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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 18, No. 1; February 2011 221

1070-9878/11/$25.00 © 2011 IEEE

Occurrence Probability of Lightning Failure Ratesat Substations in Consideration

of Lightning Stroke Current Waveforms

Shigemitsu Okabe, and Jun TakamiTokyo Electric Power Company

4-1, Egasaki-cho, Tsurumi-ku, Yokohama, Kanagawa, 230-8510, Japan

ABSTRACTTo estimate lightning stroke overvoltages exactly, the occurrence probability of lightning

stroke current waveforms must be accurately evaluated. This paper firstly formulated the

occurrence probability distribution of lightning stroke current waveforms, taking into

account the correlation between the current amplitude and the front duration. Next,

lightning overvoltages were calculated, with the current amplitude and the front duration

of lightning current as statistical parameters for Gas Insulated Switchgears (GIS’s) and

transformers in UHV substations. While overvoltages caused by back-flashovers at GIS’sare affected by the front duration of lightning current, overvoltages at transformers are

relatively less dependent on the front duration. Finally the failure rate was evaluated by

considering not only the current amplitude but also the front duration. These values were

smaller than those evaluated from the frequency of occurrence of current amplitude

alone. Further, the proposed front duration of 1.7 s was examined.

Index Terms = Lightning stroke current, current waveform, front duration,

overvoltage, failure rate, substation equipment, lightning impulse withstand voltage.

1 INTRODUCTION

ACCORDING to the latest evaluation studies [1, 2] of

assumed lightning stroke current waveforms based on actual

measurements - aimed at rationalizing the lightning protectiondesign of transmission line and substation equipment - an

appropriate front duration for each voltage class is proposed, with

the correlation between the current amplitude and front duration of

lightning stroke currents taken into account [2]. In addition, the

possibility of reducing the lightning impulse withstand voltage

(hereinafter referred to as LIWV) for UHV substation equipment is

suggested, by changing front duration to be longer, at least on a

trial calculation basis [3]. On the other hand, a decrease in the

reliability (or an increase in the failure rate) of equipment becomes

concern when the LIWV is reduced, and the accurate evaluation of

reliability, or of lightning failure rates, is an increasingly important

subject for the rationalization of insulation.

In order to appropriately evaluate failure rates and rationalize

the lightning protection design of substation equipment, it is

necessary to clarify the probability distribution of the

occurrence of lightning stroke overvoltages. To do this, the

probability of the occurrence of lightning stroke current

waveforms must be accurately estimated. For example, results

of a detailed analysis of how lightning stroke overvoltages occur

at substations, with the front duration of the assumed lightning

stroke current and the capacitance of transformers as parameters,

show that the relationship between each parameter and generated

overvoltages are not necessarily uncomplicated. The parameters

have complicated effects on the overvoltages [3]. However,evaluation of the failure rates of power stations and substations

has until now only been based on the frequency of occurrence of

the assumed lightning stroke current amplitude; the influence of

the front duration was not taken into account [4-9].

This paper formulates the probability distribution of the

occurrence of lightning stroke current waveforms with a

correlation between the current amplitude and the front duration

of measured data taken into account. This is essential for the

statistical evaluation of lightning stroke overvoltages. Lightning

stroke overvoltages are then calculated a vast number of times,

with the current amplitude and the front duration of lightning

stroke current waveforms as statistical parameters for Gas

Insulated Switchgears (GIS’s) and transformers of UHVsubstations. The statistical distributions of generated

overvoltages are obtained and the lightning failure rates are

evaluated, on a trial calculation basis. The authors have

proposed the front duration 1.7 s of the assumed lightning

current for the UHV system in Japan [2], whose duration was

derived in consideration of a correlation of the front duration

and current amplitude as a substitute of 1.0 s of the

conventional value. The newly assumed lightning stroke current

with the front durations of 1.7 s is also evaluated from the

perspective of reliability.Manuscript received on 25 March 2010, in final form 18 June 2010.



222 S. Okabe, and J. Takami: Occurrence Probability of Lightning Failure Rates at Substations in Consideration of Lightning Stroke

2 OCCURRENCE PROBABILITY OFLIGHTNING STROKE CURRENT

WAVEFORMS

The occurrence probability of lightning stroke current

waveforms is derived from observed lightning stroke current

to transmission tower data [1], with the correlation between

the current amplitude and the front duration taken into account.

Here, lightning stroke current waveforms are simulated usingramp waveforms.

First, the probability of the occurrence pi(i) of the lightning

stroke current amplitude “i” is conventionally expressed

according to the following equation, assuming the logarithmic

normal distribution [10-12] etc.

2

2

1

2

))log()(log(exp

2

1)(

ii

imi

i p

(1)

where i : the lightning stroke current amplitude [kA], m1: the

mean value of lightning stroke current amplitude [kA], and

i : the standard deviation of log (i).

Then, [2] showed that the front duration (log value) in thefixed current amplitude section was normally distributed

centered on the average regression curve - the relationship

between the current amplitude and the front duration.

Accordingly, the probability of the occurrence pfi(tf ) of the

front duration of lightning stroke current tf [s] with the

lightning stroke current amplitude i [kA] is expressed by the

following equation.

2

2

2

2

))log()((exp

2

1)(

f f

fimt z

t p f

f

(2)

where tf : the front duration of lightning stroke current [s],

z(tf ): the difference in log value between the front duration oflightning stroke current and the regression curve of the

distribution of the front duration, f : the standard deviation

of log z(tf ), and log (m2) : the mean value of the difference of

the front duration from the average regression curve (the

difference in log value). z(tf ) is obtained from the following

equation.

)

230exp(31.1loglog)(

it t z f f

(3)

The probability density of the occurrence P(i,tf ) of the

lightning stroke current waveform with the current amplitude i

and the front duration tf is derived from equations (1) to (3),

and the following equation is derived.

)()(),( f fi f t pi pt i P i

2

2

1

2

))log()(log(exp

2

1

i f i

mi

2

2

2

2

)log()230

exp(31.1loglog

f

f mi

t

(4)

The cumulative frequency of current waveforms can be

obtained by integrating the probability density of the

occurrence P(i,tf ) with respect to log (i) and log (tf ).

