Transient Grounding Impedance Research Articles

Numerical simulation and analytical calculation of the GIS transient enclosure voltage interaction with grounding system

Gas-insulated substations (GISs) are widely used in power systems due to high reliability, convenient maintenance, and small area occupation. Transient enclosure voltage (TEV) is a consequent effect of very fast transient overvoltage (VFTO) during the switching operations in GISs. It can lead to insulation degradation, damage to the secondary equipment connected with the shell of GIS, and harm to the working staff. This paper presents a correlation between a lumped-circuit model based on transmission line theory (TLT) for purpose of the simulation and an analytical approach based on the scattering matrix for the evaluation of TEVs. Furthermore, a frequency-dependent model is proposed to quantify the transient grounding impedance (TGIM) against the waves with high-frequency content such as TEVs. The interaction between the TEV and the grounding system has been investigated analytically, and experimentally based on the field measurements in other efforts. The comparisons indicate that the modified wide-band TGIM results in an improvement in comparison with the other works to track both the front and tail of a measured voltage-wave within acceptable accuracy. Furthermore, the role of the ground strap as a damping structure using a modified model on TEV profile has been scrutinized regarding the grounding system model and the rising rate of TEVs. It is indicated that considering frequency-dependent TGIM could lead to a 25% variation in the estimation of amplitude of TEVs.

Analysis of Transient Ground Impedances Based on Soil Ionization under Impulse Voltage

In this paper, the behavior of transient ground impedances in a coaxial cylindrical electrode system stressed by impulse voltages were experimentally investigated. The ground impedance-time characteristics and the voltage-current behavior for 4 types of soils were analyzed and discussed. Also, this paper modeled the electrode system into two-dimensional axial symmetry and analyzed the soil ionization growth and electric field distribution in the vicinity of the surface of the ground electrode. As a result, the ground impedance-time (Z-t) curves are dependent on the types of soils. The voltage-current (V-I ) curves for all sample soils show a ‘cross-closed loop’ of ∞-shape under ionization. When an impulse voltage is applied to the ground electrode buried in the ground, the electric field intensity around the ground electrode changes over time. It is known that transient ground impedance is dependent on the time-varying soil ionization in the vicinity of the surface of a ground electrode.

Influence of the frequency‐dependent characteristics of soil permittivity on the impulse performance of grounding electrode

In the analysis of impulse characteristics, the accuracy of the calculation results depends largely on the soil electrical parameters. However, existing calculation methods simplify or ignore the influence of the frequency‐dependent characteristics of soil permittivity on impulse performance. On the basis of the dielectric spectrum measurement results of typical soil, a finite element model that considers the frequency‐dependent characteristics of soil permittivity is developed in this paper. The influence of the frequency‐dependent characteristics of soil permittivity on impulse performance is analyzed systematically. Results indicate that a significant difference exists when the permittivity is constant. The transient grounding impedance value is highly influenced by the impulse current waveforms and the amplitude of the impulse current. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Transient grounding impedance and transient resistivity measurements using a very short current lead

This work describes a system for measurements of the transient impedance of grounding electrodes using a portable impulse generator and a new arrangement of test leads. The major innovation is the use of a physically short current reaction lead, using a transmission line with very low propagation velocity (2% of the velocity of light). The methodology for obtaining the transient impedance from the measurements is discussed and validated in field studies using a 2.4m long vertical rod. A comparison with calculated values is also presented. Additionally, the influence of the length of the potential lead is investigated using the very short current lead and the transient soil resistivity is also calculated.

침상 접지봉의 임펄스접지임피던스에 미치는 지중방전의 영향

Soil discharges near the ground rod play an important role to reduce the ground potential rise and the ground impedance and to help the fault current to spread into the earth. This paper presents the effects of soil discharges on the transient and conventional ground impedances when the lightning impulse voltage was applied to a ground rod with radial needles. The current-voltage (I-V)curves and transient ground impedance curves were calculated based on the measured current and potential traces. Soil discharge behaviors related to I-V curves and transient ground impedance curves were analyzed as a function of the magnitude of lightning impulse voltages. As a result, the soil discharges occurred near the ground electrode contribute to the reduction of conventional ground impedance and limits the ground potential rise effectively under lightning impulse voltages.

