autotransformers has been developed on the basis of the method of symmetrical components. The proposed model and obtained analytical expressions allow one to determine the currents and voltages of the phase windings of the autotransformer in a variety of asymmetrical modes of operation on the basis of a unified approach.

Simple and reliable zero sequence overcurrent protection in distribution 6–10 kV cable networks with an insulated neutral is most widely used for protection against single phase earth faults. However, protection of this type in many cases does not provide the required sensitivity to internal (inside the protected zone) faults as it must be tuned to the response current from surge transients during external faults through an intermittent arc. It is possible to increase the sensitivity if adaptive current protection is applied. However, the known methods for its implementation are only effective for stable faults through transient resistance but do not provide high dynamic stability of operation in transient conditions in case of arc intermittent earth faults that are the most dangerous for the network. Therefore, an urgent problem to be solved now is improving the principles of adaptive current protection against earth faults. To compare the efficiency of the known and proposed principles of adaptive current protection implementation taking into account the complexity of transients during earth faults through an intermittent arc in 6–10 kV cable networks, we used Matlab simulation with the SimPowerSystem and Simulink extension packages. The research into the operation algorithms of adaptive current protection against earth faults was carried out on simulation models of 6–10 kV cable networks with an insulated neutral and with neutral grounding through a high-value resistor. The studies on the simulation models have shown that the known methods of implementation of adaptive current protection against earth faults based on the use of full zero sequence currents and voltage are ineffective during intermittent arc earth faults. The authors propose a method of adaptive current protection against earth faults in 6–10 kV cable networks with an insulated neutral and with neutral grounding through a high-value resistor that provides a significant increase in dynamic stability of transient operation with arc ground faults and allows using only zero sequence current and voltage components of the operating frequency of 50 Hz as the actuating quantities. The proposed method of implementing adaptive current protection against earth faults in 6–10 kV cable networks with an insulated neutral and with neutral grounding through a high-value resistor does not only increase the sensitivity of this protection type to earth faults through transient resistence and dynamic stability of operation in transient condiitons in case of arc intermittent earth faults but also broadens the range of its possible applications

In 6 – 10 kV cable networks with isolated neutral, overcurrent protections are widely applied to earth fault protection based on components of working frequency, and in compensated cable networks based on higher harmonics of the zero-sequence current. The selectivity and stability of functioning of the former is, to a significant extent, limited by the effect on their operation of transient processes during arc earth faults, and of the latter by the instability of the level of the higher harmonics in the SPEF current, depending on the network operating mode. An adaptive method is examined for implementing the specified protections to ensure improvement of their technical capability.

In distribution 6–10 kV networks with an insulated neutral for earth fault protection, zero sequence current directional protection devices are commonly used. According to the operation data, the main disadvantage of such kind of protection is the possibility of their functioning failures in transient conditions with the most dangerous for network intermittent arc earth faults. It is known that most earth faults in 6–10 kV networks, primarily in the initial stage of insulation damage, have an intermittent arc. Operation failures of zero sequence current directional protection in case of arc faults reduce the operational reliability of the protected network and, as a result, the reliability of power supply to consumers. Nowadays, new developments of electrical power systems relay protection devices, including earth fault protection of medium voltage distribution electrical networks, are implemented only on a microprocessor base. Therefore, the selection and justification of the implementation principles of zero sequence current directional protection which can provide high dynamic stability of functioning is a relevant objective. When analyzing the dynamic stability of the functioning of zero sequence directional current protection, regarding the complexity of transients during intermittent arc earth faults in medium voltage electrical networks with an isolated neutral, the simulation in Matlab using SimPowerSystem and Simulink was carried out. This study focuses on transient currents and voltages as the main factor influencing dynamic stability of the functioning of zero sequence current directional protection. The impact of other factors, for example, the inaccuracies of the primary zero sequence current and voltage transducers, the scheme of formation of compared quantities, etc. was not taken into account in simulation models. The study has allowed determining the causes of possible functioning failures of digital current earth fault directional protection in dynamic operation modes. It has been shown that the usage of orthogonal components of fundamental frequency of zero sequence voltage and current in current directional protection devices eliminates the failure of their operation with any kind of arc earth faults. To ensure high dynamic stability of operation under the influence of transients during arc intermittent earth faults, current directional protection for this type of damage should be performed on the basis of monitoring the phase relationships of the fundamental frequency components of 50 Hz of zero sequence voltage and current, but not their full values.

