Ultra-high Voltage Grid Research Articles

Research on an Improved Convolutional Neural Network Fault Diagnosis Method for Exciter System

ABSTRACT At present, China’s power grid has entered a new stage of ultra-high voltage, long-distance, large-capacity, large-unit, AC and DC hybrid interconnection grid, and the generator excitation system has an important impact on the stability of the power system. As the core component of the excitation system, the exciter plays a vital role in the safe operation of the excitation system. In view of the above problems, this paper proposes an improved CNN exciter fault diagnosis research method. To be able to learn more abundant fault features, this article first builds a dilated convolutions which is types of deep convolutional neural network (DCNN) to expand the receptive field of the convolution kernel. Then build the Pythagorean Spatial Pyramid Pooling Layer (PTSPP) to further enhance the feature information of the extracted samples. Finally, this article will generate two-dimensional matrix samples from the collected excitation voltage and excitation current signals for model training. The experimental results show that the proposed PTSPP-DCNN method has high classification accuracy in the fault diagnosis of the exciter system. The comparison results show that the fault classification accuracy of the proposed method is higher than other deep learning methods.

Electrical-thermal coupling characteristics of pre-insertion resistors in AC filter circuit breaker for UHV grid.

The ultra-high voltage (UHV) AC/DC grid can provide a platform for sustainable power worldwide. To improve the bus voltage quality of the UHV AC system, AC filters are frequently switched into the UHV grid through circuit breakers with pre-insertion resistors. The pre-insertion resistors suppress inrush currents and operate over-voltage during switching. In this paper, we establish a macro and micro model of the pre-insertion resistor based on its temperature coefficient and micro-morphology. We simulate and analyze its electric-thermal coupling characteristics under standard closing and short-circuit faults. After the simulation model and physical comparison analysis, we find that under a usual closing surge, the electric field distribution of the pre-insertion resistor is uniform and undergoes a slight rise in temperature. However, under a short circuit fault, the temperature rise is drastic and exceeds the maximum allowable temperature, causing glassy melt in some parts of the resistor. Considering the volume ratio of each component of the resistor, a two-dimensional cross-sectional simulation model of the resistor is established to simulate the electric-thermal characteristics of the microstructure of the resistor, and insinuates that the current is concentrated in the carbon channel. That is mainly due to the uneven distribution of carbon material and may lead the local temperature to exceed the maximum allowable temperature and damage the resistor.

Belt and Road Initiative with Global Energy Interconnection

Belt and road initiative delivers an idea for Global Energy Interconnection (GEI) as globally interconnected strong and smart grid based on Ultra-High Voltage (UHV). Global Energy Interconnection is an infrastructure platform that can be used for clean energy production, transition and consumption worldwide. GEI facilitate efforts to meet global power demand with clean and green alternatives as low carbon energy. GEI is promoting integration of energy, information and transportation networks, enlarging global power trade, ensuring universal electricity service. GEI optimizes resource allocation and utilization by converting various energy sources including coal, oil, hydro, wind and solar into electricity and transmitting over long distance. It can achieve mutual support and free trade of clean energy from different regions, and maximize energy efficiency and economy by taking advantages of time-zone, seasonal and price difference. Actually, Global Energy Interconnection is a system that consist of “Smart Grid”, “UHV Grid” and “Clean Energy”. Smart Grid integrates modern smart technologies with respect to advanced power transmission, smart control, new energy integration and new energy storage. UHV Grid is mainly composed of 1000 kV (and above) AC and +-800 kV (and above) DC transmission lines, featuring long transmission distance, large capacity, high efficiency, low line loss, less land use and high security. The shaping up of GEI can be divided into three phase: domestic, intra-continental and intercontinental interconnection. UHV Grid result is visible in already tested in practice transmission lines of DC and AC voltage. Over 20 UHV networks have been built in China. There is one network longer than 2000 km in Brazil, and two networks of UHV in India.