With regard to equation (4), the constants obtained from the

observed lightning stroke waveforms to transmission towers

(the number of data N: 120 [1]) are summarized in Table 1.

Figure 1 shows the frequency distribution of the occurrence of

lightning stroke current waveforms obtained from equation (4)

Table 1. Constants related to the probability of the occurrence of lightning stroke

current waveforms [2].

Parameter UnitStatistical

value

m1: mean value of the lightning stroke

current amplitudekA 29.3

i : standard deviation of log iI log value, kA 0.28

Log(m2): mean value of the difference

of the front duration from the

average regression curvelog value, s - 0.0127

f : standard deviation of “the

difference in log value between the

front duration of lightning strokecurrent and the regression curve of

the distribution of the front

duration”

log value, s 0.135

N: number of data event 120

030

6090

120150

180

01

23

45

0

0.0010.002

0.003

0.004

0.005

0.006

0.007

0.008

Normalized

probability [%]

Current

amplitude [kA]

Front duration [s]

0.007-0.008

0.006-0.007

0.005-0.006

0.004-0.005

0.003-0.004

0.002-0.003

0.001-0.002

0-0.001

0306090120150180

0.0

0.5

1.0

1.5

2.0

2.5

3.03.5

4.0

4.5

5.0

Current amplitude [kA]

Front

duration [s

Front duration of lightning current; y=0.717ex/230

200kA,1.7s

Average regression curve; y=1.31ex/230

(b) Two-dimensional display (relationship between the current amplitude and

the regression curve of the average front duration )

(a) Three-dimensional display

Figure 1. Occurrence probability of lightning stroke current waveforms as a

function of the amplitude and the front duration.




and Table 1 both in three-dimensional and two-dimensional

displays.

3 CALCULATION METHOD FOR FAILURERATE OF SUBSTATION

To predict the failure rate with a high degree of accuracy,

the probability of the occurrence of overvoltages as well as the

probability of insulation breakdown of equipment (insulators)due to overvoltages is to be obtained, and then the failure rate

calculated by combining these two probabilities. However,

since it is difficult to evaluate the probability of insulation

breakdown of equipment at present, this study defines the

failure rate as the frequency of occurrence of overvoltages

exceeding the LIWV.

In the preceding examinations, assumptions with less validity

had to be made when calculating overvoltages, such as

assuming the current amplitude and either the front duration or

the front steepness to be independent of each other, or the front

duration constant for the lightning stroke waveform - the basic

conditions. In the previous section, the frequency distribution of

lightning stroke current waveforms was obtained based onmeasured data. So the lightning stroke overvoltage is now

calculated using the distribution of the lightning stroke current

waveform parameters such as current amplitude and front

duration, expressed by equation (4). While the frequency

distribution of overvoltages should be calculated by the analysis

of overvoltages using not only lightning stroke current

waveforms but also the combination with transmission towers

struck by lightning, the ac phases of phase conductors at the

time of lightning strokes and so on, as other types of parameter,

an overvoltage analysis in present study was made with only

lightning stroke current waveforms used as parameters because

present study aims to be a basic study focusing on the effects of

lightning stroke current waveforms.The present study is carried out for UHV transmission and

substation equipment, and as criteria for estimating the failure

rate of substations, overvoltages generated at GIS terminals

and at transformers are evaluated by comparing them with the

LIWV.

3.1 CALCULATION FLOWCHART FOR LIGHTNINGFAILURE RATE

The calculation flow for the lightning failure rate is shown

in Figure 2. First, the conditions are established and an

overvoltage analysis of a substation made using the ATP-

EMTP [13] with the amplitude and the front duration of

lightning stroke current as parameters. Next, the maximumvoltages generated are extracted from the overvoltage analysis

results in order to obtain the overvoltage distribution. The

frequency of occurrence of overvoltages here corresponds to

the probability of the occurrence of lightning stroke current

waveforms – conditions for the generation of overvoltages.

The lightning failure rate is then calculated using the

frequency of occurrence of overvoltages exceeding the

assumed LIWV.

Scripts are created to control the subsequent series

of numerous operations and finally estimate the failure rate,

including inputting lightning stroke current waveforms to the

EMTP data and instructing the execution of the EMTP

calculation, extracting the maximum overvoltages and

calculating the frequency of occurrence, creating a database of

overvoltages and their frequency, and calculating the

frequency of occurrence of overvoltages exceeding the LIWV.

3.2 ANALYSIS CONDITIONS FOR LIGHTNINGOVERVOLTAGES

The overvoltage analysis model for UHV transmission line

and substation equipment is the same as that in [3]. The

analysis conditions are listed in Table 2. The location oflightning strokes and back-flashovers were determined on the

assumption that lightning strokes hit the first transmission

tower nearest the substation and flashover occurred at the

arcing horn of the same tower. As shown in Figure 3,

Configuration I was used as the circuit configuration of a

substation in the GIS overvoltage analysis, where a circuit

breaker at the service line entrance opens and imposes a high

surge voltage on the GIS. Configuration II was used for the

transformer overvoltage analysis. This allows only a small

surge current diversion in the substation to impose a high

surge voltage on transformer terminal. Transformers were

simulated by capacitance, and a shell-type transformer with

5800 pF and a core-type with 16600 pF were used. Withregard to the flashover model, a non-linear inductance leader

development model was used, where the pre-discharge

phenomenon was simulated by non-linear inductance [14, 15].

Refer to [3] for details of the analysis model.

The lightning stroke current waveform was simulated by a

ramp waveform. Equation (4) obtained in the previous section

was used for the probability of occurrence of the current

amplitude and the front duration in Figure 1. For the analysis,

the range of the current amplitude was divided into

100 cases, from 3 kA to the max. 300 kA with 3 kA increment,

- Setting lightning stroke conditions

- Setting the analysis circuit

- Setting observation nodes

- Overvoltage analysis- Maximum overvoltage output

Probability of lightning strokes

to transmission lines taken

into account

- Absolute evaluation

- Relative evaluation

EMTP analysis

Evaluation of the

failure rate

- Inputting the current

waveform

- Instructing the execution of

EMTP

- Extracting the maximum

overvoltages

-Calculating the frequency of

occurrence

-Creating Database of

overvoltages/frequency

- Calculating the frequency

of occurrence of

overvoltages exceeding

LIWV.

