High Voltage Direct Current Ground Research Articles

The Influence of Relative Position on Ground Potential and Electric Field Based on Interfacial Charge Accumulation in HVDC

AbstractWith the development of high‐voltage direct current (HVDC) engineering around the world, there has been increasing attention to the issue of selecting site for direct current (DC) grounding electrodes since the pollution coming from ground potential and electric field. As previous study result, the distance is closer to the grounding electrode, the influence is greater for ground potential and electric field, as well as vice versa. Through proposing the analysis approach of equivalent interfacial charge accumulating, the article goes beyond this previous general‐conclusion and extracts the relative position conception from the mono‐distance for further studying on weakening their impacts on the environment. In comparison with the previous results, the research finding shows that the movement of relative position is able to achieve the best distribution of ground potential or electric field corresponding to different soil‐layered structure, furthermore, the specific mathematic formula about the best relative position has been derived by the accumulated charge model proposed. Finally, the results have been demonstrated by simulation calculation of the CDEGS software for a practical HVDC project in China, moreover, regarding soil resistivity variation related to the different seasons, the optimized relative position about 13 ~ 24 km has been adopted in the real HVDC grounding project as a suggested site. It reveals that the underlying relative position behind the mono‐distance could be applied to selecting the site of grounding electrode as reference in the future. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

Small-signal stability analysis of the asymmetrical DC operation in HVDC networks

• Stability of interconnection of monopolar and bipolar HVDC stations is analysed. • A small-signal stability model for asymmetric DC operation studies is developed. • The model is validated with EMT dynamic simulation. • The findings are useful for systematic design of the HVDC grounding systems. • A controller is designed to enhance stability during asymmetric DC operation. The increase in High Voltage Direct Current (HVDC) projects favors their future interconnection to form regional networks, but this interconnection can be hindered by the lack of standardization in station's configuration and converter's differing grounding schemes. Whilst other studies have studied the effect of Direct Current (DC) asymmetrical operation, they are limited in their scope. This paper proposes a more realistic perspective, considering stations with different topologies, permanent DC asymmetries and the influence of the grounding systems. This more complex point of departure requires the re-examination of the modeling methods, from time-domain to frequency-domain. This paper proposes a small-signal stability model of the system, which is validated with the electromagnetic transient (EMT) dynamic simulations using PowerFactory. This model provides guidelines for designing the grounding impedance and allows the proper design of a specific controller to dampen the asymmetrical DC operation mode.

Coupled flow in saturated soil around a vertical grounding rod HVDC considering underground water's action

• Coupled fluxes in soils due to current injection into HVDC ground rods are analyzed. • Phenomenon involving the fields of electromagnetism, thermal and hydraulics. • It takes into account the effect of an underground flow. • Results include temperature, velocity and pore pressure in a stratified saturated soil. The phenomenon of coupled fluxes in soils given the injection of current in HVDC (High Voltage Direct Current) ground rods is a multiphysics problem, involving different fields, including electromagnetism, thermal and hydraulics. In some regions where precipitation is intense or due to the presence of an aquifer, the saturated condition of the soil is particularly important for the analysis of the behavior of the environment, based on the active flows. In this study, the water velocity around the ground rod is investigated, as well as the pore pressure and temperature in a transient regime, considering the Joule effect, the performance of the electro-osmotic flow and the action of the underground velocity. Such phenomena are involved by temperature-dependent parameters. The study methodology is based on simulations adopting the finite element method for the coupling of physics in a saturated and stratified soil. The validation of the results for the Joule effect shows a maximum temperature error in the order of 3.3%. As a result, it was possible to infer, through a parametric sweep, the parameters that most favor the increase in temperature, velocity and pore pressure in the medium porous. The results presented serve as a reference for HVDC grounding system designs.

Dynamic physical characteristics of DC arc on arcing horn for HVDC grounding electrode line

The dynamic physical characteristics of a DC arc on an arcing horn for a high voltage direct current (HVDC) grounding electrode line are significantly different from those of the switching device arc, secondary arc, AC fault arc and pantograph-catenary arc. In this work, an experimental platform for the DC arc on the arcing horn was built, and mechanisms of the arc column short circuit and arc root movement were studied. This work further analyzes the characteristics and mechanisms of the arc motion when wind speed and direction, magnetic field and the expansion angle of the electrode are varied. Arc root movement is more likely to occur at the upper electrode. There is a competitive relationship between arc expansion and the transferring effect. The effect of wind on the arc column is greater than the effect on the arc root. The magnetic field has a significant driving effect on both the arc column and the arc root. The research results provide a comprehensive experimental basis for further probing the method of DC arc suppression, and the improvement of the arcing horn.

