This paper deals with the partial discharge inception voltage (PDIV) and breakdown voltage (BDV) characteristics of turn-to-turn insulation models for SF₆ gas-insulated transformers under steep front short pulse voltages, of which waveforms are 35/65 ns and 50/370 ns. It was found out that PDIV under nano-second pulses were 1.05_??_1.15 times higher than those under standard impulse voltages and BDV under nano-second pulses were about 1.4 times higher than those under standard impulse voltages. The ratio of impulse PDIV to ac PDIV is 1.40_??_1.48 for standard impulse voltages and 1.32_??_1.37 for switching impulses.

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A high speed grounding switch (HSGS) will be adopted in the 1000kV transmission system in Japan. HSGS is required to interrupt a current, which does not pass a current zero for 80ms, as a special duty. Therefore, new testing methods are necessary to verify the interrupting performance with delayed zeros in high power laboratories. Generally, computer simulations are efficient for the development of interruption testing methods. Interaction between the circuit and the arc plays an important role in the process of current interruption. Therefore, it is essential to take appropriate arc characteristics into account. In this paper, computer simulation technique with arc characteristics will be applied to some testing circuits and newly developed interruption testing methods with delayed zero passing will be evaluated theoretically and experimentally.

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Demand for gas insulated transformer is now increasing greatly because of the incombustible property and down-sizing effect to underground substation. Since completion of 275kV 300MVA liquid cooled gas insulated transformer in 1991, we have continued to research to develop gas insulated transformers applicable to higher voltage level up to 5000kV. As the results of the research, 500kV 300MVA liquid cooled gas insulated transformer has been developed. We introduce the abstruct of our study.

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Measurements were made of the sum of cathode-fall and anode-fall voltages, i.e. electrode fall voltage V_fall for SF6 arc at a pressure of 0.1 MPa in wide current range from 10 to 20000A. The SF6 arc was burned between electrodes of copper-tungsten, copper, iron, titanium, carbon and tungsten. For each electrode, V_fall at a given current was estimated as arc voltage at zero gap length between the electrodes, which was extrapolated from arc voltages measured in short gaps with various lengths below 3mm. The electrode fall voltage V_fall proved to be 17.5, 17.5, 17.5, 17.3, 15.2 and 13.8V for the Cu-W, Cu, Fe, Ti, C and W electrodes, respectively. The electrode fall voltage V_fall was affected by the electrode material. It was also found that the electrode fall voltage at a given electrode was independent of current in the above range.

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Transient axial distributions of post-arc temperature and its electrical resistivity were measured in a flat-type SF₆ gas-blast quenching chamber. Iron was intentionally used as an electrode material, since Fe spectral lines at wavelengths of 426 and 443nm have very much higher radiation intensities than lines of S⁺, F or Cu at temperatures below 5000K. The two-line method using these Fe spectral lines permitted to determine post-arc temperature T up to 100μs after current zero. From the temperature thus obtained, the electrical resistivity p_res of post-arc were determined. The results revealed that the p_res at the nozzle throat rose drastically with a lapse of time after current zero. Furthermore, the low- p_res region diminished from the nozzle throat to the electrodes in arc extinction process. This process of p_res was interpreted as the opening process of fictitious contacts made of thermal plasma. The opening velocity of the fictitious contacts was estimated to be more than 100m/s. The behaviours of the fictitious contacts in cases that transient recovery voltage was applied to the electrodes were obtained by solving the energy conservation equation.The results may give one of the essential facts in the arc interruption phenomena.

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Reduction of the insulation level of every equipment in converter stations is most important point for improvement of compact equipments, which tend to be higher voltage levels. Higher performance, lower discharge voltage surge arresters reduce the insulation level of the equipments. At this point of view, the high performance surge arresters are essential, In DC 500kV power systems, we reduced the protection level by approximately 25% in comparison with conventional system, it enable the protection level to be reduced 945kV/10kA Application of these surge arresters to the entrance of the DC mainline will reduce the insulation level of DC 500kV main circuit to L1WV 1, 300kV. This paper describes newly developed HVDC 500kV-600kV SF₆ gas tank type surge arresters.

