Time and Space Transition of HVDC Electric Field Distribution Based on Charge Activity in Gas-solid Composite Insulation Structures
1Aichi Institute of Technology; 2National Institute of Technology, Niihama Collage; 3Nagoya University; 4Fuji Electric Co., Ltd.
It is necessary to understand more detailed electrical insulation performance of DC electric field and charge activities in DC gas insulated switchgear (GIS) for further development of HVDC electric power apparatus and systems. In particular, the time transition characteristics of DC electric field distribution from DC-on to DC steady-state (DC-SS) could be critical but the phenomena have not yet been fully clarified.
In this paper, we investigated the time and space dependent characteristics of DC electric field from the time at DC-on to DC-SS in gas-solid composite insulation systems. The electric field distributions of epoxy-spacer in SF6 gas are calculated by the Finite Element Method (FEM) while changing wide range of the electric conductivity ratio σsp/σgas of epoxy-spacer and SF6 gas. The electric field distribution at DC-on is determined by the permittivity ratio εsp/εgas and under DC-SS by the electric conductivity ratio σsp/σgas, while in between DC-on and DC-SS
by both properties. As a result, we discussed the following three points.
1. Time transition of DC electric field in SF6 gas and charge behavior on epoxy-spacer:
We discussed the electric field and charge density distributions on epoxy-spacer in SF6 gas at time transition from DC-on to DC-SS. In particular, we focused our discussion on the time transition characteristics from the time of DC-on to DC-SS, and thus the time constant τ determined by the change of electric field stress in SF6 gas and charge activity on epoxy-spacer. As a result, we can point out that the electric field stress will be changed depending on the location and elapsed time from DC-on to DC-SS, with the certain time constant τ.
2. Effect of material properties to charge accumulations on epoxy-spacer in SF6 gas:
We quantitatively investigated the charge accumulation characteristics on epoxy-spacer while changing the electric conductivity ratio σsp/σgas of insulating materials. Especially, the electric conductivity of SF6 gas will be critical and is greatly depending on its charge supply source. It was found the evolution of surface charge density on the gas-solid interface during the time transition from DC-on to DC-SS is determined firstly by the identification of the charge supply source and secondly by the continuity equation.
3. The critical condition for the electrical insulation performance in HVDC-GIS:
We investigated the time transition of the electric field stress in SF6 gas for the electrical insulation performance in HVDC-GIS and identified the severest condition under different testing voltage (TV) application.
Finally, we quantitatively clarified that the time dependent characteristics of DC electric field in SF6 gas and charge activities are dependent on the location of the electric field space and dielectric properties of gas and solid materials.
Absolute Calibration of a ppm-precise HV Divider for the Electron Cooler of the Ion Storage Ring CRYRING@ESR
WWU Münster, Germany
The heavy ion storage ring CRYRING at GSI provides a unique possibility to test atomic structure calculations with slow exotic ion beams. In order to cool the ions and thus achieve a low momentum spread of the stored beam, CRYRING features an electron cooler, where the ion beam is superimposed with a monoenergetic electron beam. In high-precision experiments at ion storage rings the velocity of the ions is a critical quantity. For measurements at CRYRING the electron cooler determines the ion velocity and momentum spread of the ions. Consequently, a precise knowledge of the acceleration voltage of the electron beam is essential for the accuracy of the experiments.
To measure the acceleration voltage a high-precision HV divider for voltages up to 35 kV has been constructed. The design of this divider is similar to the ultrahigh-precision voltage dividers which have been developed and built in Münster in cooperation with PTB for use at the KATRIN experiment. It could be demonstrated that the precision of the divider is in the low ppm range.
In order to determine the absolute values and the stability of the scale factors of the high-voltage divider, regular calibration measurements with ppm-precision are essential. A new absolute calibration method based on a differential voltage measurement at high voltage has been developed. Here a voltage of 1 kV is added to a base high-voltage potential. This additional voltage and the high voltage divider output change are traceable to reference systems. The base voltage is monitored with a reference measuring system, which does not require ppm-precision. First results have demonstrated an absolute calibration with expanded measurement uncertainties of about 1 ppm.