Grounding System Transient Characteristics of Underground GIS Substations

The transient grounding impedance of a gas-insulated switchgear (GIS) substation was investigated through measurements on a commercial 300-kV underground substation. Based on the results, the macroscopic transient impedance of the grounding grid was clarified to be the inductance component and estimated at approximately 0.2 μ H. In the low-frequency region where the frequency is almost zero or so, the impedance is shown to be lower than the general concrete resistance value by one digit due to the complexity of grounding systems specific to underground substations. Hence, the simple model of the grounding grid can be expressed mainly by inductance. The detailed model was expressed by using a π-type equivalent circuit, where each circuit constant was calculated using the specific low resistivity of concrete. In addition, the validity of both models was confirmed by comparing the measurement results. The proposal models are applicable to the Electromagnetic Transients Program analysis on the surge phenomena within the GIS grounding system.

배전계통에 사용되는 봉형 접지전극의 접지 특성 분석

Grounding system insures a reference potential point for electric devices and also provides a low impedance path for fault currents or transient currents in the earth. The ground impedance as function of frequency is necessary for determining its performance since fault currents could contain a wide range of frequencies. In this paper, the grounding resistance, grounding impedance and transient grounding impedance are measured by using 3-point fall-of-potential method in order to analyse grounding characteristics of the copper and concrete rod grounding electrodes. An equivalent transfer function model of the ground impedance and transient grounding impedance are identified from the measured values by using ARMA method and evaluated by comparing the conventional grounding impedance.

탄소혼합물 접지전극의 접지임피던스의 특성

This paper presents characteristics of frequency-dependent grounding impedance and transient grounding impedance for the carbon compound grounding electrode used in the installation of computerized electronic equipment and lightning protection system. The frequency-dependent grounding impedances were measured by applying sinusoidal currents in the frequency range from 100 [Hz] to 10[MHz], and the transient grounding impedances were examined by subjecting the impulse current with the front-time between 1~80[]. As a result, the ground resistance of the carbon compound grounding electrode is less than that of another type grounding electrodes. The transient grounding impedance is relatively low and the conventional grounding impedance is rather lower than the ground resistance. The frequency-dependent grounding impedance of the carbon compound grounding electrode is capacitive and the grounding impedance is decreased with increasing the frequency of injected currents. Therefore in the case that the carbon compound grounding electrode is jointly used with large-scaled grounding electrodes, it is possible to reduce the high frequency grounding impedance of the integrated grounding electrode system.

배전전주용 스테인리스강판 접지전극의 접지 특성분석

접지시스템은 전기설비의 기준 전위점을 확보할뿐만 아니라 낮은 대지저항으로 고장전류나 과도 전류를 흐르게 한다. 접지임피던스는 주파수에 대한 함수로 고장이나 과도전류가 넓은 범위의 주파수 성분을 포함하기 때문에 접지성능을 평가하는데 중요한 역할을 한다. 본 논문에서는 스테인리스강판 접지전극의 접지특성을 파악하기 위하여 3점 전위강하법으로 접지저항, 접지임피던스와 과도접지임피던스를 측정하였다. 측정된 데이터를 이용하여 접지임피던스와 과도접지임피던스의 등가 전달함수 모델은 ARMA 기법으로 모델링 되었고, 규약접지임피던스로 비교하여 평가하였다.

Analysis for the Grounding Impedance of Vertical Grounding Electrodes using the Distributed Parameter Circuit Model

A grounding electrode has the transient grounding impedance characteristics against lightning surges. So the performance of grounding electrodes should be evaluated as a grounding impedance as well as the ground resistance. The frequency-dependent grounding impedance is varied with the shape and size of grounding electrode and is divided into both inductive and capacitive behaviors. This paper presents a theoretical analysis for the grounding impedance determined by the size of grounding electrode using the distributed parameter circuit model. EMTP and Matlab programs were used in calculating the frequency-dependent grounding impedances of vertical grounding electrodes. It was found that the frequency-dependent grounding characteristics of vertical grounding electrodes are characterized by the distributed parameters which are changed in the dimension of grounding electrodes.

