Ultra-high Voltage Direct Current Transmission Research Articles

Study on corrosion inhibition effect of dendritic ionic liquid polymer for corrosion protection of ultra-high voltage direct current transmission towers

Study on corrosion inhibition effect of dendritic ionic liquid polymer for corrosion protection of ultra-high voltage direct current transmission towers

Corrosion prevention performance and mechanism of slow-release molecules in micro-nano insulating coatings for ultra-high voltage direct current transmission towers

Corrosion prevention performance and mechanism of slow-release molecules in micro-nano insulating coatings for ultra-high voltage direct current transmission towers

Mono-pole blocking accident analysis caused by inaccurate measurement of optical CT in ±800 kV UHVDC transmission system under lightning strike

The ultra-high voltage direct current (UHVDC) power transmission systems usually transmit a capacity of 6-8 GW, their stability and reliability are of vital importance to the large power grid. This paper analyses a mono-pole blocking accident caused by the inaccurate measurement of optical current transformer (OCT) when the pole line of Shaanbei-Wuhan ±800 kV UHVDC transmission system was struck by lightning. Combined with electromagnetic transient simulation of the UHVDC transmission system, digital simulation and in-plant testing of the optical CT, a comprehensive understanding of the factors contributing to the accident is achieved. It reveals that optical CT measurement anomalies were significant contributors to the misoperation of protective relays, which led to the mono-pole blocking accident. Based on the simulations of optical CT under high peak currents and high di/dt conditions caused by lightning strike, the importance of proper setting of demodulation algorithm parameters in optical CT to avoid inaccurate measurements is emphasized. Through in-plant testing and steady state current accuracy verification, the reasonableness of demodulation parameters is verified. Simulation results illustrate the challenges in measuring very fast transient current accurately, which are the premise and guarantee of stable operation of UHVDC transmission systems under lightning strike.

Inhibition of Subsequent Commutation Failures in Ultra-High Voltage Direct Current Transmission Systems Using Electrochemical Energy Storage at the Sending End

Introduction: Subsequent commutation failures (SCFs) in high-voltage direct current (HVDC) systems pose a serious threat to the safe operation of hybrid AC/DC grids. Electrochemical energy storage, which is widely distributed at the sending end of ultrahigh voltage direct current (UHVDC) transmission systems, has the potential to mitigate SCFs. To fully harness the SCF-mitigating capabilities of energy storage, this article first establishes a CIGRE-HVDC standard test model incorporating electrochemical energy storage at the sending end. Method: Based on this model, the factors influencing DC commutation failures are investigated. Furthermore, the impact of rectifier-side electrochemical energy storage (EES) on inverter-side commutation failures is explored from three aspects: energy storage capacity, output magnitude, and fault conditions. It is found that rectifier-side EES absorbing power can effectively suppress inverter-side commutation failures. Finally, based on this finding, a transient active power control strategy for energy storage is designed to inhibit consecutive commutation failures and is studied on the CIGRE-HVDC standard test system. It is concluded that the optimal capacity for suppressing SCFs is between 20% and 30% of the DC capacity, and the best absorption power output is achieved with a per-unit value of 1. Result: Simulation results confirm the correctness of the proposed energy storage transient active power control strategy and its effectiveness in suppressing SCF under different fault moments, fault severities, and fault types. Conclusion: This strategy can limit the number of SCFs to three or fewer in the majority of operating conditions, facilitating rapid system recovery after faults.

Research on audible noise of ultra-high voltage direct current (UHV DC) transmission lines with a digital twin

Much attention has been paid to audible noise in the design of ultra-high voltage (UHV) direct current (DC) transmission lines because of its negative impact on residents' lives. In order to analyze audible noise, shorten the research period, and reduce experimental costs, firstly, a digital twin model for audible noise of UHV DC transmission lines is proposed. Then, the model’s structure and related principles are explained in detail. Finally, an application, as a research paradigm, uses digital twin to study the source distribution on transmission lines and influencing factors of audible noise. The A-weighted sound level of audible noise on the ground is calculated and compared with the data obtained by the BPA empirical formula. The results show that the position of the sound source tends to have uniform distribution and the intensity tends to have Gaussian distribution. We also used the model to simulate and analyze the effects of temperature and humidity on the generation and propagation of audible noise, illustrating the great value of digital twin technology in the study of audible noise. This paper not only provides a new method for audible noise research, but also provides a reference for the design of UHV DC transmission lines.

