Shigemitsu Okabe

Cross-equipment study on charging phenomena of solid insulators in high voltage equipment

This paper reports on a new cross-equipment study on the charging phenomena of solid insulators, evaluating the influence of charging on high-voltage equipment such as gas insulated switchgear (GIS), oil-immersed transformers, gas-insulated transformers and high-voltage vacuum-insulated equipment. New parameters of normalized charge density and surface resistance are introduced to consistently explain all the characteristics of the surface charge density on solid insulators and the decay time constant of the accumulated charges between the equipment. The results show that the surface charge density and the decay constant decrease as the surface resistance of the insulators is lowered. Combinations of insulating media of solid insulators and fluids (gas or liquids) are discussed for GIS and oil-immersed transformers in terms of the relation between the relative permittivity and the volume resistivity. The decay time constant of the accumulated charges is generally dominated by the insulating media of a fluid like oil which has lower volume resistivity than solid insulators in cases like transformers. However, the time constant is determined by the resistivity of the solid insulator rather than the fluid (gas) for GIS. Furthermore, it is suggested that low resistivity coatings on solid insulators are effective in suppressing the charge accumulation on the insulators. These new cross-equipment studies on the charging phenomena are an effective approach to reliable insulation design especially when downsizing high-voltage equipment

Residual charge distribution of positive surface streamer

The measuring system of charges accumulated on an insulating plate is developed with an electrostatic probe whose sensing electrode is of 0.5 mm diameter. In the inverse calculation from the probe outputs to the charge distribution, Tikhonovs regularization technique is effectively used to suppress the excessive amplification of the noise. In the case of measuring a 2 mm thick PMMA plate, the spatial resolution of the measuring system reaches 1.0 mm. With this system, the residual surface charge distribution on an insulating plate just after the occurrence of a positive surface discharge at 25 kPa (190 Torr) air is measured. On the head of a positive surface streamer, there remains 10−11 C surface charge, which satisfies the criterion of an electron avalanche-to-streamer transformation. The charge density across a streamer takes a minimum on its centre and a maximum on its sheath. This fact suggests that a quasineutral channel exists in the centre of a streamer, and positive ions remain on the sheath of it.

Insulation characteristics of gas mixtures including perfluorocarbon gas

This paper describes discharge properties of N₂ and CO₂-based gas mixtures including a perfluorocarbon (PFC) gas such as CF₄, C₃F₈ and c-C₄F₈ under non-uniform field. The mixture ratio between a base gas of N₂ or CO₂ and the additive PFC gas was fixed as 9:1; namely, 90%N₂/10%PFC or 90%CO₂/10%PFC gas mixture. The PFC gases have even smaller global warming potential (GWP) than SF₆ gas and have good insulation properties as SF₆ gas. Thus, PFC gas mixture is expected to be a SF₆ substitute without highly pressurizing the gas over the conventional pressure of 0.5 to 0.6 MPa. In this study, in order to compare the partial discharge (PD) inception voltage V_PD and breakdown voltage V_B properties between N₂ and CO₂-based gas mixtures, as well as between the additive gas of PFC and SF₆ gas, we investigated these properties of the gas mixtures with a needle to plane electrode under ac high voltage application. The gas pressure was changed from 0.1 to 0.6 MPa. As a result, it was found that V_PD and V_B characteristics of N₂ and CO₂-based gas mixtures differed considerably, especially the gas pressure dependence of V_B (so-called the N shape characteristics). V_B characteristics of N₂-based gas mixture including c-C₄F₈ proved to be excellent within the test conditions over the wide gas pressure region, showing the maximum breakdown voltage. In terms of V_PD properties, CO₂-based gas mixture had an advantage over N₂-based gas mixture due to higher V_PD. Furthermore, we discussed the synergy effects of V_PD and V_B for N₂ and CO₂-based gas mixtures using the index R_n which was defined to quantify the degree of the effect. R_n for CO₂-based gas mixture was higher than that of N₂-based gas mixture.

