kV UHVDC Research Articles

Parative research on the lightning protection of ± 800kV UHVDC transmission lines

Parative research on the lightning protection of ± 800kV UHVDC transmission lines

High‐speed main protection for multiterminal LCC‐MMC‐UHVDC based on initial wave process comparison

AbstractThe multiterminal LCC‐MMC‐UHVDC, which employs an LCC on the rectifier side and multiple FHMMCs on the inverter side, has emerged as a cutting‐edge technology. Nevertheless, distinct disparities in their protection requirements compared to those of conventional DC grids pose notable challenges in attaining the desired attributes. This paper first derives variation patterns in the time domain of initial waves under different fault conditions. By utilizing the theoretically calculated rate‐of‐change waveform for the backward current traveling wave in an external fault scenario as a reference, a main protection relay grounded in initial wave process comparison is proposed. This approach capitalizes on the disparity observed in internal faults with the theoretical waveform. To mitigate maloperations stemming from the employment of a non‐directional start‐up criterion in reverse fault scenarios, subtle noise patterns, mimicking the theoretical waveforms, are infused into the actual waveforms. This approach averts maloperations in reverse faults and obviates the need for added delays associated with directional start‐up criteria, thereby enhancing both speed and security. Case studies demonstrate that the proposed protection offers sufficient selectivity and a resistive tolerance of 600 ohms and boasts a speed of 0.2 ms, satisfying the requirements of 800 kV UHVDC systems.

Identifying faults in UHVDC transmission systems using DAE and ensemble learning

Abstract The current fault identification technique for the ultra high voltage direct current transmission system has a low success rate, the process of extracting feature qualities is complex, and it is difficult to identify high resistance grounding. This research provides a defect evaluation method for UHVDC transmission system using Deep Auto Encoder (DAE) and integrated learning. Firstly, the fault voltage signal is feature extracted using DAE, and then, a series of obtained features are feature screened by XGBoost algorithm and combined with an integrated classifier consisting of Multi-Layer Perceptron (MLP), K Nearest Neighbor (KNN), and Random Forest (RF) classifiers to identify the UHVDC faults. Finally, ±800kV UHVDC transmission line models are established in MATLAB/Simulink simulation software to simulate different types of faults. The study’s findings demonstrate that the fault diagnostic method introduced in this research may significantly enhance the poor detection success rate of current fault diagnostics methods and is more resilient to excessive resistance.

Study on corona discharge on the grounding wire of ±1100 kV transmission line during the downward lightning flash

The atmospheric space electrical field has nonlinear distribution characteristics during thunderstorm activity, which influences the ground surface electric corona of UHV transmission lines. In this paper, the corona discharge model of ground wire of ±1100 kV UHVDC transmission line is established considering the combined effect of ground lightning process and working voltage. With the combined action of thundercloud electrical field and DC voltage, the positive conductor suppresses the electric corona of the upper shied wire, and the negative conductor enhances the discharge of the upper shied wire. The corona current of the right shied wire is 54.7 μA/m larger than that of the left shied wire. When the down leader develops, the leader voltage, the line operating voltage and the thundercloud voltage work together, and the electric corona on the surface of the conductor and shied wire of the transmission line increases rapidly. The corona current increases 102 times on the left shied wire's surface and 103 times on the right shied wire's. Considering the environmental factors and the development of the downgoing leader at the same time, the degree of electric corona on the surface of the shied wire is weakened when the humidity in the space increases, but the overall value of the corona current is much higher than the thundercloud electrical field. When the wind speed increases, the degree of electric corona on the surface of the shied wire is aggravated. The corona current on the surface of the left shied wire is 37.13 μA/m, and the one of the right shied wire is 109.33 μA/m. The corona current value of ground surface increases with the increase of wind direction.

