Ultra-high Voltage DC Research Articles

Analysis of heating defects in extension rods of extra-high voltage direct current overhead transmission lines

This paper studies the overheating defect of an extension rod in a ± 800 kV extra-high voltage direct current overhead transmission line, and proposes various factors that affect the overheating of the extension rod. Establish an equivalent model of an extra high voltage DC transmission line based on equipotential point and line parameter analysis, and calculate the model parameters. The proposed model can also be applied to ultra-high voltage DC overhead transmission lines such as ± 500 kV, and has strong universality. Simulation experiments show that if the equivalent resistance value of the split wire side is reduced by 0.01Ω, the power loss on the extension tie rod can be increased to 380.951 W; if the equivalent resistance value of the jumper side is increased by 0.01Ω, the power loss on the extension tie rod can be increased to 273.41 W. Combined with theoretical analysis and simulation verification, it is concluded that the main factor affecting the overheating of the extension rod is the change in the resistance value on the jumper side caused by the increase in contact resistance. A systematic troubleshooting plan for related defect problems is proposed, which has important reference and guidance significance for such line fault problems.

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.

Preparation and characteristics of nano-MgO/polypropylene composite insulating material for ultra-high voltage DC transmission line construction

Preparation and characteristics of nano-MgO/polypropylene composite insulating material for ultra-high voltage DC transmission line construction

Adaptability analysis of travelling wavefront protection in LCC‐UHVDC transmission lines

AbstractThe existing relay protections for line‐commutated converter (LCC)‐ultra high voltage DC (UHVDC) transmission lines are insufficient in protection sensitivity and operation speed. The travelling wavefront fault distance information can be used to improve protection performance, which has been used in protection of voltage source converter ‐high voltage DC (HVDC) transmission lines. The adaptability of travelling wavefront protection in LCC‐UHVDC transmission line is analyzed in this paper. First, the conditions that need to be met for the travelling wavefront protections to be able to effectively recognize faults are analyzed. Eight applicability conditions of wavefront protection are revealed from the perspective of travelling wavefront generation and practical application requirements. Then the degree of match between the applicability conditions and the conditions of LCC‐UHVDC system is analyzed as a basis for assessing the adaptability of travelling wavefront protection in the LCC‐UHVDC system. It is pointed out that the index coefficient‐based wavefront protections are not available for LCC‐UHVDC transmission lines since the hardware sampling rate of protection device is insufficient, while first peak time of transient voltage (FPTV)‐based wavefront protection is available for LCC‐UHVDC transmission lines if it has enough anti‐noise ability. Finally, an enhanced anti‐noise algorithm is proposed for the FPTV‐based travelling wavefront protection. The adaptability of the travelling wavefront protection is verified by hardware‐in‐the‐loop test and filed recording experimental data.

Control Strategy of Suppressing Short-circuit Current VSCs Feeding into the AC Grid in a Hybrid Cascaded UHVDC System

Hybrid cascaded ultra-high voltage dc is a new dc technique with a line commutated converter cascaded with one or several voltage source converters. It is an effective solution for large capability transmission as well as improving the stability of the ac grid with multiple dc transmission systems feeding in. Baihetan-jiangsu dc project in China is the first cascaded ultra-high voltage dc project in the world. For a receiving end with a strong ac system, the short circuit current is always large and close to the maximum ac breaker breaking capability, which is a threat to the ac system fault clearance. In this paper, the issue of excessive short circuit current in a hybrid cascaded ultra-high voltage dc system is discussed and revealed. The process of the short circuit current developing is divided into 2 stages. Characteristics and the calculation method of the short circuit are proposed, and based on this, a control method is proposed, which can suppress the short circuit current during the fault clearance effectively. The parameter setting method is proposed. Finally, based on the scenario of the Baihetan-jiangsu dc project, the strategy and the parameters setting method are validated under PSCAD/EMTDC.