Self Calibrating High Voltage Divider
1Physikalisch-Technische Bundesanstalt, Germany; 2TU-Braunschweig - elenia, Germany
High voltage calibrations on metrology level require traceability down to primary national reference standards. These standards are based on the Josephson effect, hence the maximum voltage is limited to less than 10 V. To transfer these reference voltages to several hundreds of kilovolts different reference dividers are required. Several intermediate steps are needed to utilize the optimal measuring range of the utilized dividers and voltmeters. The drawback of using several dividers is the increase of measurement uncertainty within each intermediate step. Additionally, the time required for traditional traceable calibration of the high voltage reference dividers can easily exceed several weeks.
Since the PTB is one of the world's leading metrology institutes it has started the development of a new high voltage divider design. This new development includes a high voltage arm, very similar to a standard high voltage divider. However, it is possible to connect all high voltage resistors in parallel. In this parallel arrangement the scale factor can be adjusted and compared to the primary reference standard. The adjustment is implemented by including a second precision divider that is only used at low voltages below 50 V. With the aid of a Wheatstone bridge arrangement these two voltage dividers are compared to each other. The adjustment is carried out with the aid of two potentiometers. One of the potentiometers is placed in the second divider to create a precisely known scale factor. The bridge is zeroed by a potentiometer in the high voltage arm. Thus, the known scale factor of the second divider is transmitted onto the high voltage divider. After finishing the adjustment, the parallel arrangement is changed back to series for the high voltage mode. Thereby, the scale factor is set to a calculable precise value. The divider that was built as a first prototype was compared to the existing reference divider and the results are shown within this paper.
Analysis of HVDC Transmission Lines under Fault Surges Using
Benha University-Fauclty of engineering, Egypt
Traveling wave analysis process over the long High Voltages Direct Current (HVDC) transmission lines for internal and external fault surges is rarely tackled and still as a competitive research issue. Transient surges resulting from different internal and external faults, faulty equipment, overload conditions, human errors, short circuits, etc. may cause sever risk failure to the long HVDC lines. Once, a fault occurs over the HVDC transmission line, high frequency traveling waves (TW) are generated. These traveling waves propagate away from the fault position in both direction of the transmission line with approximately the light speed in a very short time. During internal faults the traveling waves can be detected at both line terminals. For external faults, the traveling wave can be detected only at one-line terminal with zero at the other terminal. Therefore, the identification method for internal and external fault surges for HVDC lines can be proposed by comparing the amplitude of the traveling waves at both ends of line. In the current paper, analysis has been investigated to develop a technique for the fault surges discrimination over long HVDC transmission line with length up to 1000 km. The proposed technique is based on the travelling wave theory and the wavelet transformation integrated with artificial neuro-fuzzy interface (ANFIS) prediction system. The developed technique can precisely predict the exact fault surge voltage. The different simulation models are developed based on EMTP/MATLAB. the proposed technique can accurately discriminate internal fault surge from external ones under various conditions. The results of the proposed technique exhibit a great convergence with the similar work in literature for surge discrimination.
Evaluation Of The Effect Of The Location Of The Corona Measurement Devices On The Corona Inception Voltage Under Hvdc Application
Vaal University of Technology, South Africa
Instruments for corona measurements form an integral part in the detection, measurement and isolation of corona discharges. IEC 60270 address the electrical method using a coupling capacitor and acquisition unit for the detection and measurement of partial discharges. However, there are also non-conventional methods available namely: the use of high frequency current transformers (HFCT), acoustic methods and optical methods for detection of corona discharges (corona camera). The different measurement methods have various advantages and disadvantages with the main difference in the location of the sensor. The coupling capacitor is connected directly to the main circuitry of the power system, the HFCT is connected in the earth connection and the acoustic sensor as well as the corona camera being placed at a safe distance away from the system. The electrical method using a coupling capacitor is the only method that can be calibrated to measure the apparent charge according to IEC 60270 while the HFCT measure the discharge current by means of electromagnetic coupling. The corona camera visualizes, locates the corona discharge point and displays the partial discharge intensity by detecting the ultraviolet radiation photon count rate. In this paper, the aim is to determine how the location of the corona measurement sensor influences the measured corona inception voltage under HVDC application. The tests were conducted in a high voltage laboratory using a corona cage. The test methodology was based on gradually increasing the voltage until the corona inception voltage is reached for both positive and negative polarities. The results show a difference in the measured inception voltage for the different methods and that the location of the corona measurement devices has an impact on the measured corona inception voltage. The results indicate that connecting the corona measurement device directly to the main circuitry of the system as is the case with the electrical measurement method using a coupling capacitor will be the most sensitive method and result in the lowest measured corona inception voltage. The results are analyzed and the reasons for the differences explained.