Building a New-Type Power System to Promote Carbon Peaking and Carbon Neutrality in the Power Industry in China

Power industry is the largest carbon emission sector in China. Therefore, this industry will play an important role in achieving the carbon peaking and carbon neutrality goals (“dual carbon” goals) for the whole society. For this purpose, it is critical to balance four relationships, i.e. relationship between carbon mitigation and energy security, carbon peaking in the short term and carbon neutrality in the long term, the coordinated carbon mitigation in power industry and other industries, and technical feasibility and economic efficiency. This paper proposes the China’s Energy Interconnection scenario to promote carbon peaking and carbon neutrality of the society, which can be divided into three stages, i.e. peaking carbon emissions before 2030, rapid emission reduction during 2030–2050, and comprehensive carbon neutrality before 2060. It is expected that the power system will achieve zero emissions before other industries and contribute negative emissions afterwards, providing emission space for carbon neutrality in other industries across the society. Looking into the future, China’s electricity demand will continue to grow. Clean energy will become the main power source, the use of coal-fired power will be gradually reduced, and gas-fired power will undertake the function of peak-load regulation. Faster progress will be made in forming an overall grid pattern with ultra-high voltage (UHV) grid as the backbone and featuring “west-to-east and north-to-south power transmission, a balanced energy mix, and transnational power interconnection.” Finally, this paper discusses a new-type of power system characterized by a high proportion of electricity generated from clean energy, high ratio of electric and electronic equipment, and a high proportion of power transmission and reception, together with summer and winter high load periods, and points out the challenges faced by the new-type power system with a high proportion of renewable energy sources in terms of flexibility, safety, and economic efficiency, and proposes corresponding policy recommendations.

Investigations on Pressure Fluctuations in the S-Shaped Region of a Pump–Turbine

Hydraulic pumped storage is a special power generation and electricity shortage technology, which is usually operated with thermal power and nuclear power units, and plays a key role in ultra-high voltage and smart grid. Pressure fluctuations are the main reasons for the instability of the S-shaped region of pump–turbines, which seriously affects their lifespan and operation stability. To reveal the mechanism and propagation law of pressure fluctuations in the S-shaped region as well as numerical simulations at the turbine, the braking and the reverse pump operating conditions of a pump–turbine were carried out. Numerical results were validated using the performance experiments, and the generation mechanism and propagation law of pressure fluctuation were analyzed in detail. The analyses show that high-amplitude pressure fluctuations mainly occur in the braking and reverse pump operating conditions. Under the braking condition, a 0.49-fn low-frequency pressure fluctuation was captured, which is caused by the rotation of the backflow in the vanes. Under the reverse pump condition, a 0.19-fn low-frequency pressure fluctuation was confirmed, which is caused by the periodic rotation of the vortex between the vaneless space. This study has important guiding significance for practical engineering application.

Geomagnetically Induced Current Calculation of High Voltage Power System With Long Transmission Lines Using Kriging Method

Calculation of geomagnetically induced current (GIC) flowing through power system during the geomagnetic storm has attracted more attention recently. However, for high voltage power systems with transmission lines over hundreds or even thousands of kilometers, the earth model and geomagnetical field generally vary significantly. So, it is essential to take them into consideration using limited earth survey sites and geomagnetic observatories. To address this problem, a Kriging method is introduced in this paper to make earth model and geomagnetical field interpolations. It has the characteristic of spatial autocorrelation by considering not only the distances between predicted points and training points but also the distances between training points themselves. Finally, a case study of the Central China 1000 kV ultra-high voltage (UHV) grid is carried out to illustrate the applicability and effectiveness of the proposed method.

Has the inter-regional transmission grid promoted clean power development? A quantitative assessment on China’s electricity sector

Inter-regional transmission has always been taken as an important measure to better the power mix in China, however, not all the people think it effective in boosting the development of clean power because quite many intensively invested transmission lines are not fully occupied. To objectively evaluate the impacts of the introduction of inter-regional transmission lines, especially the ultra-high voltage (UHV) grid, may clarify the disputes and benefit China’s electricity policy reform. This work simulates an equilibrium model of China’s electricity market through the mixed complementarity problems (MCP) method, to investigate the impacts of the construction of the UHV transmission lines on the deployment of clean power and reduction of carbon emissions under different scenarios. The results indicate that the introduction of the UHV grid has increased the competition in both power generation and consumption sides, but it has limited contribution to mitigate the curtailment of clean power and carbon emissions, and there may exist the hysteresis effect of the UHV grid. We propose the following suggestions to better the development of China’s power system. (1) With regard to electricity market coupling, it is more appropriate to firstly connect the regional market in the central and eastern China. (2) Promoting the installed capacity of clean power in a short-run is a necessary auxiliary method to effectively reduce the carbon emissions.