Study on the minimum polarity distance of suspension tower for UHVDV transmission line in ultrahigh altitude area

The design experience of ± 800 kV UHVDC transmission line in 3500 m and above is less. Based on the basic design conditions of ± 800 kV DC transmission line project, the minimum polarity distance of the suspension tower in an ultrahigh altitude area is investigated and the effects of the electromagnetic environment, insulator string length, V-string angle string and air gap on polarity distance are analyzed. Finally, the minimum polarity distance of the suspension towers under different altitudes, V-string angle string and icing area conditions are calculated, which provides technical support for the tower design of UHVDC transmission lines in ultrahigh altitude areas.

Research on Electromagnetic Scattering Influence of Transmission Towers on Medium Wave Antenna Based on the Characteristic Mode Theory

Electromagnetic scattering from UHV transmission towers is a major factor affecting the safety and stability of the surrounding antenna signal system. In this paper, the electromagnetic scattering effect of ±800 kV UHV DC transmission tower on the medium-wave antenna is investigated based on the characteristic mode theory (CMT). The simulation model of the tower and antenna is established, and the mode selection is carried out according to the percentage of the contribution of the characteristic mode to the total electromagnetic scattering. The effects of electromagnetic scattering under three conditions, namely, the number of towers, the distance between towers and antennas, and different frequencies, are investigated separately. The simulation results show that as the number of towers increases from 1 to 3, it leads to an increase in the electromagnetic scattering impact by about 49.5%. The shape distortion of the antenna’s directional map becomes more pronounced and is accompanied by the extension with the direction of the power line. The distance between the tower and the antenna is shortened from 500 m to 125 m, resulting in the growth of the influence of electromagnetic scattering by about 36.4%, and the directional gain of the antenna increases along the direction of the transmission line. As the frequency increases from 600 kHz to 1400 kHz, it leads to the rise of electromagnetic scattering effect of about 32.7% and the antenna directional map becomes more complicated. The research results will provide technical support for developing protective measures against electromagnetic scattering from UHV DC transmission towers to medium-wave antennas.

Study on debugging of lightning impulse voltage waveform for large capacity ±800kV converter transformer in high altitude area

The Qinghai-Henan ±800kV UHV DC power transmission project is an UHV channel specially constructed for clean energy delivery, ±800kV converter transformers of the project are assembled and tested in Xining at an altitude of 2500 m, full-wave lightning impulse test is one of the main means to evaluate converter transformer insulation performance, its valve side lightning impulse voltage waveform parameters often fail to meet IEC60076-3 standards because of the large winding to the ground capacitance. In order to effectively assess the lightning tolerance of converter transformer, a simulation model of impulse test circuit of ±800kV converter transformer was built based on the existing 6000kV/560kJ impulse voltage generator and ATP-EMTP simulation software, three kinds of optimization methods are proposed, which include increasing the wavetail resistance, increasing load capacitor, and connecting the wave head in parallel with different inductors, the results show that the single optimization method can not meet the waveform requirements. Therefore, a combinatorial optimization test method of increasing the wavetail resistance and connecting the wave head in parallel with different inductors is proposed, which is completed and verified in the full-wave lightning impulse test on the valve side of Qinghai ±800kV high-end converter transformer, the results show that the optimized test waveform parameters meet the standard requirements.

Altitude Correction Method for Audible Noise of ± 800 kV UHVdc Overhead Transmission Line and its Engineering Application

With the improvement of the voltage level of DC overhead transmission lines (OHTLs), the audible noise (AN) caused by the corona discharge of conductors is an increasingly serious problem. AN has become a critical parameter that determines the selection of conductors, line structures and specifications for UHVdc OHTLs in high-altitude areas. To study the altitude correction method of AN for UHVdc OHTLs, we erected 6 × 720 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 6 × 900 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> conductors on DC full-scale test lines located at altitudes of 4,300 m and 50 m, respectively. We carried out an experimental study on AN over a long period and obtained its distribution law at high and low altitudes and its altitude-related increment. At the same time, on four DC reduced-scale test lines located at 4,300 m, 3,400 m, 1,700 m and 50 m above sea level, 6 × 95 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> conductors were erected for experiments. The law that the AN of the DC OHTL shows a nonlinear change with increasing altitude is obtained. Based on the combination of full-scale and reduced-scale tests, an altitude correction method of AN for UHVdc OHTLs considering the influences of both altitude and electric field strength on the conductor surface is proposed. From 0 to 4,300 m above sea level, when the maximum electric field strength on the conductor surface is 19 kV/cm, the increase in AN calculated with the formula suggested by this article is only 55% of the value calculated with the method recommended by the EPRI. The altitude correction formula of AN proposed herein is applicable to the design of UHVdc OHTLs in high-altitude areas, which can greatly save line investment while meeting the requirements of environmental protection.

