Earth Fault Research Articles

Evaluation of the Operating Modes of the Urban Electric Networks in Dushanbe City, Tajikistan

Currently, energy saving has become an acute problem all over the world. Due to the rapid development of both the energy and information technology sectors, as well as an increase in the electricity demand, the electric distribution system is facing problems caused by stricter requirements for electricity quality, reliability, efficiency, and sustainability. Therefore, the use of energy-saving technologies, both the improvement of existing ones and the application of new ones, is one of the main reserves for electricity saving in power supply systems. This study is devoted to the evaluation of the operating modes of transformer substations of the 6–10 kV electric distribution network in Dushanbe city, based on the results of control winter measurements and the dependence of the low coefficient of active power in transformer substations. It has been observed that, with a low coefficient of active power in transformer substations, when transmitting electricity from substation busbars to transformer substations of a final consumer via overhead transmission lines (an average length of 5 km), the voltage loss exceeds the maximum permissible values. When transmitting electricity via cable lines, the voltage losses are within the limit of 5%. However, a low active power factor may be the reason for an increase in capacitive currents, followed by the command of single-phase earth fault currents. Due to the low active power factor, the increase in voltage losses and useful power losses leads to the inefficient operating modes of the transformer substations.

Behavior of Residual Current Devices at Earth Fault Currents with DC Component.

Low-voltage electrical installations are increasingly saturated with power electronic converters. Due to very high popularity of photovoltaic (PV) installations and the spread of electric vehicles (EV) as well as their charging installations, DC-AC and AC-DC converters are often found in power systems. The transformerless coupling of AC and DC systems via power electronic converters means that an electrical installation containing both these systems should be recognized from the point of view of earth fault current waveform shapes. In such installations, various shapes of the earth fault current may occur-a DC component of a high value may especially flow. The DC component included in the earth fault current influences the tripping threshold of residual current devices (RCDs)-the devices which are mandatory in certain locations. This paper presents results of the AC-type, A-type, and F-type RCDs sensitivity testing under residual currents of various compositions of the DC component. This testing has shown that the DC component may both degrade and improve the sensitivity of RCDs. Moreover, unexpected positive behaviors of RCDs in some circumstances under DC residual current is discussed. Therefore, recognizing the real sensitivity and behavior of RCDs from the point of view of the DC component is important for effective protection against electric shock, in particular, in PV installations and EV charging systems. The research results provide a new insight into the real behavior of RCDs in modern power systems and, consequently, the safety of people.

Условия полной компенсации токов однофазного замыкания на землю в кабельных сетях напряжением 610 кВ с заземлением нейтрали через дугогасящий реактор

To improve the reliability of power supply of consumers, a significant part of 6–10 kV cable networks (in Russia about 20 %) operate with resonant neutral grounding via arc suppression coil (ASC) (single-phase earth fault capacitive current compensation). This neutral grounding mode provides suppression of arcing earth faults accompanied by dangerous overvoltage and reduction of current at the point of fault by compensating the capacitive component of the elementary frequency. Field experience of operation of 6–10 kV compensated cable networks has shown that the reason of the decrease of the efficiency of the neutral grounding mode is an increase of the residual current at the point of fault. It happens due to the active component of the elementary frequency and higher harmonic components that are not compensated via ASC up to the values when long time arc duration and transition of the earth fault to the short circuit become possible. The existing solutions of the problem of full compensation of the current of a single-phase earth fault are not sufficiently developed and substantiated. Thus, the solution of the problem to increase the efficiency of grounding the neutral of the network via ASC can be achieved by developing and implementing methods and devices of the full compensation of the earth fault current, including the active component of the elementary frequency and higher harmonic components. To achieve the terms and conditions of full compensation of the current components of a single-phase earth fault, the classical theory of electrical circuits has been used. To verify the validity of the results obtained, simulation modeling in the Matlab software package has been carried out. For an electrical network with neutral grounding via ASC, the authors have obtained a general mathematical description of the terms and conditions of full current compensation at the point of earth fault, including higher harmonic components and transient components. The authors have defined the requirements for accuracy to determine the moment of a single-phase earth fault and the voltage value of the damaged phase when insulation fault occurs. Possible options of algorithms to compensate higher harmonics in the current of stable earth fault are proposed. The obtained general mathematical description of the terms and conditions of full current compensation at the point of a single-phase earth fault in 6–10 kV compensated cable networks allows us to substantiate existing methods and develop new ones to compensate both the total current at the point of fault and its components.

