Chen Weijiang

Development of Full Frequency Bandwidth Measurement of VFTO in UHV GIS

In order to study the characteristics of very fast transient overvoltage (VFTO) in ultra-high voltage (UHV) gas-insulated switchgear (GIS), the authors have developed a full frequency bandwidth measurement system based on a porthole-type capacitive voltage divider. Three kinds of measurement circuits for the measurement system are studied and the influence of cable length on measurement results is analyzed in this paper. The calibration of the measurement system using step response was conducted. The results show that the measurement system of the capacitive voltage divider integrated with an impedance converter has a bandwidth ranging from 0.003 Hz to 300 MHz at least. It can be used to measure the full VFTO during DS operation perfectly. A full-scale UHV DS (disconnect switchgear) is used to establish the test circuit. The measurement system with good anti-electromagnetic interference ability was constructed. Four different measurement locations are chosen on which to install the measurement system. An approximate total of 3000 closing and opening operation tests were conducted. Due to the wide frequency band of the measurement system, it is possible to obtain the full VFTO waveform accurately and conveniently. According to the waveforms, the key parameters of VFTO, such as trapped charge voltage, peak value of VFTO, breakdown characteristic of DS, frequency spectrum of transient waveform, and the overvoltage coefficient could be obtained.

Influence of the Switching Speed of the Disconnector on Very Fast Transient Overvoltage

During the switching of disconnectors in gas-insulated switchgear (GIS), repeated strikes occur in the contact gap and lead to very fast transient overvoltage (VFTO). Severe VFTO may bring damage to transformers and other equipment. The switching speed of the disconnector is one of the important factors which influences VFTO. There are still different opinions on this factor. Very little quantitative study was done before this paper. In this paper, the whole repeated strike process associated with the switching operation of the disconnector was simulated and analyzed. The polarity and the probability characteristics of breakdown voltage were analyzed. The impacts of the switching speed of the disconnector on VFTO were studied by statistical methods. Comparisons were made between simulated and measured results. The simulation result shows that within a certain range of speed, disconnectors with a lower switching speed generate lower trapped charge voltage after opening operation and lower VFTO magnitude during the opening and closing operations.

Test Study on Corona Onset Voltage of UHV Transmission Lines Based on UV Detection

Corona discharge is being detected by UV imaging detection technology at home and abroad in recent years. This technology is used in the corona tests of bundled conductors in this paper. In order to further research the corona characteristic, optimize geometry parameters and diameter of sub-conductor, and increase corona onset voltage of transmission lines, corona tests of three mode conductors which are placed inside the outdoor corona cage are conducted. Corona cage could be used to simulate the corona activities on transmission lines under a low voltage and different conditions in an effective and economical way. Photon which was created by UV light as a result of corona discharge on conductors is detected by the UV detection apparatus. The photon number within unit interval, namely photon counting rate is adopted as the parameter of quantifying the intensity of corona discharge. According to the apparent change of photon number, corona onset voltage can be judged. All tests are conducted under almost same atmosphere condition. Using the method, corona onset voltage is acquired. The results indicate that the tests have a good repeatability, in other words, repeat same test twice can acquires same result. The corona onset voltage can be acquired exactly from the curve of applied voltage vs. photon counting rate. Therefore UV detection apparatus can not only used to find discharge point exactly, but also applied on corona discharge research and live detection for power equipments. The method using in this paper is proved that is a new available method.

Simulation for secondary arc caused by single-phase earthing in UHV transmission system

Secondary arc caused by single-phase earthing fault in transmission system is with great uncertainty. To compensate the shortcomings of arcing tests, numerical modeling and simulation method has become an important feasible tool to investigate and predict the features of secondary arc. The simulation model for secondary arc in transmission system was established in the paper. Secondary arc caused by single-phase earthing fault in UHVAC demonstration project was taken as an example, to simulate arc movement, arc current waveforms, and the duration time of secondary arc. The simulation results conform to the measured data in artificial test, which validates the simulation model of secondary arc being fit for the engineering application.

Repeated Strike Process During Disconnector Operation in Ultra-High Voltage Gas-Insulated Switchgear

Very fast transient over-voltage (VFTO), induced by disconnector operations in gas-insulated switchgears, has become the limiting dielectric stress at ultra-high voltage levels. Much work has been done to investigate single-strike waveforms of VFTO. However, little study has been carried out investigating the repeated strike process, which would influence VFTO significantly. In this paper, we carried out 450 effective experiments in an ultra-high voltage test circuit, and conducted calculations through the Monte Carlo simulation method, to investigate the repeated strike process. Firstly, the mechanism of the repeated strike process is proposed, based on the experimental results. Afterwards, statistical breakdown characteristics of disconnectors are obtained and analyzed. Finally, simulations of the repeated strike process are conducted, which indicate that the dielectric strength recovery speed and polarity effect factor have a joint effect on VFTO. This study enhances the understanding of the nature of VFTO, and may help to optimize the disconnector designed to minimize VFTO.

A Study on the Protection to Prevent Overhead Insulation-covered Conductors from Lightning-caused Breakage in 1OkV Distribution Networks

The lightning-caused breakage mechanism of insulation-covered conductors is analyzed in the paper; two kinds of protective methods are concluded as guiding mode and blocking mode. The structure, characteristics, applied guidance and field application of six devices based on two kinds of protective methods are introduced summarily.