Calculation study of ion flow field in ±800 kV transmission lines under the effects of air pressure, temperature, humidity and improvement measures

In line operation, Ultra-High Voltage Direct Current (UHVDC) transmission lines must meet the corresponding electromagnetic environment requirements; the ion flow field is a crucial environmental index of ultra-high-voltage DC transmission lines. Under the combined effects of humidity, air pressure, and temperature, the ion flow field is easily influenced. A calculation method for the ion flow field of UHVDC transmission lines that considers the combined effects of humidity, air pressure, and temperature is proposed to investigate the impact on the ion flow field. Based on this, when the humidity exceeds 60%, the influence of the charging characteristics of airborne droplets on the ion flow field is considered to align the calculations more closely with actual conditions. An improved finite element method is used to calculate the ground synthetic electric field strength and ion flow density of the ±800 kV line under these combined effects, and its effectiveness is validated through empirical testing. The results show that the synthetic electric field strength under the positive line decreases with increasing humidity, peaks at 90% humidity under the negative line and reaches a minimum at 50% humidity for the negative line. When considering the charging characteristics of droplets with humidity above 60%, the maximum synthetic electric field strength under the positive line is increased by 5.46%, and under the negative line by 5.5%, while the ground ion flow density remains largely unchanged. Finally, in conjunction with the electromagnetic environmental parameter limits for UHVDC transmission lines promulgated by China, improvement measures are proposed for the ground ion flow field.

UHVDC transmission line diagnosis method for integrated community energy system based on wavelet analysis

With the large-scale development of renewable energy power, China has faced with the challenges of the reverse regional distribution of wind and solar resources and power load, as well as the intermittency and randomness of renewable energy power. Therefore, China is vigorously developing ultra-high voltage direct current (UHVDC) transmission technology to solve the problem of insufficient flexibility caused by the uncertainty of renewable energy and the fluctuation of multi-energy loads in integrated community energy systems. UHVDC plays an increasingly pivotal role in the west-east transmission system in China’s power system due to its high transmission capacity and long transmission distance. Once the fault occurs in the ultra-high voltage direct (UHVD) transmission line, quick and accurate fault location identification is of great significance. Hence, this paper proposes a UHVDC transmission line diagnosis method based on wavelet analysis for integrated community energy systems. Wavelet transform (WT) is used to decompose the transient signal on a multi-scale, and then power systems computer-aided design (PSCAD) software is utilized for simulation calculation to obtain the singular spectrum entropy of each layer and facilitate wavelet transformations for signal denoising with advanced tools such as MATLAB. The prediction results can distinguish outside the rectification side fault, within the rectification side fault, and outside the inverter fault with an accuracy of 100%. A large number of simulations demonstrate that combining singular spectrum entropy with support vector machines (SVM) has emerged as a robust technique for integrated community energy systems, suggesting its potential as a standard method in UHVDC transmission line diagnosis. This study is of significant reference for realizing the complementarity of multiple types of power supply and ensuring a reliable power supply.

A method for calculating space charge distribution in ultra-high voltage direct current transmission lines based on the transport of diluted species theory

The occurrence of corona discharge in ultra-high-voltage direct current transmission lines results in the generation of space charge, which significantly affects the nearby electromagnetic environment. To investigate the micro-physical mechanisms of discharge and provide better guidance for the calculation of the ground ion current density and total electric field, a method based on the theory of the transport of diluted species is proposed. This method enables the visualization and analysis of the movement process of space charge in air medium, as well as the calculation of the total electric field below the transmission lines. To validate the effectiveness of this algorithm, experimental tests of the total electric field are conducted on actual transmission lines. The experimental results demonstrate that the algorithm not only highlights the evident differences in the transient microscopic processes between positive and negative corona discharges but also accurately calculates the total electric field below the transmission lines. In addition, considering the weather conditions in the Chongqing region, relevant recommendations are provided under the influence of wind speed, aiming to offer protection guidelines for individuals engaged in activities near transmission lines.