Дистанционное определение места замыкания на землю на кабельной линии 6–10 кВ с использованием ее моделей

As a rule, electrical values of the transient process that occurs during insulation breakdown are considered to determine the fault location in case of short-term self-eliminating and arc intermittent single-phase earth faults in distribution cable networks of 6–10 kV. The known methods for remote earth fault location based on monitoring the parameters of transient currents and voltages, do not always provide sufficient accuracy to determine the fault location. It is due to the influence of various random factors (the value of the total capacitive current that depends on the network operation mode, the moment the fault occurs, etc.). Thus, the task to improve the accuracy of the remote fault location algorithm under the influence of these factors, as well as the speed to determine the fault location is relevant. Considering the complexity of transient processes in case of single-phase earth faults in 6–10 kV cable networks, simulation in Matlab software package is carried out to study the effectiveness of the algorithm of the operation of a device for remote earth fault location based on the above method. A method to determine remotely the location of an earth fault is proposed. It makes possible to ensure sufficient accuracy and speed based on the analysis of the factors that affect the measurement accuracy and in-depth studies on simulation models of 6–10 kV cable networks in Matlab. It is shown that the total effect of various factors that distort the measurement in case of remote fault location using a controlled line model can cause errors in measuring the distance up to the point of the fault equals to 10–15 % with close earth faults (about hundreds of meters away the busbars of the power source) and no more than 5–10 % at the distances a few kilometers away. The method to determine remotely the fault location using the models of a controlled transmission line provides an increase in the accuracy and speed of measuring the distance to the earth fault location in 6–10 kV cable networks. It is more promising to apply when one develops devices for remote earth fault location.

On the Expediency of Laying Cable Power Lines with a Voltage of 6–35 kV Outside Settlements Instead of Overhead Power Lines

. It is noted that at present, 10 and 35 kV overhead power transmission lines are being laid outside the settlements of the Republic of Belarus on reinforced concrete vibrated (10 kV) and centrifuged (35 kV) poles that are characterized by low reliability and damaging the environment (on account of alienation of land for poles, the need to make a wide clearing for laying in the forest, obstructions by poles and wires of lines to the operation of agricultural machinery, the danger of electric shock to personnel and the public). It is possible to avoid these disadvantages if, instead of overhead lines, power transmission cables with cables insulated by cross-linked polyethylene are used which are characterized by a very low failure flow parameter. Contrary to the prevailing opinion about the higher cost of cable power transmission lines compared to overhead ones of the same rated voltage, it turned out that, taking into account reliability, the cost of electricity lost in the lines for a year, damage to the environment and to the power system caused by the need to perform more expensive emergency repairs (as compared to a planned one), laying cable lines with three-core and single-core cables of a voltage of 10 and 35 kV instead of overhead cables outside the populated area is fairly justified. In this connection, the laying of three-core cables is more preferable. It should be also borne in mind that with an increase in the length of cable lines, the capacitive earth fault current increases, to compensate for which additional devices are needed to be installed in power centers, viz. arc-extinguishing reactors or resistors, accounting for the cost of which (up to 22 % of the cost of one kilometer of cable line) does not significantly affect the conclusions we have drawn regarding the effectiveness of using 6–35 kV cable power lines in an unpopulated area instead of overhead ones, however.

Resolving Current Transformer Partial Saturation Problem: An Analytical Approach to Design the Flux Equalizing Winding

Stray fluxes passing through the current transformer (CT) core produced by adjacent bus-bars and conductors or eccentric installation of primary conductors results in asymmetric magnetic flux distribution along the core. This may lead to exceeding the magnetic flux from its appropriate saturation level in some parts of the core, which is called partial saturation (PS) phenomenon. Under such a condition, CT error increases, and some relays maloperations may take place. This paper proposes a novel analytical approach to design flux equalizing winding (EW) for resolving such a problem. Moreover, PS occurrence in a core balance CT (CBCT) is analyzed using the finite element method (FEM), and its negative influence on a sensitive earth fault (SEF) relay is scrutinized as a case study. Simulation studies reveal that the proposed approach can prevent PS-related undesirable effects in a real power system.

On the use of the mutual correlation of cosmic rays penetrating the earth’s crust with geoacoustic emission

The elastic deformation energy accumulated at the edges of a fault in the earth’s crust in a seismically active area can be released with a small external impact, causing vibrations that propagate in the form of a sound wave through the lithosphere and can be detected on its surface. As a trigger effect that causes such a vibration, an ionization can be used that is created in the deep lithosphere by particles of the penetrating component of cosmic rays. This idea was once proposed in the number of theoretical studies. An experiment to test this hypothesis was started at the cosmic ray facility at the Tien Shan Mining Scientific Station. As a result, short-term sporadic acoustic emission signals were recorded by highly sensitive microphone detectors of the station. Presumably, the origin of this emission can be associated with seismic processes occurring in the area of a deep earth fault, located directly under the station. A statistically significant temporal correlation has been found between acoustic emission and high-energy cosmic ray muon events up to 100 TeV. If the further research in this direction is confirmed, then the effect of stimulated acoustic emission from a seismically active region of the earth’s crust may be of interest for solving the problem of short-term earthquake prediction.