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Partial discharge (PD) may take place due to residual defects like metallic particles or microprotrusions inside SF₆ gas insulated power apparatus such as GIS, leading to the failure breakdown. Thus, PD measurements are very effective and promising technique to diagnose GIS under operating conditions. However, the signal of PD occurring in SF₆ gas is very weak and susceptible to external noises which mainly consist of PD in air. Thus, it is important to distinguish the PD in SF₆ gas more sensitively from the external noises.
From the above points of view, we have been investigating characteristics of electromagnetic wave spectrum radiated from PD in SF₆ gas and in air using wide-band antennas. In this paper, we propose “phase gate control method” in order to reveal the dependence of the polarity of ac voltage on the electromagnetic wave spectrum. Using the phase gate control method, an attempt is made to separate electromagnetic spectrum caused by PD occurring in SF₆ gas from that in air. We also investigate the frequency range where PD spectrum in SF₆ gas alone can be detected under noisy conditions. On the basis of these results, we discuss the minimum detectable PD charge in SF₆ gas as a function of measuring frequency.

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The recent trend in electric power consumption shows a steady increase and concentration in large cities and industrial districts. This indicates strong need for a long-distance and enclosed-type power transmission line with large capacity and high reliability. A hybrid gas-insulated transmission line (H-GIL), which has the combined features of an XLPE cable and a conventional gas-insulated transmission line (GIL), has the potential to be applicable to such use.
In the present work, we analyze the required dimension and the current capacity of H-GIL on the basis of insulation and thermal design for various parameters determining the structure. We also report an experimental study on flashover characteristics for a simple electrode model of H-GIL. The results show that the H-GIL can be realized with almost the same sheath diameter and current capacity as a conventional GIL. The experiment has also clarified that the insulation reliability of the H-GIL is much higher than that of a GIL. These properties show that the H-GIL has enough feasibility applicable as a long-distance and reliable transmission line with large capacity especially in urban areas.

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The waveform of a standard lightning impulse greatly differs from those of actual lightning surges coming on GIS. This raises a problem on the equivalence of the standard lightning impulse. This report describes the effect of the voltage wave-forms on an insulating performance in SF₆, gas gap subjected to fast oscillating impulse voltages simulating actual lightning surges, and the evaluation of V-t characteristics by applying the equal area criterion.

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This paper describes the development of surge arrester of GIS for non-effectively ground system. The surge arresters are developed by the improvement of the manufacturing of the praseodymium ZnO elements. The improvements include the new additive and the new process of production . Reference voltage per unit thickness attained 300V/mm owing to the fine-grain ZnO.
Therefore the residual voltage of the arrester reduced 25% for 154kV class arresters and 15% for 66kV. It is important to check the long duration thermal stabilty and the temporary over voltage discharge capabilty. The long duration thermal stabilty contained 3D-electric field analysis and the voltage distribution test with the aid of the optical-electrical transducer.
The surge arresters have lower residual voltages that attain lower insulation level for GIS.

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It is important to improve the sensitivity of partial discharge (PD) sensors for diagnostics of high voltage electric apparatus. However when noise similar to the PD signal is picked up by sensors, false detection will be made. PD pulses have about one nanosecond rise time in the SF₆ gas insulation. These pulses propagate in the apparatus to PD sensors in the form of the voltage oscillation, the high frequency current, and the electromagnetic radiation. We succeeded to detect the periodic damping waveforms by PD sensors installed at the GIS. These waveforms have information of the PD.
In this paper, we investigated the characteristics of three types of PD sensors, by putting high frequency signals in the form of sine wave and pulse into test apparatus. We found that these sensors can detect some 100MHz high frequency signals and one nanosecond rise time pulse similar to PD signals in SF₆. Frequency characteristics of waveforms detected by the PD sensors are influenced by a test circuit.