Experimental and theoretical study on the influence of bundle number on corona loss characteristics of ultra high voltage corona cage positive/negative DC conductors at high altitude

The corona loss generated by the conductor is one of the important factors in the design of ultra-high voltage (UHV) direct current transmission lines. This article is based on the high-altitude (2200 m) ultra-high voltage corona cage experimental platform built in Xining City, Qinghai Province. For the LGJ400-35 conductor used in the typical ± 800 kV DC transmission lines from Qinghai to Henan Province, positive and negative polarity corona experiments were conducted on LGJ400-35 conductors with 4, 6, 8, and 10 bundle under surface field strength conditions of 14–22 kV/cm. For the first time, corona loss data of bundle conductors in positive/negative polarity ultra-high voltage DC transmission lines were obtained, Fit the prediction formula for corona loss of transmission lines under different bundle numbers and surface electric field strengths of wires. And build a model in COMSOL Multiphysics software for simulation analysis, fitting the correction factor for the impact of wind speed on wire corona loss. The research results in the article can provide data reference and support for the construction of ultra-high voltage direct current transmission lines in high-altitude areas.

Failure analysis of strain clamps on a ± 800 kV transmission line

During X-ray testing of a ± 800 kV ultra-high voltage direct current transmission line tension clamp, it was found that the internal steel core had broken. To investigate the cause of the steel core fracture, the failed clamp was dissected and analyzed, including chemical material testing, mechanical performance testing, fracture morphology inspection, and analysis of the compression process of the tension clamp. A preliminary judgment was proposed on the cause of the internal steel core fracture of the tension clamp, And verification experiments and simulation analysis were conducted to preliminarily determine the cause of the steel core fracture, and it was ultimately concluded that the main reason for the steel core fracture was the incorrect compression sequence of the large cross-section wire tension clamp.

Fault Identification of UHVDC Transmission Based on DF-AD and Ensemble Learning

High-resistance ground faults are difficult to detect with existing ultrahigh voltage direct current (UHVDC) transmission fault detection systems because of their low sensitivity. To address this challenge, a straightforward mathematical method has been proposed for fault detection in UHVDC system based on the downsampling factor (DF) and approximation derivatives (AD). The signals at multiple sampling frequencies were analysed using the DF, and the AD approach was used to generate various levels of detail and approximation coefficients. Initially, the signals were processed with different DF values. The first, second, and third order derivatives of the generated signals were calculated by the AD method. Next, the entropy features of these signals were computed, and the Random Forest-Recursive feature elimination with cross-validation (RF-RFECV) algorithm was used to select a high-quality feature subset. Finally, an ensemble classifier consisting of Light Gradient Boosting Machine (LightGBM), K Nearest Neighbor (KNN), and Naive Bayes (NB) classifiers was utilized to identify UHVDC faults. The MATLAB/Simulink simulation software was used to develop a ±800 kV UHVDC transmission line model and perform simulation experiments with various fault locations and types. Based on the experiments, it has been established that the suggested approach is highly precise in detecting several faults on UHVDC transmission lines. The method is capable of accurately identifying low or high resistance faults, irrespective of their incidence, and is remarkably resistant to transitional resistance. Furthermore, it exhibits excellent performance in identifying faults using a small sample size and is highly reliable.