Research on Optimal Control of PFC of Thermal Power Units in DC UHV Grid

By analyzing the change of grid frequency during direct current(DC) ultra high voltage(UHV) faults this year, it is found that the use of conventional primary frequency control(PFC) technology in thermal power units cannot suppress the occurrence of extreme frequency of the power grid, and the conventional PFC method can no longer adapt to the frequency control of the power grid. By analyzing the control signals and control logic of the unit participating in the frequency regulation of the power grid, the PMU(phasor measurement unit) frequency signals that are closely connected to the power grid are introduced, and the second-order differential feed-forward is used to achieve overshoot compensation, which makes the frequency modulation action faster and more accurate, ensuring the power grid’s safe and stable operation.

Recognition Algorithm of Transient Overvoltage Characteristic Based on Symmetrical Components Estimation

The recognition of transient overvoltage characteristics is the premise of disturbance compensation of the transient overvoltage. Based on that, the recognition algorithm of transient overvoltage characteristics based on symmetrical components estimation was proposed. The generation mechanism of the transient overvoltage in gas insulated switchgear (GIS) was analyzed. Then, the transient overvoltage was measured via the capacitive sensor method. The three-phase voltage of ultra-high voltage grid was asymmetrical when the transient overvoltage appeared. At present, the asymmetrical three-phase voltage was decomposed into the superposition of a symmetrical positive-sequence component, a negative-sequence component, and a zero-sequence component via the symmetrical components estimation to build the superposition model. The model was decomposed via the trigonometric identity and the modified neural network of the least mean square learning rule was used to estimate the parameter vector of the characteristic quantity of the transient overvoltage in real time. The feasibility of the proposed algorithm was verified via comparing the simulation of the proposed algorithm and the algorithm based on dp transformation. The experimental results show that the proposed algorithm has the advantages of a small operand, high detection precision, and fast action.

Uncertainty Quantification of Geo-Magnetically Induced Currents in UHV Power Grid

Geo-magnetically induced currents (GICs) have attracted more attention since many Ultra-High Voltage (UHV) transmission lines have been built, or are going to be built in the world. However, when calculating GICs based on the classical model, some input parameters, such as the earth conductivity and dc resistances of the grid, are uncertain or very hard to be determined in advance. Taking this into account, the uncertainty quantification (UQ) model of the geo-electric fields and GICs is proposed in this paper. The UQ of the maximums of the geo-electric fields and GICs during storms is carried out based on the polynomial chaos (PC) method. The results of the UHV grid, 1000 kV Sanhua Grid, were presented and compared to the Monte Carlo method. The total Sobol indices are calculated by using the PC expansion coefficients. The sensitivities of geo-electric fields and GICs to the input variables are analyzed based on the total Sobol indices. Results show that the GICs and geo-electric fields can be effectively simulated by the proposed model, which may offer a better understanding of the sensitivities to input uncertain variables and further give a reasonable evaluation of the geomagnetic threat to the grid.

Intelligent Control Technology of Ultra-High Voltage Grid

In order to ensure the stability and economy of ultra-high voltage grid in construction, we need to research the intelligent control method of ultra-high voltage grid. Using current method in ultra-high voltage grid construction, there is a problem of poor stability. Therefore, this paper proposed an intelligent control method of ultra-high voltage grid. This method analyzed the transmission capacity of power grid and electromagnetic loop operation, and used the genetic algorithm to compute the optimization model, finally analyzed the stability of the power frequency voltage completing the intelligent control of ultra-high voltage grid. Experimental results show that this method has high practical value.

