Gas-insulated Lines Research Articles

Technical Viability of Retro-Filling C3F7CN/CO2 Gas Mixtures in SF6-Designed Gas Insulated Lines and Busbars at Transmission Voltages

Sulphur hexafluoride (SF6), the most popular dielectric medium adopted in compressed gas insulated equipment, has been identified as a highly potent greenhouse gas. This has led to increased interest in finding a more environmentally friendly replacement candidate. In this paper, the technical viability of C3F7CN/CO2 gas mixtures was assessed as a potential retro-fill solution for existing SF6-filled gas insulated lines (GIL) and busbars (GIB). A reduced-scale coaxial prototype was developed to establish the breakdown strength of 20% C3F7CN / 80% CO2 and 16% C3F7CN / 84% CO2 gas mixtures in direct comparison with pure SF6 under the standard lightning impulse (1.2/50 μs). Breakdown results demonstrate that a mixture of 20% C3F7CN / 80% CO2 exhibits comparable insulation capability to pure SF6 in coaxial geometries with similar field uniformity to GIL/GIB. This initial finding has led to the construction of a full-scale GIB demonstrator rated for 420/550 kV. Type tests according to IEC 62271–204 showed that the 20% C3F7CN / 80% CO2 gas mixture has passed all the required voltage levels as SF6. The research findings in this paper are an encouraging step towards a technically viable SF6-free retro-fill solution for existing GIL/GIB installed for the 400 kV transmission network in the UK.

Synergistic effect of SF6/N2 gas mixtures on surface partial discharge under DC voltage

With the development of high-voltage direct-current (HVDC) transmission, DC gas insulated lines (GIL) with SF 6 /N 2 gas mixtures as insulating medium have been successfully adopted. Space is typically the weak link of equipment insulation where surface discharge is one of the common discharge forms in power equipment. Therefore, research on the surface discharge characteristics of SF 6 /N 2 under DC voltage can contribute to insulation design of SF 6 /N 2 insulated DC GIS, GIL and other power equipment. In this paper, a DC high-voltage experimental system is established to study partial discharge (PD) along insulation surfaces. The characteristics at the initial discharge and the discharge development stages are analyzed in detail. Additionally, the synergistic effect of mixed gas is investigated based on the discharge characteristics and the synergistic effect coefficient is introduced. The experimental results show that the synergistic effect of mixed gas partial discharge inception voltage (PDIV) under high pressure is more obvious compared to low pressure and the discharge quantities of mixed gas is smaller than that of pure SF 6 at the discharge development stage. This study on the multiple influencing factors of synergistic effect is expected to provide important insight for exploring the SF 6 /N 2 discharge mechanism and helps decreasing the usage SF 6 .

Novel spacer coated with functionally graded ZnO film for HVDC gas insulated line

To achieve a more uniform electric field (E-field) distribution on a spacer surface in HVDC gas insulated lines (GILs), novel spacers were fabricated by depositing ZnO films onto spacer surfaces using magnetron sputtering method. Through measurement, the deposited ZnO films of different sputtering times were found to have nonlinear surface conductivities, which can relax the E-field distribution. An iterative method was applied to optimize the sputtering time distribution in the radial direction of spacer, and an almost uniform E-field distribution was achieved after five iterations. For fabricating the optimized spacer, a baffle with a designed gap was placed above the rotating spacer to control the sputtering time at different radial positions. From the E-field simulation results, it can be seen that the introduction of a novel spacer with a 30 min-sputtered ZnO film can greatly reduce the E-field concentration around the triple junction of HV electrode, air and spacer. By grading the sputtering time distribution in an optimal way, a further E-field strength reduction is obtained inside the GIL. Flashover tests were conducted under DC and polarity reversal voltages, and the optimized spacer shows the best insulation performance, followed by the 30 min-sputtered spacer, and finally the conventional spacer. By applying such novel spacers, higher compactness and reliability can be achieved in HVDC GILs.

