Impulse Grounding Research Articles

Suppression of the atmospheric over-voltages in grounding neutral conductor low-voltage grids

This study reports the issues related to over-voltages in low-voltage distribution networks characterised by high specific soil resistance. A detailed description of real case observed on GSM Transmitter Glavica – Vrgorac is reported. The transmitter is supplied via the distribution cable from SS 10/0.4 kV/kV Podprolog. Since specific soil resistance and corresponding transmitter grounding resistance are quite high, over-voltages were flowing towards the substation grounding system via the cable neutral conductor, instead through the transmitter grounding system. In this case, authors applied an innovative approach. The solution is based on the theory considering the impulse grounding behaviour rather than the classical static response. Instead of modelling the horizontal grounding electrode as Ohmic resistance, the electrode capacitance and inductance are incorporated (adopted Telegrapher's equation). The following solution arisen: approximately in the middle, 100 m long part of the existing over-head cable was replaced by the buried cable. Instead of the neutral conductor from the cable (which was disconnected), non-insulated Cu wires (2 × 50 mm2) were applied. The novelty is in the application of a non-insulated neutral conductor as the lightning grounding. The validity of solution is verified via extensive in situ measurements.

An EMTP Comprehensive Model of Tower Grounding and its Simulation Analysis

For the purpose of designing the grounding device of the poles and towers an improved EMTP model is put forward in this paper. The impulse characteristics of the tower grounding under impulse currents are simulated and analysed based on circuit theory. Only the non-linear effects caused by soil ionization are considered in the current studies. And the interactions deriving from the coexistence of segmented grounding electrodes have been neglected. Consequently, the calculated results of the impulse grounding resistance of the complex grounding device such as the tower grounding are not enough accurate using the traditional models. The soil ionization effects are simulated through the non-linear resistance. The current-controlled voltage sources are introduced in the model to take into account the interactions of the segmented grounding electrodes. Then the comprehensive model of the tower grounding is established. Comparison of simulation results with experimental data implies that the tower grounding model proposed in this paper is more accurate and effective than the traditional model.

Grounding Resistance Characteristics of Gravity Foundations of Offshore Wind Turbines in the Intertidal Zone

Compared with onshore wind turbines, offshore wind turbine develops very quickly in recent years due to less land cover, higher wind speed and steady wind direction. At present, gravity foundations of offshore wind turbines are widely applied in Rudong intertidal wind farm in China. However, up to now, few attentionsare paid to grounding of gravity foundations. In this paper, grounding resistance characteristics of gravity foundations of intertidal wind turbines were calculated and analyzed systematically. Various impact factors, including depth of seawater, resistivity and thickness of clay and gravel layers, and size and depth of pile pumped into ground, were considered. Analytical results show that grounding resistance is affected significantly by depth of seawater. If seawater depth is less than 0.3 m, power-frequency and impulse grounding resistances in intertidal zone of gravity foundation offshore turbine will, in some conditions, exceed limited value specified in standards. As seawater depth increases, both power-frequency and impulse grounding resistances decrease rapidly. Moreover, the two types of grounding resistances increase with increase of soil resistivity of clay and gravel layers, and decrease with increase of clay layerthickness, although these effects become weaker as depth of seawater increases. Structure size of gravity foundation, including its diameter and driven depth, has limited effect on resistances. These results are of great significancefor grounding design of gravity foundations of offshore wind turbines.

Wavelet-based Analysis of Impulse Grounding Resistance–Experimental Study of the “A”-type Grounding System

This article presents the results of experimental investigation concerned with the “A”-type grounding system's impulse characteristics, which are utilized for grounding of high-voltage transmission line towers. The investigation results point to the presence of high-frequency components in voltage and current signals that are not caused by tower footing response to the impulse current. Additionally, both surge generator coupling and measuring circuit remain their main source. High-frequency components of signals are extracted from voltage and current signals by means of the discrete wavelet transform. In addition to being external influence results, the extracted components have very low energy content as well as short duration. The discrete wavelet transform analysis application has validated the existence of external influences.

Nonlinear impulse current spreading and electrical breakdown in soil

The results of laboratory experiments on the impulse current spreading from the spherical electrodes and the breakdown occurring in quartz sand of various humidity at the impulse voltages of 20–40 kV are presented. It is shown that, at the current densities on the electrode above some critical value, a sharp nonlinear decrease of the impulse grounding resistance occurs, an ionization-superheating instability develops, and thus the current contraction takes place and a plasma channel in the soil occurs. A method is proposed to determine the critical strength of the ionization electric field. It was discovered that, at long discharge gaps in humid sand, the breakdown develops with a long time delay, similarly to thermal breakdown.

Monte Carlo method for estimating backflashover rates on high voltage transmission lines

This paper presents a novel Monte-Carlo based model for the analysis of backflashover rate (BFOR) on high voltage transmission lines. The proposed model aims to take into the account following aspects of the BFOR phenomenon: transmission line (TL) route keraunic level(s), statistical depiction of lightning-current parameters (including statistical correlation), electrogeometric model of lightning attachment, frequency-dependence of TL parameters and electromagnetic coupling effects, tower geometry and surge impedance, tower grounding impulse impedance (with soil ionization), lightning-surge reflections from adjacent towers, non-linearity of the insulator strings flashover characteristic, distribution of lightning strokes along the TL span and power frequency voltage. In the analysis of the BFOR, special attention is given to the influences emanating from the insulator strings flashover characteristic and lightning statistics. The model could be applied to the transmission line as a whole or some of its portions, e.g. first several towers emanating from the substation or several towers crossing a mountain ridge.