A novel pilot differential protection for HVDC transmission line based on the differential voltage

The HVDC transmission system has played an important role in modern power system. In traditional HVDC transmission line protection, the differential protection is invulnerable to high fault resistance, but its time delay is too long due to the distributed capacitance current. In order to solve this problem, a novel pilot differential protection is proposed in this paper. The compensating voltage on the setting point on the line can be calculated by using the voltage and current at two terminals of the line, separately. The difference value of the compensating voltages calculated by the voltage and current at rectifier side terminal and the inverter side terminal are defined as the differential voltage in this paper. When external fault occurs, the compensating voltage on the setting point calculated by the rectifier side terminal and inverter side terminal should be almost the same, so the differential voltage should be relatively small. However, when internal fault occurs, the compensating voltage on the setting point calculated by the two terminals are different, so the differential voltage should be relatively large. The analysis proves that the differential voltage can be used to identify the fault. A ±800 kV UHVDC system is modeled for simulation in PSCAD/EMTDC, and the correctness and effectiveness of the proposed protection method is verified.

Study on technology and performance of ZnO resistor for ± 1100kV DC lightning arrester

In order to improve the stability of ± 1100kV UHV DC transmission line, the ± 1100kV DC arrester has strict parameter requirements on aging characteristics, consistency and residual voltage of zinc oxide resistor. The performance of zinc oxide resistor was studied by adjusting the ball milling method, grinding method, side insulation material and body formula of zinc oxide resistor. The results show that the strict processing and surface treatment of the zinc oxide resistor can effectively improve the electrical performance parameters of the zinc oxide resistor, which has important application reference value for the research of UHV DC transmission line.

Research on the key technology of lightning protection for ±1100kV UHV DC transmission lines

In recent years, in order to meet the huge demand for electricity brought about by economic development, many ultra-high voltage project lines in China have been built one after another, alternating vertically and horizontally, gradually forming a large-scale high-voltage trunk line network. transmission project in the world, there is a lack of relevant standards and operational experience. Lightning strikes as the most profound influence on the stable operation of the current transmission line. This paper conducts an in-depth study on how to protect the ±1100kV UHV DC transmission line from lightning, combining lightning protection technology and geographical factors to develop a series of theoretical analysis and verification, hoping to provide some reference value for the national lightning protection technology research.

Baihetan - Jiangsu engineering soil and water conservation online monitoring data system

Baihetan - Jiangsu ± 800kV UHVDC project can improve the power supply stability of the hydropower station, which has very important economic significance. The construction of power transmission projects will affect the water and soil conservation situation along the site. Currently, the commonly used monitoring methods are based on manual data collection with low accuracy and a large workload. Therefore, the government vigorously promotes the development of intelligent water conservancy and strives to improve the digitalization degree of water and soil monitoring systems. In this paper, the traditional chain pin method was improved, using the photosensitive chain pin and ultrasonic chain pin instead of the ordinary chain pin. Then calculated the soil erosion modulus by collecting the soil loss index of the measured point, and uploading it to the cloud for analysis by the radio transmission device, so that the monitor system obtained accurate soil loss data. Finally, the system is applied to the baihetan-Jiangsu soil and water conservation monitoring project, and the monitoring data of soil and water conservation of the two stations are obtained. Combined with the local meteorological conditions, we can analyze the causes of soil and water loss, and give the corresponding prevention measures.