An integral backstepping controller design for compensated distribution networks with rapid earth fault current limiters in bushfire prone areas

AbstractThis paper presents a nonlinear integral backstepping controller (I‐BSC) design scheme for a T‐type residual current compensation (RCC) inverter used in compensated distribution networks with rapid earth fault current limiters (REFCLs). The major control task for the proposed scheme is to reduce the fault current to a value suitable for the mitigation of powerline bushfires due to single phase‐to‐ground faults in compensated distribution networks. The key distinct feature of the proposed I‐BSC over the traditional backstepping controller (T‐BSC) is that it introduces an integral action for analyzing the dynamic of the tracking error which minimizes its steady‐state value and ensures better dynamic performance. In order to prove the global asymptotic stability of the RCC inverter with the proposed integral backstepping controller (I‐BSC), the Lyapunov function‐based theory is used. Finally, the performance of the I‐BSC is analyzed on the MATLAB/Simulink environment and compared with a T‐BSC. The performance of both I‐ and T‐BSCs is assessed in terms of transient behaviors of the injected current to the neutral, fault current, and line‐to‐ground voltage of the faulty phase to ensure the standard operational criteria for self‐extinguishing powerline bushfires. Simulation results clearly demonstrate that both controllers fulfill operational standards for REFCL‐compensated networks though the I‐BSC archives better transient behaviors while comparing with the T‐BSC. Results from the processor‐in‐loop (PIL) validations are also included to further justify the applicability of the newly proposed scheme in the real‐time environment.

Performance of Restricted Fault and Bias Differential Protection Against Earth Fault on a Transformer

In this paper, the effectiveness of restricted earth fault and bias differential protection against earth fault on of transformer winding is presented. Earth fault is one of the failures that occurs on a power transformer winding. Its magnitude depends on how the neutral is connected and the location of fault. For a transformer which is connected to Neutral Earth Resistance (NER), as the fault moves towards its neutral, the magnitude of fault current decreases and the detection of fault diminishes, thus limiting percentage of winding that can be protected. To overcome this problem, restricted earth fault is employed. This paper will show that based on its principles, restricted earth fault protection is more sensitive and provide better coverage for the star winding against earth fault especially resistance earthed.

Повышение электробезопасности обслуживающего персонала в условиях неполных однофазных замыканий на землю

The results are given related to the analytical studies of the causes of accidents occurrence and abnormal operating modes of electric receivers in the medium-voltage networks. It is established that the main cause of damage is the single-phase earth fault mode. It is established that the main cause of damage is a single-phase ground fault. It is found that a long single-phase ground fault increases the likelihood of transition to a more severe phase-to-phase short circuit. It is accompanied by immediate shutdown and long downtime of the equipment, large economic losses, as well as the appearance of touch and step voltages that are dangerous for people. It is identified that the earth fault current in a network with an isolated neutral will decrease in proportion to the decrease in the coefficient of the share of the transient resistance in the total earth fault current. Analysis of the operability of protection against single-phase earth faults is carried out on the example of a section of 6 kV distribution network of the coal mining enterprise JSC Polosukhinskaya Mine. Experimental studies of the efficiency of the functioning of non-directional current protection against single-phase earth faults were carried out. In the event of an incomplete single-phase earth fault, a drop in the zero-sequence current in the protected line invariably leads to a critical decrease in the sensitivity coefficient. There is a violation of the selectivity of the protection action, as a result of which the probability of electrical injury to the maintenance personnel increases. Boundary conditions for stable operation of relay protection and automation systems in case of incomplete single-phase earth faults are established. Definition of these conditions allows developing new algorithms for the operation of non-directional current protection in order to improve the electrical safety of the operating personnel.

Performance of Backfill Material for Grounding System under High Voltage Condition

Grounding (or earthing) play the main role to in order to protect electrical equipment, building and life from lightning, switching and earth fault occurrences. A good grounding system must be able to dissipated this unwanted charge in very fast time and this can be achieved by reducing the grounding resistance. Introduction to backfill material to be filled into grounding pit is widely implemented in order to lowering the resistance of grounding system. This paper investigates the performance of marconite, cement and sand as part of backfill materials for ufer type grounding system under alternating high voltage and impulse voltage condition. The purpose of high impulse voltage experiments is to determine the performance of above said materials under transient condition while the alternating high voltage experiment to observed the behaviour backfill material under leakage or fault current occurrence. The findings reveal that performance of grounding system may significantly change from the expected result at design stages.