Epoxy insulator with surface graded-permittivity by magnetron sputtering for gas-insulated line

To reduce the electric field concentration induced flashover in AC gas insulated line (GIL), a novel concept of surface functionally graded material (SFGM) is proposed in this paper, and the continuously graded high-A layer was fabricated by depositing BaTiO 3 on the insulator surface. The iterative method was introduced to optimize the thickness distribution of the BaTiO 3 layer. After four iterations, the layer thickness distribution almost reaches its optimal solution, and the maximum electric field strength is reduced by approximately 70% compared with that of the conventional insulator. To fabricate the optimized SFGM insulator, a rotatable baffle with the designed gap was placed above the insulator during the sputtering process. For comparison, a uniformly sputtered insulator with the 4μm BaTiO 3 layer and a discretely graded insulator with four gradients were prepared. Both electric field simulations and experiments were performed to evaluate the electrical performance of different insulators. The results show that, compared with the conventional insulator, the sputtered insulators, especially the optimized SFGM insulator, can significantly improve the uniformity of the electric field distribution along the insulator surface under a 50 Hz voltage. Accordingly, the discharge inception voltage and flashover voltage of the insulator both increased significantly. Under negative standard lightning impulse voltage (1.2/50 μs), the sputtered insulators show insulation performance comparable to that of the conventional insulator.

A Dual-Slot Barrier Sensor for Partial Discharge Detection in Gas-Insulated Equipment

This paper reports on the development and testing of a novel barrier sensor for UHF Partial Discharge (PD) detection in gas-insulated equipment. The sensor features a unique dual-slot planar antenna backed by an air-filled cavity. The dual-slot arrangement allows different parts of the antenna to resonate at different frequency ranges in the UHF spectrum. As a result, the sensor exhibits broader bandwidth and higher sensitivity than other barrier sensors. Finite Element Analysis simulations have been used to optimize the sensor design. Furthermore, testing using a specially made PD test rig, GTEM cell testing and testing on a gas-insulated line section in the high voltage laboratory, have validated the simulation results and the capabilities of the sensor. The Dual-Slot Barrier (DSB) sensor exhibits a bandwidth of 0.3 – 2.0 GHz with a mean effective height of 13 mm, and an effective height above 2 mm for 90% of the frequency range. The sensor can be used with both wideband instruments, such as oscilloscopes, and narrow band instruments such as frequency downconverters. Additionally, its optimized dimensions and unique replaceable sealing attachment ensure maximum compatibility for retrofitting on a wide range of equipment.

The Attenuation and Propagation Law of Ultrasonic Wave in UHV Gas Insulated Line

Gas insulated transmission line (GIL) is widely used for power transmission in special geographical environment such as river crossing, mountain crossing and city tunnel crossing scenarios. Because of its totally enclosed structure, it is difficult to locate the fault position when the internal breakdown discharge occurs. Fault location by using the ultrasonic wave aroused by discharge is an efficient method for GIL maintenance. This paper reports the experimental and theoretical results of attenuation and propagation law of ultrasonic signal in GIL. An ultra-high voltage (UHV) GIL model was set up to simulate the geometry and material conditions of real GIL. The common lead breaking signal was adopted to generate ultrasonic source, and the propagation velocity and attenuation coefficient of ultrasonic wave were obtained. The experimental results show that the attenuation coefficient of ultrasonic wave on GIL linear unit is about 0.234 dB/m, and its propagation velocity is about 3.3 km/s. But the attenuation level of ultrasonic wave is much higher when it passes through the epoxy insulators or expansion joints. The validity of the method is verified by the creeping discharge test on the GIL model. In order to analyze the propagation path of ultrasonic wave, the relationship between the path and time is calculated and the shortest propagation time is obtained. The results of this research provide support for the fault location of GIL by ultrasonic wave.

