UHVDC Transmission Research Articles

Fragility analysis of an ultra high voltage transmission tower in saline soil subjected to wind loads

The wind resistance of transmission towers is not only affected by wind load, but also by service environment. This study uses the world’s second Ultra High Voltage Direct Current transmission project – Xinjiang ±800 kV Tianzhong Line UHV DC transmission project – to develop a fragility analysis method for transmission towers in saline soil under wind loads to investigate the change of wind loads fragility of transmission towers in long-term service in the saline soil environment. It develops a tower-line-foundation (3 T-2L-F) system model considering soil-structure interaction. In addition, this research addresses the durability damage of the transmission tower using field investigation data and material degradation models and analyzes the influence of various durability damage components on the natural vibration mode of the basic of 3 T-2L-F model. Finally, it builds the structure-wind samples utilizing a Latin hypercube sampling method and explores the time histories analysis, the pushover analyses, and the time-varying fragility analyses considering the uncertainty of materials and wind loads. The findings indicate that the 3 T-2L-F model accurately simulates the actual situation of the transmission tower. The fragility of a transmission tower subjected to wind loads is proportional to the degree of material damage and the strength of wind loads.

Design of Energy-efficient and Energy-efficient Slag Discharge Device for Large-bore Drilling Rigs

Micro pile hole is a kind of bored pile with a diameter of 100~400mm, which has been widely used in the field of transmission line engineering in recent years, and its hole formation usually uses DTH (Down-the-Hole) drilling rig. When DTH drilling rigs operate, the slag discharge method is to use the compressed air provided by the air compressor to blow the muck directly from the bottom of the hole to the ground. However, during the slag discharge process, it is difficult to blow out the muck due to the large cross-sectional area of the annular space between the hole wall and the drill pipe, especially after a certain drilling depth, and it is even more difficult to discharge if wet soil is encountered. Therefore, there is an urgent need to develop a machine for auxiliary slag discharge to improve the efficiency of slag discharge and reduce working energy consumption. In this paper, a barrel slag discharge device connected to the end of the drill pipe and following the drill bit to the bottom of the hole is designed, which can collect and store the rock slag that is difficult to discharge directly outside the hole, achieve a good slag discharge effect, and can reduce the energy consumption of the air compressor and reduce the drilling cost. Through the Fujian UHVDC transmission line project, the reliability and practicability of the barrel slag discharge device are verified.

Research on Intelligent Identification of Magnetizing Inrush Current based on Empirical Modal Decomposition

The accurate identification of the converter transformer magnetizing inrush current can ensure the safety of the operation of the UHV DC transmission system. In the differential protection of the converter transformer, it is extremely hard to distinguish between the excitation inrush current generated when the transformer is closed and the Fault current due to internal fault, which leads to frequent misoperation of the transformer protection to the extent that the transformer access fails. This paper proposes a current identification model based on empirical modal decomposition (EMD) and probabilistic neural network (PNN) algorithm with particle swarm (PSO) optimization. The EMD algorithm decomposes the current waveform, and the eigenmode functions (IMFs) of the three highest correlation bars are selected. The energy, cliffs, and standard deviation of the IMFs are extracted as the original waveform eigenvectors.

A novel pilot protection method for UHVDC transmission lines based on U-I characteristics

As one of the principal protections of UHVDC transmission lines, pilot protection requires strict synchronization and small data flow by exchanging the current on both terminals under the limitation of DC protection action rapidity. To solve this problem, this paper finds that the DC voltage and current on both terminals of the line have different plane distribution characteristics when faults occur internally and externally in the system area, and then advances a novel pilot protection method for UHVDC transmission lines based on U-I characteristics. By setting up the U-I plane with DC voltage and current as variables, analyzing the dynamic trajectory of the real-time captured U-I plane, constructing protection criteria based on the U-I plane on the rectifier side and inverter side separately, and finally converting the judgment results into state information. Both terminals of the line identify the fault location by exchanging the longitudinal comparison of state information. This protection project only utilizes the instant tracks of voltage and current in the rectifier side and inverter side at the time of fault to judge. The principle is simple and can quickly locate faults. The state messages of the two terminals are not modified by the data asynchrony on both terminals. This protection method is convenient for setting, has high sensitivity, and can quickly identify interior and exterior faults. The results are testified by theoretical analysis and simulation verification.

