HVDC Ground Electrodes Research Articles

Research on the arcing horn with active arc extinguishing function for HVDC grounding electrode line

AbstractThe grounding electrode line is an important part of the Ultra high voltage direct current (UHVDC) transmission system. It acts as the channel to conduct the unbalanced current and the grounding current.The arcing horn consists of a pair of electrodes installed in parallel at both ends of the insulator string, which provides a discharge channel for the fault current by an air gap to protect the insulators. In this paper, the voltage and current distributions of the arcing horn were studied when the grounding electrode line was struck by lightning. It is found that the current and voltage gradually decrease along the direction from the converter station to the earth electrode. An active arc extinguishing method based on variable resistance for arcing horn was proposed. An electromagnetic transient simulation model of the grounding electrode line is established based on the electrical parameters of a ±800 kV UHVDC transmission system. It is found that the absorbed energy of the arc extinguishing unit needs to be at least 31.2 and 49.6 kJ when the lightning current is 20 and 50kA, respectively. According to the calculation results, the arcing horn with arc extinguishing unit was developed and tested. The test results verifies that the designed arcing horn can meet the requirements of engineering application.

Calculation of DC current distribution in AC power system near HVDC grounding electrode in subregional layered soil

The DC current from the grounding electrode of a HVDC system may result in great ground potential differences in a large area, and may flow into the widely distributed systems with dispersed grounding points such as AC power grid which may disturb the systems. Soil resistivity is the most important factor that affects the current distribution. The heterogeneous soil has a subregional layered structure on the whole. In this paper, a numerical approach coupling the boundary element method with complex image method is developed to calculate the current fields in the subregional layered soil. By using indirect boundary element method, the subregional layered soil is turned into a uniformly layered soil in which the electric field is calculated with the help of complex image method. The calculation scale is much smaller than that just using the boundary element method, and the complex image method can be extended to calculate the electric field in soil with complex structure. Then, an equivalent circuit network in the complex soil is extracted and is coupled with the equivalent circuit of AC power grid, with which the DC current distribution in an AC power grid around a HVDC grounding electrode is analyzed. The results show that even in the area where the ground electrode is located, the ground potential gradient in the subregional layered soil is different from the result in the uniformly layered soil, which leads to a difference in the current flowing through the neutral point of the substation. The closer the DC grounding electrode or the substation is to the interface of the subregional layered soil, the greater the effect on the ground potential around the DC grounding electrode and the DC current flowing through the substation.

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.

Calculation of Corrosion Depth of Wind Power Generators and AC Substations Induced by HVDC Grounding Electrode

At present, grounding potential rise is often used as the evaluation index to assess the electromagnetic intererence induced by HVDC grounding electrodes. It will lead to too stringent standards, for HVDC grounding electrodes are considered as long-term operation. Actuallly, HVDC grounding electrodes operates only in the test run stage, planned maintence and forced outage. In this paper corrosion depth is adopted as the evaluation index to assess the electromagnetic intererence. The method to calculate the corrosion depth of wind power generators and AC substation is described. The corrosion depth of windfarms and AC substation nearby Yanmenguan converter station is derived. The effects of material properties, distance and soil resistivity on corrosion depth are also studied.

Development of Multi-layer Soil Model for Yanmenguan Converter Station Based on Wenner and Magnetotelluric Method

In order to assess the effects of Yanmenguan HVDC grounding electrode on buried metal facilites, it is necessary to build up the soil model of Yanmenguan converter station. Soil resistivity was measures by Wenner and magnetotelluric method. Then the soil structure of Yanmenguan converter station was inversed based on the experimental data. It was shown that the soil structure of Yanmenguan converter station was three-layer horizental soil. The resisitivity of three layers was 101.7·m, 1037.5·m and 76.4·m. The thickness of three layers was 400m, 4600m and infinite.

Evaluation of impressed potential on buried pipeline near HVDC grounding electrode considering polarization effect

Evaluation of impressed potential on buried pipeline near HVDC grounding electrode considering polarization effect

Adjustment of a crustal geoelectric model from Commissioning data of HVDC ground electrodes: A case study from the Northeastern Paraná Basin, Brazil

Small-scale near-surface galvanic scatters are known to be a bane when using natural field EM data to get quantitative information about the subsurface resistivity structure. Specifically, on magnetotelluric measurements, the effect of local surficial inhomogeneities is almost inevitable and produces a static shift in the measured apparent resistivities over the whole range of sampled frequencies. We present here the methodology used to account for static shift effects and make a correct interpretation of the resistivity distribution in a region beneath the northeastern part of the volcanic-covered Paraná basin, SE Brazil. Within this region are located the ground electrodes of the Rio Madeira HVDC transmission system (Bipoles I and II). The available data includes geophysical soundings made for the electrodes site selection before their installation and data acquired in the subsequent commissioning measurements after their construction (electrode resistance, soil surface potentials and pipe-to-soil potentials). Iterative calculations for the simultaneous adjustment of the measured electrode resistance and the local geoelectric model were developed, based on the commissioning data of the electrode. The accuracy of the adjusted geoelectric model was confirmed by the excellent fit obtained for the measured and calculated pipe-to-soil potentials at Gasbol, the Bolivia-Brazil gas pipeline located 26 km away from the electrode.