Prediction of Breakdown Voltage of Equipotential Live-Line Work Air Gaps in Transmission Lines: A Computational Discharge Model

Equipotential live-line work (EPLW) is an indispensable technology to ensure the power supply. Due to the huge risks of the strong electric field, the distance of the EPLW air gap should be greater than the minimum approach distance (MAD). MAD is determined by calculating the hazard probability according to the characteristics of the switching overvoltage and the 50% breakdown voltage of EPLW gaps. Currently, the 50% breakdown voltage is obtained by a great many actual-size discharge tests, which costs a large amount of time and workforce. In this article, a computational discharge model was presented to solve the problem. First, the discharge process of EPLW air gap was analyzed based on the test experience. Subsequently, according to each stage of the discharge process, a computational discharge model for predicting the 50% breakdown voltage of EPLW gaps was presented. Finally, the computational discharge model was applied to EPLW air gaps of ±800 kV UHVDC and 1000 kV UHVAC transmission lines. The calculated results agreed well with the test results, and the errors did not exceed 6%. This research can provide theoretical support for the transmission line design and lineman safety protection.

Research on structural type of two-way connection fittings based on valve hall

Based on the research on the structural type of valve hall two-way connection fittings, with the sustained and rapid economic growth, the power demand of various industries increases. While accelerating the construction of UHV backbone power grid, improving the transmission capacity and transmission quality of the existing power grid has become a top priority. In ± 800kV UHV DC transmission project, the rated current is up to 6250A, and the current carrying capacity of valve hall fittings in converter station increases accordingly, which puts forward higher requirements for heat resistance and reliability of fittings. Therefore, it is very necessary to study the operation reliability of valve hall fittings in converter station. In this paper, two structural types of two-way connection fittings are analyzed. The temperature field of two-way connection fittings is simulated by using the electric thermal coupling finite element analysis method. At the same time, the high current temperature rise test is carried out, and the temperature at the key position of two-way fittings is measured by thermocouple. The high current temperature rise test and simulation results show that when 6250A current is applied, there is uneven shunting between the two specimens. Under the condition of meeting the temperature rise limit, the flow through of aluminum strand connection mode is more uniform than that of aluminum row connection mode. The test results are consistent with the simulation results, which provides a reference for the subsequent design of valve hall fittings.

Research on continuous current characteristics and operating load characteristics of EGLA in UHVDC lines

In order to ensure the safe and stable operation of UHVDC transmission system, line arresters with series gap (EGLA) are often installed on UHVDC lines in engineering. When the line is struck by strong lightning, the series gap of EGLA will be broken down and the lightning will flow through the SVU. After lightning flows, EGLA needs to cut off the continuous current in time and reliably to restore the high resistance state. Therefore, the research on the characteristics of continuous current and operating load of EGLA is related to the design and selection of EGLA parameters, which has important engineering significance. However, due to the lack of UHVDC experimental platform, the relevant research is less, resulting in the lack of basis for determining SVU parameters. Based on the ±800 kV UHVDC transmission line from northern Shanxi to Nanjing in Jiangsu Province, this paper uses the method of control variables to simulate and study the characteristics of EGLA under different lightning strike modes, line positions and transmission capacities, and draws the conclusion that compared with back flashover on the tower, the peak value of EGLA under lightning shielding failure is smaller but the absorbed energy is larger, and the influence of transmission capacity is more obvious. When the lightning shielding failure on the middle tower of the line, the absorbed energy of EGLA is the largest and the continuous current is smaller.

Bidirectional field–circuit coupling analysis of converter transformer inter‐tap short‐circuit faults in on‐load tap changers

Converter transformer inter-tap short-circuit faults in on-load tap changers have occurred successively in recent years, resulting in severe accidents and enormous losses. Due to the lack of an effective model and credible arc-in-oil conductance parameters, related fault analysis has remained a challenge. This paper develops a bidirectional field–circuit coupling method for modelling inter-tap faults, in which the electromagnetic field and electric circuit are directly coupled and simultaneously solved. After verification based on on-site arcing tests, the Schavemaker model is applied to calculate the non-linear arc conductance. Taking a ±800 kV UHVDC system as a study case, inter-tap faults in a 415 MVA/500 kV converter transformer are simulated and analysed. Under an inter-tap fault, the flux distribution is significantly distorted, with large radial leakage fluxes near the shorted turns, and the circulating current can reach tens or even hundreds of kiloamperes. Further calculation results for faults between various taps indicate that leakage flux distortion and the change in the transformation ratio are the main factors influencing the short-circuit current. These representative findings effectively reveal the variation characteristics of the leakage flux and the short-circuit current as well as the non-linear relationship between fault severity and tap position under inter-tap faults.

