Jun Takami

Observational Results of Lightning Current on Transmission Towers

Lightning parameters, particularly the amplitude and the front duration of lightning stroke current, are the basic requirements for lightning protection design, and identifying lightning stroke current waveforms in detail is essential for rational lightning protection design. Between 1994 and 2004, Tokyo Electric Power Company directly measured the current waveforms of lightning strokes on 60 transmission towers (mainly 500-kV transmission lines) and obtained 120 data sets, including waveforms exceeding 100 kA. This paper quantitatively evaluates the characteristics of each lightning waveform parameter calculated from the observed waveforms as well as correlations between parameters, together with comparisons with previous data. In terms of the correlation between the current amplitude and the front duration, the front duration corresponding to the maximum rate of rise is defined, and the relationship that greater current amplitude results in a longer front duration is formulated for the first time. Quantifying the relationship of the current amplitude and the front duration is very useful to rationalize lightning protection design

Characteristics of Direct Lightning Strokes to Phase Conductors of UHV Transmission Lines

Since transmission towers are taller and interphase distances are longer for ultra-high voltage (UHV) transmission lines than for conventional transmission lines, it is important to know the features of direct lightning strokes to phase conductors. With this in mind, the features of direct lightning strokes to UHV transmission lines were observed and 81 datasets were obtained for seven years between 1998 and 2004. Of the 81 datasets, 79 recorded strokes from negative lightning, and the characteristic features of these direct lightning strokes were quantitatively evaluated by Electromagnetic Transients Program analysis of the current amplitude, front duration, and stroke duration. The frequency distribution of lightning strokes to each phase conductor was different from the estimated values obtained from conventional shielding models. Based on the cumulative frequency distribution of the current amplitude observed in this study, a method is proposed for calculating direct lightning stroke current as part of designing lightning protection

A Simplified Model of Corona Discharge on Overhead Wire for FDTD Computations

A simplified model of corona discharge on overhead wire has been proposed for propagating surge computations using the finite-difference time-domain method. The radial progression of corona streamers from the wire is represented as the radial expansion of cylindrical conducting region whose conductivity is several tens of microsiemens per meter. Two wire radii are considered: 5 and 2 mm, in order to simulate two experimental con- figurations by Noda. The critical electric field on the surface of a 5-mm radius wire for corona initiation is set to E0 = 1.8 or 2.9 MV/m. For a 2-mm radius wire, it is set to E0 = 2.2 MV/m. The critical background electric field necessary for streamer propagation is set to Ecp = 0.5 MV/m for positive voltage application, and Ecn = 1.5 MV/m for negative voltage application. The computed waveform of radial current (including both conduction and displacement currents) agrees well with the corresponding measured waveform. Also, the computed relation between the total charge (charge residing on the wire and emanated corona charge) and applied voltage (qV curve) agrees well with the corresponding measured one, except for relatively low applied voltages. Additionally, the increase of coupling between the energized wire and another one nearby due to corona discharge is well reproduced.

Reliability evaluation with weibull distribution on AC withstand voltage test of substation equipment

For the development of a ldquoshort-duration AC withstand voltage testrdquo, an insulation specification of substation equipment, there is a precise method of reliability evaluation using a Weibull distribution function. Regarding this method, there remains a subject of handling coexistence of multiple voltage levels. This paper first defines the two reliability evaluation methods, ldquoindependence methodrdquo; and ldquoaccumulation methodrdquo, applying to Weibull evaluation for coexistence of multiple voltage levels in relation to their physical meanings. Next, the influence of the Weibull parameter values are examined on the cumulative fault probabilities and test voltages calculated using these methods. When the time shape parameter a>1, the accumulation method gives higher values than the independence method; When a=1, the two methods give the same values; When a<1, the former gives lower values than the latter. Then, appropriate reliability evaluation methods are investigated for various insulation media and insulation structures of substation equipment from the viewpoint of inception and development mechanisms of dielectric breakdown and partial discharge. According to the result of engineering evaluation of the presently available data, the independence method may be appropriate for both gas insulated switchgear and oil-immersed transformers.