Development of Hvdc Xlpe Cable System for Lcc and Vsc Type
LS cable & system Ltd., Korea, Republic of (South Korea)
HVDC transmission systems have been employed all over the world for long-distance power transmission lines to interconnect power grids between the continents and for short distance between neighbour countries. Besides, the global market requires a HVDC power cable system with large power transmission capacity bringing about the economic benefits. In this paper, our development plan has been described in detail for HVDC XLPE cable system for various voltage ratings; insulation material at laboratory, miniature cable for the evaluation of characteristics and finally the design of full scaled power cable system including necessary joints. We started with developing ±80kV cable system in 2010 and then the necessary efforts will be continued for ±525kV for several years more. For this purpose, various materials have been employed for the cable insulation depending on the type of converters; Nano composite compound by adding Nano inorganic filler for LCC type and commercially available compound for VSC. LCC and VSC type full scaled cables have been developed for those four operating voltages such as ±80kV, ±150kV, ±250kV and ±320kV. Furthermore, the development of ±525kV for LCC and VSC types has been now undergoing and would require a couple of years more.
Insulation Solutions for HVAC to HVDC Conversion of a High Voltage Transmission Overhead Line: the L7 Tower Case Study
1Cardiff University, United Kingdom; 2National Grid, United Kingdom
This paper presents two insulation solutions for an AC to DC power transmission line conversion. The purpose is to maximise active power of both solutions, by maximising voltage. The method includes High Voltage Direct Current (HVDC) scheme selection, insulation dimensioning for HVDC energisation in polluted environments and switching surge overvoltage clearance calculation.
The intended readers are HVDC Outdoor Insulation Scientists and Engineers, and Transmission System Operators (TSOs), looking for economically viable solutions to increase the power transfer capability of the power system. They will benefit by learning a proposed comprehensive framework method to convert an AC transmission line to DC.
This method has been implemented for the case study of a 132 kV line to line double AC circuit supported by a typical transmission tower. The theoretical and simulation results show in detail that a substantial increase of the power transfer capability of the line may be achieved. However, further investigation is needed.
Free Moving Particles in Gas-Insulated Lines Under DC Conditions – Basic Properties, Specific Effects and Countermeasures
Siemens AG, Germany
High-voltage gas-insulated lines (GIL) are well-established solutions for high-current AC power transmission in several installations worldwide. The high reliability of AC GIL is also depending on the clean assembly conditions and the technical design to handle small-sized free moving particles inside the high voltage compartment. For this reason, AC GIL is using particle traps which provide low electric field areas which capture particles. Under DC conditions, particle movement is different from AC. Due to that fact, the design of particle traps for DC GIL needs to consider different design criteria than for AC GIL. This contribution reports about particle movement in DC electric fields, their specific physical phenomena and potential countermeasures to ensure the full dielectric performance of gas-insulated lines under service conditions. For the investigation of the basic particle behaviour, an experimental test set-up was created with a particle injector, to achieve an optimised test procedure. As an additional feature a high-speed video system was used for observation of the particles.
Optimized particle trap design for DC GIL is an essential measure to eliminate the influence of free moving metallic particles. These particle traps must fulfil specific requirements, such as a high rate and speed of trapping and must provide areas of very low electric field for both polarities. Because of the different behaviour of the particles at positive and negative polarity, two types of particle traps are recommended. At positive voltage particles show a standing motion at the grounded enclosure side and at negative voltage particle show a standing motion on the high-voltage conductor side. This has been observed in most cases. A bouncing motion of the particle between conductor and enclosure can occur at both polarities. Therefore, particle traps shall be located at the enclosure and at the conductor side. For the conductor a specific design is recommended to trap the particles moving at the conductor surface. The investigations presented here show specific DC effects of free moving metallic particles in gas insulated lines, such as polarity effects, the effect that particles are attracted towards higher field strengths and the effect of reactivation of particles after polarity reversal.