A New Principle of Distance Protection for the UHV GIL-Overhead Hybrid Line Based on Frequency Domain Lossless Transmission Line Equation

The Gas-insulated Line (GIL) is a technical equipment that allows power transmission underground at high voltage level. The UHV GIL-overhead hybrid transmission line is an essential way for power transmission in complicated landscapes. Because of the distributed capacitance of the UHV transmission line and the difference of the structure and the electrical parameters between GIL and overhead transmission line, the distance protection based on uniform lumped parameters cannot be applied to this hybrid transmission line directly. In order to overcome the shortcomings, this paper studies and proposes a new principle of distance protection for the UHV GIL-overhead hybrid transmission line based on frequency domain lossless transmission line equation, with the distributed capacitance accounted. Then, the paper presents a setting scheme for this distance protection with the hybrid line parameters considered. Finally, tests are carried out on the Power Systems Computer Aided Design (PSCAD) platform, and the accuracy and reliability of the distance protection method proposed are verified by simulation results.

High-Resolution Temperature Distribution Measurement of GIL Spacer Based on OFDR and Ultraweak FBGs

The temperature distribution on the gas-insulated line (GIL) spacer affects surface charge accumulation. However, the spacer temperature distribution is hard to measure because the conventional electric sensors are large dimensions, hard to multiplex, and sensitive to electromagnetic interference (EMI). In this article, a distributed temperature measurement system of GIL spacer based on the optical frequency domain reflectometry (OFDR) was developed to solve the problems. In this system, ultraweak fiber Bragg grating (FBG) was adopted to replace single-mode fiber due to its higher signal-to-noise ratio. The demodulation method was described for compensating the nonlinear frequency tuning errors induced by the unstable tunable laser. In addition, the relationship between the spacer temperature and the wavelength shift was determined in the calibration experiment. Moreover, the temperature distribution on a 126-kV GIL spacer was measured by the proposed OFDR system. The data processing method for 3-D surface temperature on cone-type spacer was also introduced. High-resolution temperature distribution on the spacer surface was successfully obtained using OFDR system, which is unable to accomplish with the conventional electric temperature sensor.

A Non-Intrusive Electrical Discharge Localization Method for Gas Insulated Line Based on Phase-Sensitive OTDR and Michelson Interferometer

To overcome the high cost and low-spatial resolution of the electrical discharge localization systems for the gas insulated line (GIL), a non-intrusive distributed optical fiber sensing system based on the phase-sensitive optical time domain reflectometer (phi-OTDR) and the optical fiber Michelson interference is proposed. In the system, the Michelson interference provides a trigger signal since it has a better sensitivity and a higher frequency response, and the phi-OTDR localizes the ultrasonic vibration induced by a discharge. First, the principle of the proposed discharge localization system is introduced. Then, the bandwidth and multi-points distributed sensing performances of the system are investigated based on the ultrasonic signal induced by a piezoelectric transducer. Furthermore, the localization results of experiment using gas insulated switchgear spacer discharges indicate that the sensing distance of the sensing system can reach 1 km and the positioning accuracy of the discharge is smaller than 15 m. Thus, the system meets the requirement of the GIL discharge localization. Contrary to the traditional GIL discharge methods, the system has several advantages, such as non-intrusive, better spatial resolution, lower cost, and greatly simplified structure.

Multiphysics coupled modelling in HVDC GILs: Critical re-examination of ion mobility selection

Surface charging of spacers is one of the main factors restricting the industrialization of DC gas insulated lines (GILs). The effects of ion mobility, gas pressure and temperature on surface charge behavior are important issues to be clarified. In this paper, a selection method of ion mobility is reviewed and reconsidered. A multiphysics-coupled model considering ion mobility, species of ions, temperature, and gas pressure is investigated. A mathematical model of ion mobility under low and high electric fields is presented and the critical electric field strengths are calculated, based on the current density vs electric field strength. The results show that the two critical electric field strength have different characteristics with gas pressure and temperature. The ion mobility in SF 6 gas has a completely different calculation model in high electric field and low electric field, and these two ion mobilities are equal only at a specific temperature and gas pressure. This paper can provide a reference for the selection of ion mobility in DC GILs under conditions of temperature and pressure.