Analysis Method of Primary Frequency Modulation Capability of Power System Based on GRNN

One of the most important key parameters in power network operation is power frequency. The power network will be affected by various disturbances in operation, and the frequency of the power network will show different fluctuations under different disturbances. Once the frequency of the power network deviates from the normal operation level, the power quality of the power network will decrease, and the power equipment will be in an abnormal operation state. With the development of power network, the scale of distributed microgrid with high uncertainty such as new energy is expanding increasingly, the proportion of UHVDC transmission lines is increasing, and the structure of power network is changing, which makes the regulation of power network frequency facing greater challenges. In this paper, relying on the mechanism of integrated load of power network and the participation of generator set in primary frequency modulation, the primary frequency modulation capability of 60 s power network under different frequency disturbance conditions is analysed, and the training and simulation prediction of GRNN generalized regression neural network prediction model are completed by using MATLAB software. By predicting of the generator set under different disturbances, the parameters related to the primary frequency modulation capability of the power network, such as the maximum amplitude of the active power fluctuation, the time when the maximum amplitude occurs and the time when the power network is restored to stability, are obtained to provide more accurate dispatching support for the dispatching side.

Topography Optimization and Buckling Test of Tension Plates in UHVDC Transmission Lines

Based on the dynamic response of a typical UHVDC transmission line section after ice-shedding and the topology optimization of the tension plate, topography optimization of the tension plate to maximize buckling load is carried out to obtain the optimized schemes with stiffener. Considering the optimization results and manufacture cost, three optimized schemes of the tension plate with stiffener, whose buckling load enhances more than 40% with a bit of weight increase, are proposed. Reduced-scale modeling method for tension plates was presented and the buckling tests of the reduced-scale original and one optimized tension plates were performed. It is demonstrated that the buckling loads of the tension plates measured by the tests are consistent with those by the numerical simulations, which verifies the correctness and efficiency of the numerical simulations for the nonlinear buckling and the optimization of the tension plate. The optimized tension plates with stiffener are efficient and can be utilized in real transmission lines.

Research on Energy Dissipation Technology and AC Overvoltage Suppression of Sending End in UHVDC Transmission

For suppress the amplitude of overvoltage from HVDC sending terminal, an overvoltage suppression scheme based on the principle of controllable energy dissipation was proposed. The overvoltage mechanism of a weak AC system under the fault of the DC system was analyzed, and the method and operating principle of the CSREDD were proposed. Prototype of the develop CSREDD was tested, which proved correctness about the design. The controllable energy dissipation device has been successfully applied to the ±800kV Lu-Gu UHVDC transmission project. The operation results show that the controllable energy dissipation device can effectively suppress the AC overvoltage.

Construction of a Digital Twin Model for Loss Metering in UHVDC Transmission Systems Based on Deep Learning

Construction of a Digital Twin Model for Loss Metering in UHVDC Transmission Systems Based on Deep Learning

Analysis of Protection Methods of UHVDC Transmission Lines

Analysis of Protection Methods of UHVDC Transmission Lines

Research and application of high voltage measurement technology in DC UHV preventive test

With the state’s high attention to clean energy and the requirement to reduce the loss of power grid transmission lines, DC UHV has developed rapidly in recent years. It refers to DC transmission and related technologies with voltage levels of ±800 kV (±750 kV) and above. In the power system, DC high-voltage transmission is inseparable from power equipment, but the higher the transmission voltage level, the higher the voltage of DC withstand voltage preventive test. Before the transmission of UHV DC transmission lines, if the DC voltage withstand device is 800 kV or above, the voltage withstand test shall not be carried out for the high-voltage line. The traditional direct comparison method using voltage divider cannot be used to measure the DC output voltage of DC UHV withstand voltage device on site. This paper analyzes the method of full coverage of DC UHV system in limited voltage range and the method of out of range calibration using scale factor. In order to solve the problem of voltage traceability in the DC high voltage generator, especially the voltage traceability of the field DC UHV device, when the voltage output range of the calibrated DC high voltage generator exceeds the measurement range of the standard, the standard value of the voltage calibration point beyond the range can be determined by comparison and obtained through the linearity test. The comparison with the standard shall be conducted at a voltage not lower than 20% of the rated voltage of the calibrated DC high voltage generator, and compared to the maximum voltage of the standard voltage divider. The comparison is carried out at a (a ≥ 2) voltage point. When comparing at a point, the highest voltage level is equal to the upper limit of the standard. Under a voltage point, according to the mathematical relationship between the voltage standard value of the tested device and the current flowing through the linear load resistance, the fitting function can be obtained through the linear regression method. For b (b ≥ 4) calibration points beyond the measurement range of DC standard voltage divider, the standard value can be “extrapolated” from the linear load resistance current and the fitting function Yang (2002). Through comparison and verification, the method is feasible, which is of great significance to promote the compilation of calibration specification for DC high voltage generator.