Conjugate observations of power line harmonic radiation onboard DEMETER

The effects of power line harmonic radiation (PLHR) on near-Earth space have been debated for many years. Since some ultrahigh voltage AC and DC power projects were added to the Chinese power grid, increasingly numerous radiations at 50 Hz and their harmonics have been caught by the microsatellite DEMETER in the geomagnetically conjugate topside ionosphere during the daytime of winter and spring. Distribution of SYM-H index has been studied, showing geomagnetic disturbances may be unnecessary for the occurrence of PLHR. The propagation of PLHR between hemispheres without much attenuation illustrates that PLHR can be amplified when crosses the equatorial zone. Eliminating seasonal and diurnal factors, dependence of PLHR propagation on the change in plasma density is disclosed. The abrupt variation of electron density gradient may create a favored condition for PLHR waves propagating between hemispheres.Graphical

Suppression to concurrent commutation failure of UHVDC with hierarchical connection mode based on improved dc current prediction

AbstractFor the ultra high‐voltage dc (UHVDC) transmission with hierarchical connection mode (HCM) at the inverter side, local commutation failure (LCF) at one layer after the ac fault may cause concurrent commutation failure (CCF) at the non‐fault layer. The inter‐layer couplings at the ac and dc sides add the difficulty to suppress the CCF. The fault instant may decide the dc current variation during the commutation, which affects the suppression effect. This paper shows the mechanism of the CCF including the inter‐layer ac and dc coupling, and proposes a control method for the inverter at the fault and non‐fault layers. First, an analytical expression among the dc current, ac voltage, and extinction angle of both layers is newly derived to find the dominant factors to the CCF. Second, to improve the calculation accuracy, a three‐point sampling function using Newton interpolation is newly proposed to predict the dc current. Finally, a coordinated control strategy based on the constant extinction area, the overlap‐arc area, and the dominant factor of the CCF is proposed to adjust the firing angle and suppress the CCF. The simulation results using the PSCAD/EMTDC software are given to verify the control effect of the proposed method against the CCF.

Analysis of Corrosion Causes of Ring Connection Fittings of ±800 kV Transmission Line

The ring-linked fitting used in an ultra-high voltage DC transmission line project has suffered serious local corrosion after only one year of operation. Based on the macroscopic inspection, chemical composition analysis, galvanized layer quality analysis, metallographic organization analysis, scanning electron microscopy analysis, and energy spectrum analysis, the physical and chemical examination of the ring coupling fixture was carried out to analyze its corrosion causes and explore the corrosion mechanism. The results show that the galvanized layer of the rusted area was damaged by wear and tear due to external factors in the process of transportation, lifting, and installation. And the electrochemical corrosion led to the serious local corrosion of the fixture within a short period of operation. It is recommended to send someone to visually check the shape of the galvanized layer of the metal before the ring-linked fixture is put on the tower and to review the thickness of the zinc layer at the end of the fixture with a handheld zinc layer thickness tester if necessary.

Active power balance capability of security and stability control system (SSCS) against outage of UHVDC

The aim of the security and stability control system (SSCS) is to maintain active power balance and frequency stability. For large SSCS, it is difficult to find the state space and the transition rates of the paths. The flat design yields more complex topology. Considering the random outages, the ultra-high-voltage dc (UHVDC) has more capacity levels and states. The existing evaluation cannot quantify the power balance capability of the SSCS. This paper newly quantifies active power balance capability of the multi-level SSCS with partial or complete outage of the UHVDC. An incidence matrix is proposed to describe the topology of the SSCS. A recursive solution to calculate the transition rates is proposed. The equivalent downward transition rate is newly defined. The increasable generation and the transition rates of the UHVDC at any initial state are newly included. The numerical analysis shows that the incidence matrix and recursive algorithm reduces the calculation effort for the practical SSCS with multi-level, redundancy component, and flat design. The active power balance capability of the SSCS is decided by the increasable power, the reduced generation and the downward transition rate.

DC Commutation Failure Analysis for the Sending-Side System Fault in Hybrid UHV Half-Wavelength and UHV DC Transmission System

Commutation failure (CF) caused by the failure which occurs on the sending side of the ultra-high voltage (UHV) half-wavelength AC (HWAC) transmission system is studied. Firstly, a hybrid UHV HWAC and UHV DC transmission system model is built, and the essence of CF is revealed regarding the influences of various factors on occurrence probability of CF. Then, the mechanism of CF of the UHV DC transmission system caused by the fault at the HWAC line in the hybrid system is explored. The steady-state operation characteristics are obtained through characteristic analysis according to the quasi-steady-state model of the HWAC line, and a three-phase short-circuit fault is assumed on the sending-side of a HWAC line and its voltage characteristics are deeply analyzed. In the actual power system, the specific situation of DC CF caused by the fault of the HWAC line is verified and analyzed based on a large quantity of transient stability calculations. Comparative analysis of the effects on UHV DC lines of the faults which occur on different locations of a HWAC line is presented. The conclusions can provide a useful reference and reliable technical support for the development of HWAC system and maintain power grid security and stability.

