Eiichi Zaima

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. >

Observation of current waveshapes of lightning strokes on transmission towers

Considerable cost reductions have been achieved through rationalizing insulation design for substation equipment based on the application of high-performance surge arresters and improvement in analysis techniques. For further rationalization in insulation design and cost reduction, it is important to evaluate the lightning stroke current waveshape in detail. Some studies to observe current waveshapes have been conducted before, but they were ones conducted on independent towers and do not provide many data in a large-current region. So, the authors have been observing lightning stroke currents larger than 9 kA on 60 transmission towers (mainly 500 kV lines) since 1994. In this paper, they present some lightning parameters for insulation design of substations, drawn from current waveshape data, which have been collected so far. In addition, they report on the relationships among the peak value, the front steepness and the front duration, which heavily affect the insulation design of substations.

Observation and analysis of lightning surges at substations connected with UHV designed transmission lines

On UHV-class transmission lines with sections of 500 kV class insulation, lightning observations have been made to study the generation and the propagation of overvoltage into substations. Voltages and currents were recorded at substations and at junction-tower footings during three flashovers on the UHV system. These observations were then matched to EMTP calculations by adjusting the average soil resistivity and the average conductor height in the propagation model. It is also found that the attenuation due to corona of voltages was small because it is expected to be minimal for 8-conductor bundles.

Historical review of high voltage switchgear developments in the 20th century for power transmission and distribution system in Japan

In Japan during the 20th century, developments of high voltage transmission and distribution apparatus advanced tremendously and circuit breakers were the center of these developments. And as a result, 1100 kV SF/sub 6/ gas insulated switchgear (GIS) for ac transmission systems and also 500 kV dc GIS for dc transmission systems have become available. Vacuum circuit breakers (VCBs) have also been actively developed and a 168 kV two break VCB and 100 kA VCB are already available. This paper describes these developments and future trends in technology.

Study of Lightning Surge Overvoltages at Substations Due to Direct Lightning Strokes to Phase Conductors

Accurate predictions of lightning surge overvoltages are essential to power equipment insulation design. Recent observations of lightning strokes to ultra-high-voltage designed transmission lines confirmed direct lightning strokes caused by shielding failure and found phenomena unexplainable by conventional shielding theories. However, there are few detailed studies of direct lightning surge overvoltages. This study assumed direct lightning stroke currents based on observational data and performs electromagnetic transient program analysis of the gas-insulated switchgear (GIS) and transformer overvoltages at substations to study the basic characteristics of overvoltages due to direct lightning strokes and evaluate lightning protection design. Consequently, the maximum GIS overvoltages were found during back-flashovers, and the locations of maximum overvoltages from direct lightning strokes and back-flashovers differ. Direct lightning strokes may be more severe than back-flashovers for transformers. This paper also studied the overvoltage generation mechanism and showed the relationship of the maximum voltage to lightning stroke current and transformer capacitance.

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. >

Application of dynamic arc equations to high frequency arc extinctions in SF/sub 6/ gas circuit breakers

Mayr-type arc equations were applied to the analysis of high frequency arc extinctions in SF/sub 6/ gas blast circuit breakers. Parameters for the arc equations were obtained from the measured waveforms of high frequency arc conductance. They were calculated in combination with circuit equations. The results gave a good explanation of the measured results. There are a number of examples of applying Mayr-type arc equations to one current zero point in short circuit current interruption. Such equations were applied to high frequency current arc extinctions involving several current zero points, and the results are presented. >

Development of suspension-type arresters for transmission lines

A suspension-type surge arrester has been developed by incorporating ZnO elements into the shed of a conventional suspension insulator. This arrester is realized through the development of new techniques such as high-performance ZnO elements, airtight incorporation of ZnO elements into the shed of a conventional insulator, the protection gap, and pressure relief. Both single units and strings of arresters are subjected to contamination, pressure relief, and other tests to confirm that they satisfy the specifications. Long-term reliability was demonstrated in a long-term outdoor test with energization. >

Lightning Surge Overvoltages at Substations Due to Backflashover With Assumed Lightning Current Waveforms Based on Observations

Recent lightning current observations have suggested that a front duration of 1.7 s is more appropriate than the conventional 1.0 s for the lightning protection design of ultra-high voltage (UHV) systems in Japan. This study examined overvoltages at gas-insulated subsystems (GIS) and transformers in a UHV (the rated voltage of 1100 kV) substation during backflashovers, with the front duration of lightning stroke current waveforms taken as a parameter. The dependence of overvoltages on the front duration of the lightning stroke current is significant at GIS and relatively small at the transformer. Generally speaking, the shorter the front duration is, the higher the voltage will be. The transformer capacitance, ranging from 0 to 1000 pF, equivalently increases the distance to the surge arrester, and a voltage increase was observed at the transformer terminal. However, the steepness of the intruding surge waveform is reduced and the voltage increase suppressed at 1000 pF and over. The effect of overvoltages on insulation design was also evaluated and the results thereof indicated the possibility of reducing the lighting impulse withstand voltage.

Evaluation of degradation in polyimide by recovery voltage measurement

Polyimide (PI) has been widely used for insulation in motors, integrated circuits, electric wire, cable, and so on, due to its high mechanical strength, thermal stability, chemical stability, and radiation resistance. We have examined the degradation of API (all-aromatic polyimide) and TPI (thermoplastic polyimide) soaked in H/sub 2/O or KOH by measuring recovery voltage. The peak value of the recovery voltage was found to depend on the soaking time. We have developed a theoretical equation of recovery voltage, and have found that the peak value of recovery voltage is a function of the dielectric constant. By measuring the recovery voltage, it is possible to diagnose the degradation of API and TPI.