Analysis of Unit Commitment of Coal-fired Units in a Power Base of Cross-area DC Transmission Lines Under Dual Carbon Targets

China has announced achieving a carbon peak before 2030 and neutralization by 2060. Cross-area UHVDC transmission provides a feasible way for the new energy base to transmit power to the load center on a large scale. Usually, the dispatching of coal power units of DC transmission line power base is divided according to the equal proportion of unit capacity, this paper takes all the units as a group and proposes a unit commitment solution to realize lower carbon emission. As the capacity of new energy in the power base increases, this paper also considers the generation margin for new energy to do the optimization simulation. The results show that by operating coal-fired units as a group, it operates in a more economic and environmentally friendly way.

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.

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.

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.

A numerical analysis model for inception and propagation of positive upward leader from UHV DC transmission lines

As the upward leader from the transmission line plays a vital role in the interception of lightning downward leader, the proper description of upward leader characteristics is beneficial for improving the evaluation accuracy of lightning performance for UHV DC transmission lines. Based on the leader discharge theory, a novel numerical analysis model to simulate the inception and propagation process of positive upward leader from the transmission line was proposed. The model-based calculation results show good agreement with the data of laboratory long spark discharge, natural lightning observations and operation experience of transmission lines, which verifies the validity of this model. Moreover, the influence of operating voltage, protection angle and tower height on the upward leader characteristics for the typical UHV DC transmission line was analyzed. The results show that the lightning interception ability of upward leader from the positive polar conductor is enhanced with the increase of voltage level, protection angle, and tower height, which theoretically illustrates that installing the positive polar conductor closer to hillsides, adopting the negative protection angle and reducing the tower height are effective lightning protection measures for UHV DC transmission lines.

Frequency control strategy for sending-end of hybrid cascaded UHVDC system with large-scale wind power access

For the frequency overrun problem caused by the high proportion of wind power in the sending grid of hybrid UHVDC transmission system, the coordinated frequency control strategy of wind power and hybrid UHVDC system in the sending system is proposed. Firstly, the additional frequency control strategy considering primary frequency control and inertia characteristics is introduced in the wind turbines and the line commuted converter (LCC) station respectively, and the virtual inertia control strategy is introduced in the modular multilevel converter (MMC) station. Secondly, the action rules and control strategy of wind power and hybrid UHVDC system coordination involved in the power grid are designed. At the same time, the mechanism of frequency control strategy is analyzed​ based on the frequency response model of wind power and UHVDC integrated system. Finally, the simulation results indicate that the proposed frequency control strategy can enhance the inertia and primary frequency capability of the AC system at the sending end, and effectively improve the frequency stability problem of the AC system under different operating conditions.

Fault Diagnosis Method for UHVDC Transmission Based on Deep Learning under Cloud-Edge Architecture

Aiming at the problem of fault diagnosis after the UHVDC system fails, a deep learning-based UHVDC fault diagnosis method under the cloud-edge architecture is proposed. First, based on the edge computing framework of the “cloud” + “edge terminal,” a four-layer fault diagnosis structure including the data integration layer, edge prediction layer, cloud diagnosis layer, and human-computer interaction layer is constructed. Then, a fault data set is constructed by finding effective information that can fully reflect the DC fault in the huge power grid environmental information, and the data set is screened, processed by classification feature fields, and linearly normalized. Finally, a deep convolutional generative adversarial network (DCGAN) is constructed by introducing a deep convolutional neural network (DCNN) into the traditional generative adversarial network (GAN) for data training and DC fault diagnosis. In addition, the corresponding process is given. The proposed method and the other three methods are compared and analyzed by simulation experiments. The results show that the method proposed has the highest accuracy and smallest error loss value of 95.6% and 0.18, respectively. It has the highest diagnosis accuracy under different fault types, and its performance is better than the other three comparison methods.

