Large-capacity Power Transmission Research Articles

Research on relay protection technology of high temperature superconducting cable system

High-temperature superconducting cables have many advantages such as low line loss, large transmission capacity, small space occupation, and environmental friendliness, which provide a highly efficient, compact, reliable, and green way of power transmission for power grids, and have great potential for application in large-capacity power transmission and urban power grid expansion and reconstruction. However, high-temperature superconducting cables have a complex structure, which is quite different from traditional cable lines in terms of operating conditions and fault mechanisms, and the requirements for the secondary system are also more stringent. Relying on the project of China Power Grid, “Research and Demonstration Application of Superconducting Transmission Technology in Urban Distribution Networks”, the thesis carries out an in-depth research on relay protection technology adapted to the access of high-temperature superconducting cables.

Study on sound transmission characteristics and positioning strategy of gas insulated transmission line shell

Gas insulated transmission line (GIL) is widely used for large-capacity power transmission in complex environment, and has the characteristics of long distance and totally enclosed structure. When the insulation fault occurs inside GIL, its metal shell structure makes it more difficult to monitor through optical, electrical and other physical quantities. Considering the rapidity and accuracy of positioning, the propagation of sound waves along the GIL sheath is a commonly used method in GIL on-site PD testing and running state online monitoring for fault positioning. This paper establishes a high-pressure GIL acoustic propagation simulation model based on pipe acoustics theory, analyzes the frequency dispersion and mode distribution of acoustic waves along the GIL shell, determines an appropriate fault location strategy based on the propagation velocity and attenuation of the selected acoustic wave mode, and carries out experiments to verify it. The results of the research show that the low order bending wave F(1,1) propagated along the GIL shell has good positioning characteristics, and the propagation velocity conforms to the dispersion curve distribution. By taking into account the attenuation and delay of the acoustic wave passing through epoxy insulators or expansion joints in the fault localization strategy, the localization accuracy can be improved by 60%. And this positioning strategy was verified to be effective in the GIL insulation fault localization test conducted in the field.

The Improved DC Breakdown Strength Induced by Enhanced Interaction between SiO2 Nanoparticles and LLDPE Matrix.

Direct current (DC) power transmission systems have received great attention because it can easily integrate many types of renewable energies and have low energy loss in long-distance and large-capacity power transmission for electricity global sharing. Nanoparticles (NPs) have a positive effect on the insulation properties of polymers, but weak interaction between NPs and polymer matrix greatly decreases the effort of NPs on the enhancement of insulation properties, and thereby limits its engineering application. In this work, grafting strategy was used to link the modified NPs and polymer matrix to improve their interactions. Silica NPs (SiO2-NPs) were modified by 3-(methacrylyloxy) propyl-trimethoxysilane (MPS) to introduce highly active groups on the SiO2-NPs surface, followed by the pre-irradiated linear low-density polyethylene (LLDPE) being easily grafted onto the MPS modified SiO2-NPs (MPS-SiO2-NPs) in the melt blending process to obtain LLDPE-g-MPS-SiO2-NPs nanocomposites. Fourier-transform infrared (FT-IR) spectrum and X-ray photoelectron spectroscopy (XPS) confirm the successful incorporation of MPS into SiO2-NPs. Transmission electron microscopy (TEM) verifies that the modified SiO2-NPs exhibits more uniform distribution. The rheology result shows that the interaction between MPS-SiO2-NPs and LLDPE significantly improves. More importantly, the LLDPE-g-MPS-SiO2-NPs nanocomposites displays superior DC breakdown strength to that fabricated by conventional modification methods. When the addition of MPS-SiO2-NPs is 0.1 wt%, the highest DC breakdown strength values of 525 kV/mm and 372 kV/mm are obtained at 30 °C and 70 °C, respectively, and high DC breakdown strength can be well maintained in a wide loading range of NPs.

High-Temperature Superconducting Cable Optimization Design Software Based on 2-D Finite Element Model

With the increasing electricity power demand in major cities, the existing conventional power cables is difficult to meet the requirements of high-density and large-capacity power transmission. Compared with conventional power cables, high-temperature superconducting cables can significantly increase transmission capacity, reduce power losses, and save land occupation, which have great potential in future urban high-density power transmission applications. However, the process of designing a complete set of HTS cable is very complex, and many factors need to be considered comprehensively, such as current distribution, AC loss calculation, thermal stability analysis. In this case, a HTS cable design software platform is under development to simplify design process and improve the work efficiency of designers. Based on the software, design schemes of three-phase 10kV/1.5kA HTS cable with two different structures are given. A 2-D AC loss finite element model of HTS cable is established and verified. The simulation results show that at low voltage level, the HTS cable with three-phase coaxial structure has good performance, and can save more land, which is more suitable for the expansion of urban power grid in the future.