Script controlSetting conditions

Figure 2. Calculation procedures for substations’ failure rates based on

probability of occurrence of lightning stroke current waveforms and EMTP

overvoltage analyses.




and the range of the length of the front duration was divided

into 50 cases, from 0.1 s to the max. 5.0 s with 0.1 s

increment. Since the stroke duration has only a small effect on

lightning surge overvoltages, it was constant for 70 s. This

has been conventionally used in lightning protection design in

Japan [9]. 5000 cases of patterns for lightning stroke current

waveform analysis were prepared. The analysis used ATP-

EMTP [13].

3.3 EVALUATION CONDITIONS FOR LIGHTNINGFAILURE RATE

3.3.1 PROBABILITY OF LIGHTNING STROKES TO

TRANSMISSION LINES

The probability of lightning strokes to transmission lines

(Nt) could be worked out in detail using electrogeometric

modeling with the help of computers. However, the following

simplified method is used in this study because it has been

adopted by the conventional design guidelines for back-

flashovers [9]. Regarding the relationship between the

transmission tower height “h” and the frequency of lightning

strokes, the frequency of lightning strokes is proportional to h

if the tower is assumed to be a stand alone-structure. If,

however, in the case of horizontally long structures such as

transmission lines, this is proportional to, as in studies based

on electrogeometric models. This relationship holds good as

long as the height is up to about 100 m.

The probability of lightning strokes to transmission lines

(Nt) is based on the information above and the actual

occurrence of 43 lightning strokes per 100 km of transmission

line when the height of transmission towers was 25 m, with an

IKL of 30 to 35 (which more or less corresponds to the

calculation results using the electrogeometric model.)

35302543

IKLh Nt [times / 100 km / year] (5)

Here, the transmission tower height h is 110 m, the average

height of UHV transmission towers. As for IKL (Mesh

covering 15 minutes of latitude and longitude: about 25

km×27 km), the lightning stroke density (1 km×1 km) where

UHV designed transmission lines pass through was obtainedusing a lightning positioning and tracking system (LPATS)

and converted into IKL. In consequence, IKL = 40 was used.

3.3.2 RANGE OF ASSUMED LIGHTNING STROKE

LOCATIONS

With regard to the location of lightning strokes, as referred

to in Section 3.2, only the top of the first transmission tower is

considered as a lightning stroke location for the overvoltage

analysis since the present study is a basic one focusing mainly

on the effects of lightning stroke waveforms. Severe lightning

stroke overvoltages are generally generated when back-

flashover conditions occur close to the substation, and

therefore the spans including one of the first towers areassumed as the range of the location of lightning strokes for a

rigorous evaluation, taking the attenuation of surge due to

corona and other factors into account. Also since the surge

current is shunted two ways and back-flashovers will be less

likely in the case of lightning strokes in the middle part of the

span, it is appropriate to assume a range within about a quarter

of the span length on both sides of the transmission tower.

Consequently, the range of the location of assumed lightning

strokes was set to a total length of 200 m on the both sides of

the first tower for transmission lines designed for UHV.

Table 2. Analysis conditions.

L i g h t n i n g s t r o k e

p h e n o m e n a

LocationUpper phase back-flashover due to a lightning

stroke to the first transmission tower

Lightning stroke

current

Ramp waveform, Current amplitude: 3 - 300 kA,

Front duration: 0.1 - 5.0 sStroke duration : 70 s

Lightning stroke

impedance400

ac phase ac voltage superimposed

T r a n s m i s s i o n l i n e

Flashover Non-linear inductance model based on the leader

method

Transmission

line8 phase Semlyen model

Transmission

tower

4-story transmission tower model (constant

determined by actual measurement), Grounding

resistance: 10

Gantry 2- story model, Grounding resistance: 4

Corona effect Not considered.

S u b s t a t i o n

All GIS Double bus - 4 bus tie system - 4 circuits - 4 banks

C

i r c u i t Configurat

ion IOpen circuit breaker at the service line entrance

Configuration II 1 circuit1/4 bus1 transformer

GISSingle phase distributed constant circuit, Lossless

line, Surge impedance: 91

Circuit breakerIn closing: same as the GIS

In opening: -type capacitance simulation

TransformerCapacitance simulation

- Core-type 16600 pF

- Shell-type 5800 pF

Bushing Capacitance simulation: 300 pF

Surge arresterModel taking fast transient current characteristics

into accountV10kA= 1550 kV

Figure 3. Circuit configuration of UHV substation subject to lightning surge

overvoltage analysis.

Service lineentrance

Circuit breakerat the service line

Circuit configuration I :Overvoltage at a GIS analyzed with thecircuit breaker at the service line entranceopened

Circuit configuration II :Overvoltage at a transformer analyzed withone transformer in one circuit(Solid line in the figure)

Transformer

Open

Close

(CB is 18m away SA)

(Tr. is 12m away SA)




0306090120150180210240270

0

1

2

3

4


Front duration

[s]

200kA,1.7s : 2137kV

200kA,1.0s : 2622kV

Figure 5. Distribution of overvoltages generated at the terminal of GIS’s

with changes in current amplitude and front duration of lightning stroke

current waveforms.

3.3.3 FREQUENCY OF OCCURRENCE OF

LIGHTNING STROKE CURRENT IN RANGE OF

ASSUMED LIGHTNING STROKE LOCATIONS

With the frequency of occurrence Pi of lightning stroke

current waveforms IL(i p,tf ), with current amplitudes and front

durations mentioned in the previous section as parameters,

given by equation (4), the frequency of occurrence of

lightning strokes in the range of assumed lightning stroke

location P(i, tf ) can be obtained by the following equation (6).

In the equation, L is the range of assumed lightning stroke

locations, L = 200 m for UHV transmission lines.