Study on the Interference Distribution Characteristics of the HVDC Grounding Electrode Current with Buried Pipelines Based on MoM and FEM

It is very important to calculate the interference of high voltage direct current (HVDC) grounding current with pipelines accurately and take proper protective measures to ensure energy transmission safety. There is still a lack of systematic research into related prediction methods that consider the nonlinear polarization on the interface between the soil and pipelines. In this paper, a methodology is proposed for calculating the coupling voltage and current on buried pipelines induced by grounding currents that accounts for nonlinear polarization based on the method of moments (MoM) and the finite element method (FEM). The validity of the proposed mathematical model is verified by a scale experiment. The interference distribution characteristics under different parameters are analyzed using the proposed method. The results show that the relationship between the maximum leakage current density and the electrode grounding current satisfy the linear function. The corrosion area becomes more concentrated as the grounding current increases. The corrosion area range increases substantially as the distance increases, whereas the influence degree decreases substantially. Both the maximum leakage current density and corrosion risk area are positively correlated with the soil resistivity.

Measurement and Safety Criteria of Step Voltage of High Voltage Direct Current Grounding Electrode

Grounding electrode is an indispensable part in HVDC transmission projects. During its design and safety evaluation, step voltage is a crucial aspect. By reviewing and sorting out the present standards involving step voltage measurement and its safety limit, this paper illustrates the contradiction between the measurement methods and the safety evaluation. Reasons for the contradiction are explained by analyzing the principal of step voltage measurement and safety evaluation. The Open-circuit step voltage measurement method and the measurement method with a human body equivalent resistance paralleled are compared to find out their advantages and disadvantages. Suitable circumstances and corresponding safety criteria of these two methods are put forward. Method proposed in this paper can be applied to step voltage measurement and safety evaluation of HVDC grounding electrode, providing reference when revising the related standards.

A Review about the Modeling and Simulation of Electro-Quasistatic Fields in HVDC Cable Systems

In comparison to high-voltage alternating current (HVAC) cable systems, high-voltage direct current (HVDC) systems have several advantages, e.g., the transmitted power or long-distance transmission. The insulating materials feature a non-linear dependency on the electric field and the temperature. Applying a constant voltage, space charges accumulate in the insulation and yield a slowly time-varying electric field. As a complement to measurements, numerical simulations are used to obtain the electric field distribution inside the insulation. The simulation results can be used to design HVDC cable components such that possible failure can be avoided. This work is a review about the simulation of the time-varying electric field in HVDC cable components, using conductivity-based cable models. The effective mechanisms and descriptions of charge movement result in different conductivity models. The corresponding simulation results of the models are compared against measurements and analytic approximations. Different numerical techniques show variations of the accuracy and the computation time that are compared. Coupled electro-thermal field simulations are applied to consider the environment and its effect on the resulting electric field distribution. A special case of an electro-quasistatic field describes the drying process of soil, resulting from the temperature and electric field. The effect of electro-osmosis at HVDC ground electrodes is considered within this model.

Calculation model for the earth potential of HVDC ground electrode based on image recognition of surface-layer soil moisture

Changes in soil electrical parameters can affect the distribution of earth potential in high-voltage direct-current (HVDC) ground electrodes when climatic conditions changes. This paper proposes a model to describe the relationship between surface-layer soil moisture and surface-layer soil resistivity under short-term climatic influence, by using image recognition technology. Based on the relatively stable resistivity of lower soil layers, a soil model more reflective of the actual operating conditions is established for soils near the ground electrode, and a finite element method is adopted to calculate the earth-surface potential (ESP). The experimental results indicate the following: (1) Compared with other measurement methods, image recognition of surface soil resistivity is a low-cost, real-time, online, and accurate method; and (2) changes in surface-layer soil moisture affect both ESP and step voltage. These effects are large in the case of high resistivity for the soil layer where the ground electrode is buried. This large fluctuation in step voltage particularly results in a potential safety hazards during ground electrode operation. Therefore, in order to ensure personal safety and obtain more accurate electrical parameters, it is necessary to consider the effect of natural climate on the soil surface resistivity. Finally, the value of the step voltage can be observed using image recognition, this also provides a new method for the safety monitoring of the DC ground electrode.