Preparation and preliminary characteristic evaluation of epoxy/alumina nanocomposites

Epoxy/alumina nanocomposites were newly prepared by dispersing 3, 5, and 7 weight % nano-scale boehmite alumina particles in a bisphenol-A epoxy resin. Dispersion of alumina particles in the nanocomposite specimen was observed by scanning electron microscopy (SEM). Flexural properties, dynamic viscoelasticity, permittivity, partial discharge (PD) resistance, and electrical breakdown time were investigated for the nanocomposite specimens in comparison with those of an epoxy resin without nanofillers. The following results were obtained. The nanocomposite specimens keep high transparency similar to the pure resin. Alumina particles with a size below about 50 nm are homogeneously dispersed in the epoxy matrix. Due to dispersion of the nanofillers in the specimens, flexural properties, PD resistance, and electrical breakdown time are improved. The nanocomposites exhibit almost no change in glass transition temperature and permittivity.

Interruption characteristics of double-break vacuum circuit breakers

Since the majority of vacuum circuit breakers in use are of the single-break variety, there have been few studies of double-break vacuum circuit breakers. This report describes our investigations of the interruption characteristics of the double-break vacuum circuit breaker. In this investigation, the post arc current and recovery voltage distribution ratio were measured in order to clarify the relationship between them. The results show that a bias of the voltage distribution ratio occurs when the post arc current value becomes imbalanced. In addition, the measurements of the post arc current for circuit breakers with spiral contacts are very scattered as compared with those with axial magnetic field contacts. Normally, the double-break vacuum circuit breaker consists of two contacts of the same type. However, when two spiral contacts are used, the scattering of post arc current causes a current imbalance. As a result, a bias of the voltage distribution ratio may occur. It appears to be preferable to use double-break vacuum circuit breakers with axial magnetic field contacts.

Evaluation of Interruption Capability on Various Gases

SF6 gas is widely used as interrupting medium for disconnecting switch and circuit breaker in power system whose voltage is above 72kV or medium voltage. Global warming effect of SF6, however, is well known and emission of this gas is strictly limited by regulation. As for research to find out substitute for SF6 gas, many works have worked out. Gleitzes [1] compared with SF6 and N2 and found out characteristic of dv/dt that SF6 was about three times larger than that of N2 Mr. Grant [2] examined dv/dt to the mixture ratio of N2-SF6 at the total pressure 5bar, 6bar, and 7bar of di/dt=9 A/ μ s for SLF interruption performance. It was shown that interruptible dv/dt became N2-SF6 gas mixture of 65% to 90% had higher than pure SF6 gas, and this will be explained by synergistic effect. Also for He-SF6 gas mixture its rate varied from 25%,50% to 75% and interruptible dv/dt characteristic were observed higher for gas mixture compared with pure SF6. Leel[3] also carried out experiment and showed that the higher the mixture ratio of SF6 with N2 and He higher the interruptible current and that of He mixture was always inferior to that of N2 Mr. Nakagawa [4] measured di/dt, which could interrupt the mixed gas when increasing the pressure of N2 with a constant SF6 pressure of 3 bars. As a result, if the pressure of SF6 was the same, it was shown that increasing of total pressure became higher, interruption performance would be smaller. As mentioned before interruption and insulation abilities with various gas and their mixture have been studied by many researchers, but research especially for interruption was not done sufficiently.