전력계통 저주파수 진동억제를 위한 회전형 변압기 속도제어에 관한 연구

Recently, an energy transition is taking place around the world in the direction of increasing the proportion of renewable energy in order to reduce carbon emission. Accordingly, the problem of unbalanced distribution of power resources and loads has been raised, and the studies on asynchronous grid connection and AC-DC hybrid grid connection based on long distance large capacity power transmission technologies such as HVDC and HVAC are in the spotlight. Such a complex system structure may threaten the stability of the system so consideration of system operation is essential. In particular, Low Frequency Oscillation (LFO) is considered to be a factor that adversely affects power system stability, such as a torsion of a generator shaft or a decrease in generator output. LFO occurring under certain conditions in the power system are characterized by slow attenuation or gradually amplification. As a solution to suppress such LFO, there are PSS (Power System Stabilizer) of a generator system, FLC (Frequency Limit Control) of DC power system, and FACTs (Flexible AC Transmission system) facilities. However, these conventional studies have problems such as control difficulties and high investment costs. In this paper, a LFO suppression method using Variable Frequency.

Research status of the external insulation pollution flashover of high voltage transmission line

In recent years, China’s electric power industry has made great progress, especially the achievements of ultra high voltage and ultra high voltage power transmission and transformation technology have attracted worldwide attention. At present, China has put into operation power transmission projects such as ±660kV yin-East DC extra-high voltage, ±800kV upward, Yunnan-Guangzhou DC extra-high voltage and 1000kV Dongnan Jindong-Jingmen AC extra-high voltage. These long-distance and large-capacity power transmission projects provide strong technical support for the realization of the national strategic goal of “west power transmission to the east and North power transmission to the south”.With the rapid industrialization and environmental deterioration, various industrial and natural pollutions deposited on the insulators of high-voltage transmission lines are becoming increasingly serious. In severe weather, such as fog, dew, drizzle and snowmelt, the external insulation strength will be greatly reduced. Therefore, it is very important and necessary to thoroughly study the pollution flasking of uHV and UHV insulators and comprehensively consider the influence of China’s complex geographical environment on their external insulation characteristics, which is one of the key issues related to the safe and stable operation of power grid.

A Reclosing Scheme of Hybrid DC Circuit Breaker for MMC-HVdc Systems

With the increasing demand for large capacity power transmission over long distance, the use of overhead lines is required, which greatly increases the probability of temporary faults. After the faulty line is isolated by hybrid dc circuit breakers (HCBs), the power supply can be restored immediately by reclosing them in the event of temporary fault. However, fragile semiconductor switches will undergo two consecutive faults after the reclose of HCBs when permanent fault occurs. Therefore, a reclosing scheme is proposed in this article to avoid the nonselective reclose of HCBs, whose core principle is an identification method of the instantaneous and the permanent faults. By controlling semiconductor switches in HCB, traveling wave signal is injected into the dc line. Then, whether the fault exists on the line can be determined by the difference in propagation characteristics of the injected signal. The effectiveness of the proposed reclosing scheme is verified by both point-to-point and four-terminal high-voltage dc (HVdc) test systems based on modular multilevel converter built in PSCAD/EMTDC platform. Extensive simulations show that it is robust to fault resistance, etc., and has no identification dead zone. Besides, it has little impact on the healthy part of the system even the permanent fault occurs.

Key technologies of ultra-high voltage hybrid LCC-VSC MTDC systems

The key technologies of ultra-high voltage hybrid LCC-VSC MTDC systems are investigated, focusing on the design of system configurations, converter topologies and the control and protection system. A double converter per pole of VSC connection is proposed along with the design of a 5000 MW VSC valve to develop a ± 800 kV/5000 MW large-capacity power transmission. The hybrid MMC topology capable of clearing the DC faults and the control strategy are developed to effectively improve the reliability in case of overhead line faults. The control and protection system of the LCC-VSC MTDC system is introduced to offer flexible operations under both normal and abnormal conditions, which includes voltage/current margin-based coordination, converter switch-in and switch-out, re-connection and drop-off of a third station. Simulations of an LCC-VSC MTDC system based on the LCC-VSC MTDC project are performed.