Fault Location Algorithm Based on Characteristic- Harmonic Measured Impedance for HVdc Grounding Electrode Lines

It is still a significant challenge to accurately locate faults of grounding electrode lines in high-voltage direct current (HVdc) transmission systems, especially high resistance faults and remote faults. In this article, a novel fault location approach based on characteristic-harmonic measured impedance (CHMI) is proposed. This method works well under high resistance faults and remote faults. Considering that there are abundant and steady characteristic harmonics in electrical signals of grounding electrode lines, the CHMI is defined first according to the equivalent circuit of the grounding electrode system. Then, the CHMIs of the two grounding electrode lines under faults are analytically derived. On the basis of that, the fault branch is identified by the amplitude ratio of the two CHMIs. Furthermore, since both of the CHMIs of the two lines are relevant to fault distance and fault resistance, the precise fault distance is obtained by solving two fault location functions simultaneously and eliminating fault resistance. The proposed algorithm is finally validated through numerous simulations covering various fault scenarios carried out in PSCAD/EMTDC.

DC Current Distribution in Both AC Power Grids and Pipelines Near HVDC Grounding Electrode Considering Their Interaction

The dc current injected into the earth from the grounding electrodes of an HVdc system could flow into the ac electric power grids and gas pipelines through their grounding points. The dc current distribution either in ac power grids or in pipelines would be affected by each other. In this paper, a numerical approach to simulate the dc current distribution in ac power grids and gas pipelines considering the interaction is put forward. The approach is validated by field test. The dc current distribution in ac power grids and gas pipelines is discussed based on the calculation result in a real case. It is found that the effect of ac power grids on the current distribution in gas pipelines is greater than the effect of gas pipelines on the current distribution in ac power grids.

Evaluation of the Interference Effects of HVDC Grounding Current on a Buried Pipeline

The stray current created by the grounding current of HVdc ground electrode will accelerate the corrosion of the oil and gas pipeline when the HVdc is operate in single-pole mode. The magnitude of the stray current and the pipe-soil potentials (PSP) offset are indexes to evaluate the corrosion situation of the pipeline. An algorithm of the stray current and PSP of an oil pipeline generated by the Gaoqing ground electrode of Zhalute-Qingzhou ±800-kV ultrahigh voltage direct current project is put forward, the influence of potentiostat output, defects of insulation coating, and the insulation joint are considered for the first time. The stray current and PSP of the pipeline is calculated. The correctness of the algorithm is verified by the comparison of the calculation results and the monitoring data. The results show that the magnitude of the stray current and the PSP offset of the pipeline near an oil transmission station is the largest, the PSP offset is beyond the limit value prescribed by the national standard, and the potentiostat cannot provide an effective protection for the pipeline when the HVdc is operated in single-pole mode with the transmission load reaches to 2500 MW.

The influence of coke aging on electrical performance of double rings DC grounding electrode

The long-time work of the HVDC grounding electrode leads to the degradation of electrical performance, which not only affects the normal operation of the grounding electrode, but also may affect the safe and stable operation of the HVDC system. It is an important task to study the effect of the aging of coke bed and the corrosion of grounding electrode for the operation and maintenance of grounding electrode. Aiming at Xintian grounding electrode of Tianguang HVDC project, which has been running for 17 years, this paper analyzes the structure of double rings grounding electrode and the mechanism of aging of the coke, derived the algorithm with the consideration of coke bed to calculate the electrical performance indexes of ring grounding electrode, then use the design date of Xintian grounding electrode to verify it, finally, analyze the effect on the electrical performance of double rings grounding electrode which contributed by the aging of coke. The research results and conclusions can be used as reference for maintenance and overhaul of double rings grounding electrodes.

Numerical Simulation and Field Test of the Transient Temperature Rise of HVdc Grounding Electrodes

This paper investigates the temperature rise of vertical/horizontal high-voltage direct current (HVdc) grounding electrodes by simulations and field tests. Based on a field-circuit coupling current distribution model and a three dimensional (3-D) finite element model, a comprehensive methodology is proposed to analyze the transient temperature rise. Since soil resistivity varies with temperature around the electrodes, the temperature-dependent soil resistivity is tested, and then approximated by an empirical formula. Moreover, two practical tests are implemented to validate the simulation model. By ignoring the underground water fluctuations, the calculation results match test results nicely, which demonstrates that the terminal effect of the vertical electrode significantly affects the temperature distribution. But for a horizontal grounding electrode, the current distribution is more uniform than the vertical one. This study provides valuable references for design of HVdc grounding electrodes.