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The application of SF₆, -gas-immersed transformer is increasing rapidly instead of oil-immersed transformer, for prevention disasters and nonflamability of substations. Authors have developed middle-capacity SF₆-gas-immerced transformer with sheet winding as far as 66kV/20MVA class, to save the expert skill for disk winding and to reduce kind and number of parts and product lead time.
Firstly this paper describes the essential techniques for middle-capacity SF₆-gas-immersed transformer of analysis and measuring results of prototype about current density and leakage magnetic flux distribution at the edge and insulation characteristics at the edge and layers of sheet winding, and secondly, the good results of three-phase verification-type transformer.

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Gas insulated transformer is suited for urban underground substations because of its non-flammable and non-explosive nature and compact installation space and its use has been growing in urban area. However, to meet the demand for large capacity unit of gas-cooled type transformer, cooling difficulty caused by small heat capacity and thermal conductivity of SF₆ gas should be solved. A 275kV•300/3MVA gas-cooled gas-insulat ed shell form transformer has been developed by use of a new cooling structure and it is now under a long term energizati on test. This paper describes main features of the transformer as well as cooing and insulation characteristics obtained by component models of trans-former and a full scale model transformer.

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The paper describes the improvement in the insulation characteristics of a disconnecting switch for DC500kV gas insulated switchgear, It is effective to coat metallic enclosure with dielectrics to prevent metallic particles from lift-off. From tests with a small size model, it was found out that the insulation characteristics of the operating rod made with FRP was influenced by not only the total rod length, but the inserted length in high voltage shield. The gas insulated disconnecting switch for DC500kV gas insulated switchgear was developed. This equipment uses an operating rod with a round-shaped cross-section to improve insulation characteristics. This equipment has an efficient dielectric characteristics. This disconnecting switch is now under the long-term insulation test for the evaluation of long term insulation reliability.

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In Japan, construction of 1000kV (UHV) transmission lines is planned to cope with the increase of electric power demand expected. On the line, after the fault current is interrupted by circuit breakers, the arc caused by electrostatic induction current remains for a long time because of its high voltage. To re-energize the fault line after the arc extinction, a new circuit breaker reclosing system which has high-speed grounding switches (HSGS) installed at both ends of the line is employed.
If, while the HSGS is interrupting an electromagnetic induction current, a ground fault takes place in another energized line, causing a short-circuit current including the dc component to flow, the large dc component is superposed on HSGS current, producing a zero shifting state with no pass through the zero point for long time. Such zero shifting durations are estimated to be up to about 80ms. Therefore HSGS are required to interrupt this delayed zero current as a special duty. This requirement is met by a newly developed puffer interrupting chamber allowing a long time pressure rise through optimizing the exhaust and residual volume of the puffer cylinder and utilizing the effect of pressure rise due to arc.

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This paper describes a cubicle type gas-insulated switchgear(C-GIS), which employs a composite insula-tion that combines a solid insulatoin with a low-pressure SF₆, gas insulation to reduce the size of the equipment.
There are two types of the composite insulation structure: one type of structure has solid insulat-ions arranged in a gap so that their surfaces cross electric lines of force perpendicularly; and in the other type the solid insulation solidly supports the electrode so that its surface is parallel to the electric lines of force. It is possible to combine these structures.
With the perpendicular type, the flashover voltage is significantly improved over that of the metal electrodes because of the covering insulation. In the parallel type which involves a creepage insula-tion, the electeic field strength components are so set that the normal component is equal to or larger than the tangential component, making the field strength uniform to improve the flashover voltage.
To deal with the creeping discharge, a boundary point of the insulation layer in contact with metal is peovided with a projection on the metal side, or with a curved portion on the insulation layer side to avoid an increase in the electric field strength due to triple-junction.
In an non-uniform electric field, the composite insulation can stabilize corona when the field utilization factor U is 0.2 or less, improving the flashover voltage.
The 66/77-kV C-GIS applying these composite insulations has achieved a significant reduction in size.