100000),(

L N P t i P t i f [times / route / year ] (6)

3.3.4 ESTIMATION OF THE LIGHTNING FAILURE

RATE

To evaluate the failure rate, the frequency per year of

occurrence of overvoltages generated in the substation

equipment (GIS’s and transformers) exceeding the set LIWV is

calculated. The frequency of occurrence of individual

overvoltages can be expressed by the frequency of occurrence P

in equation (6) of lightning stroke current within the range ofassumed lightning stroke locations - the condition for generating

overvoltages. The lightning failure rate can be calculated by

integrating the frequency of occurrence of overvoltages

exceeding the LIWV assumed in advance, taking the probability

of lightning strokes to transmission lines into account.

4 OVERVOLTAGE ANALYSIS RESULTS

4.1 DISTRIBUTION OF OVERVOLTAGESGENERATED AT GIS’S

In the Circuit Configuration I, Figures 4 and 5 show

examples of overvoltage waveforms and the distribution ofovervoltages in three-dimensional and two-dimensional

displays, respectively, generated at the GIS terminals and with

changes in lightning stroke current waveform parameters. As

shown in Figure 4, in the case of GIS’s, the maximum value

appears at the spike waveform of wavefront, the peak value

varying with changes in the front durations of lightning stroke

current waveforms.

The part where overvoltages generated are negative valuesin Figure 5 is the area where back-flashovers do not occur.

The boundary area with positive values indicates the boundary

between the presence and absence of back-flashovers. In the

figure, the critical points of back-flashover occurrence are

almost linear. These critical characteristics can be almost

linearly approximated with respect to the current amplitude

and the front duration, suggesting a condition in which the

front steepness is almost constant. However, when the current

amplitude is around 50 kA, back-flashovers do not occur

because of lack of time to generate back-flashovers resulting

from shorter front duration less than approx. 1 s. While

back-flashovers partly occur in the range 500 kV to 1000 kV,

late in the wave tail of the lightning current, there is littlelightning stroke current flow to phase conductors and the

voltage increase is small.

According to the distribution of generated voltages and the

contour lines of overvoltages, in the area where the front

duration is about 0.5 s or longer, the higher the current

amplitude, and the shorter the front duration, the higher the

voltage tends to be. While the area of occurrence of

overvoltages between 1500 kV and 2000 kV is wide, the area

between 1500 kV and 1600 kV was found to be the widest of

all after detailed checking. This was due to the suppression of

0 1 2 3 4 5-1000

0

1000

2000

3000

Time [ ]

V o l t a g e [ k V ]

t f : 1.0 s

t f : 2.0 s

t f : 3.0 s

s

Figure 4. Examples of overvoltage waveforms generated at GIS’s with

changes in front duration of lightning stroke current with amplitude of

200kA. The time 0 is defined as the time of the lightning to a tower and ac

phase voltage is superimposed.

030

6090

120150

180210

240270

0

2

3

5

-1000-500

0500

1000150020002500

30003500

4000

Voltage [kV]

Current amplitude

[kA]

Front

duration

[s]

3500-4000

3000-3500

2500-3000

2000-2500

1500-2000

1000-1500

500-1000

0-500

-500-0

-1000--500

or below

(b) Two-dimensional display





generated overvoltages, by surge arresters, to the residual

voltage level of the surge arresters. Conversely, when the

front duration is shorter than about 0.5 s, overvoltages

decrease. This is because back-flashovers occur at the wave

tail, after lightning stroke current peaks, as a result of front

durations shorter than the time required for back-flashovers to

occur. It is also thought that the voltage increase due to pre-

discharge has an impact on decreasing the surge suppression

effect of surge arresters [3].

Under the analysis conditions established in the present

study, the highest overvoltage was 3760 kV, with a current

amplitude of 300 kA and a front duration of 0.1 s; a

combination of the highest current amplitude and the shortest

front duration.

4.2 DISTRIBUTION OF OVERVOLTAGES

GENERATED AT TRANSFORMERS

In the circuit configuration II, overvoltages generated at the

primary terminal of shell-type transformers and core-type

transformers were analyzed and calculated. Figures 6 and 7

show examples of lightning stroke overvoltage waveformsand the distribution of overvoltages, respectively, generated at

the primary terminal of shell-type transformers. As shown in

Figure 6, the maximum value is generated during the

wavefront of overvoltages; however, there is no steep spike as

observed in the case of GIS’s and changes in overvoltages

with changes in the front duration of lightning stroke current

waveforms seem small.

The part where generated overvoltages are negative values in

Figure 7 is the area where back-flashover does not occur and the

boundary area to positive values indicates the boundary between

the presence and absence of back-flashovers. These critical

characteristics of back-flashover generation are almost identical

to those of GIS’s. The characteristics of generated overvoltagesare also the same as those at GIS’s in that the voltage area

corresponding to the residual voltage of surge arresters around

1550 kV is the widest.

The way back-flashovers occur is strongly affected by the

front duration in the case of both transformer and GIS.

However, the overvoltage distribution after back-flashovers

occur is different to that of GIS’s. While it would be natural

to conclude that the contour line of overvoltages after back-

flashovers occur shows a positive correlation between the

current amplitude and the front duration (a straight line

from the right bottom to the top left in the figure) - similar

to critical characteristics of flashover generation - the

contour line of overvoltages at transformers is closer to being in parallel with the front duration axis than in the

case of GIS’s. Overvoltage waveforms are steep during the

wavefront due to the effect of the front duration of

lightning stroke current and the maximum voltage

appearing at the peak during the wavefront of waveforms.

However, if a large capacitance, such as a transformer, is

connected to the line, the impedance to steep surge voltage

is reduced and the influence of the front duration of

lightning stroke current decreases. The generated voltage at

transformers increases as if charging the entire substation, so

the overvoltage is regarded as being determined by the size of

the charge from a lightning surge.