New aspects of UHF PD diagnostics on gas-insulated systems

Gas insulated switchgears (GIS) have been increasingly introduced as main substation equipment since the late 1960s thanks to their high reliability, safety, and compactness. The UHF method to diagnose partial discharge (PD) is broadly employed in the field as an advanced insulation diagnostic technology. There are three primary factors related to the PD signal detection characteristics using this UHF method; namely the frequency spectrum of the PD signal itself inside GIS, the propagation characteristics of the PD signal inside the GIS tank, and the detection characteristics of the sensor measuring the PD signal. Accordingly, to further improvement of the diagnostic technique based on the UHF method, the PD phenomenon itself and its propagation characteristics should be clarified and the measurement system refined. From these perspectives, the present study reviewed the recent and latest findings concerning the PD phenomenon and its propagation characteristics inside GIS as well as the PD measurement system verification methods and diagnostic technologies, and summarized typical examples. Firstly, with regard to the PD phenomenon, measurement up to the high frequency band exceeding 10 GHz (even 30 GHz) clarified that the rise time of the PD current waveform is several tens of picoseconds, shorter than previously known. In the case of PD in micro-defects inside the epoxy insulator, however, the rise time is relatively long, in the order of nanoseconds, and a crack is the most critical defect. Subsequently, with regard to UHF PD diagnostics/monitoring technology, a novel technique using PD current waveform characteristics has been developed, and a more advanced PD diagnostic algorithm has been established by clarifying the influence of the shape and GIS internal structures on the electromagnetic wave propagation characteristics. Less uncertain and simplified calibration and verification technologies are also proposed both for a single UHF sensor and the entire UHF measurement system. These new technologies and further advanced studies in future are expected to make the UHF method more convenient and sophisticated.

Dielectric properties of biodegradable polylactic acid and starch ester

In order to examine the applicability of biodegradable polymers to the fields of electrical insulation, several dielectric properties of two typical biodegradable polymers, polylactic acid (PLA) and starch ester (SE), are examined. A fairly larger amount of space charge is accumulated in both polymers in comparison to low-density polyethylene (LDPE). This seems partly due to the presence of hydroxyl and carbonyl groups in these polymers. Permittivity and conductivity are higher in SE than PLA that has the values close to those of LDPE. The dielectric breakdown strength is lower in SE and is higher in PLA than LDPE. As for the resistance to photodegradation by ultraviolet photons, SE is stronger than PLA, although the two are much inferior to LDPE.

Fundamental insulation characteristics of high-pressure CO 2 gas for gas-insulated power equipment - effect of coating conductor on insulation performance and effect of decomposition products on creeping insulation of spacer -

Currently, environmental problems such as global warming are important issues, and SF6 has been identified as a greenhouse gas with a long atmospheric lifetime. Therefore, in the long term, it is preferable to reduce the amount of SF6 used as an insulating gas. It is thus important to discuss the possibility of using more environmentally friendly gases as alternative insulation for gas-insulated apparatus. In this paper, we describe the fundamental insulation breakdown characteristics of high-pressure CO2 gas at gas pressures of up to 1.0 MPa under simulated practical conditions, including the insulation breakdown characteristics of a high-voltage conductor with an insulating coating and the effect of decomposition products on the creeping insulation of a spacer. With the aim of enhancing insulation performance, we discuss the effect of the type of insulating coating on insulation performance. The breakdown electric field was increased by 20% by coating the conductor. It was verified that the application of an insulating coating is a practical method for enhancing the insulation performance of high-pressure CO2 gas. It was also verified that the decomposition products have only a slight effect on the creeping insulation of the spacer except for when there is heavy pollution on the insulating spacer. However, if a large amount of decomposition products is expected to be deposited on the insulating spacer during operation, which may cause a severe interruption to the current, it will be necessary to consider this factor in the insulation design.

Reignition surges at reactor current interruption in cable-system GIS

A 150 MVA reactor current interruption test using a GIS (gas-insulated switchgear) on an actual 275 kV cable system and a laboratory high-frequency current interruption test using a 300 kV SF/sub 6/ gas circuit breaker were conducted. The results of these tests and their analyses, made simultaneously, permitted (1) the calculation of reignition surges and the detection of high-frequency arc extinctions caused when the reactor current was interrupted by a GIS on a cable system and (2) an evaluation of voltage escalation caused at high-frequency arc extinctions. >