Lightning protection on the large crossing section of single circuit ±800kV UHVDC and double circuit of 500kV transmission lines erected on the same tower

For ±800kV UHVDC project, due to the limitation of corridor, some sections need ±800kV and 500kV AC lines to be erected on the same tower. When at the large crossing section ±800kV DC transmission line and 500kV line are on the same tower, the height of the tower is further increased, and the lightning area is increased, so it is important to research the lightning protection and adopt proper lighting protection measures. In this paper, the lightning protection scheme is determined by the criteria that the safe operation years of large crossing section of UHVDC project under lightning overvoltage shall not be less than 50 years. Based on Chizhou large crossing section that is erected in the mode of UHVDC and EHVAC transmission lines on the same tower of the UHVDC project from Baihetan to Jiangshu, the lightning shielding failure outage rate and back flashover outage rate were given, and the flashover outage rate while lightning stroke on the grounding wire in the middle of the large crossing section. Based on the results of the lightning overvoltage calculated above, the lightning protection measures were proposed.

Investigation on dielectric properties of epoxy resin impregnated paper composites for UHV DC wall bushing

The DC wall bushing is a crucial equipment connecting the AC and DC power grid, which withstands the full voltage and full current of the system during operation and has drawn widespread attention for its insulation problem. As the main insulating material of the DC wall bushing, the dielectric performance of the epoxy resin impregnated paper composites is important to the service performance of the bushing. In this paper, a sheet sample of epoxy resin impregnated paper composites obtained from an 800kV UHV DC bushing was characterized by scanning electron microscopy (SEM). The dielectric properties of epoxy resin impregnated paper composites were tested by the broadband dielectric spectrometer under different test voltages and temperatures. A simulation model of an 800 kV DC wall bushing was established based on COMSOL for electric thermal coupling field calculation. It is found that the electric field distribution in the radial direction of the bushing reverses as the temperature gradient increases, which can be attributed to the impact of temperature on the dielectric characteristics of epoxy resin impregnated paper composites.

Research on the standardization of lightning arresters layout at AC side of ±800kV UHVDC converter station

UHVDC converter station is the key position of UHVDC power system. Once lightning damage accident occurs, it may cause large-area power outage and have a very serious consequence. Therefore, UHVDC converter station requires reliable lightning protection measures. At present, the electrical distance between AC side arrester and protected equipment in UHVDC converter station has not been standardized. This paper will give the critical value of safe electrical distance between AC side arrester and protected equipment by simulation calculation. Firstly, the lightning intruding wave overvoltage level on the equipment on the AC side of UHVDC converter station under typical operation mode will be given by simulation. Secondly, the critical value of safety distance between AC side arrester and protected equipment of converter station will be studied and determined. Thirdly, for the condition that the distance between the AC side arrester and the protected equipment of UHVDC converter station exceeds the critical value of safety distance, the strengthening measures of lightning protection of UHVDC converter station are proposed. The research results will provide data support for the standardized layout of lightning arresters in UHVDC converter stations.

Overvoltage and insulation coordination study for UHVDC transmission system that is hierarchically connected to 500kV and 1000 kV AC grids on line side

Overvoltage and insulation coordination of converter station is one of the key technologies for UHVDC transmission project. The current level was upgraded to 6250A form 5000A, and the converter station was hierarchical access to 500kV and 1000kV AC grid in Shanghai Temple-Shandong and Ximeng-Taizhou DC project. We analyzed the major changes brought on insulation coordination, and calculated the level of protection and equipment insulation at each point. The design for insulation coordination was proofed to be reasonable through simulation calculation. The result of the research could provide important reference for equipment development of Shanghai temple - Shandong and Ximeng - Taizhou ±800 kV UHVDC project.