Computation of Total Electric Field Considering Natural Wind Under High-Altitude UHVDC Transmission Lines

With the rapid development of ultra high-voltage DC (UHVDC) transmission technology, the total electric field problem in high-altitude dc transmission lines is more important. Therefore, this article proposes a calculation model of the ion flow field at high altitude based on the natural wind model and the gas motion theory. First, the initial charge density is assumed, and then, Poisson’s equation is solved by the finite-element method (FEM). The measured wind speed and altitude are introduced into the natural wind model and the ion mobility model, and the natural wind speed and the ion mobility of each node are obtained. Subsequently, the current continuity equation is calculated by this information and the upstream FEM. When the calculation results do not meet the steady-state convergence conditions, the charge density on the conductor surface is iteratively corrected. The calculation model is verified by the measurement results of the ±800-kV UHVDC transmission line at the National Engineering Laboratory for Ultrahigh Voltage Technology (Kunming) of the China Southern Power Grid. The research shows that with the increase of altitude, the ion mobility gradually becomes larger, and the total electric field at ground level also increases. Due to the influence of wind, the peak value of the total electric field at ground level is obviously shifted, and the maximum peak values are on the downwind side. This model in this article can accurately calculate the total electric field of UHVDC transmission lines at high altitude considering the natural wind and provide a reference for the design of transmission lines.

Time-Frequency Domain Conversion of UHVDC Transmission Line Corona Current Based on Fourier Transform

In the power system, Ultra-High-Voltage (UHV) transmission is an effective means to improve and solve the imbalance between power supply and demand. However, with the continuous improvement of the voltage level, the adverse effects caused by the electromagnetic environment of the transmission lines are also growing. Audible noise produced by corona discharge of transmission lines has seriously interfered with the daily life of residents near the transmission lines, which can not meet the requirements of protecting residents and environmental friendliness. In this article, the background noise in the time-domain waveform of corona current is removed by wavelet analysis. The time domain waveform of corona current is transformed into the frequency domain by a fast Fourier transform. Through the analysis of the spectrum of corona current, it is found that the corona current attenuates with the increase of frequency, and most of the energy is concentrated in the first 1.5 MHz frequency band. The relationship between each frequency band of corona current and audible noise is obtained by the multiple linear regression, and the regression model is proved to be well fitted by testing. The experimental analysis shows that the algorithm in this article is 8% better than the traditional finite element method, and is closest to the actual measurement value, which proves that the actual effect of the research contained in this article is more effective. These achievements are helpful for the further study of the electromagnetic environment of UHV transmission lines, which is conducive to promoting the vigorous development of the UHV DC transmission technology in China, allocating resources more reasonably and reducing environmental pollution, and meeting the strategic requirements of energy and environment in China.

Distribution characteristics for initial time delay of streamer of shielding spheres under operating impulse voltage at high altitudes

With continuous promotion of construction of ultra high voltage DC projects in high-altitude areas, it is of important theoretical and practical engineering significance to study long gap discharge characteristics at high altitudes. As the initial and key stage for development of long air gap discharge, the initial stage of streamer reflects the randomness of long gap discharge to a certain extent, and time delay characteristics are important indicators to reflect the randomness. In this paper, we build a photoelectric detection system based on the photomultiplier tube and photoelectric integrated sensor, carry out an operating impulse discharge test of long gap of the shielding sphere-plate with different diameters at altitudes of 50 m and 2200 m respectively, obtain the space field strength, optical power, discharge current and voltage of the whole process of discharge development, and analyze the characteristics of boost time delay and statistical time delay of streamer based on the photoionization model and Weibull Distribution model. According to study results, the average value and dispersibility of initial time delay of streamer increase with the increase of altitude and electrode diameter; the boost time delay t 0 of streamer decreases with increase of altitude and increases with increase of electrode size; the statistical time delay t s of streamer can properly comply with Weibull distribution, and its characteristic parameter has a clear physical meaning. The dispersibility of statistical time delay will increase with altitude and electrode size, and compared with electrode size, altitude has a greater impact on the dispersibility of statistical time delay. The study results of this paper have important guiding value for selection and optimization on clear distance of the valve hall of a converter station in high altitude areas.