Single-phase earth faults are the predominant type of damage in distribution 6-10 kV cable networks, and are often the root cause of power failures to consumers, accompanied by significant economic damage. Therefore, reliability for about 50% of consumers of industrial and urban power supply systems depends on the technical perfection of protection against earth faults. The currently used approach to the design of protection and signaling of earth faults, based on the application of the existing concept of selectivity of considered protection, does not always ensure the achievement of the main goal – increasing the reliability of power supply to consumers. To improve the reliability of power supply, new technical solutions are needed that provide not only selective detection of the damaged connection for all types of single-phase earth faults, but also the recognition of the most dangerous for the network and the protected connection faults types for automatic selection the most effective protection action (signal or shutdown). Within the framework of the existing approach, the design of protection against earth faults with the specified properties is possible only on the basis different methods of its implementation in networks with different neutral grounding modes, which is associated with the complication of protection circuit, its design and exploitation. A new approach has been proposed that provides universal technical solutions for protection and signaling of earth faults for 6–10 kV cable networks with various neutral grounding modes. To implement the proposed approach, universal adaptive current protection and universal admittance protection based on the control of the capacitance of zero sequence loop of the protected connection have been developed.

About half of the electricity generated in Russia is distributed through electric cable networks of medium voltage of 6-35 kV [1]. 6-10 kV networks account for more than 99% of total length of medium voltage cable networks [2]. The predominant type of damage in these networks is single phase earth fault (SPEF), often arcing. SPEFs are often the primary cause of outages of cable lines and electric motors. Therefore, the reliability of power supply to consumers largely depends on the technical perfection (selectivity and stability of operation [5]) of protection against earth faults.

A method for performing overcurrent protection against earth faults in 6–10 kV cable networks with insulated neutral and with neutral grounding through a high-resistance resistor providing adaptation to the influence of main variable factors - transients during arc faults and transient fault resistance is provided. The analysis of the effectiveness of the proposed protection, carried out on mathematical models of 6–10 kV cable networks in Matlab, confirmed the high dynamic stability of its operation in transient modes at arc earth faults and sensitivity for stable earth faults through transient resistance. The proposed method provides an extension of the area of possible application of adaptive current protection against earth faults in 6–10 kV cable networks.

Zero sequence current protections are most widely used for protection from single-phase ground faults in distributed 6 – 10 kV cable networks operating with an isolated neutral. Multiparametric operating principles for current protections are proposed which enhance their sensitivity in stable operation and during the most hazardous arc intermittent ground faults.

Devices based on the method of absolute measurement of the level of higher harmonics in zerosequence currents of connections of a protected object are the most commonly used for protection from earth faults in compensated 6- to 10-kV cable networks of industrial and urban power-supply systems. However, many years of operating experience have shown that the technical refinement (the selectivity and the sensitivity) of overcurrent protection from earth faults based on the method of absolute measurement of higher harmonics is not always sufficient. One of the reasons for the low technical refinement of these protection devices is the inaccuracy in estimating the maximum and minimum possible higher harmonics levels in the earth-fault current of compensated 6- to 10-kV cable networks, which are required for the selection of the operating current settings and the sensitivity determination. The main factors influencing the maximum and minimum higher harmonics levels in the earth-fault current and their ratio were revealed on the basis of the computational experiments using the imitation models for 6- to 10-kV cable networks, the limiting values of these levels were determined, an determination of the harmonics-level instability in the fault current for cable networks with different total load compositions was given, and its interrelation with the fluctuations of the total coefficient of harmonic voltage components on 6- to 10-kV buses of network power centers was established. The conditions of applicability and the area of application of overcurrent protection from this type of failure based on the use of higher harmonics of the zero-sequence current for different objects of 6- to 10-kV compensated cable networks were determined taking into account the above estimates. Specifications for a method for selecting the operating current settings were proposed.

Single-phase earth faults (SPEFs) are the predominant type of electrical breaks in the cable networks of medium (6–10 kV) voltage and are often the principal cause of accidents involving significant economic losses. Improving the reliability of electrical supply and reducing of the losses in case of SPEFs are possible only with the help of significantly reducing of the determination time for fault location. Solving this problem is particularly relevant for cable networks of 6-10kV of urban power supply and for complex configuration of power supply cabling of enterprises of certain industries (e.g., ferrous and nonferrous metallurgy, pulp and paper, automotive, and others).

For protection against single-phase earth faults (SPEF) in medium voltage 6–10 kV distribution networks, operating with isolated neutral or neutral grounding through high-resistance resistor, simple and reliable overcurrent zero sequence protections (OZSP) have been most widely used [1], [2], etc.]. Overcurrent protections based on higher harmonics (HH) of zero sequence current 3i 0 of protected connection (OZSPHH) have received the widest application in compensated networks [3], [4]. The disadvantage of all OZSP performances is a significant effect on the efficiency of its operation of zero-sequence transient currents with arc intermittent earth faults (AISPEF), limiting selectivity and sensitivity and, therefore, possible application scope of the above protection [5], [6], etc.]. The main OZSPHH disadvantages are significant influence on the conditions of its selectivity and sensitivity of HH level instability in the current of compensated network limiting possibilities of its application, as well as protection blocking for AISPEF, to prevent unnecessary trips. At the same time, it is known that the occurrence of AISPEF accompanied by dangerous overvoltages for network is also possible in compensated networks with large compensation deviations possible in real operating conditions [7].

The questions of realization single-phase ground faults protection devices and selective signalling system, based on transient process electric magnitudes usage in 6-10 kV electric networks with neutral ground different operating conditions are presented.