Interfacial E-field self-regulating insulator considered for DC GIL application

The non-uniform electric field (E-field) distribution is typically considered a key factor in triggering surface flashover inside gas insulated lines (GILs). To improve the E-field distribution along GIL insulators, this study proposes the concept of an interfacial E-field self-regulating (IER) insulator. The capability of the IER insulator with a resistivity-nonlinear (ρ-nonlinear) skin layer to regulate the E-field distribution is validated via theoretical analysis. In this paper, cone-type insulators are coated with epoxy (EP)/SiC composites to fabricate EP/SiC coated IER insulators. Next, an electrical simulation model is built to calculate the E-field distributions and energy losses of the insulators. As the coating thickness or SiC content increases, the E-field regulation function of the EP/SiC coated insulator becomes increasingly significant with increasing leakage current and energy loss. Under DC voltage, nearly 24 hours is required for the E-field distribution along the conventional insulator to reach the steady state. When the SiC content in the coating layer is increased, the transient time of the EP/SiC coated insulators decreases because of the reduced volume resistivity. Furthermore, coating thickness measurements and DC flashover tests are conducted. The coating thickness of the EP/SiC coated insulator is found to increase with increases in the SiC content because of the higher viscosity of the liquid-state coating. The DC flashover voltage of the EP/SiC coated insulator is found to be higher than that of the conventional insulator, with the flashover voltage increasing with the SiC content in the coating layer.

Multiphysics coupled modelling in HVDC GILs: Critical re-examination of ion mobility selection

Surface charging of spacers is one of the main factors restricting the industrialization of DC gas insulated lines (GILs). The effects of ion mobility, gas pressure and temperature on surface charge behavior are important issues to be clarified. In this paper, a selection method of ion mobility is reviewed and reconsidered. A multiphysics-coupled model considering ion mobility, species of ions, temperature, and gas pressure is investigated. A mathematical model of ion mobility under low and high electric fields is presented and the critical electric field strengths are calculated, based on the current density vs electric field strength. The results show that the two critical electric field strength have different characteristics with gas pressure and temperature. The ion mobility in SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> gas has a completely different calculation model in high electric field and low electric field, and these two ion mobilities are equal only at a specific temperature and gas pressure. This paper can provide a reference for the selection of ion mobility in DC GILs under conditions of temperature and pressure.

Surface coating affecting charge distribution and flashover voltage of cone-type insulator under DC stress

In order to scale down the dimensions of gas insulated line (GIL) insulators and to improve their reliability at ultra high voltage (UHV), considerable research has been conducted to raise the flashover voltage of the insulators. Usually, a non-uniform electric field distribution is an important factor triggering flashovers. Under AC and impulse voltages, the application of functionally graded materials (FGM) with spatial distribution of dielectric permittivity (e-FGM) has been shown to be an effective solution. But at DC, the electric field distribution depends not only on the conductivity of the dielectrics but also the charge distribution on the surface. Based on the cone-type insulator, this paper reports on the surface charge accumulation and DC flashover of epoxy (EP)/graphene (GR) coated insulators in air. The EP/GR coated insulators with different filler amounts were prepared to test their surface charge accumulation and flashover characteristics. The results show that the uniformly distributed homopolar surface charge is helpful in reducing the peak electric field thereby raising the flashover voltage. The 0.1% EP/GR composite has the slowest surface potential decay process. Accordingly, the 0.1% coated insulator has the highest flashover voltage among the other EP/GR coated insulators and uncoated insulator. The bipolar surface charge regions caused by wire-type metal particles distort the electric field distribution. Flashovers of the 0.1% coated insulator occurs at lower voltage than the uncoated insulator when a wire-type particle attaches to the surface.

Field-dependent charging phenomenon of HVDC spacers based on dominant charge behaviors

Spacers are key components that are used to support high voltage conductors in gas-insulated substations or gas-insulated lines. The analysis of the surface charge patterns on spacers remains a difficult task, which requires a comprehensive understanding of the physical mechanism of the gas-solid interface charging phenomenon. In this letter, we reported a field dependent property of surface charge accumulation patterns on spacers under DC stress. We verified this finding through experiment, and further, we put forward a field-dependent charging model based on dominant charge transport behavior under different electric fields. It was found that the charging characteristics of the spacer are dominated by the Ohmic conduction from the volume below an electric field of 2.5 kV/mm. When the electric field stress is higher than 2.5 kV/mm, the charging property of spacers is dominated by the enhanced gas ionization according to Townsend's law. The correctness of this model was verified by surface charge measurement results in literature studies, and a method for determining the dominant mechanism of charge accumulation under different electric fields was proposed.