Under the analysis conditions set out in this study, the

highest overvoltage at shell-type transformers was 2219 kV,

with a current amplitude of 300 kA, and a front duration of

0.3 s. Compared with voltages generated at GIS’s, changes

in voltage are smaller on the whole, and the maximum voltage

is about 59 % of the maximum voltage at GIS’s. Overvoltages

generated at core-type transformers are shown in Figure 8.

0 2 4 6 8 10-1000

0

1000

2000

3000

Time [ ]

V o l t a g e [ k V ]

t f : 1.0 s

t f : 2.0 s

t f : 3.0 s

s

Figure 6. Examples of overvoltage waveforms generated at shell-type

transformers with changes in front duration of lightning stroke current with

amplitude of 200kA. The time 0 is defined as the time of the lightning to a

tower and ac phase voltage is superimposed.

0

30

60

90

120150

180210

240270

01

23

4

-1000

-500

0

500

1000

1500

2000

2500

Voltage [kV]

Current amplitude

[kA] Front duration

[s]

2000-2500

1500-2000

1000-1500

500-1000

0-500

-500-0

-1000--500

or below



Figure 7. Distribution of overvoltages generated at primary terminal of shell-

type transformers with changes in current amplitude and front duration of

lightning stroke current waveforms.

0306090120150180210240270

0

1

2

3

4


Front du ration

[s]

200kA,1.0s : 1741kV

200kA,1.7s : 1800kV




Since the capacitance of transformers is larger, the influence

of the front duration becomes smaller than on shell-type

transformers. Accordingly, the maximum overvoltage is

comparatively lower and the overvoltage characteristics with

changes in current waveforms are flatter. The highest

overvoltage at core-type transformers was 2070 kV, with a

current amplitude of 300 kA, and a front duration of 0.6 μs.

While the highest overvoltage at GIS’s appeared with the

shortest front duration of lightning stroke current, the highestovervoltage at transformers of both types appeared in

conditions other than the shortest lightning stroke current

front duration.

As described above, the characteristics of overvoltages

shows that neither the increased current amplitude nor the

shorter front duration of lightning stroke current necessarily

lead to a simple increase in overvoltages. So as far as

lightning stroke current conditions in studies of lightning

failure rates are concerned, it will be necessary to carry out

statistical evaluations of front durations of lightning stroke

current, taking into account their correlation with current

amplitudes.

5 EVALUATION OF LIGHTNING FAILURE

RATE OF SUBSTATION

5.1 FAILURE RATE OF GIS’S

Based on the overvoltages at GIS’s with back-flashovers

analyzed in the previous section, overvoltages exceeding the

LIWV are defined as insulation failures here. The frequency

of occurrence of overvoltages is obtained by calculatingequation (6). The failure rates of GIS’s are obtained by

calculating the frequency of occurrence of overvoltages

exceeding the specified LIWV and integrating them, which

are expressed in the per year per route (line). The results of

the calculation of the failure rate of GIS’s are shown in Table

3. Figure 9 shows the relationship between the failure rate of

GIS’s and the LIWV. In the UHV field-test equipment, the

test voltage for GIS’s is set to 2250 kV [16] and the failure

rate is very low at 8.41×10-6 [times/route /year]. (Hence the

frequency of failure (MTBF: Mean Time Between Failure) is

about once in 119000 years per route.) If the LIWV is reduced

to 2100 kV, 1950 kV, and 1800 kV, with the lightning

impulse withstand voltage level specified by IEC [4] as aguide, the failure rate is increased to 2.5, 6.5 and 18.9 times,

respectively, from the baseline of the failure rate with the LIWV

at 2250 kV; however, the failure rate remains at 1.59×10-4

[times/route/year] even if the LIWV is reduced to 1800 kV.

While this is limited to the failure due to back-flashover, the

frequency of occurrence is about once in 6290 years per route.

Evaluating the failure rate conventionally, based only on the

frequency of occurrence of the assumed lightning stroke current

amplitude of 200 kA (the cumulative frequency of occurrence of

0.3 % of the current amplitude exceeding 200 kA), would mean

that failures occur once in about 700 to 1000 years [9]. The

failure rate mentioned above is considerably lower than this. In

the case of an LIWV set to 2250 kV, the evaluation based on the

frequency of occurrence of generated overvoltages resulting in

MTBF of 119000 [year, route/time] is significantly different

from the conventional evaluation based only on the frequency of

occurrence with the current amplitude resulting in MTBF of 700

to 1000 [year, route/time]. This is an significant difference from

the point of view of reliability to promote electrical insulation

rationalization.

A high assumed failure rate with the conventional method is

mainly due to not taking into consideration the frequency of

occurrence of the front duration of the assumed lightning

stroke current. In the present analysis, the overvoltage of 2622

kV is generated by the conventionally assumed lightning

stroke current waveforms with a current amplitude of 200 kAand a front duration of 1.0 μs. Figure 10 shows the

overvoltage distribution overlaid with the density distribution

of occurrence of lightning stroke current. The failure rate

conventionally used was evaluated using only the current

amplitude, regardless of the front duration. Consequently,

overvoltages generated under the condition of 200 kA or

higher are assumed to result in failure in all cases as shown by

the hatched area surrounded by a light red line in Figure 10a.

On the other hand, in the case of evaluation using overvoltages,

these overvoltages depend on the influence of the current

030

6090

120150

180

210240

270

01

23

4

-1000

-5000

500

1000

1500

2000

2500

Voltage [kV]

Current amplitude

[kA]

Front duration

[μs]

2000-2500

1500-2000

1000-1500

500-1000

0-500

-500-0

-1000--500

or below

0306090120150180210240270

0

1

2

3

4


Front duration

[μs]

200kA,1.７μｓ: 1616kV

200kA,1.0μｓ: 1592kV

Figure 8. Distribution of overvoltages generated at primary terminal of core-

type transformers with changes in current amplitude and front duration of

lightning stroke current waveforms.






0306090120150180210240270

0

1

2

3

4


Front duration

[s]

200kA,1.0s

0306090120150180210240270

0

1

2

3

4


Front duration

[s]

200kA,1.0s

200kA,1.7s

amplitude and the front duration. Therefore, the evaluation of

overvoltages exceeding 2622 kV with a front duration of 1.0 s

using the conventional assumed lightning stroke current

waveform results in the hatched area surrounded by a light red

line in Figure 10b. The area surrounded by a light red line in

Figure 10b is determined by overvoltages with a front duration

of 1.0 s. The light red line here is the 2622 kV contour line.