Improvement of Dynamic Reactive Power Characteristics of New-type Condenser by Excitation Parameters

The new-type condenser has the capability of dynamic reactive power compensation under subtransient, transient and steady-state, which could effectively solve the problem of insufficient dynamic reactive power reserve under the background of large-scale development of UHVDC transmission. In this paper, the excitation system parameters of the condenser are taken as the research object. First, the basic principle of new-type condenser and the excitation system model are introduced. Then, an effective gain of reactive current is defined as the quantitative index of the dynamic reactive power characteristics of the new-type condenser, and the effect of excitation system parameters on the dynamic reactive power characteristics of new-type condenser is studied by frequency domain trajectory sensitivity analysis. Finally, the simulation model of the condenser grid connected system is built based on PSCAD/EMTDC, the steady-state reactive power output level and transient response characteristics of new-type condenser with different excitation parameters are studied, and the optimization direction of the condenser excitation system parameters is determined. The simulation results show that reducing k A, k p, k D, T m and increasing k I T D, T Z are beneficial to improve the steady-state reactive power output of the condenser. Decreasing k A, k p, k D and increasing k I, T D, T m, T Z are beneficial to improve the transient response characteristics of the condenser.

Study on Triggering Characteristics and Induced Breakdown Rules of SF₆ Gap Switch Plasma Jets at Extremely Low Working Voltage

The SF₆ gap switch has obvious advantages in the flexible control very fast switch of the controllable arrester used in hybrid UHVDC system. However, the plasma jet ability in SF₆ atmosphere at high pressure is greatly restricted, which is easy to cause trigger failure. The triggering characteristics and induced breakdown mechanism are still unclear during the plasma jet-triggered process of the SF₆ gap switch at extremely low working voltage. In this study, a plasma jet-induced breakdown research platform based on two-stage continuous surface-triggered gas gap switch (GGS) was established to investigate time-space distribution evolution characteristics, the critical triggering conditions parameters, and working voltage functioning rules of the SF₆ gap. The jet plasma has a high-brightness-like conical shape, and its initial jet velocity can reach more than 2 km/s. Increasing capacitance value and charging voltage of the secondary chamber in SF₆ gap switch trigger cavity, its creeping arc current is increased, and the time delay and dispersion will be reduced. Moreover, the geometric characteristics (including jet area, jet radial and axial length, and jet velocity) and electrical parameters of the jet plasma increase gradually. Increasing the working voltage of main gap in SF₆ gap switch, the plasma jet velocity lift phenomenon appears at induced breakdown end of SF₆ gap switch. With the continuous increase of the working voltage, the lift effect is more significant. The purpose of this study is to solve the key problem of plasma-induced breakdown failure of the SF₆ gap switch, which will guide the design of fast self-recovery energy discharge protection switches for hybrid UHVDC transmission systems.

Accurate Modeling Simulation and Experimental Study of Hybrid Multi-Terminal UHVDC Transmission System

This study researches on the Wudongde hybrid multi-terminal Ultra High-Voltage Direct Current Transmission (UHVDC) project (referred to as Wudongde transmission project in short) from the electromagnetic transient model and simulation and establishes an accurate simulation model of the hybrid multi-terminal UHVDC transmission system with the Power Systems Computer-Aided Design (PSCAD). The hybrid multi-terminal UHVDC transmission system consists of the primary system and the control system. The simulation model of the primary system adopts the parameters of the Wudongde transmission project, and the simulation model of the control system refers to the control system benchmark model of HVDC transmission and 9-terminal DC grid in the International Council on Large Electric Systems (CIGRE). According to the characteristics of the hybrid multi-terminal UHVDC control system, the benchmark model of HVDC transmission and 9-terminal DC grid control system in the CIGRE is modified accordingly, and a simulation model of the control system applicable to the hybrid multi-terminal UHVDC transmission system is established. The accuracy and credibility of the model are verified through the comparison of the recording waveform and the simulating waveform of the steady-state and transient operation. The simulating and field recording waveforms show that the model can accurately simulate the Wudongde transmission project and be used as an effective tool for further studying the electromagnetic transient characteristics of the hybrid multi-terminal UHVDC transmission system.