The Research on the Impulse Characteristics of the Extended Grounding Electrode in the Layered Soil

The composition of the tower's grounding system is always with several layers, for single-layer structure of the tower in high soil resistivity area, it is the use of partial replacement of soil to reduce the tower grounding resistance, also the analysis object is equivalent to the hierarchical model, Therefore, the study of layered soil structure tower grounding impulse characteristics is more representative. To study the impact properties of the layered structure of the soil grounding, In this paper, through the establishment a double-layer soil model structure, the impulse characteristics of the soil structure of grounding electrode is studied through simulation, resulting of the quantitative relationship between several high resistivity soils partially replacing low resistivity soil area size and impulse grounding resistance of elongation grounding. The results show that the impulse grounding resistance decreases as the replacement area of low resistivity soil increases and converges to a certain value, which indicates the impulse characteristics of extended grounding electrode in double-layer soil model increases with the expand of the upper level low resistivity zone and approaching that of single layer with low resistivity soil. The results have a reference value for use of local soil replacement method to reduce tower impulse grounding resistance in high soil resistivity areas and the determination of soil replacement plan.

Conformation Optimization Based on Impulse Grounding Resistance for Ground Electrode of Power Lines

Reducing tower ground’s impulse grounding resistance can prevent the power lines from lightning effectively. The concept of impulse grounding resistance is described. Calculation formula of the impulse grounding resistance for ground electrode in different Conformations is derived with impulse coefficient method. The conformation of the ground electrode is improved according to the theory of impulse grounding resistance is decreased when the effective area of the electrode is reduced, using ETAP for simulation of the improved ground electrode. The results show that the impulse grounding resistance of the improved ground electrode can be reduced to 41.1% of the original, and safety margin is larger in practical engineering.

Research of Lightning Over-Voltage and Simulation Program of Substation Based on EMTP

High voltage substation is the junction station of electrical power system. In order to conduct lightning over-voltage simulation and analysation of couple of substations rapidly and intuitively, a serialization supposed simulation program of substation is developed based on the research of EMTP kernel and data format. Model of impulse grounding resistance and improved leader progression is included in the program, which enhance the accuracy as much as possible and satisfying the demand for emulation of different type and voltage level. Through the simulation of some 500kV high voltage substation, the influence to lightning over-voltage of different system operating plan and lightning strike spots is analyzed, which verifies the validity and convenience of the program.

Measurement of Impulse Grounding Resistance

A kind of measurement method of impulse grounding resistance is proposed. The measurement method of impulse grounding resistance is also analyzed and discussed theoretically. The hardware and software scheme of measurement system is given. The method of simulation of thunder current and transformation calculation is better than the method of work-frequency measurement by the analysis and comparison. By using the method, the thundering course can not only be simulated , but also the impulse grounding resistance can be calculated directly in the scene. In the end, a simulation result is given. Simulation result show that the method proposed in the paper is effective and feasible.

Lightning Impulse Performances of Grounding Devices Covered With Low-Resistivity Materials

The low-resistivity material (LRM) is widely used to decrease the power-frequency grounding resistances of grounding devices for transmission lines in the regions with high soil resistivity. The study about the influence of the LRM on impulse performances of grounding devices cannot be found in literature. Experimental results of lightning impulse properties of grounding devices covered with LRM were presented in this paper. The influences of impulse current, geometrical dimensions of grounding devices, and soil resistivity on impulse grounding resistance and impulse coefficient of grounding devices with the LRM coverings are still the same with those without the LRM coverings. The impulse grounding resistance decreases from about 25% to 45% when the LRM covering is used. The fitting formulae to calculate the reducing ratio of impulse grounding resistance of different grounding devices are provided. On the other hand, the influence of LRM on the effective impulse length of grounding electrodes is discussed.

Laboratory investigation of impulse characteristics of transmission tower grounding devices

Reducing grounding resistance of a transmission tower grounding device is an important method to improve performance of transmission lines against lightning. Principles of impulse characteristics of grounding devices were discussed. Impulse experiments using grounding device models were systematically performed to analyze the influence of different factors on the impulse characteristics of grounding devices. During the experiments, soil resistivity was changed in the range of 100 to 5103 /spl Omega//spl middot/m. Effects of different parameters on impulse grounding resistance and impulse coefficients of different transmission tower grounding devices were discussed. Formulae to calculate impulse coefficients and power frequency grounding resistance of different grounding devices were obtained.

Influence of transmission line tower grounding impedance to the line flashover rate

AbstractThe paper presents a mathematical model for analysing impulse characteristics of complex grounding grids in uniform and two‐layer soil. The model is based on the grounding grid equivalent circuit with lumped parameters, described by a system of differential equations. The effect of the mutual couplings of the grounding grid elements can be comprised by the so‐formed model. The established model has been successfully checked experimentally for the square grounding grid. A grapho‐analytical approach to the evaluation of the transmission line tower impulse grounding impedance model is described. The proposed model is convenient for practical application. It is incorporated into the present lightning overvoltage computation program. The methodology of the line flashover rate estimation is illustrated at the example of 400‐kV overhead lines.