This area is small and, further, is far from the area where the

probability of occurrence of lightning stroke current is high andtherefore the frequency of occurrence is low. The higher the

current amplitude, the longer the front duration becomes.

Therefore, in the area of large currents such as the one with a

current amplitude of 200 kA, there is a large gap between an

evaluation with one parameter shown in Figure 10a, and an

evaluation with two parameters shown in Figure 10b.

The evaluation of the failure rate with overvoltages is more

essential, and the front duration has a large impact on

overvoltages. Therefore, evaluations of failure rates with the

overvoltage level using two parameters, the current amplitude

and the front duration of lightning stroke current, are more

rigorous than evaluations of them using the conventional

method, where only the frequency of occurrence of the currentamplitude is used.

Meanwhile, in the present study, the evaluation is made

using the maximum value of generated overvoltages.

Waveforms are not evaluated by being converted into standard

waveforms [17]. Under the conditions in the present study, if

lightning overvoltages generated at GIS’s are converted into

standard lightning impulse waveforms, they will be lower.

Thus it is anticipated that the evaluation of waveforms will

lead to a lower failure rate.

5.2 FAILURE RATE OF TRANSFORMERS

For shell-type transformers, as with GIS’s, the calculation

of the lightning failure rate is shown in Table 4, and the

relationship between the failure rate and the LIWV is shown

by the solid line in Figure 11. The higher the LIWV, the lower

the failure rate, and in the present calculation if the LIWV is

set to 2250 kV, no overvoltages in excess occur, resulting in a

failure rate of zero. The failure rate at the test voltage of 1950

kV [16] for transformers in the UHV field-test equipment was

7.94×10-5 [times/route/year]. It was 3.99×10-4

[times/route/year] for the lowest LIWV of 1675 kV on a trial

calculation basis studied in [3]. Expressed by MTBF, the

frequencies of occurrence of lightning failure are about 12600

years and about 2500 years per route, respectively, in the case

of LIWV of 1950 kV and 1675 kV. In comparison with the

case of GIS’s in Figure 9, the failure rate of GIS’s is higher in

the range of 1800 kV or higher, and if a comparison is made

based on the test voltages of the UHV field-test equipment for

transformers (LIWV: 1950 kV) and GIS’s (LIWV: 2250 kV),

the failure rate of transformers is about 9 times that of GIS’s.

Qualitatively, as described in Section 4.2, transformers have

large capacitance and therefore smaller increases in

overvoltages caused by a shorter front duration compared with

GIS’s; the contour line is closer to being in parallel with the

front duration axis, as is clearly evident when Figures 7 and 8

Table 3. Calculation of the lightning failure rate of GIS’s.

LIWV

Failure rate

[times/route/year]

(MTBF [year])

Relative failure rate

compared with LIWV of

2250kV

1550 1.48×10-3 (676) 176

1675 5.14×10-4 (1950) 61.1

1800 1.59×10-4 (6290) 18.9

1950 5.44×10-5 (18400) 6.47

2100 2.13×10-5 (46900) 2.53

2250 8.41×10-6 (119000) 1.00

2400 3.13×10-6 (319000) 0.30

1600 1800 2000 2200 24000

0.5

1.0

1.5

[10-3

]

Lightning impuls withstand voltage [kV]

F a

i l u r e r a t e

[ c i r c u i t s / r o u t e / y e a r ]

Lightning impulse withstand voltage [kV]

Figure 9. Relationship between lightning impulse withstand voltage (LIWV)

for GIS and lightning failure rate.

(a) Distribution of overvoltages generated at 200 kA or higher

Figure 10. Distribution of overvoltages at GIS with respect to the current

amplitude and the front duration of lightning stroke current, and density

distribution of occurrence of lightning stroke current waveforms.

(b) Distribution of overvoltages exceeding 2622 kV




0306090120150180210240270

0

1

2

3

4


Front duration

[s]

200kA,1.0s

200kA,1.7s

0306090120150180210240270

0

1

2

3

4


Front duration

[s]

200kA,1.0s

200kA,1.7s

are compared with Figure 5. As shown by a light red line in

Figure 12, the overvoltage distribution for 1741 kV

generated in the case of a current amplitude of 200 kA and a

front duration of 1.0 s has a contour line distribution more

like that with the single parameter - the current amplitude -shown in Figure 10a than the contour line (light red line) of

overvoltages at GIS’s shown in Figure 10b. This results in a

higher failure rate, because the area is wider and includes

the range where the density of the probability of occurrence

is high. As for the core-type transformers described next,

while the counterpart of Figure 12 is Figure 13, since the

dependency on the front duration is even smaller, the

contour line is more closely parallel to the axis of the front

duration of the lightning stroke current supposed by Figure

8, and the evaluation result is close to the that based on the

conventional method using only the distribution of the

current amplitude. However, in the case of transformers,

overvoltages are suppressed to the residual voltage level ofsurge arresters, even when lightning stroke currents are large

and the failure rate is almost zero if the LIWV is set to 2250

kV or higher.

The calculation of the failure rate of core-type

transformers is shown in Table 5, and the relationship

between the failure rate and the LIWV is shown by the

dotted line in Figure 11. Since overvoltages at core-type

transformers are lower than those at shell-type transformers,

the failure rate is about half.

6 RELIABILITY WITH ASSUMEDLIGHTNING STROKE CURRENT

WAVEFORM

The study has clarified that the front duration of lightning

stroke current has a significant impact on overvoltages and

that the effect of the front duration is not simple. It is therefore

more essential, and more appropriate, for the reliability to be

evaluated by generated overvoltages rather than by thefrequency of occurrence of lightning stroke current

waveforms This section, therefore, evaluates the reliability

ensured in the context of the assumed lightning stroke current

based on the generated overvoltage level. With regard to the

assumed lightning stroke current waveforms, a comparison is made

between a waveform with 200 kA and 1 / 70 s in the study of

Table 4. Calculation of the lightning failure rate of shell-type transformers.