Research on Total Electric Field Prediction Method of Ultra-High Voltage Direct Current Transmission Line Based on Stacking Algorithm

Research on Total Electric Field Prediction Method of Ultra-High Voltage Direct Current Transmission Line Based on Stacking Algorithm

Numerical Calculation of Direct Current Breakdown Voltage for Complex Short Air Gaps under Varying Altitudes

With the increasing construction of ultra-high voltage direct current transmission in high-altitude environments, it becomes crucial to explore the impact of altitude on the short-gap discharge characteristics. To this end, this study proposes a finite element simulation and calculation method for the direct current (DC) breakdown voltage of actual complex short-gap structure under varying altitudes based on the Townsend criterion and the streamer criterion. The method is verified by combining the discharge experiments in Beijing, Qinghai, and Tibet. The results demonstrate that this method can effectively calculate the DC breakdown voltage of gaps between damping capacitors with a relative error range of -3.64% to 0.2%. The research results provide new insights for insulation evaluation of short-gap structures under varying altitudes.

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.

An Improved Calculation Method Based on Newton–Raphson Method for Ion Flow Field of UHVDC Transmission Lines at High Altitude

The fast and accurate solution of the total electric field and ion current density is crucial for the electromagnetic environment of ultra high voltage direct current (UHVDC) transmission lines. In this article, an improved method based on Newton–Raphson method (NRM) and upstream finite element method (UFEM) is proposed to calculate the ion flow field under complex conditions. According to the gas motion theory, the corona-onset electric field, ion mobility, and recombination coefficient are obtained at high altitude and put into the ion flow field model in this article. Subsequently, the charge density is used to solve Poisson’s equation, and the current continuity equations are calculated by UFEM. After that, the charge density of the conductor surface is updated based on NRM. These three parts are iterated repeatedly until the convergence conditions are met. According to the test results of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 800-kV UHVDC transmission line at high altitude, the validity and rationality of the proposed method are verified. The result demonstrates that due to the reduction of atmospheric pressure at the high altitude, the corona-onset electric field decreases, and the ion mobility and recombination coefficient increase, which makes the total electric field and ion current density at ground level increase. Compared with the traditional methods, the improved method is insensitive to the initial value of the surface charge density. Moreover, this proposed method can effectively improve the convergence ability and achieve the stable and fast calculation of the ion flow field at high altitude.

Multi-time scale stochastic production simulation under VHVDC long-term contract trading electricity quantity constraint

A two-layer multi-time scale stochastic production simulation framework is constructed to account for the long-term contract electricity quantity of ultra-high voltage direct current (UHVDC) transmission. On the upper layer, based on the characteristics of load demand and renewable energy output extracted from the historical operating data, monthly and daily production simulation models are carried out considering the seasonal characteristics of hydropower during a high-water period and low-water period to optimize the distribution of contract electric quantity sending through UHVDC transmission in the target year or month. According to the DC transmission electric quantity optimized by the daily production simulation in the upper layer, together with the forecast scenario, the lower layer of the framework provides the optimization of day-ahead scheduling and intra-day rolling dispatch in the implementation process. The day-ahead dispatch optimization makes full use of the adjustment capability of transmission and optimizes the DC transmission electric quantity correction. Its compensation is based on the result of the daily production simulation, then the correction will be returned to the upper layer to restart the optimization of the remaining UHVDC contract electric quantity of the subsequent period and its distribution plan. Combined with the day-ahead DC transmission plan, the intra-day rolling optimization is carried out to adjust the output of the unit using more accurate forecasting scenarios. The distributionally robust optimization model is used in the lower layer to convert an uncertain problem into a deterministic quadratically constrained quadratic programming (QCQP) problem according to the form of an uncertain distribution set. Then the QCQP problem is further converted into a linear programming (LP) problem by using the reformulation linearization technique (RLT). A test system with the energy composition and distribution referring to a real provincial power grid in northwest China is established for verification. The results show that the proposed method can effectively improve the economics of system operation and the accommodation of renewable energy based on ensuring security.