Adaptive Under-Frequency Load Shedding Control Strategy of Power Systems With Wind Turbines and UHVDC Participating in Frequency Regulation

Represented by wind turbines and ultra high-voltage DC (UHVDC), the power-electronic interfaced power sources participate in fast frequency control, which has a significant impact on the power system frequency. However, the conventional under-frequency load shedding (UFLS) scheme doesn’t take into account the impact above, resulting in unreasonable load shedding after a large loss of generation. To this end, this article proposes an adaptive UFLS control strategy of power systems with wind turbines and UHVDC participating in frequency regulation. Firstly, based on the virtual inertia control model and the primary frequency control model of wind turbines and UHVDC, the study establishes the simplified frequency response model of the power system considering the participation of wind turbines and UHVDC in frequency regulation. Furthermore, the impact of the active power response characteristics of wind turbines and UHVDC participating in frequency regulation on the magnitude of the active power deficiency is comprehensively analyzed. Thus the precise estimation of the magnitude of the power deficiency can be achieved, which provides technical guidance for multi-stage UFLS. Finally, simulation results demonstrate that the proposed UFLS strategy is capable of reflecting the power system frequency more objectively after a large loss of generation event. In addition, the proposed UFLS strategy outperforms the conventional UFLS strategy in terms of shedding less amount of load when the same desired effect of frequency recovery is achieved.

Research on overvoltage characteristics and arrester operation under typical failure of Ultra High Voltage DC project

Accurately predicting the overvoltage level under typical faults and operating condition in the UHVDC (Ultra High Voltage DC) transmission project is not only beneficial to the insulation coordination design of the system, improving the security and reliability of the system, but also helping to reduce the project cost. Based on the circuit transient simulation platform, this paper builds a UHVDC transmission system simulation model, analyzes the typical overvoltage sources, compares the overvoltage characteristics of the system under 3 types and 12 typical operating faults including the inverter side, rectifier side, and DC line side, and analyzes the operation of the arrester under fault conditions. Results show that when the midpoint of the line is ground faulty, an overvoltage of 1791kV will appear at the midpoint of the non-faulty DC line, which is the most serious overvoltage of the DC line, and when a ground fault occurs at the outlet of the DC line on the rectifier side or the inverter side, the overvoltage amplitude of the DC valve top on the corresponding side is the highest, which respectively are 1324kV and 1190kV. The research results provide a reference for the insulation coordination of the UHVDC transmission project.

Study on the lightning shielding characteristics of the crossover area of UHV/HV transmission lines

Due to space limitations of transmission corridors, ultrahigh voltage (UHV) lines inevitably cross over other lines. UHV lines in a crossover section have a lightning shielding effect similar to that of a lightning conductor of lower lines. While, whether a 500-kV line below a crossover section meets the lightning protection requirements is unknown. Therefore, a calculation method for the shielding failure trip rate of transmission lines in a crossover section in a three-dimensional space is proposed. The calculation of the shielding failure trip rate of the 500-kV line without a ground wire below the UHV line was performed. The results revealed that the shielding failure rate of the lower line was positively correlated with the length of the lower line and was negatively correlated with the crossing angle and length of the upper line. The maximum traversal lengths for a 500-kV line without a ground wire below a UHV AC or DC line were 200 m and 150 m, respectively. These findings provide a scientific basis for lightning protection design of ultra-UHV line crossing areas.

Pollution morphology characteristics on a superhydrophobic surface and its pollution flashover voltage in DC electric field

To solve the pollution problem of insulators on ultra-high voltage DC transmission lines, superhydrophobic coatings were presented and their pollution deposit morphology and pollution flashover voltage in electric field were studied by an artificial pollution deposit system. The experimental results show that the pollution on the surface of superhydrophobic coating is distributed in clusters, which inhibits the chain distribution on the surface of silicone rubber and improves the pollution flashover voltage. Secondly, the competition mechanism of the pollution flashover on the superhydrophobic coating surface was found. This competition mechanism can lead to the failure of the insulation performance of the superhydrophobic coating in severely polluted environments. Finally, a pollution flashover model of superhydrophobic coating is proposed to explain the competition mechanism, which is affected by pollution and the discrete droplets. The conclusions of this study can provide an experimental and theoretical basis for the application of the superhydrophobic coating in the field of UHV DC external insulation.