Temperature characteristics and influence of water-saturated soil resistivity on the HVDC grounding electrode temperature rise

The temperature rise process has always been the main factor for the safe operation of high-voltage direct current (HVDC) grounding electrodes, and the soil resistivity is the key parameter affecting the temperature rise of the grounding electrode. In some region where rainfall is high or groundwater is abundant, the soil moisture content at the electrode site may be very high or saturated. Therefore, studying the temperature characteristics of soils with saturated water content (water-saturated soil) for grounding electrode heating is particularly important. In this study, the resistivity measurement of temperature rise was conducted on different types of water-saturated soil. On the basis of this measurement result, an electro-thermal coupling finite element model of grounding electrode in water-saturated soil was established. Then the reliability of the coupled model is verified through an experiment of the vertical electrode in water-saturated soil. The comparison of experimental and simulation results shows that the maximum temperature error is 3.5%. Finally, the influence of water-saturated soil resistivity on practical engineering application was analyzed. Under the condition of water-saturated soil, the saturation of electrode temperature rise curve gets more pronounced as the current injection time increases. Thus, the temperature rise rate of the actual grounding electrode is lower than the calculation result without considering the water-saturated soil resistivity. Therefore, using the temperature characteristics of water-saturated soil resistivity to simulate the temperature rise of the grounding electrode can provide a theoretical basis for the grounding electrode to work under water-saturated soil conditions.

Current interference of HVDC ground electrode to buried pipelines and its personal safety distance

When DC ground electrode of a high-voltage direct current (HVDC) transmission system is in the stage of early operation or in the case of failures and maintenance, an instantaneous large current will be generated and bring great potential safety hazards to the nearby buried oil/gas pipelines and facilities and operators. In order to ensure the safety of persons near the buried pipelines, we investigated the current interference of the HVDC ground electrode to buried pipelines and its personal safety distance (hereinafter referred to as a safety distance). Firstly, the model simulating the electromagnetic interference to buried pipelines was established by means of the numerical simulation technology. Then, the model was used to calculate the safety distance corresponding to eddy current interference of HVDC ground electrode to buried pipelines under different soil resistivities, pipeline lengths, pipeline anti-corrosion insulations, earth currents of ground electrode and pipeline sizes. Finally, the influence laws of above-mentioned factors on the interference degree of HVDC ground electrode were analyzed. And the following research results were obtained. First, the influence of pipeline length on the interference degree of interference of the HVDC ground electrode is great. The longer the pipeline, the greater the safety distance. When the pipeline is equal to or longer than 600 km, however, the safety distance is basically the same. Second, the effect of pipeline coating on the interference degree of HVDC ground electrode is great. As the coating surface resistivity increases, the safety distance increases gradually. Third, for multi-layer soil structures, the maximum single-layer resistivity can be taken as the overall resistivity. And correspondingly, the calculated safety distance obtained is the longest, and the evaluation result is more conservative. In conclusion, the safety distance diagrams of the pipelines with 4 lengths which are obtained based on the calculation results can be used as reference in the design of HVDC ground electrodes and pipelines, as well as the basis for selecting the safety distance of HVDC ground electrode or buried pipeline to be constructed.

Effect of Coke Bed on the Electrical Performance of HVDC Ground Electrode

To prevent the high voltage direct current ground electrode from eroding, it is usually wrapped by a coke bed, which may affect the electrical performance of the electrode, such as current distribution and step voltage. In this paper, the effect is analyzed by a numerical method, which couples moment method with circuit theory by regarding the coke bed as several resistors connecting the ground electrode with soil. The method is simple and efficient. The calculated results are in good agreement with the measured ones. The current distribution, ground resistance. and step voltage of several typical electrodes with and without coke beds are calculated. The effect of the coke bed on the above parameters is summed up.

Calculating current and temperature fields of HVDC grounding electrodes

Current field calculation based on the resistance network method (RNM) and temperature field calculation based on the finite volume method (FVM) can be used to evaluate the performance of high-voltage direct-current (HVDC) grounding electrodes. The main idea of the two methods is to transform an electric and temperature field problems to equivalent circuit problems by dividing the 3D soil space near the grounding electrode into a suitable number of contiguous and non-overlapped cells. Each cell is represented as a central node connecting to the adjacent cells. The resistance network formed by connecting all the adjacent cells together can be solved to calculate the current field. Under the same conditions, the results calculated by the RNM are consistent with the result by CDEGS, a widely used software package for current distribution and electromagnetic field calculation. Based on the finite volume method, the temperature field results are also calculated using time domain simulation.

Electrical and electromagnetic investigations for HVDC ground electrode sites in India