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.

Study on DC Bias Magnetic Properties of Transformer with Different Structures

When the HVDC works in mono-polar operation or bipolar unbalanced operation, DC flows through the neutral point and causes DC bias magnetic of transformer around HVDC grounding electrode, which influences the operation security of transformers. Beside highly saturated core, it also causes the distortion of exciting current, increasing harmonics and leakage magnetic flux, and thus the losses, which in turn results in the structures of transformer overheating and insulation aging. Based on the principle of transformer DC bias magnetic and UMEC of transformer in PSCAD/EMTDC, electromagnetic transient simulation platform of transformer DC bias magnetic is built. Considering the effect of DC bias magnetic on different structure of transformer, three kinds of typical transformer that has been widely applied are selected including three-phase three-limb transformer, three-phase five-limb transformer, three-phase bank transformer. After comparing the differences between these three kinds of transformer, related parameters are set such as the core structure and mode of winding connection and so on.

The effect of soil model on earth surface potential and reasonable selection method for soil model

The Green's function of three-dimensional combined-layer soil model is firstly derived using simulated image method. Based on the specified soil model, the influence of soil model on the earth surface potential (ESP) is analyzed for different soil structure configurations. It is found that difference in soil model only changes the absolute value of ESP, but the influence of ESP difference can be ignored under certain conditions. And the physical mechanism for the effect of soil parameter change on ESP is explained theoretically. One of the challenges that have not been solved effectively is how to establish the proper soil model to calculate DC current flowing through the neutral point of transformer accurately. For this challenge, the criteria and threshold value for selecting the soil model reasonably are introduced. Therefore, the arbitrariness and blindness of selecting the soil model can be avoided. The results are useful for calculating accurate results, which can further provide theoretical references for selecting substations near the HVDC ground electrode.

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 DC Current Distribution in AC Power System Near HVDC System

When dc current is injected into the earth through the grounding electrodes of an HVDC system, transformers in an ac system may be under dc bias if the dc currents flowing through the transformers are large enough. In this paper, a circuit model is presented to calculate the dc current distribution in an ac power system caused by the ground return current of a dc system. A complex image method is used to calculate the ground potential rise of each substation in multilayer earth. An indirect boundary element method is used to take into account the effects of mountains, rivers, and sea. The circuit equations are used to take into account the effects of the transmission lines. By using the method, the dc current distribution and the effectiveness of restraining measure in an ac power system are analyzed. The method and the results are not only useful for analyzing the influence of the ground return current from an HVDC ground electrode on ac power systems, but also useful for analyzing the effectiveness of the restraining measure.

ITAIPU HVDC ground electrodes: interference considerations and potential curve measurements during Bipole II commissioning

A simplified method is presented to measure the grounding resistance of adjacent electrodes which can be of use in other situations, when ground electrodes are composed of many elements or when an existing electrode is extended. The opportunity of remeasuring electrode potential curves showed that local gradients can change, but in a relatively small range, +or-10%. For applications like the design of mitigation procedures or the prediction of corrosion currents, this range is deemed to be insignificant, not only because the verified range is small, but also because of the tendency of the changes to compensate. The tests in the irrigation system changed somewhat the formula previously used for evaluating touch potentials in irrigation pipes. The adopted mitigation criteria concerning electric shocks to immersed persons are explained together with suggestions for further work on evaluating body currents in the case of large DC gradients in water near energized metal parts. >

Computer aided evaluation of pipeline current near toroidal HVDC ground electrodes

The effect of the return current of an HVDC toroidal electrode on underground metallic installations such as pipelines and cables is evaluated. In order to quantify the effect of HVDC electrodes on the buried conductor, an equivalent model is first presented that takes into account the electrical properties of the pipeline and its surrounding. In particular, infinitely long pipelines with different coatings are considered. The pipelines are assumed to be continuously buried in homogeneous soil and are located in the vicinity of the HVDC electrode. It is found that the voltage gradient in the vicinity of the electrode results in return currents in grounded conductors, the effects of which can extend several kilometers. Protective coatings reduce stray current from local voltage disturbances but have limited effect against distant electrodes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evaluation of the voltage distribution around toroidal HVDC ground electrodes in n-layer soils

A method for evaluating the electrical voltage and the current density in the immediate vicinity of a toroidal grounding installation of DC substations is presented. In order to guarantee that voltage rises generated during operation are within safe limits and considering the need to reduce the dimensions and costs of grounding installations, it is necessary in certain cases to improve the soil model in the area affected by the electrode. Electrostatic theory is used to generate a voltage solution for the Laplacian equation for n-layer soil models, and computed voltage profiles are shown in the vicinity of an actual electrode. Theoretical results for a two-layer soil model are also presented, along with measurements carried out on a reduced-scale electrode.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>