Optimal Coordination Voltage Control of Hybrid AC/DC Grids Considering Control of HVDC System

With the wide application of UHVDC in long-distance and large-capacity power transmission, the current power grid has developed into a complex AC-DC deep coupling system, which poses a serious challenge to the voltage safety control of the system. Based on the voltage sensitivity of AC/DC system, this paper presents a voltage optimization control method for integrated coordination of AC/DC system control variables. In this control method, a linear constrained quadratic optimization model with minimum control cost is constructed to coordinate the control variables of AC/DC system and correct the voltage amplitude deviation. The potential of UHVDC system to participate in voltage control is studied by optimizing the control quantity of rectifier side and inverter side converters. The simulation results of AC/DC hybrid system show that the proposed optimal control method can effectively coordinate the control quantity of AC/DC system and ensure the voltage operation in a safe range.

Overvoltage Study on UHV AC Half Wave Length Transmission System

In China some resource bases in the west are located at a distance of about 2800~3000km from the load centers in the east. AC Half Wave Length transmission in UHV stage is an alternate for the long distance and large capacity power transmission. A study model of half wave length line transmission system was established. The power frequency overvoltage, switching overvoltage and grounding fault overvoltage were studied using EMTP. The study result shows that the highest overvoltage occurs during grounding faults. The amplitude and waveform of overvoltage during grounding fault are affected by fault type, fault location as well as the characteristic of the protection system. The use of metal oxide arresters to limit overvoltage is discussed. The parameters and configuration of metal oxide arresters is suggested.

Voltage Distribution–Based Fault Location for Half-Wavelength Transmission Line with Large-Scale Wind Power Integration in China

Large-scale wind farms are generally far away from load centers, hence there is an urgent need for a large-capacity power transmission scheme for extremely long distances, such as half-wavelength transmission lines (HWTLs), which can usually span thousands of kilometers from large-scale wind farms to load centers. An accurate fault location method for HWTLs is needed to ensure safe and reliable operation. This paper presents the design of a modal voltage distribution–based asynchronous double-end fault location (MVD-ADFL) scheme, in which the phase voltages and currents are transformed to modal components through a Karenbauer transformation matrix. Then, the modal voltage distributions along transmission lines are calculated by voltage and current from double ends. Moreover, the minimums and intersection points of calculated modal voltages from double ends are defined as the fault location estimation. In order to identify incorrect fault location results and reduce calculation errors for the correct ones, air modal and earth modal voltage distributions are applied in the fault location estimations. Simulation results verify the effectiveness of the proposed approach under different fault resistances, distances, and types. Lastly, a real-time digital simulator (RTDS)–based hardware-in-the-loop (HIL) test is undertaken to validate the feasibility of implementing the proposed approach.

Research and application of UHV power transmission in China

The power demand increases rapidly in China; however, the areas of huge power demands are of long distance from most areas of abundant energy resource in the country. Therefore, China put in great effort to develop ultrahigh voltage (UHV) power transmission systems to optimise its energy allocation. This includes (i) systematically developing key technologies such as overvoltage suppression, external insulation design, and electromagnetic environment control, and (ii) developing key equipment such as UHV alternative current (UHVAC) transformers, circuit breakers, and series compensated equipment, and UHV direct current (UHVDC) converter transformers, smoothing reactors, converter valves, and DC transmission control and protection systems. Eight AC UHV projects and 11 DC UHV projects have been built in China, which play an important role in the optimal allocation of energy. Plus there are one more UHVAC and three more UHVDC transmission projects in construction, while UHVAC gas-insulated lines will be applied in the UHVAC line crossover the Yangtze River and the ±1100 kV UHVDC power transmission technology is in development. Here, the development and application of UHV power transmission technologies in China are described, some main challenges the UHV projects encountered are discussed, and experiences obtained from the operation of UHV systems are introduced. It is concluded that China obtained mature experience in developing, constructing, and operating UHV systems and successfully realised long-distance, large-capacity power transmission, and the UHV power transmission technology has become an important measure for energy allocation in large areas.

Study on the characteristics of half-wavelength transmission line with digital simulation and dynamic simulation

AC Half-Wavelength transmission in UHV stage is an alternate for the long distance and large capacity power transmission, because of its unique characteristics. The characteristics half-wavelength ...

