Ground Fault Current Research Articles

Effects of High Fault Currents on Ground Grid Design

Due to increased load demands and reduced incentives to build new transmission lines, energy companies are increasing power flows on the existing transmission assets, which will increase the fault current levels (for both three-phase and phase-to-ground faults) throughout the power system. New generation sources to be added at the transmission and distribution network will increase fault current intensities. It is crucial for the users of industrial facilities to be aware of increased ground-fault current magnitude at the service entrance and of the actual condition of the grid. The protection that ground grids provide against step and touch potentials is only good up to the expected level and duration of ground-fault currents, as originally communicated by the electric utility in the design phase. In addition, thermal and mechanical stresses to the customer's ground grid and ground grid connections can increase the grid's resistance to ground and, at the same time, fault potentials. In order to prevent these problems from occurring, a ground grid assessment, utilizing field and utility updated data, should be carried out on a regular basis. This paper will illustrate a European Committee for Electrotechnical Standardization (CENELEC) approach to ground grid design, aimed to maximize the electrical safety under ground fault. In addition, case studies will be included, showing how high fault currents have damaged ground grids and what repairs are possible.

Verification of operation of MCCBs for one-lone ground fault current on TN grounding system

Verification of operation of MCCBs for one-lone ground fault current on TN grounding system

Methods to Determine Overhead Ground Wire Sizes in Distribution Systems

This paper presents an approach to analyze the overhead ground wire (OHGW) sizes in distribution system focus on the i2t of line to ground fault current. For line to ground fault, the fault current and duration time when fault take place as well as the relay operating time have been concerned. The Line Type 1, OHGW not connected to the neutral of substation power transformer, give the short circuit current magnitude which is less than the current from Line Type 2, OHGW connected to the neutral of substaion power transformer. So the Line Type 1 gives the smaller OHGW sizes than Line Type 2. To find the suitable OHGW sizes, the fault current magnitude, the duration time when fault take place and the relay operating time have to be considered. For the short circuit current is not greater than 3 kA, the 25 mm2 can be used. For the short circuit current is not greater than 7 kA, the 35 mm2 should be used for distance of 500 m from the substation, 25 mm2 for line beyond that location should be used. For the short circuit current is not greater than 12 kA, the 50 mm2 should be used for 500 m from substation, 35 mm2 should be used between 500 m-3 km and 25 mm2 for line beyond that location should be used.

Identification of the Faulted Distribution Line Using Thyristor-Controlled Grounding

For ungrounded systems, identifying the line experiencing a single-phase-to-ground fault is very difficult because an ungrounded system produces very low fault current. This paper presents a novel method that can help to overcome this difficulty. The idea is to temporarily convert an ungrounded grounded system into a grounded system through a controlled grounding of the system neutral. The result is a controllable ground fault current that is large enough for identifying the faulted line and yet small enough not to cause system problems. Theoretical analysis, computer simulation and lab experiments verified the effectiveness of the proposed method. The proposed method also applies to high-resistance grounded and resonant grounded systems.

Grounding the Neutral of Electrical Systems Through Low-Resistance Grounding Resistors: An Application Case

It seems common knowledge that three-phase short-circuit currents are the worst possible scenario for electrical systems. In reality, single-phase ground-fault currents may be significantly more intense than three-phase fault currents. With the occurrence of high single-phase fault currents, severe damage could be caused to the iron cores of electric machines included in the zero-sequence fault loop. The method of neutral low-resistance grounding will be discussed by applying a step-by-step calculation procedure to an actual case, in order to properly size the grounding resistor, thereby limiting the fault current.