매설지선의 길이와 전류인가점에 따른 접지임피던스특성

본 논문은 임펄스전류가 인가된 매설지선의 과도접지임피던스특성에 관한 것으로 길이가 10, 30, 50 [m]인 3개의 매설지선에 대하여 임펄스전류의 인가점을 파라미터로 하여 과도접지임피던스와 규약접지임피던스를 측정하고 분석하였다. 임펄스전류의 상승시간에 대한 규약접지임피던스는 과도접지임피던스와 비슷한 경향이었다. 과도접지임피던스는 임펄스전류의 인가위치에 영향을 크게 받으며, 짧은 시간영역에서 과도접지임피던스는 매설지선의 길이에 매우 의존적이다. 임펄스전류가 인가된 10 [m]의 매설지선은 용량성 특성이었으나 30 [m]와 50 [m]의 매설지선은 유도성 특성을 나타내었다. 접지도선은 매설지선의 중앙점에 접속하는 것이 바람직한 것으로 밝혀졌다.

임펄스전류의 인가위치에 따른 심매실 봉상 접지전극의 과도접지임피던스 특선

본 논문은 임펄스전류의 인가위치에 따른 심매설 봉상 접지전극의 과도접지임피던스와 규약접지임피던스 특성에 관하여 기술하였다. 과도접지임피던스는 심매설 접지전극의 길이 및 임펄스전류의 상승시간과 인가위치에 강하게 의존하며, 접지전극의 자체 인덕턴스는 짧은 시간 영역에서 접지시스템의 과도접지임피던스에 크게 영향을 미치는 것으로 나타났다. 심매설 봉상 접지전극의 자체 인덕턴스의 저감은 과도접지임피던스 특성의 개선에 결정적인 역할을 한다. 접지도선의 길이는 최대한 짧게 하여 접지전극의 상단에 접속하는 것이 효과적이다.

가변주파수 및 임펄스 전류를 이용한 과도접지임피던스 분석

접지전극의 형태, 크기, 재질 및 임펄스전류의 형상에 따라 변화하는 과도접지임피던스는 접지시스템의 성능에 중요한 영향을 미친다. 본 논문은 기존의 실효임펄스임피던스가 실제 과도접지임피던스를 얼마나 효과적으로 나타내고 있는가에 대한 분석과 보다 효과적인 과도접지임피던스 평가방법에 대하여 실험적인 결과를 제시하였다. 실험에 사용된 접지전극은 저항성분과 인덕턴스를 포함한 모의 접지전극과 1[m] 탄소접지봉, 9[m] 일반동봉 그리고 40[m] 매설지선이며, 가변주파수 전류와 임펄스전류에 의한 접지임피던스를 각각 측정하였다. 각각의 값을 비교 했을 때 기존의 실효임펄스임피던스의 값보다는 전압의 최대값과 전류의 최대값을 이용하여 과도접지임피던스를 구한 임피던스 값(Z₁)이 등가주파수에 해당하는 가변주파수 전류에 의해 구해진 접지임피던스에 근접함을 확인하였다.