Generic approach to calculation of short circuit currents in power transformers

Power transformer in service is exposed to various voltage and current stresses. The ability to withstand short circuit is an essential requirement for power transformers. There are different types of short circuit: single - phase to earth, double - phase with or without simultaneous earth fault and three - phase short circuit. These various short circuit conditions result in different stress conditions for the windings. Mesh analysis and symmetrical components are the two methods most commonly used for determining the magnitude of short circuit currents. In this paper, both methods will be presented with results compared on a real-case transformer. Also, a generic scheme using the symmetrical components approach is designed in order to standardize the short-circuit currents calculation for all power transformer types and to reduce the time required for obtaining results.

MODELING OF A SINGLE-PHASE EARTH FAULT IN DETERMINING THE LOCATION OF DAMAGE USING LINEAR ALGEBRA THEORY

The article is devoted to the analysis of emergency modes in a network with an isolated neutral of 6(10)-35 kV.. The purpose of the work is to develop a mathematical model of an electrical network with a voltage of 6(10)-35 kV when calculating a single-phase earth fault to determine the location of damage using linear algebra theory. Tasks: to analyze the emergency mode in a network with an isolated neutral; to develop a methodology for calculating an emergency event taking into account the parameters of the transient process to determine the location of accidents in the network. The result of the proposed solutions is a rational determination of the single-phase earth fault current parameter, taking into account the values of the longitudinal and transverse components of the network replacement circuit. The study of the replacement circuit and the calculation of the circuit parameters using the decomposition theory of the matrix determinant show good dynamics of accounting for all parameters of the electrical network in the event of an emergency ground fault. According to the calculated values of the frequencies under consideration, the intrinsic resistance «Z» of each of the circuits of the resulting substitution circuit is determined. After determining the intrinsic resistance of the earth fault circuit taking into account the transient resistance, the dependence Z = f(ω) is calculated to determine the distance to the location of the defect in power transmission lines in remote devices for fixing emergency events installed in distribution networks of 6(10)-35 kV.

A Simplified Approach to Calculate the Earth Fault Current Division Factor Passing Through the Substation Grounding System

The main aim of designing a safe grounding system is to provide a low impedance path to the flow of the earth fault currents without exceeding the operational constraints and equipment limits that will ensure electrical continuity. One of the most significant and well-known parameters to design a safe grounding system in power systems is the exact determination of the maximum earth fault current division factor. This paper presents a simplified and accurate method to calculate the earth fault current division factor in different states of the earth fault occurrence within and the vicinity of the substation under study. In the proposed method, a hybrid overhead-cable line, the impact of the frozen soil, mutual coupling between phase conductors and guard wires, grounding system resistance of adjacent substations, different tower footing resistances, the impact of the phase conductor impedance, and different spans in transmission lines can be considered. In addition, a closed-form formulation is also developed for the occurrence of the earth fault on the transmission line. Finally, details of the analysis results of this study have been compared with other methods in the literature. The validity and accuracy of the proposed approach also have been assayed and confirmed in details.

A nonlinear model predictive controller design for residual current compensation inverters in rapid earth fault current limiters to mitigate powerline bushfires

This paper presents a nonlinear model predictive control scheme for residual current compensation inverters in rapid earth fault current limiters to inject appropriate current to the neutral so that the fault current is compensated in distribution networks. The proposed nonlinear model predictive controller is designed to limit the fault current within a level that powerline bushfires do not ignite due to single line-to-ground faults on power distribution systems in bushfire prone areas. The nonlinear model predictive controller is designed for a T-type residual current compensation inverter in rapid earth fault current limiters based on its dynamical model where the control objective is to inject current for compensating the fault current. The control law for the residual current compensation inverter is obtained by solving an optimal control problem while using the concept of the receding horizon control scheme. The performance of the controller is evaluated through standard software-based computer and processor-in-loop simulations where results are benchmarked against backstepping and sliding mode controllers in terms of compensating both faulty phase voltage and fault current.