New ANN-Based Algorithms for Detecting HIFs in Multigrounded MV Networks

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Application of two new ANN-based algorithms for arcing high impedance fault (HIF) detection in multigrounded medium-voltage (MV) distribution networks is presented in this paper. The paper provides an evaluation of two new structures of artificial neural networks (ANNs) that may be used for reliable HIF detection in multigrounded as well as isolated, compensated, and grounded via small resistance distribution grids. The results obtained by use of both neural nets are presented. The performance was tested using data obtained from staged HIFs in real MV network as well as from Electromagnetic Transients Program–Alternative Transients Program simulations. A small number of necessary neurons in developed ANNs, short measuring sliding data window, and easy interpretation of obtained output signals are the main advantages of the proposed approach. Satisfactory results of ANN performance were observed for all examined HIF cases in which the ground fault current was greater than 16 A. The selected ANNs of best performance show high reliability and immunity to transients resulting from switching operations in protected feeders and from capacitor bank switching. </para>

Verification of the Wavelet-Based HIF Detecting Algorithm Performance in Solidly Grounded MV Networks

The application of the wavelet-based algorithm for arcing high-impedance fault detection medium-voltage (MV) distribution networks is presented in this paper. This paper describes continuation of research on HIF detection with particular reference to algorithm application in solidly grounded MV networks. The results obtained by use of the improved version of the algorithm are presented. The algorithm performance was tested using data obtained from staged HIGFET in a real MV network as well as from EMTP-ATP simulations. Satisfactory results of the algorithm performance were observed for all examined HIF cases in which the ground fault current was greater than 3 A root mean square. The improved algorithm also proved to be more immune to transients generated during switching operations in protected feeders and to capacitor bank switching.

Ground-fault fast transients in modern unit-connected generators

In this paper the ground-fault overvoltages in the circuits of unit-connected generators during interrupted arcing ground faults in the stator winding of the generator with the neutral grounded through the ground-fault neutraliser were analysed. Comparative simulation tests of ground-fault voltages and currents for units with the biggest difference in parameters at the neutralisation coefficient equal to 0.6 were carried out. The currents in all the branches of the equivalent scheme of the unit, the voltage drops on its particular elements and the overvoltage danger were determined. The real changes in the resistance of the ground-fault arc in the main insulation the stator windings and the real mechanism of its quenching were taken into account.

Ground Fault Protection for Bus-Connected Generators in an Interconnected 13.8-kV System

This paper is based on modifications implemented in the protection and grounding systems of a large paper mill and describes selective ground fault protection for a 13.8-kV system with multiple bus-connected generators, synchronous bus ties, and utility interconnections. The ground fault current in the system is reduced from the existing 3400 A to 500 A, and a hybrid grounding system is implemented for each of the generators. As the ground fault currents are reduced to limit the fault damage, the sensitivity and selectivity of the ground fault protection become important. Directional ground fault relays with coordinating pickup settings are applied to achieve this objective. The new platform for directional elements (numerical relays) drives its performance from sequence impedance measurements

Development of Arcing Horn Device for Interrupting Ground-Fault Current of 77 kV Overhead Lines

This paper describes an arcing horn device for interrupting the ground-fault current of 77-kV overhead lines. The main characteristics of the device are as follows. The arcing horn device interrupted the ground-fault current 445A within a half cycle without replacement ten times. The arcing horn device did not interrupt short-circuit current. However, even when a short-circuit current of 10 kA flowed through the interruption portion, the device did not explode and damage any surrounding apparatus, such as insulators and conductors. The arcing horn device flashovered perfectly in the interruption portion when a lightning impulse voltage of 1400 kV was imposed under dry, water-sprayed and contaminated conditions. The arcing horn device was installed on a 77-kV overhead line belonging to the KANSAI Electric Power Co, Osaka, Japan, and was found to interrupt the ground-fault current.

MV Generator Low-Resistance Grounding and Stator Ground Fault Damage

Most of the in-plant medium-voltage generators are grounded through resistors ranging from 200 A to 400 A. In some unusual situations, the resistors may even be as high as 1200 A. low-resistance grounding is a preferred choice for a medium-voltage power distribution system. However extensive generator stator ground fault damages had been reported due to the prolonging generator neutral ground current, which would continue to flow even after the main and field breaker opened. This ground fault current could continue to flow for as long as 5 seconds depending on the generator open-circuit time constant T'/sub do/. As a result, the lower the grounding resistor i.e. higher the generator neutral current, would lead to the higher stator ground fault damages. An IEEE Working Group has completed its study and made recommendations for medium-voltage generator grounding in a multiple source industrial environment. Based on the considerations of transient over-voltage, ground fault damages and ground fault protection, the working group suggests a few variations of grounding systems, which basically are low-resistance grounding systems during normal operating conditions, and the generator would either be on high-resistance grounded or be switched to a high-resistance grounded system from a normal low-resistance grounded system when a ground fault occurs in the generator stator. The purposes of this paper are (1) to examine the generator transient over-voltage and currents under the low-resistance ground fault conditions, and also to evaluate their corresponding stator ground fault damages, and (2) to establish an acceptable maximum system ground fault level. For comparison purpose, three versions of low-resistance grounding systems have been considered and they are: a low-resistance grounding system with a neutral breaker at the generator (neutral breaker system); a low-resistance grounding system with the generator neutral low resistor being switched to a high-resistor after a stator ground fault (hybrid system); and a low-resistance grounding system similar to the current practice (tradition system). The simulation study is conducted with the aid of the Electromagnetic Transient Program (EMTP). An experimental analog generator model is also used to verify the simulation results.