Influence of strike object grounding on close lightning electric fields

Using the finite difference time domain (FDTD) method, we have calculated vertical electric field Ez, horizontal (radial) electric field Eh, and azimuthal magnetic field Hϕ produced on the ground surface by lightning strikes to 160‐m‐ and a 553‐m‐high conical strike objects representing the Peissenberg tower (Germany) and the CN Tower (Canada), respectively. The fields were computed for a typical subsequent stroke at distances d′ from the bottom of the object ranging from 5 to 100 m for the 160‐m tower and from 10 to 300 m for the 553‐m tower. Grounding of the 160‐m object was assumed to be accomplished by its underground basement represented by a 10‐m‐radius and 8‐m‐long perfectly conducting cylinder with or without a reference ground plane located 2 m below. The reference ground plane simulates, to some extent, a higher‐conducting ground layer that is expected to exist below the water table. The configuration without reference ground plane actually means that this plane is present, but is located at an infinitely large depth. Grounding of the 553‐m object was modeled in a similar manner but in the absence of reference ground plane only. In all cases considered, waveforms of Eh and Hϕ are not much influenced by the presence of strike object, while waveforms of Ez are. Waveforms of Ez are essentially unipolar (as they are in the absence of strike object) when the ground conductivity σ is 10 mS/m (the equivalent transient grounding impedance is several ohms) or greater. Thus, for the CN Tower, for which σ ≥ 10 mS/m, the occurrence of Ez polarity change is highly unlikely. For the 160‐m tower and for σ = 1 and 0.1 mS/m, waveforms of Ez become bipolar (exhibit polarity change) at d′ ≤ 10 m and d′ ≤ 50 m, respectively, regardless of the presence of the reference ground plane. The corresponding equivalent transient grounding impedances are about 30 and 50 Ω in the absence of the reference ground plane and smaller than 10 Ω in the presence of the reference ground plane. The source of opposite polarity Ez is the potential rise at the object base (at the air/ground interface) relative to the reference ground plane. For a given grounding electrode geometry, the strength of this source increases with decreasing σ, provided that the grounding impedance is linear. Potential rises at the strike object base for σ = 1 and 0.1 mS/m are some hundreds of kilovolts, which is sufficient to produce electrical breakdown from relatively sharp edges of the basement over a distance of several meters (or more) along the ground surface. The resultant ground surface arcs will serve to reduce the equivalent grounding impedance and, hence, potential rise. Therefore, the polarity change of Ez near the Peissenberg tower, for which σ is probably about 1 mS/m, should be a rare phenomenon, if it occurs at all. The equivalent transient grounding impedance of the cylindrical basement is similar to that of a hemispherical grounding electrode of the same radius. For the 160‐m tower and for hemispherical grounding electrode, the transient grounding impedance is higher than its dc grounding resistance for σ = 10 and 1 mS/m, but lower for σ = 0.1 mS/m. For the 553‐m tower, the transient grounding impedance of hemispherical electrode is equal to or larger than its dc resistance for all values of σ considered.

임펄스전압에 의한 수증방전특성의 분석

이 논문은 반구형 수조에 설치되어 있는 침전극과 구형전극에 표준뇌임펄스전압을 인가하였을 때 나타나는 수중 방전현상과 절연파괴특성을 나타낸다. 이 논문의 목적은 뇌서지에 대한 과도접지임피던스와 관련된 기본적인 특성을 파악하는데 있다. 인가전압의 극성과 물의 저항률에 따른 방전광을 촬영하였고 절연파괴전압의 의존성을 측정하였다. 침전극과 구형전극의 끝단에서 스트리머코로나가 발생하였고 접지된 수조를 향하여 단계적으로 진전하였다. 저항률에 따른 절연파괴전압은 V자 형태를 나타내며, 구형전극의 절연파괴전압-시간곡선이 침전극보다 높게 나타났다.

인하도선의 길이가 과도접지임피던스에 미치는 영항

본 논문은 인하도선의 설치에 따른 접지시스템의 임펄스전류에 대한 과도임피던스 특성을 다루었다. 임펄스전류를 심매설 봉상 접지전극과 그리드 접지전극에 인가하였을 때 임펄스전류의 상승시간과 인하도선의 길이에 따른 전위상승과 실효임펄스접지임피던스를 측정하고 분석하였다. 과도접지임피던스는 접지전극의 형상과 크기 및 임펄스전류의 파형과 인하도선의 길이에 강하게 의존하며, 인덕턴스가 최소화되도록 인하도선을 설치하는 기법이 과도 접지임피던스의 저감을 위한 중요한 요소이다.

