Tatsuo Kawamura

Multistory transmission tower model for lightning surge analysis

A multistory transmission tower model to be used in multiconductor analysis by EMTP (Electromagnetic Transients Program) is proposed. The circuit parameters of the model are determined based on the measurement of voltages across the insulator strings on an actual 500 kV transmission tower. The multiconductor lightning surge analysis considered enables one to predict the waveform of each insulator voltage, which is useful in determining the phase and the instant of a back flashover. The new tower model is recommended in Japan to be used in the new multiconductor analysis of EHV and UHV class double circuit transmission lines. The new tower model and conventional tower models are compared in the analysis of minimum back flashover lightning currents at a UHV transmission line. The new model gives about 20% smaller current. >

Experimental evaluation of a UHV tower model for lightning surge analysis

An experimental investigation was performed on a UHV tower model for the EMTP multiconductor calculation of lightning overvoltage at substations associated with back-flashover at an adjacent transmission tower. The various lightning surge response characteristics were measured on an actual UHV tower, and parameters of a multistory transmission tower model that can reproduce voltages across the insulator strings, voltages of the crossarms, and voltages of the power lines were determined. A value of 120 /spl Omega/ was determined as the surge impedance at each section of the multistory tower model, which closely agreed with the tower surge impedance measured for the UHV tower alone. >

Pressure Dependence of DC Breakdown of Contaminated Insulators

The effect of pressure on dc breakdown voltage of contaminated porcelain insulators and glass models was investigated in the range of 13 to 100 kPa. The study revealed that flashover voltage decreases nonlinearly with lowering of pressure. Arc characteristics under different pressure conditions were studied and the theoretical basis of low pressure flashover is discussed.

Japanese Method of Artificial Pollution Tests on Insulators

Salt pollution on outdoor insulators has been a serious problem in Japan, and steady efforts have been made to prevent flashover faults on polluted insulators. As a part of such efforts, a simple method of artificial pollution test has been developed to evaluate the flashover characteristics of polluted insulators, especially those with large dimensions. In this test, a salt solution of various concentrations containing a certain amount of kaolin is prepared and sprayed on the entire surface of the insulator to give a required density of salt deposit and sufficient moisture.

Step Response of Measuring Systems for High Impulse Voltages

The generalized impulse voltage measuring system is analyzed as a complete entity using the step response approach. The analysis shows that the response time of the system is not determined correctly by conventional methods and a new method is presented. Revisions to the conventional methods are proposed which will enable the true response time of the system to be determined. Five separate measuring systems are investigated experimentally using both techniques and the predicted and measured values of the conventionally measured response time are shown to be in very good agreement. As a result of the investigation, certain specific suggestions are made for revisions to existing standards for this type of measurement.

Internal winding failure due to resonance overvoltage in distribution transformer caused by winter lightning

In this paper, we describe the internal winding failures of the no. 3 distribution transformer at Katayamazu substation caused by winter lightning. Almost the same internal winding part of the transformer was damaged in January 1997 and November 2001. Therefore, detailed investigations of the cause of failures were carried out. From the investigations on the measurement of the frequency characteristics of transformer windings and detailed lightning surge analysis used by Electromagnetic Transients Program (EMTP), it was found that resonance overvoltages were generated by resonance phenomena between the surge waveform passing through the transformer and the natural frequency characteristics of the transformer winding. This finding was used to improve the winding form of tap windings and install a surge protection device between tap windings. After the improvement of winding structures, it was clearly shown that the internal stress of tap windings was reduced and the breakdown probability of the damaged part was significantly reduced.

Optoelectronic Measurement of Partial Arcs on a Contaminated Surface

In this paper, characteristics of a partial arc on a contaminated surface containing NaCl, measured by optoelectronic techniques, are described. Streak photographs using a solid state image sensing device clarified propagation characteristics and temporal variations of the diameter of a partial arc. A spectroscopic measurement revealed that most of the light emitted from the arc consists of the Na spectrum, and the temperature of the arc column is lower than that of a free-air arc. The calculated results on physical characteristics of the partial arc are discussed together with the experimental results.

A High Quality Voltage Divider Using Optoelectronics for Impulse Voltage Measurements

By applying an optical measuring system, which consist of light emitting diode, optical fiber light guide and photomultiplier, a high quality shielded resistance divider for impulse voltage measurements, whose response time is less than 5 ns, has been obtained. The characteristics have been verified experimentally.

Transient Impulse Breakdowns of SF6 Gas in Inhomogeneous Electric Fields.

In this paper, we describe the dielectrics and predischarge developments of gaseous SF6 stressed by nonoscillating and oscillating impulse voltages in inhomogeneous fields caused by a metallic protrusion. The breakdown voltage-pressure curves and the formative time lags to breakdown were measured in the gas-pressure range extending from 0.05 to 0.3 MPa. The predischarge processes were simultaneously investigated through measurements of the current induced by moving space charges and photomultiplier records. The predischarge propagates into the gap with a stepwise leader, and the basic physical mechanisms leading to electrical breakdown are the same for nonoscillating and oscillating impulse voltage waveforms. While for the other, the quickly changing macroscopic electric field due to space charges in the gap affects the leader propagation under the oscillating impulse voltage waveform. In the case of negative polarity, the predischarge growth processes at the increasing and decreasing parts of the oscillating wave tail are not very similar. It is inferred that the breakdown at the decreasing part of the oscillating impulse voltage originates from ion depletion due to recombination.

Lightning Surge Analysis in a Multi-Conductor System for Substation Insulation Design

Analysis of back flashover lightning surges coming into a substation is important to understanding of substation insulation co-ordination. These surges are usually analyzed for a single conductor system, but in this paper, they were analyzed with EMTP for a multi-conductor UHV system consisting of two ground wires and six phase conductors. It was clarified that lightning surge voltages on the phase conductor decreased due to negative reflection of the lightning surge at a tower, and the electrical coupling between the ground wire and phase conductor. The multi-conductor system was applied to surge analysis for the line entrance circuit of a gas insulated substation.