Transmission Capacity of Gas-Insulated Power Lines Laid in Earthen Trenches

One of the main problems in electrical supply for large cities is the absence of corridors for overhead transmission lines. This problem can be solved by using gas-insulated lines (GIL) laid in earthen tranches. With earthen deposition, the transmission capacity of GIL will be affected by the depth of deposition and by the temperature and thermal resistance of the soil. Amethod is proposed for calculating the long-term permissible current of an underground GIL that takes the external conditions into account. Calculations for a specific GIL design are reported and the results are analyzed.

Multiphysics Modeling for Transient Analysis of Gas-Insulated Lines

In this paper, two discretization techniques are combined to present a coupled electro-thermal model for the transient analysis of gas-insulated transmission lines (GILs). The analysis of the thermal problem is based on the volume-element-method, representing heat transfer equations on a 2-D axisymmetric GIL model and determining temperature distribution over space and time. For the electrical formulation, the finite-difference time-domain method is adopted considering constant parameters calculated at the frequency of interest. The coupled electro-thermal problem in the case of transient conditions is solved by means of a bidirectional procedure. A set of test cases is imposed on the electric part, and transient analysis is conducted while considering the line coupled electro-thermal performance.

Microscopic charge provision at interfaces of gas-insulated (HVDC/HVAC) systems

Charge generation at the interfaces of gas-insulated switchgear or gas-insulated lines is mostly studied from a macroscopic point of view. Integral current measurements are a common method to determine the average inception field strengths and intensities of micro discharges. However, because of the lack of spatial resolution and measurement sensitivity, these experiments do not provide deeper insights on the microscopic processes. By applying surface potential measurement techniques, studying the charge generation of single interface protrusions and drawing conclusions relative to their spatial distribution have become possible. This paper presents a spatially resolved analysis of charge-generation processes at rough aluminum electrodes and insulating interfaces. The results reveal a highly inhomogeneous charge generation at the interfaces even for electric fields below 6 kV/mm. Analyzing the charge generation from a macroscopic perspective has been demonstrated to possibly underestimate the local charge generation by up to three orders of magnitude. Using sulfur hexafluoride (SF 6 ) as the insulation gas at 0.45 MPa and Al 2 O 3 -filled epoxy resin insulators, the inception field strength for charge generation at the insulating interface is measured to be 3 kV/mm. For a technical insulation system that includes both a rough aluminum electrode and an insulating interface, significant discharge intensities are observed at a 5-kV/mm electric field. From these results, micro discharges are expected to be highly relevant for dimensioning of gas-insulated devices.

Electromagnetic Situation Near Gas-Insulated and Cable Power Lines

Gas-insulated and XLPE cable power lines can be used as high-power outputs from power plants and deep inputs of power to big cities. When laying power cables in a populated area, it is important to ensure a low level of electromagnetic field. The magnetic-flux density for specific designs of gas-insulated and XLPE cable lines is calculated for different shield-grounding methods and arrangements of cables. The calculated results are analyzed.

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 Gas-Insulated Line Equivalent Model and Simplified Model

The gas-insulated line (GIL) is one technical solution to allow the transmission of electricity underground at a high voltage level, yet its equivalent model is quite complicated. Based on an examination of the geometrical structure of the GIL and the way the metallic enclosure is grounded, this paper analyzed the electromagnetic and electrostatic coupling among the inner conductors and the metallic enclosures of the three phases. Then, the paper proposes a modeling method for the widely-used short-distance GIL based on the PI-model (the model consisting of two lumped admittance at each terminal and a lumped impedance in between). The GIL parameters were later simplified with the coupling effect of the metallic enclosure considered, and a simplified PI-model was produced. Finally, the proposed PI-model and its simplified version were built on the Power Systems Computer Aided Design (PSCAD) platform, and their effectiveness verified by simulation results.