Nonlinear Transient Mathematical Model of Large-Capacity Synchronous Condenser Based on Time-Varying Reactance Parameters

Large-capacity synchronous condenser (SC) is an essential dynamic reactive power compensation device in an Ultra-high voltage direct current (UHVDC) transmission system. The particular transient operating state and structural characteristic of SC lead to a severe magnetic saturation effect during transient operation. To accurately describe the dynamic characteristics of SC, it is urgent to establish a transient mathematical model considering the magnetic saturation effect. Therefore, this paper proposes a nonlinear transient mathematical model for SC based on time-varying reactance parameters and gives a calculation method for time-varying reactance parameters. The mathematical model proposed in this paper is verified by taking a 300MVar SC as the research object. This condenser is connected to the power grid at the sending end of the UHVDC transmission system. The proposed mathematical model, the traditional fixed-parameter transient mathematical model, and the saturated standard SG model used in the power system were used to simulate the voltage generated by dynamic reactive power compensation characteristics of SC during a drop failure. Compared with the fault test results, the accuracy of the proposed mathematical model is verified. The effect of magnetic saturation on the dynamic reactive power compensation characteristics was analyzed, respectively. In addition, the accuracy of the proposed mathematical model is closer to the actual motor state, which is helpful for further research on the operation analysis and state monitoring of SC.

Static electric field exposure decreases white blood cell count in peripheral blood through activating hypothalamic–pituitary–adrenal axis

ABSTRACT With the development of ultra-high-voltage (UHV) direct-current (DC) transmission, the health risk from the static electric field (SEF) generated by UHV DC transmission lines has drawn public attention. To investigate the effect of SEF exposure on white blood cell (WBC) count, mice were exposed to 56.3 kV/m SEF. Results revealed that total WBC count and lymphocyte count significantly decreased and serum levels of corticotropin-releasing hormone, adrenocorticotropic hormone and corticosterone (CORT) significantly increased after the exposure of 7d and 14d. All indices above recovered after the exposure of 21d. Analysis showed that the exposure of 7d and 14d could activate hypothalamic-pituitary-adrenal (HPA) axis. The increased CORT could bind to the glucocorticoid receptor (GR) in lymphocytes, and then promote the migration and apoptosis of lymphocytes. After the exposure of 21d, the magnitude of HPA axis activation declined through CORT-mediated negative feedback and the regulation of stress-related neural circuitry, so WBC count recovered.

Direct current volt-ampere characteristics of long arc on ice surface

With the continuous construction of ultra-high voltage transmission lines, the problem of long insulator string ice flashover in the icing area is becoming more and more prominent. At present, only the volt-ampere characteristics of short arc on the ice-surface have been studied in the previous research. However, the volt-ampere characteristics of the long arc on ice-surface are significantly different from the short arc, so the flashover characteristics obtained from the short insulator string aren’t suitable for the long string under icing conditions. Therefore, in this paper, the volt-ampere characteristics of direct current long arc on the ice-surface are measured by using ice column in a multi-functional artificial climate chamber. The results show that the arc length has an effect on the volt-ampere characteristic constant A, exponent n and expression. In addition, the dynamic volt-ampere characteristics and the static volt-ampere characteristics are redefined and studied, which lays a foundation for the flashover voltage prediction of long iced insulator string. The research of this paper has important theoretical guiding significance for the external insulation design of ultra-high voltage direct current transmission lines in icing areas.

Stability Behavior and Optimization of Tension Plates in Transmission Lines in Heavy Ice Zones

Finite element model of a typical ultra-high voltage direct current transmission line section, in which buckling collapse of the tension plate took place during operation, is established. In the finite element model, the tension plate and the connected hardware fittings are discretized with 3D solid elements and the sub-conductors, spacers, clamps and insulator strings with spatial beam elements. Dynamic responses of the line after ice shedding are numerically simulated and the maximum loads on the tension plate in the cases with different ice thicknesses and ice-shedding are obtained. Buckling behaviors of the tension plate are analyzed and the conditions leading to buckling collapse are discussed. Finally, the topology optimization of the tension plate is performed and two optimized schemes of the tension plate are proposed based on the optimization result and manufacture cost. Efficiency analysis demonstrates that the buckling loads of the optimized tension plates increase more than 20% nearly without increasing weight.