Effect of Voltage on Thomson-coil Actuator for Fast Mechanical Switch
1Energy Internet Research Institute Tsinghua University, China; 2Department of Electrical Engineering, Tsinghua University, China
The fast mechanical switch equipped with the Thomson-coil actuator is one of the key devices in hybrid HVDC circuit breaker. And electromagnetic damping technology can solve the problems of high-speed damping opera-tion without adding additional mechanical parts. In order to make the open-ing/closing operation process as unified as possible, the driving and damping voltages are the same in the previous research. In this paper, the effects of different combinations of driving and damping voltages on the travel charac-teristics have been studied. Voltage combinations are divided into three types. The first type is to change the driving and damping voltage at the same time; the second is to keep the damping voltage constant and change the driving voltage; the last type is to keep the driving voltage constant and change the damping voltage. In order to ensure that the contacts system can move to the isolating distance in 3ms, the driving voltage should not be re-duced. At the same time, in order to ensure a better damping effect, the buffer start time range should be as large as possible. Therefore, it is sug-gested that the damping voltage is 2%~4% lower than the driving voltage. A fast simulation method based on equivalent circuit method, which avoids lots of test operations, has been proposed in this paper. It can provide reliable references for the application and improve the feasibility and reliability of fast mechanical switch in engineering.
"The DC Dielectric Characteristic Study on The Epoxy and The Air Insulation Materials for The Development of MVDC Switchgear"
1HYUNDAI ELECTRIC, Korea, Republic of (South Korea); 2HAN YANG UNIVERSITY, Korea, Republic of (South Korea)
This paper deals with the DC dielectric characteristic of the epoxy and the air insulation materials for the deduction of insulation design standards for the development of MVDC switchgear. First of all, after the international standards related to DC voltage test are investigated, the test electrode poles and the applied voltage conditions are discussed. In the experiments, DC voltage test, the LI(Lightning Impulse) voltage test, and superimposed lightning impulse voltage test are conducted and the results are analyzed about each insulation material such as epoxy and air. In the case of the solid insulation such as epoxy, the experiment contains the results of the applied time od DC voltage at the insulation material and variable DC voltage level in order to secure long time dielectric reliability under the operation condition. In the case of the air insulation, it compares the results of superimposed lightning impulse voltage tests with those of lightning impulse voltage tests which are specified on AC switchgear in IEC 62271. As the results of the experiments, the applied time of DC voltage doesn’t greatly affect the solid and air insulation performance in contrast with XLPE cable. It is caused by the property of the insulation such as hetero or homo space charge. If the DC voltage is getting higher, it is identified that the lightning voltage level is getting lower on the solid and air insulation performance.
Electromagnetic Interference Mitigation in a High Voltage Power Line Inspection Robot
University of kwaZulu-Natal, South Africa
A challenge in developing any electronic system to be operated near high voltage power lines, is achieving reliability in environments associated with electromagnetic interference. In this paper, success was achieved in the characterization and mitigation of the effects of the interference that an inspection robot would experience during proximity dielectric breakdown, electromagnetic coupling and electric arcing to a robot chassis. The source (electric arc) tested was found to couple to communication lines with a magnitude disruptive to Universal Serial Bus communications. Shielding was adequate to mitigate this interference, but when arcing occurred to the chassis itself, it needed to be controlled to maintain the integrity of the shield.
Study of Deep Dielectric Charging Characteristics and Suppression Method under Space Irradiation Environment
1State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, China, People's Republic of; 2Qian Xuesen Laboratory of Space Technology, Beijing , China, People's Republic of
With the rapid increasing interest on the space exploration, the reliability of the spacecraft becomes a very important problem. The space solar power station (SSPS) is inevitably exposed to space plasma, energetic particles radiation, extreme temperature, cosmic rays, etc. Energetic electrons can penetrate through the aluminium shield and deposit in the deep-layer of insulating materials, leading to partial accumulation of space charges and high electric field. Electrostatic discharge (ESD) occurs when the maximum electric field of insulating materials exceeds a certain threshold, resulting in deterioration of the insulating material and even the failure of the entire electronic equipment. Deep-layer dielectric charging has been the key scientific issue for developing high-voltage and high-power spacecraft technology. In this paper, a physical model is established to simulate the deep-layer charging characteristics of ethylene-tetra-fluoro-ethylene (ETFE) under FLUMIC spectrum electron irradiation, based on the processes of carriers’ transport and deposition of charge and energy. Two operating conditions, i.e. typical GEO condition and extreme GEO condition with varied flux enhancement, are studied. In addition, the possibility of suppressing the deep dielectric charging properties of ETFE by the addition of nano-boroncarbide (nano-B4C) is also investigated. The calculation results show that the maximum electric field in the deep-layer of ETFE rapidly reaches 108 V/m under extreme GEO space environment. Electrostatic discharge is easily to take place as it exceeds the breakdown threshold. It is found that the time spent to reach the maximum value of potential and electric field is less than one minute under extreme space radiation environment. Furthermore, the addition of nano-B4C can suppress the deep dielectric charging properties of ETFE to a large extent by introducing more shallow traps. This provides a potential approach on suppressing the deep dielectric charge accumulation.