Protection of 6-kV TPP Auxiliary Grids with Two Neutral Ground Modes from Single-Phase Ground Faults

A 6-kV power plant auxiliary grid with low ground fault currents is considered, which can operate in modes with insulated or compensated neutral depending on the connection of the working or backup auxiliary supply element. A high-sensitivity zero-phase sequence current protection from single-phase ground faults is suggested, which operates on commercial-frequency capacitive ground fault current and is suitable for the two kinds of auxiliary grid.

Implementation of Fuzzy Logic Controller for HVDC Using Genetic Algorithm

This paper presents an optimal tuning method for Fuzzy Logic Controller (FLC) of current controller for HVDC using Genetic Algorithm (GA). GA is probabilistic search method based on genetics and evolution theory. The scaling factors of FLC are tuned by using real-time GA. The proposed tuning method is applied to the scaled-down HVDC simulator at Korea Electrotechnology Research Institute(KERI). Experimental result shows that disturbances are well-damped and the dynamic performances of FLC have the better responses than those of PI controller for small and large disturbances such as ULTC tap change, reference DC current change and DC ground fault.

Determination of the reduction factor for feeding cable lines consisting of three single-core cables

The paper presents an original analytical procedure for the determination of the ground fault current distribution in the cases when a feeding line is composed of three single-core cables. The procedure takes into account the existence of all three cable sheaths as the return path for the ground-fault current. The reduction factor for these types of cables as given by the cable manufacturer takes into consideration only the existence of the metal sheath of a faulted single-core cable. As a consequence of that, the estimations of the safety conditions (step and touch voltages) on the grounding systems of the supplied stations are too severe. For the high-voltage cables, a certain, sufficiently low reduction factor achieved by an increase of the sheath cross-section can be demanded from the manufacturer. In both cases, too conservative values for the reduction factor lead to unnecessary expenditures. The quantitative analysis performed in this paper shows that the reduction factor for three single-core cables belonging to the same line is significantly lower in comparison to the situation when each of these cables is treated separately.

Directional ground-fault indicator for high-resistance grounded systems

Locating ground faults is a difficult and challenging problem for low-voltage power systems that are ungrounded or have high-impedance grounding. Recent work in pilot signals has renewed efforts in developing fault location methodologies. This paper presents a method for directional ground-fault indication that utilizes the fundamental frequency voltages and currents. Although the ground-fault current is small and usually less than the load currents, the fault has zero-sequence components that distinguish it from the load. Signal processing techniques are used to identify and compare the fault signals to determine the fault direction. The process takes advantage of the currents flowing from the distributed grounding capacitance. An experimental microprocessor-based directional indicator unit is tested in an industrial power distribution system. Directional indication of ground faults is applied near tap-off branch circuit connections. Promising results from field test conducted in a harmonic-noisy setting are presented. Directional indicator units simplify the search process on large networks, thus reducing the time and effort necessary to locate and remove the fault, and thereby significantly reduces the probability of a second ground fault with its destructive currents.