Design and Application of Insulator Detection Robot System for UHVDC Transmission Line

In recent years, with the vigorous construction of the ultra-high voltage grid, the line operating voltage level has become higher and higher. In order to ensure the safe operation of the transmission line, the line insulator string has a length of several tens of meters, and traditional manual testing has been unable to meet the requirements. With the continuous maturity of wireless communication technology, electromagnetic compatibility technology, sensor technology and other comprehensive technologies, it is necessary to develop an insulator string detection robot to replace artificial insulator detection work on UHV transmission lines. By means of wireless sensor communication technology, electromagnetic compatibility technology, video image technology and many other technologies, the insulator detection robot designed in this paper has realized automatic detection of zero resistance value of insulator of UHV transmission line. In addition, the real-time monitoring of insulator resistance value can be carried out through wireless communication to achieve the purpose of timely alarm. In addition, the robot's motion status and the insulator's field status can be observed in real time through video image acquisition. The insulator detection robot designed in this paper realizes the automatic detection of the zero value of insulator of the UHV DC transmission line. It can adapt to the UHV AC DC transmission line with different voltage levels by changing the length of the climbing arm, and at the same time adapt to the climbing detection of the string insulator of tension tower and the vertical insulator of the straight tower. The problem that the insulator string of the UHVDC transmission line can not be automatically detected by electric charge has been solved. It greatly improves the efficiency of insulator detection and the automation level of power transportation maintenance, reduces the cost of human transportation, and creates higher economic and social benefits.

An UHV Grid Security and Stability Defense System: Considering the Risk of Power System Communication

An ultra high voltage (UHV) ac and dc interconnection will become the foundation of China’s future smart grid. Due to the wide spread of interconnected regions, the distance between control stations will increase dramatically. Therefore, the communication system’s reliability and real-time performance will become increasingly crucial. However, failures of the communication system, such as interruptions, latency, and bit error, are inevitable. This paper uses the UHV grid security and stability defense system (SSDS) as an example to analyze its requirements for communication and the impact of communication failure on the system’s performance. The effect of communication latency on the power system’s stability is analyzed quantitatively and qualitatively. Based on this analysis, a framework of an UHV grid SSDS considering the risk of the communication system is proposed. A preliminary power system and communication system co-simulation tool is developed to perform a case study. The case study demonstrates that communication latency in the UHV grid changes the control strategy’s effectiveness due to a delay in executing the control strategy. Furthermore, communication latency will negatively affect the power grid’s stability.

Impact of the EHV Power System on Geomagnetically Induced Currents in the UHV Power System

Earlier calculations of geomagnetically induced currents (GICs) in Chinese power grids have mainly concentrated on the highest voltage-level system whose geomagnetic risk is considered to be the largest, thus ignoring secondary voltage-level systems. With the 1000-kV system being newly added to China's power system, it is significant to figure out the interaction between GIC in the 500 kV [extremely high voltage (EHV)] system and GIC in the 1000 kV [ultra-high voltage (UHV)] system. Based on the North China-Central China-East China Power Grids, this paper establishes a single-voltage grid by only considering the 1000-kV system and a dual-voltage grid by considering the 1000- and 500-kV systems and investigates GIC in these two grids by developing their GIC “Full-node models.” The impact of the 500-kV system on GIC in the 1000-kV system is analyzed. GIC risks in the UHV and EHV systems are assessed by comparing calculated GIC data with monitored values of GIC. The results show that the impact of the 500-kV system on GIC in the 1000-kV system is obvious, both the EHV and the UHV grid have a high GIC risk. So calculating GIC in the UHV system and in the EHV system should utilize GIC modeling methods for multivoltage power grids.

A Coordination Control Method of Generator Out-of-Step Protection Based on WAMS Transient Stability Prediction

The construction of ultra-high voltage grid and the input of large generating units make the out-of-step protection become increasingly important. Traditional out-of-step protection device can only judge from local perspective, no consideration to the global situation. In order to solve this problem, a coordination control method of generator out-of-step protection based on WAMS transient stability prediction is proposed, the coordination between grid side out-of-step protection and the generator out-of-step protection is achieved. Using the self-memory trajectory to predict the stability of grid, calculate excess energy of system based on EEAC theory, and utilize convexity-concavity of phase diagram method which is not affected by the change of network structure to judge the stability of system. Utilize double forecast strategy to improve the stability of the system further.