Simple Method to Visualize Surface and Space Charges by Specially Processed Colour Pigments
HTW Berlin, Germany
HVDC applications are getting globally more and more important for the energy transport in the high voltage networks.
To design apparatus properly it is important to understand the space and surface charges that are accumulating under DC stress in certain areas. Existing methods to measure or visualize such charge accumulations like “Pres-sure Wave Propagation Method”, “Thermal Step Method”, “Pulsed Electro Acoustic Method” or “Electro Static Volt Meters” show often limitations applied to full size apparatus. Often the resolution or specimen thickness is limited or the application can be rather time consuming or complex. A simple method to visualize surface charges is known as “Lichtenberg’s dust figures” is known since hundreds of years but rarely practiced as the best working chemicals (yellow sulphur and red lead) are not health save. In this paper a fresh view is given to charge visualization by colour pigments. It is shown that different pigments attach to positive or negative charges. The application of this method is easy and takes just a view seconds. E.g. tree type structures can be visualized with a resolution of less than 1mm. Hover the value charge magnitude cannot be obtained. The surface charge behaviour on different materials like Epoxy, PE-HD, Silicone is shown. In addition, the possibility of space charge visuali-zation on specimen cross sections is discussed.
Simulation of Partial Discharge Influenced by Space Charges in Silicone Rubber
1State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, China; 2State Key Laboratory of Advanced Power Transmission Technology,Global Energy Interconnection Research Institute Co. Ltd., Beijing, China; 3XIAN XD Transformer Co. Ltd., Xi’an, China
As the fast development of modern city and industry, electric energy consumption shows an increasing tendency in recent years. More power cables of high-voltage and extra-high-voltage class are widely used to meet the demand of sustainable growth of grid capacity. However, in power cable system, statistics show that above all failures in recent years, the faults of cable accessories can make up more than 70%. One of the main reasons for the high failure rate is the performance degradation of insulating materials, which is strongly influenced by the partial discharge occurrence. Hence, as an effective way to evaluate the performance of insulating dielectrics, modelling of partial discharge is a significant issue to cable accessories. The paper aims to give a simplified model of partial discharge based on space charge transport along the silicone rubber which is adopted as the main insulating material of cable accessories containing one spherical air void, considering partial discharge happens when gas ionization by charge collision under AC applied voltage. The model contains the physical processes of charge injection and extraction at the interface between electrode and dielectric material, space charge transport along the material, charge exchange at the interface between dielectrics and the spherical void. Additionally, the influence of gas conductivity under various temperature and electric field is also concerned. The parameters of applied voltage and the size of void are analysed in this paper. The simulation results indicate that the range of discharge phase concentrates on the first and third quadrant of one AC cycle where the voltage amplitude ascends. With the increase of applied voltage and the size of void, the magnitude of charge also rises. Higher applied voltage and larger void size contribute to the accumulation of space charges in the void and accelerate gas ionization process. Therefore, the probability of partial discharge in the interior of silicone rubber increases as well. The model which links the partial discharge phenomena with space charge transport along the material and the interface between solid and gas provides the other efficient way to reflect the condition of partial discharges in insulating material.
Analysis of Lightning Overvoltage Based on Gronding Method of Lightning Arresters in Wind Farm
1Hankyong National University, Korea, Republic of (South Korea); 2Korea Electric Power Research Institute, KEPCO, Korea, Republic of (South Korea)
This study simulates lightning strikes and a wind farm using the simulation tool EMTP-RV and analyses the effects of lightning overvoltage. It compares the lightning overvoltage for two grounding methods of lightning arresters-common ground and connected ground. For the simulations, the ground impedance of the lightning arrester is selected according to the earth resistivity and lightning frequency. The study also analyses the resulting magnitude of the overvoltage when corona is the lightning strikes. Thereby, it suggests a method of reducing the lightning overvoltage through various simulations, depicting various conditions such as the change of ground impedance and the grounding method of the lightning arrester.