Ground-fault currents in unit-connected generators with different elements grounding neutral

The results of the analysis of zero-sequence currents flowing in generator neutral and at ground-fault location during the generator normal operation and during ground-faults in the stator windings are presented. The influence of different methods of grounding the generator neutral on these currents in the primary circuits for different parameters of generators, transformers and additional capacitance to ground of the generator breakers was determined. This influence was analyzed for generators with ungrounded neutral, for generators with the resistance in neutral and for generators with ground-fault neutralizer (GFN) grounding the generator neutral. The analysis has been done for real resistance of the breakdown channel during ground-faults along the whole length of generator stator winding. It has been found that the applied method of grounding the generator neutral, the parameters of generator and transformer, the additional capacitance to ground of the generator breakers, level of fault resistance and ground-fault location have a substantial influence on currents flowing in the generator neutral and at ground-fault location in the breakdown channel in the main insulation of a generator stator winding.

Ground-Fault Currents in Unitconnected Generators with Different Elements Grounding Neutral

The results of the analysis of zero-sequence currents flowing in generator neutral and at ground-fault location during the generator normal operation and during ground faults in the stator windings are presented. The influence of different methods of grounding the generator neutral on these currents in the primary circuits for different parameters of generators, transformers, and additional capacitance to ground of the generator breakers was determined. This influence was analyzed for generators with ungrounded neutral, for generators with the resistance in neutral, and for generators with ground-fault neutralizer (GFN) grounding the generator neutral. The analysis has been done for real resistance of the breakdown channel during ground faults along the whole length of the generator stator winding. It has been found that the applied method of grounding the generator neutral, the parameters of generator and transformer, the additional capacitance to ground of the generator breakers, the level of fault resistance and ground-fault location have a substantial influence on currents flowing in the generator neutral and at ground-fault location in the breakdown channel in the main insulation of generator stator winding.

A Finite-Element Method Solution of the Zero-Sequence Impedance of Underground Pipe-Type Cable

In order to provide adequate circuit protection for underground pipe-type cable systems from ground fault currents, it is important to be able to determine the zero-sequence impedance of pipe-type cables. For better knowledge of the impedance, a more accurate method needs to be developed. In this paper, we present a numerical method for computing the zero-sequence impedance of a pipe-type cable. This method is developed based on a finite-element analysis. Special attention is paid to the nonlinear B-H characteristic of the steel pipe, and an iterative procedure is employed for determining the permeability varying in the steel pipe. To validate the numerical method presented, measurements are made for the zero-sequence impedance at different current levels. A good agreement is observed between the numerical results and the measurement data.

Analysis of very-high-resistance grounding in high-voltage longwall power systems

The application of very sensitive ground-fault protection in underground coal mines was demonstrated in the early 1980s for low- and medium-voltage utilization circuits (less than 1 kV), but its commercial application did not occur until the advent of high-voltage utilization circuits on longwalls in the late 1980s. With these high-voltage systems (greater than 1000 V), the Mine Safety and Health Administration initially required a maximum ground-fault resistor current limit of 3.75 A for 4160 V systems and 6.5 A for 2400 V systems in 101-c Petitions for Modification. However, more recent Petitions for Modification have been required to limit maximum ground-fault resistor currents to 1.0 A, or even 0.5 A. Standard practice in other industries generally requires high-resistance grounding to be designed so that the capacitive charging current of the system is less than or equal to the resistor current under a ground-fault condition. The intent of this practice is to prevent the system from developing some of the undesirable characteristics of an ungrounded system, such as overvoltages from inductive-capacitive resonance effects and intermittent ground faults. Shielded cables, which have significantly more capacitance than their unshielded counterparts, are required for high-voltage applications in the mining industry. Thus, with the long cable runs of a high-voltage longwall system, capacitive charging currents may exceed grounding-resistor currents under ground-fault conditions. An analysis of a typical 4160 V longwall power system that utilizes very-high-resistance grounding (grounding-resistor-current limit of 0.5 A) is performed to determine whether or not potential problems exist.

Behaviour of grounding loop with bentonite during a ground fault at an overhead line tower

The results of a single-phase short-circuit experiment at a 35 kV overhead line are presented. During the experiment, the grounding loops, backfilled with bentonite and waste drilling mud, were exposed to real ground fault currents. As a consequence of the increased thermal stresses of bentonite and waste drilling mud, only short time increases of the resistance of the grounding loops were registered. It is shown that, independently of the manner of the neutral point grounding in 35 kV and 110 kV networks, as a backfill material, bentonite retains its positive characteristics even after being exposed to real ground fault currents.