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This paper presents an acoustic based location scheme of corona discharge in oil-filled transformer. Four supersonic-sensors were used, and the three dimensional coordinates of corona discharge location in the transformer could be estimated from three time differences between those sensors. AC high voltage was applied to a needle-plane-electrode configuration in a model transformer, and the acoustic sound wag initiated. In this scheme, FFT processing and high-pass digital filtering were applied to discriminate the corona discharge signal and others, and to extract the dominant frequency components. First, the traveling time differences between sensors were roughly evaluated from the time difference of each onset time of sound signal and those were corrected by the mutual correlation factor approaches. The coordinates were computed in reasonable accuracy. Moreover, some discussions about the discrete errors were described.

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Electric contact members are classified by the form such as a point, a line, and a plane. Particularly, the point contact member which makes a small circluar contact spot is basis of the contact form. Also, the plane contact member will make many contact spots which are different in size and distributing over the contacting portion. Authors have been developing the contact model to measure the conatact resistance. In this model, it is available to select the size, the number, and the arrangement of contact spots by the macroscopical contact spots which are formed with some metal electrodes arranged in electrolyte solution of dilute sulfuric acid. In this paper, the fundamental characteristics of the contact resistance caused by the constriction of current are investigated with 1 to 3 contact spot models. Then the relations between the conatact resistance and the distribution of the contact spots are examined with 2 to 9 multi-contact spot models. As the results, it is cleared that the proposed contact model presents the successful experimental method for the analysis of the conatact resistance.

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Power systems may experience nonlinear resonances due to quasi-periodic disturbances from the viewpoint of oscillation. These circumstances are suspected to trigger more complicated nonlinear phenomena related to the dynamics of the power system. In this paper, the possibility of the dynamic stability problem caused by Hopf bifurcation under the condition that there exists nonlinear resonance in power systems is revealed by means of the Intrinsic Multiple-Scale Harmonic-Balancing method. Analysis focuses not on simple bi-furcation parameters such as damping, but on the auxiliary parameters such as drifting of the frequency of disturbance. The results suggest the occurrence of perturbed Hopf bifurcation in a power system experiencing nonlinear resonance, which can cause long-term oscillations to influence dynamic stability of power systems.

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This paper presents an accurate mathematical model for a thyristor controlled series capacitor. Conventional models assume that the TCSC has no loss and neglect the presence of resistance component of its equivalent impedance. The mathematical equations considering the effect of resistance component of TCSC were developed using Lapalace Transformation by separating the thyristor on and off equivalent circuits. They represent the inherent slow response of the TCSC by solving the differential equations of the TCSC circuit every on/off period. Anti-parallel thyristors and the firing circuits were also simulated correctly. It is more accurate in determining the transient response of TCSC voltages and currents than previous models. The conventional models have a limitation to operating regions, while the developed model is valid for both the capacitive and inductive regions of the TCSC operation as well as the resonant region. The proposed model is also capable of incorporating unstable operating conditions of TCSC. Simulations by the proposed model were conducted and compared with experimental results to check the validity of the model. Effects of resistance on the TCSC performances have been also investigated.

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Autoparametric resonance is a phenomenon in which one oscillation mode gets unstable through interaction with another mode. There is a possibility that a low-frequency oscillation gets unstable because of the resonance in longitudinal power systems. The physical cause of the resonance is load, and the resonance gets stronger as load increases. Damping torques of generators are effective to suppress the resonance. We show that if there is certain amount of damping torque, a longitudinal system gets stable irrespective of the number and capacity of generators. Since the interaction between the modes arises from voltage deviation at load buses, it is useful to make the voltage deviation small. We clarify how automatic voltage regulators (AVR) mitigate the resonance. The AVRs reduce the strength of the resonance to one tenth of that for the system where the internal voltages are assumed constant. High gain AVRs make the mode unstable, however. Power system stabilizers (PSS) stabilize the mode, and simultaneously provide some damping necessary to suppress the resonance. Lastly, we verify through simulations that AVRs and PSSs really stabilize the system.

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