High Voltage Direct Current (HVDC) power transmission systems require setting up of specially designed ground electrodes at terminal ends of the transmission line to close the circuit with an earth return path. The design parameters of these electrodes need the information about the electrical conductivity structure within a radius and depth of several km of the site in order to ensure that the injected current penetrates deep enough into the earth. Further, detailed conductivity structure, up to 100–200 m depth, of the electrode site covering an area of less than a sq. km is also needed to ensure safe limits for the step and touch potential at the site. Electrical and electromagnetic methods are very useful tools for this purpose. However, artificial source variants such as deep direct current (DC) resistivity sounding pose logistic problems for target depths greater than a couple of kilometers and in inaccessible areas. We have employed magnetotelluric (MT) and electrical resistivity tomography (ERT) tools to investigate the detailed deep and shallow electrical conductivity structure, respectively, of several potential sites in India for a ±800 kV, 6000 MW HVDC multi-terminal system. Investigations of a site in Assam revealed the presence of highly resistive crustal rocks at the depth of about 1.3 km beneath a thick pile of conductive sediments, rendering the site unsuitable even though the shallow conductivity was favorable. At another site identified after analyzing available geological and geophysical data, we inferred the presence of conductive structure up to at least 4.0 km depth. ERT investigation at this site revealed a favorable conductive structure except for the presence of a 20-m-thick near-surface resistive layer. This information has been useful for the electrode design. We suggest that a strategy suitably combining MT and ERT is useful in the selection of ground electrode sites.

Influence of Coke Bed on HVDC Grounding Electrode Heat Dissipation

This paper analyzes the influence of coke bed on the heat dissipation around high voltage direct current grounding electrode. By using a method coupling moment method with boundary element method, the influence of coke bed on the leakage current distribution is analyzed. Then, the affected current distribution in the soil is achieved. By using a finite difference method, the temperature distribution caused by the affected current flow is achieved. Some verification samples are also presented in this paper.

Numerical analysis of potential distribution between ground electrodes of HVDC system considering the effect of deep earth layers

The ground potential differences due to the ground return current of a long-distance high-voltage direct-current (HVDC) system may make the grounded transformers in AC substations under DC bias. Complex image method is used to calculate the potential distribution in multilayer earth between ground electrodes of an HVDC system. The effects of the deep earth layers, including the crust, the mantle and the core, on the potential distribution along the earth's surface are analysed. Analysis results show that although the effects of the deep earth layers on the potential distribution near the ground electrode can be neglected, it should be taken into account for the ground potential rise over tens of kilometres away from the ground electrode, which is important for analysing the transformer's DC bias due to the ground return current. The method and the results are useful for analysing the influence of the ground return current from HVDC ground electrode on systems that use grounding technique.

Analysis of the toroidal HVDC grounding systems in horizontal multilayer soils

A Fredholm integral equation of the first kind has been developed for analyzing the performance of the toroidal high-voltage direct current (HVDC) grounding system buried in the horizontal multilayer soil. The method of moment is used to solve the equations. The formulation of potential in the earth due to a circular current is proposed in closed form by means of conception of the complex resistivity and depth, which leads to the saving of the computational time greatly. The numerical results show the good agreements with those in other papers and the measured data

Effects of toroidal HVDC ground electrode on earth‐return circuits

AbstractThis paper presents a method of evaluation of currents and potentials, excited by conductive effects of high‐voltage direct‐current (HVDC) transmission system, along two parallel earth‐return circuits such as pipelines and cables buried in the vicinity of a toroidal ground electrode. It is assumed that the system considered is linear and that the earth is an isotropic, homogeneous medium of finite conductivity. Conductive coupling between earth‐return circuits is taken into account, whereas the reaction of the conductors' currents on the electrode current is disregarded. The transmission line model of the conductor with earth‐return, a segmental linear approximation of the curve of the primary earth potential distribution along the conductor and the concept of superposition have been used in the method. It should be pointed out, that the method does not require the time consuming numerical integration. The technical applications of the method are illustrated by examples.

Transient Analysis of Heat Dissipation Due to a HVDC Ground Electrode

The temperature of the soil surrounding a High Voltage Direct Current (HVDC) ground electrode was computed at various points in time using finite difference methods. The response of temperature, for various ground electrodes, was computed using these same techniques. The results were then compared with those results obtained experimentally in a laboratory at Ecole Polytechnique.

Transient analysis of heat dissipation due to a HVDC ground electrode

The temperature of the soil surrounding a high-voltage direct-current (HVDC) ground electrode was computed at various points in time using finite difference methods. The temperature response for various ground electrodes was computed using these same techniques. The results were then compared with results obtained experimentally in a laboratory. It is suggested that peak temperature calculations may be desirable for periodic short-term operation of a HVDC ground electrode. Since the surrounding soil temperatures may not have reached a steady state, a transient analysis may be necessary.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

HVDC ground electrode heat dissipation in an N-layer soil

The temperature distribution in an N-layer soil, due to high-voltage direct-current (HVDC) injection via ground electrodes, was computed using finite-difference methods. The temperature profiles for various ground electrode shapes buried in a two-layer soil were computed using these same techniques. The results obtained were then compared with results obtained experimentally in the laboratory. A sensitivity analysis was performed on the effect of the height of the top layer of a two-layer earth model. The computed results agree very well with those obtained experimentally. The effect of varying the height of the top layer of a two-layer earth was then investigated. The computations indicate that the temperature can be predicted as a function of this height. >