Normalized Dynamic Time Warping Distance Based Auxiliary Restraint Scheme for Restricted Earth Fault Protection of Converter Transformers

Converter transformers are one of the most important and costly equipment in High Voltage Direct Current (HVDC) transmission systems. They require sensitive and reliable protections. However, the maloperation of restricted earth fault (REF) protection of converter transformers caused by unloaded energization of converter transformers has been reported. Aiming at this problem, the saturation and measurement distortion of the current transformer on neutral line (CTN) and the resulting maloperation mechanism of REF protection are analyzed firstly. Then, an auxiliary restraint scheme of REF protection of converter transformers is proposed exploiting the similarity between the waveforms of the zero-sequence current on neutral line and the calculated zero-sequence current from measured phase currents under magnetizing inrush and fault conditions. The proposed auxiliary restraint scheme is based on normalized dynamic time warping distance (DTWD), which has strong robustness against CT saturation. The proposed auxiliary restraint scheme is verified by simulations and practical data recorded on-site.

Experimental Registration and Numerical Simulation of the Transient Overvoltages Caused by Single Phase Intermittent Arc Earth Fault in 35 kV Network With Isolated Neutral

The paper presents the results of experimental registrations and numerical simulations of the transient overvoltages caused by single phase intermittent arc earth fault in 35 kV network with isolated neutral. Experimental laboratory investigation is performed in order to implement appropriate arc fault model for arcs with low and discontinuous current. Single phase to earth arc fault is created in field in 35 kV network. Numerical model of the system is created in EMTP-ATP software. Overvoltage waveshapes and overvoltage factors are calculated and compared to measured values. Good agreement between the measured and calculated results is achieved.

Remote determining the location of a single-phase earth fault in 6-10 kV networks based on voltage sensors

The article considers a method for determining the location of a single-phase earth fault by voltage values measured in steady state on the damaged phase. A voltage sensor based on a capacitive voltage divider is presented.

METHOD OF CONTROL OF TECHNICAL CONDITION OF INSULATION OF ELECTRICAL EQUIPMENT OF THE GENERATOR-TRANSFORMER UNIT

Significant wear of the main electrical equipment of the power supply scheme (synchronous generators, complete shielded power lines, transformers) of thermal and nuclear power plants, implementation of the policy of extending the service life of existing power units, and gradual implementation of equipment repair strategies according to technical condition determines the relevance of development of new and improvement of existing systems for monitoring the technical condition of electrical equipment in general and its insulation characteristics in particular. Insulation diagnostics, relay protection devices and information-measuring systems currently operated on power units do not allow tracking changes in electrical insulation parameters of electrical equipment such as insulation resistance and capacitance relative to ground and tangent angle of dielectric loss detection that makes detection of defects in isolation at an early stage of their development impossible. Relay protection devices can be insensitive to single-phase earth faults of the stator winding near the neutral and with a symmetrical decrease in the insulation parameters of the stator winding, as well as malfunction when starting units due to voltage asymmetry in generator phases. The method of control of technical condition of electrical equipment insulation in the generator-transformer unit circuit is proposed, which is based on combined superimposition of direct current and alternating current with frequency less than industrial, control of discharge of precharged capacitor and control of alternating current flow with Rogovsky coils. The proposed method will detect both abrupt and gradual changes in the technical condition of the insulation of electrical equipment of the “generator-transformer” unit, improve the quality of planning of repair work and speed up the search for faulty elements in case of damage.

A new approach to improving the sensitivity of earthing relay and reducing the over-voltage in 6 kV grid of open-pit mines

Purpose. Earthing relays are utilized very commonly in open-pit mines for disconnecting the failure part of 6 kV electric system from earthing incident. Much previous research is mentioned on searching either an optimal method for improving the effect of relays or improvements of reducing the overvoltage caused by grounding separately. The paper presents a new approach to not only improving the relays sensitivity but also to limiting the over-voltage. A new algorithm is also proposed to control automatic connection of an additional resistor in order to achieve both above mentioned purposes of 6 kV earthing relay. Methodology. The propose controlling diagram is implemented on PROTEUS to get 3U0 signals. The main components appeared when 6 kV feeders were faulted to earth. Afterward, the detecting signal is utilized as input for simulation diagram in MATLAB to get and analyze the wave forms of responded zero sequence voltage. Findings. The paper presents a new algorithm utilizing a resistor that is auto-controlled to auto-connect the open-triangle coil of 6 kV measurement transformer at the moment of earth fault. This automatic connection aims to increase the relay sensitivity as well as reduce the overvoltage. The finding results are sensitivity factors of relay and voltage magnitudes of healthy phases, which will be compared with corresponding values when there is no connection of resistor to prove the effectiveness of the method. Originality. A useful method is proposed for increasing the earthing current to enhance the sensitivity of earthing relay. Practical value. The proposed control diagram could be used to design a device aiming to improve the safety of 6 kV grid in open pit mines when earth faults occur.