Thyristor Aging-State-Evaluation Method Based on State Information and Tensor Domain Theory

The thyristor is the key device for the converter of the ultra-high-voltage DC (UHVDC) project to realize AC–DC conversion. The reliability of thyristors is directly related to the safe operation of the UHVDC transmission system. Due to the complex operating environment of the thyristor, there are many interrelated parameters that may affect the aging state of thyristors. To extract useful information from the massive high-dimensional data and further obtain the aging state of thyristors, a supervised tensor domain classification (STDC) method based on the adaptive syn-thetic sampling method, the gradient-boosting decision tree, and tensor domain theory is proposed in this paper. Firstly, the algorithm applies the continuous medium theory to analogize the aging state points of the thyristor to the mass points in the continuous medium. Then, the algorithm applies the concept of the tensor domain to identify the aging state of the thyristor and to transform the original state-identification problem into the state classification surface determination of the tensor domain. Secondly, a temporal fuzzy clustering algorithm is applied to realize automatic positioning of the classification surface of each tensor sub-domain. Furthermore, to solve the problem of unbalanced sample size between aging class data and normal class data in the state-identification domain, the improved adaptive synthetic sampling algorithm is applied to preprocess the data. The gradient-boosting decision tree algorithm is applied to solve the multi-classification problem of the thyristor. Finally, the comparison between the algorithm proposed and the conventional algorithm is performed through the field-test data provided by the CSG EHV Power Transmission Company of China’s Southern Power Grid. It is verified that the evaluation method proposed has higher recognition accuracy and can effectively classify the thyristor states.

Research on Consistency of Transient Response Characteristics of Current Transformers for DC application in DC Engineering

In ultra high voltage(UHV) DC system, the transient characteristics for fault current monitoring of current transformers for DC application (DCCTs) and the consistency of response characteristics when different types of DCCTs are used for differential protection have become important factors affecting the safe and stable operation of DC system. In this paper, transient characteristics of all-fiber DCCTs and shunt DCCTs mainly used in DC system were studied, the main parameters of the transient characteristics including delay time and maximum peak instantaneous error were tested, and the key parameters influencing the response consistency of DCCTs were analyzed. The results show that the maximum peak instantaneous errors of all-fiber DCCTs and shunt DCCTs can meet ±5% limit requirement, meeting the demand for protection application in DC system. The delay time is the main factors affecting the maximum peak instantaneous error of DCCT. The longer the delay time is, the greater the maximum peak instantaneous error is, which may even exceed the limit value of ±5%. If necessary, the delay time of DCCTs participating in differential protection can be compensated. The research results provide a theoretical basis for preventing the malfunction of protection device caused by the inconsistent response characteristics of DCCTs in DC system.

UHVDC-Technology Future of India Electricity Transmission

it's proposed to use highly complex grid controllers to include power grids into one super- grid that may acquire large penetration of inexhaustible powers, without compromising power quality, active and reactive power flow, and voltage and facility stability. The super-grid constructed with ultra- high voltage DC (UHVDC) and flexible ac transmission systems (FACTS) together with dedicated ac and dc interconnectors with intelligent systems applications to supply a wise Integrated Super-Grid. DC interconnectors will segment the whole continent's power systems into five large asynchronous segments (regions). Noncontemporary divisions will prevent ac fault propagation between sections while allowing power exchange between different parts of the super-grid, with minimum difficulty for grid code unification or harmonization of regulatory regimes across the mainland as each segment maintains its accord . a sensible Integrated wattage Super-Grid powered by these technologies is critical in supporting sustained economic process and development; established on the keystone of renewable energy and utilizing over 600GW immeasurable potential of Africa's clean and renewable hydroelectric, photovoltaic and alternative energy as a little of a extensive energy comingle of traditional and complementary energy resources.