The number of chained lumped-circuits for the physical analogy of half-wavelength power transmission lines

Half-wavelength AC power transmission (HWACT) technology is a mode for long-distance and large-capacity power transmission, has broad application prospect in ultra-distance point-to-point transmission mode. In order to implement physical analogy of HWACT lines, in general, the equivalent lumped-circuits consisting of some chained π-type circuits or T-type circuits are used in laboratory. The number of the chained circuits is the most important aspects in the physical analogy of HWACT lines. In this paper, the voltage waveform along π-type equivalent circuit is analysed for different number of the chained circuits. Next, the relationship between the number of chained circuits and the wave frequencies is discussed based on the relative error of the cascaded matrix (relative to the transfer matrix). For 50Hz, 32 chained π-type circuits can ensure the oscillation at the end of the waveform is small enough and the sending-to-receiving voltage ratio is approximately 1. The greater frequency is, the more the number of chained circuits required to establish good equivalence between the lumped-circuits and the transmission line. These works are instructive for the physical analogy of HWACT lines.

Single pole‐to‐ground fault location method for MMC‐HVDC system using active pulse

Modular multilevel converter HVDC (MMC-HVDC) systems have been commissioned in China, America and Europe. With the demand of long-distance and large-capacity power transmission, the MMC-HVDC systems using overhead lines get more attention. When a single pole-to-ground permanent fault occurs, the fault current is very small, therefore if the fault location is determined, the fault can be cleared and the HVDC system can revert to normal operation without interruption. MMC-HVDC can quickly control the work state of sub-modules (SMs), therefore, it is possible to generate travelling wave pulses propagating to the line by controlling SM, without the need of additional equipment. So in this study, a fault location method for MMC-HVDC using active pulse is proposed. Meanwhile dichotomising recursion singular value decomposition is used to detect the arrival time of active pulse. The proposed method does not require additional equipment and the method is not influenced by transition resistance. And fault location can be repeated since the pulse can be sent repeatedly. Computer simulations are performed and the results show that the proposed method can locate the grounding point of MMC-HVDC reliably and with increased accuracy relative to existing methods.

Investigation of a Hybrid HVDC System with DC Fault Ride-Through and Commutation Failure Mitigation Capability

A hybrid HVDC system that is composed of line commutated converter (LCC) at the rectifier side and voltage source converter (VSC) in series with LCC at the inverter side is studied in this paper. The start-up strategy, DC fault ride-through capability, and fault recovery strategy for the hybrid HVDC system are proposed. The steady state and dynamic performances under start-up, AC fault, and DC fault scenarios are analyzed based on a bipolar hybrid HVDC system. Furthermore, the immunity of the LCC inverter in hybrid HVDC to commutation failure is investigated. The simulation results in PSCAD/EMTDC show that the hybrid HVDC system exhibits favorable steady state and dynamic performances, in particular, low susceptibility to commutation failure, excellent DC fault ride-through, and fast fault recovery capability. Results also indicate that the hybrid HVDC system can be a good alternative for large-capacity power transmission over a long distance by overhead line.

The progresses of superconducting technology for power grid last decade in China

With the increasing development of renewable energy, it is expected that large-scale renewable power would be transported from the west and north area of China to the east and south area. For this reason, it will be necessary to develop a wide-area power grid in which the renewable energy would be the dominant power source, and the power grid will be faced by some critical challenges such as long-distance large-capacity power transmission, the stability of the wide-area power grid and the land use problem for the power grid. The superconducting technology for power (STP) would be a possible alternative for the development of China`s future power grid. In last decade, STP has been extensively developed in China. In this paper, we present an overview of the R&D of STP last decade in China including: 1) the development of high temperature superconducting (HTS) materials, 2) DC power cables, 3) superconducting power substations, 4) fault current limiters and 5) superconducting magnetic energy storage (SMES).

Development of a 10 kA HTS DC Power Cable

The new energy revolution which will be dominated by renewable energy will need to develop a corresponding new power grid. Based on the characteristics of renewable energy resources, and the stability problem of AC network, it was proposed that the DC-based transmission grid should be developed in the future. The high Tc Superconducting (HTS) power cable will be a competitive candidate for large-capacity power transmission of renewable energy. In this paper, the progress of the development of a 360 m/10 kA HTS DC power cable is presented, and the prospects of HTS cable is discussed.

Development of HTS AC Power Transmission Cables

Along with the rapid growth of the national economy in China, the electric utility is faced with an ever rising demand for electricity and the problem to solve large capacity power transmission. Under the support of the High Technology Research & Development Program of China, a 1.5 kA/10.5 kV 10-m long 3-phases HTS transmission power cable and a 75-m long, 3-phase, 10.5 kV/1.5 kA HTS power cable have been developed successfully by the Institute of Electrical Engineering, Chinese Academy of Sciences. The 75-m long HTS power cable operates now at a local live distribution grid of 6.6 kV to supply electric power for Changtong's factory in Baiyin, Gansu Province. The cable operates stably and reliably over 7000 hours since December 2004.