LIWV

Failure rate

[times/route/year]

(MTBF [year])



1950kV

1550 5.64×10-4 (1770) 7.11

1675 3.99×10-4 (2510) 5.02

1800 2.50×10

-4

(4000) 3.151950 7.94×10-5 (12600) 1.00

2100 8.43×10-6 (119000) 0.11

2250 0.00 () 0.00

2400 0.00 () 0.00

1600 1800 2000 2200 24000

2.0

4.0

6.0

[10-4

]

Lightning implus withstand voltage [kV]

F

a i l u r e r a t e

[ c i r c u i t s / r o u t e / y e a r ] Shell-type transformers

Core-type transformers

Lightning impulse withstand voltage [kV]

Figure 11. Relationship between lightning impulse withstand voltage

(LIWV) for transformer and lightning failure rate.

Figure 12. Distribution of overvoltages at shell-type transformer with respect

to the current amplitude and the front duration of lightning stroke current and

density distribution of occurrence of lightning stroke current waveforms.

Figure 13. Distribution of overvoltages at core-type transformer with respect

to the current amplitude and the front duration of lightning stroke current and

density distribution of occurrence of lightning stroke current waveforms.

Table 5. Calculation of the lightning failure rate of Core-type transformers.

LIWV

Failure rate

[times/route/year]

(MTBF [year])



1950kV

1550 3.57×10-4 (2800) 7.30

1675 1.91×10-4 (5240) 3.91

1800 1.14×10-4 (8770) 2.32

1950 4.89×10-5 (20400) 1.00

2100 0.00 () 0.00

2250 0.00 () 0.00

2400 0.00 () 0.00




the test voltage for the UHV field-test equipment (hereinafter

referred to as the conventional waveform), and a waveform with

200 kA and 1.7 / 70 s, as proposed in [2] (hereinafter referred

to as the proposed waveform). It should be noted that the study

of reliability in this study was done under specific analysis

conditions. In addition, for the actual test voltages, LIWV, it is

necessary to consider not only the back-flashover analysis

results but also the impact of direct lightning strokes on phase

conductors with shielding failures [18] and overvoltages inother lightning surge time areas, such as disconnector surges.

With regard to GIS’s, according to Figure 10b the overvoltage

generated by the conventional waveform is 2622 kV and the

frequency of occurrence of overvoltages exceeding this is 2.27×10-

6 %. Therefore, when LIWV is set to 2622kV generated from the

conventional waveform, calculating the failure rate by integration

using equation (6), 5.45×10-7 [times/route/year] (MTBF: 1.83×106

[year, route/time]) is obtained. This failure rate is shown by the area

surrounded by a light red line in Figure 10b. On the other hand, the

overvoltage generated by the proposed waveform is 2137 kV and

the frequency of occurrence of overvoltages exceeding this is

6.80×10-5 % with a lightning failure rate of 1.64×10-5

[times/route/year] (MTBF: 61000 [year, route/time]), which isshown by the area surrounded by a blue line in Figure 10b. In

comparing the contour lines of both cases, in the case of the

proposed waveform as shown in Figure 10b, generated

overvoltages are lower than with the conventional waveform, and

the probability distribution of occurrence based on the proposed

waveform extends to the range where the probability of occurrence

is higher and the frequency of occurrence of overvoltages with the

proposed waveform is as high 30 times as that with the

conventional waveform. This means that the reliability from the

point of view of lightning protection design decreases. However,

the absolute value of the frequency of occurrence is extremely

small compared to the assumed failure rate of 1.0×10-3 to 1.5×10-3

[times/route/year] (MTBF: about 700 to 1000 [year, route/time]) -an evaluation value with a cumulative frequency of 0.3 % which

exceeds the conventionally assumed value of 200 kA, based only

on the current amplitude distribution frequency. Therefore, the

lightning stroke waveform proposed in [2] maintains a high

reliability from the engineering point of view, namely, in the

evaluation the failure rate is basically zero.

Similarly, with regard to shell-type transformers, according to

Figure 7, the overvoltage generated by the conventional

waveform under the present analysis conditions, is 1741 kV, and

the frequency of occurrence of overvoltages exceeding this is

1.29×10-3 % and the failure rate is 3.10×10

-4 [times/route/year]

(MTBF: 3230 [year, route/time]). The overvoltage generated by

the proposed waveform was 1800 kV and the frequency of

occurrence of overvoltages exceeding this was 1.04×10-3 % with

a lightning failure rate of 2.50×10-4 [times/route/year] (MTBF:

4000 [year, route/time]). These failure rates correspond to the

areas surrounded by a light red line and a blue line in Figure 12,

respectively. Unlike the case of GIS’s, the required reliability of

the proposed waveform is to some extent better than that of the

conventional waveform and the failure rate is reduced by about

20%. In either case, the reliability conventionally assumed with

only the current amplitude (assumed failure rate MTBF: about

700 to 1000 [year, route/time]) is adequately ensured. As for the

core-type transformers, while the overvoltage generated by the

proposed waveform is low at 1616 kV, the lightning failure rate

was 2.50×10-4 [times/route/year] (MTBF: 4000 [year,

route/time]), which is shown by the area surrounded by a blue

line in Figure 13, accidentally just the same as that of shell-type

transformers, because generated overvoltages were generally

lower. The overvoltage generated by the conventional waveform

is 1592kV and the failure rate, the area surrounded by a light red

line in Figure 13, is estimated as about same as that calculated bythe proposed waveform.

As for a UHV transmission system, making the front duration

of the assumed lightning stroke current 1.7 s instead of 1.0 s

is considered appropriate, based on the properties of lightning

current [2]. It was found from this study that lightning failures

continue to be basically zero, as seen in changes in the required

reliability with a revised front duration.

7 CONCLUSIONS

The frequency of occurrence of lightning stroke current

waveforms was formulated from the lightning stroke currentwaveform parameter characteristics based on observed

lightning data, and an evaluation of the failure rate of UHV

substations was attempted in terms of the probability of

occurrence of lightning overvoltages exceeding the lightning

impulse withstand voltage (LIWV). The results of this study

are summarized below.