3-D FDTD Computation of Lightning-Induced Voltages on an Overhead Two-Wire Distribution Line

Lightning-induced voltages on a 738-m long overhead two-wire line have been computed using the 3-D finite-difference time-domain (3-D FDTD) method for solving Maxwells equations. The 3-D FDTD method employed here uses a subgrid model, in which spatial discretization is fine (cell side length is 0.9 m) in the vicinity of overhead wires and coarse (cell side length is 4.5 m) in the rest of the computational domain. The overhead wires having radii of some millimeters are simulated by placing a wire having an equivalent radius of about 0.2 m (≈0.23 × 0.9 m) in the center of an artificial rectangular prism having a cross-sectional area of (2 × 0.9 m) × (2 × 0.9 m) and the modified (relative to air) constitutive parameters: lower electric permittivity and higher magnetic permeability. Induced-voltage peaks computed at different points along the line for the return-stroke speed of 130 m/μs and ground conductivity of 3.5 mS/m agree reasonably well with the corresponding voltage peaks measured in the rocket-triggered lightning experiment of Baker et al., in 1996.

FDTD Simulation of Lightning Surges on Overhead Wires in the Presence of Corona Discharge

A simplified model of corona discharge for finite-difference time-domain (FDTD) computations has been applied to analyzing lightning surges propagating along overhead wires with corona discharge. The FDTD computations simulate the experiments of Inoue and Wagner . In Inoues experiment, a 12.65-mm radius, 1.4-km-long overhead wire was employed, and in Wagner s experiment, a 21- or 25-mm radius, 2.2-km-long overhead horizontal wire was employed. The critical electric field on the surface of the 12.65-mm-radius wire for corona initiation is set to E0 = 1.4, 2.4, or 2.9 MV/m, and those for 21- and 25-mm-radius wires are set to E0 = 2.2 and 2.1 MV/m, respectively. The critical background electric field for streamer propagation is set to Ecp = 0.5 MV/m for positive voltage application and Ecn = 1.5 MV/m for negative voltage application. The FDTD-computed waveforms (including wavefront distortion and attenuation at later times) of surge voltages at three different distances from the energized end of the wire agree reasonably well with the corresponding measured waveforms. Also, the FDTD-computed waveforms of surge voltages induced on a nearby parallel bundled conductor agree fairly well with the corresponding measured waveforms.

Improved method of calculating lightning stroke rate to large-sized transmission lines based on electric geometry model

The characteristics of the lightning shielding of large-sized transmission lines have been calculated based on an electric geometry model (EGM) proposed by Armstrong and Whitehead. However, the characteristics of lightning strokes to large-sized transmission lines observed in recent years differed from calculation results, hence the present study was conducted to improve the conventional calculation method. First, the relationship between the striking distance and the lightning current was reviewed. Specifically, the air gap discharge characteristics based on test data with complementing the large gap region were applied. In addition, the return stroke velocity distribution obtained by actual measurement was newly applied. Second, the distribution of lightning waveforms was reviewed; the distribution of peak values of lightning current with the distribution of front duration was used in this study. With these improvements, the calculated lightning stroke rate to power lines and ground wires differs less from actual observations than that calculated by the conventional method.

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.

Basic study of fitting method for base curve extraction in lightning impulse test techniques

The IEC, which prescribes lightning impulse test techniques for electric power equipment, is about to introduce the k-factor method for evaluating the overshoot of impulse waveforms. Its procedure specifies that waveforms are to be fitted with a double exponential function to derive a base curve. Some derived waveforms, however, deviate upwards from the central lines of the recorded waveforms in the wavefront area. This paper examines a more rational method of extracting a base curve by considering the relationships between equivalent electric circuits and the solutions of their governing equations. A trial calculation with the new methods gives results with lower crest values and slower rises than the existing method.

Study on a field data of secondary arc extinction time for large-sized transmission lines

Secondary arc extinction time is an important factor, which influences system stability because it directly affects the reclosing time. However, few reports exist on a field data of the secondary arc extinction time on transmission lines. This paper describes the investigation results of secondary arc extinction characteristics by analyzing voltage wave shapes during the lightning faults on 550 kV transmission lines. Recovery voltages and secondary arc currents on each transmission line during faults were calculated to consider correlation with the secondary arc extinction time. The secondary arc extinction time is estimated by the existing formula, the secondary arc current and the recovery voltage required for one second reclosing are evaluated.