접지전극의 형상에 따른 과도접지임피던스

This paper presents the transient behaviors of grounding system impedance against lightning impulse currents. The dynamic characteristics of the deeply driven ground rod and grounding grid subjected to the impulse current were investigated. The transient ground impedance strongly depends on the configuration and size of grounding electrodes and the shapes of impulse currents, and the inductance of grounding electrodes has a significant affect on the transient ground impedance of grounding system Also, the deeply-driven ground rod was effective to reduce the transient ground impedance in time domain of miceroseconds.

A theoretical study on the consequence of a direct lightning strike to electrified railway system in Sweden

Direct lightning strike to a single-track electrified railway system in Sweden is modeled in this paper. Using this model, the induced voltages in each of the nine conductors at heights varying from 0.5 m (tracks) to 10 m above the ground are estimated. The effect of the finitely conducting ground is included using a time domain expression for the transient ground impedance that has better early time and late time behavior. The main interconnection between the conductors and the flashover strength of the supporting insulators is included in the simulations. A simple model for the arc channel during flashover of the insulators and the ionization of the soil around the pole foundations is also included in the model to assess the possible realistic surge voltage distribution in the system. It is shown in the paper that finite ground conductivity, interconnections between the conductors, arcing phenomena of insulation flashover and grounding of the poles decide the voltage/current distribution in the conductors. Simulations have been also carried out to determine the voltages on the lines and across the rails as function of distance from the point of strike as it could be a necessary data for deciding the possible future protection schemes. It was found that for a lightning stroke of 31 kA peak, large common mode and differential mode surges exist on the lines which could create excessive voltages between the line and neutral of the transformer and might pose a threat to the various low voltage equipments used for telecommunication, signaling and control.

봉상 접지전극에시 전류유입위치에 따른 임펄스 접지임피던스의 특성

본 논문은 전류유입위치에 따른 봉상접지전극의 과도 및 실효임펄스임피던스의 특성에 관한 것으로 뇌격전류를 인가하여 실규모 접지봉에 대한 시간영역에서의 성능을 평가하였다. 임펄스전류가 가해진 봉상접지전극의 과도 접지임피던스는 접지저항보다 높게 나타났으며, 접지전극의 길이가 길어짐에 따라 접지저항과 실효임펄스접지임피던스는 감소되었다. 또한 실효임펄스접지임피던스는 짧은 시간범위에서는 급격하게 증가하였다. 접지저항의 저감은 접지시스템의 임펄스임피던스 특성의 개선에 결정적인 역할을 한다. 임펄스전류를 접지봉의 하단에 인가하였을 때 접지봉 전위의 파두부에 고주파의 진동 파형이 포함되고 실효임펄스임피던스는 다른 경우보다 높게 나타났다.

On the influence of conductor heights and lossy ground in multi-conductor transmission lines for lightning interaction studies in railway overhead traction systems

Railway overhead traction system is a classic example of scattered conductor configuration, where tracks and other wires form multi-conductor transmission lines (MTLs) with large variation in conductor heights above ground and they are spread across regions having different soil conditions. Lightning transient analysis in such systems has not received much attention earlier. Here we analyze the influence of conductor heights and lossy ground on the induced voltages in a two conductor MTLs for the case of a direct lightning strike. For transient analysis, modified time domain transient ground impedance expressions having better early and late time behavior was used. The dependence of transient ground impedance on conductor heights and ground resistivity are presented and discussed. The early time transient ground impedances are unaffected by ground resistivity but their decaying nature is highly dominated by ground resistivity. It is found, if one of the conductors is close to ground (a rail) and if it is at large vertical distance from struck conductor (an auxiliary power line), then with increasing ground resistivity the peak induced voltages in the conductor close to ground initially increase, then decrease and finally tend to remain constant (within 100–10,000 Ω m). This phenomenon is opposite to that compared to conductors that are close to each other with minimum vertical separation (two auxiliary power lines), where the peak induced voltages increase with increasing ground resistivity. The study focuses mainly to access when a mutual coupling due to system geometry or due to ground losses becomes dominant in determining induced effects from lightning in MTLs, which could be an important contribution to the lightning interaction studies for electrified railway systems.