Impacts of UHV on Short Circuit Currents in China's Central 500kV Grid and Its Countermeasures

Ultra-high voltage (UHV) projects are making power system growing lager and resulting in sharp increase of short circuit currents in some area especially in the vicinity of UHV access points. The paper presented the damage of short circuit current increases and common methods to limit fault current. Taking the construction and development of UHV grid in a province of central China as background, the over short circuit currents in 500kV Grid within UHV were showed and several countermeasures including choosing proper UHV access scheme, implementing UHV substation lower voltage bus partition running, installing reactors, optimizing power grid network based on UHV connection point were proposed and then verified by PSASP simulation. The results show those measures are useful to reduce the short circuit current to a reasonable level.

Comparison of Treatment Method of Plane Nodes in Truss Structure of Transmission Tower

With the advance of construction of the ultra-high voltage grid, more and more attention is paid to transmission tower design while taking into account the economic and security. In the transmission tower structure, there are usually plane nodes, accompanied with a diaphragm or a cross arm. The treatment method of plane nodes has direct effect on the accuracy of result in solving the stiffness matrix for analyzing the tower structure. Several treatment methods of plane nodes are compared by ANSYS and MATLAB programming respectively. Empirical results show that the computational results are more close to the actual situation with "Plus virtual bar method". Thus, it demonstrated that above-mentioned method is more suitable for the independent developed tower calculation, which can provide a good reference for future tower calculation.

Construction of UHV AC test base of SGCC

SUMMARYIn order to research ultra high voltage (UHV) transmission technology systematically, State Grid Corporation of China launched the UHV AC test base project to provide strong technical support for UHV grid construction. The facilities including UHV power supply, UHV test lines, equipments qualified field, electromagnetic environmental laboratory, climate laboratory, UHV corona cage, impulse test field were built at the test base. Much useful experience had been achieved through the construction and research in the fields of UHV electromagnetic environment, external insulation characteristic, equipment technical requirements, equipments manufactory, electrical design, construction, test, and operation. It accelerated the construction of the UHV pilot project of China. Copyright © 2011 John Wiley & Sons, Ltd.

Research on the feature extraction of DC space charge behavior of oil-paper insulation

As the ultra high DC voltage grid comes into an important stage in the ‘Twelfth Five-year Plan’ of China, the requirement on the operation safety of power equipment needs to be improved. Oil-paper insulation plays a significant role as the main insulation material of many key power equipments. The space charge behaviors of oil-paper insulation not only affect the dielectric strength, but also relate to the stability and security of power transmission facilities. Until now, the research on the space charge behaviors of oil-paper insulation remains on the stage of objective description and phenomenon explanation, thus it is very urgent to make some feature extractions on the test results in order to further investigate the DC space charge behaviors of oil-paper insulation. In this paper, based on the large numbers of test data in the first stage, data mining and feature extractions were performed on the DC space charge behavior of oil-paper insulation. The variation laws of some key characteristic parameters obtained in different experiment conditions such as the total charge injection amount, apparent charge mobility, and trap energy distribution were analyzed. In addition, the distribution of trapping level inside oil-paper was calculated by using the PEA test results based on the mechanism of the isothermal decay current, which realized the approximate calculation of trap depth by charge decay characteristics. What’s more, the exponential injection and decay equations of DC space charge changing with time were acquired for the first time, and the physical meanings of related parameters were defined. This has laid a foundation for the prediction and simulation of space charge in the oil-paper insulation.

Ultra High Voltage Transmission in China: Developments, Current Status and Future Prospects

The developments and current status of ultra high voltage (UHV) alternating current (AC) and direct current (DC) transmission in China were reviewed in this paper. The UHV transmission historical developments in the past twenty years; the demand of development UHV transmission; the research results of the UHV key technologies including electromagnetic environment, over-voltage and insulation coordination, lightning performance, live-line working and equipment manufacture recent years in China; the design parameters and construction of UHV AC, UHV DC test and demonstration transmission lines, UHV AC, DC test bases and state grid simulation center were introduced. The suggestions at the aspects of UHV transmission future design, construction, commercial operation and maintenance were simply presented and discussed. The review and discussion are important for the safe and reliable operation of UHV grid in China and can be a reference for other countries that want to develop UHV AC and DC transmission.