1) Based on the lightning stroke current waveform

parameter characteristics obtained from observed lightning

data, the probability (density) distribution of occurrence of

lightning stroke current waveforms with the correlation

between the current amplitude and the front duration taken

into account was formulated.

2) Using, as parameters, lightning stroke currentwaveforms with the correlation between the current amplitude

and the front duration taken into account, overvoltage analysis

was conducted in 5000 cases each for GIS’s, shell-type

transformers and core-type transformers, respectively. With

regard to GIS’s, overvoltages generated during back-flashover

are suppressed by surge arresters. In general, however, the

higher the current amplitude and the shorter the front duration,

the higher the overvoltage generated. Conversely, when the

front duration was shorter than about 0.5 s, overvoltages

decreased. This is regarded as being caused by delayed back-

flashover and the decreased surge suppression by surge

arresters of the voltage increases caused by pre-discharge.

With transformers on the other hand, overvoltages during back-flashover were less dependent on the front duration, and

generated overvoltages were lower than those at GIS’s, due to

the their large capacitance.

3) As a result of the trial calculation of the lightning failure

rate of GIS’s, in the case where a test voltage of 2250 kV for

the UHV field-test equipment is set as the LIWV, the failure

rate is 8.41×10-6 [times/route/year] (MTBF: 119000 [year,

route/time]) under the analysis conditions of the present study.

If the LIWV is reduced to 2100 kV, 1950 kV, and 1800 kV,

the failure rate is increased to 2.5, 6.5 and 18.9 times,




respectively, from the baseline with the LIWV at 2250 kV;

however, the failure rate remains at 1.59×10-4

[times/route/year] (MTBF 6290 [year, route/time]) even if the

LIWV is reduced to 1800 kV.

4) As a result of the trial calculation of the lightning failure

rate of transformers under the conditions of the present study,

in the case of the calculation for shell-type transformers, if the

LIWV is set to 2250 kV, no overvoltages in excess occur,

resulting in a zero failure rate. The failure rate at the test

voltage of 1950 kV of the UHV field-test equipment was

7.94×10-5 [times/route/year] (MTBF: 12600 [year,

route/time]), and 3.99×10-4 [times/route/year] (MTBF: 2500

[year, route/time]) even when the LIWV was reduced to 1675

kV. Since overvoltages generated at core-type transformers

are lower than those at shell-type transformers, the failure rate

is about half that of shell-type transformers.

5) From the point of view of reliability, in the evaluation

of the assumed lightning stroke current waveforms used for

the lightning protection design, the assumed lightning stroke

current waveform with 200 kA and 1.7 / 70 s proposed in

[2] ensures a lightning failure rate reliability for GIS’s at

1.64× 10-5 [times/route/year] (MTBF: 61000 [year,

route/time]) or less, and a lightning failure rate reliability for

transformers - both shell-type and core-type - of 2.50×10-4

[times/route/year] (MTBF: 4000 [year, route/time]) or less.

The assumed lightning failure rate, based only on a current

amplitude of 200 kA, for setting the test voltage of UHV

field-test equipment, is 1.0×10-3

to 1.5×10-3

[times

/route/year] (MTBF: about 700 to 1000 [year, route/time]).

Comparing the failure rates, the assumed lightning stroke

waveform proposed has adequate required reliability with a

basically zero failure rate.

The probability distribution of occurrence of lightning

stroke current waveforms formulated in the present studywould be useful not only for the lightning protection design of

substation equipment and for countermeasures against

lightning damage to power transmission lines and structures,

but also for analyzing phenomena dependent on the current

amplitude and the front duration of lightning stroke current.

The authors would expect it to be widely used. While the

lightning failure rate was evaluated on a trial calculation basis

using statistical methods, overvoltage analysis could be

conducted in the future using a combination of all possible

parameters, such as transmission towers struck by lightning

and the ac phases on phase conductors, and applying the

method of calculating lightning stroke rate to transmission

lines [19]. Those should increase estimation accuracy ofovervoltages and reliability.

While the relationship between GIS’s and transformers and

the LIWV was referred to in the present study, for the actual

test voltages, LIWV, it is necessary to consider not only the

back-flashover analysis results but also the impact of direct

lightning strokes on phase conductors with shielding failures

[18] and overvoltages in other lightning surge time areas, such

as disconnector surges. In addition, it should be noted that the

study of reliability in this paper was done under specific

analysis conditions.

REFERENCES[1] J. Takami, and S. Okabe, “Observational Results of Lightning Current on

Transmission Towers”, IEEE Trans. Power Delivery, Vol. 22, pp. 547-556,

2007.

[2] S. Okabe, and J. Takami, “Evaluation of Improved Lightning Stroke Current

Waveform Using Advanced Statistical Method”, IEEE Trans. Power Delivery,

Vol. 24, pp. 2197-2205, 2009.

[3] J. Takami, S. Okabe, and E. Zaima, “Evaluation of Lightning Surge

Overvoltages at Substations Using Assumed Lightning Stroke Current

Waveforms Based on Observations”, IEEE Trans. Power Delivery, Vol. 25, pp.

2958-2969, 2010.

[4] IEC 60071-1-2006, “Insulation co-ordination, Part 1: Definitions, principles and

rules”.

[5] IEC 60071-2-1996, “Insulation co-ordination, Part 2: Applications guide”.

[6] IEEE Std 1313-1-1996, “IEEE Standard for Insulation Coordination-

Definitions, Principles, and Rules”.

[7] IEEE Std 1313-2-1999, “IEEE Guide for Application of Insulation

Coordination”.

[8] The Japanese Electrotechnical Committee, Standard for Test Voltage, JEC-

0102-1994, 1994 (in Japanese).

[9] Sub-committee for Power Stations and Substations, Lightning Protection

Design Committee, “Guide to Lightning Protection Design of Power Stations,

Substations and Underground Transmission Lines”, CRIEPI Rep., No. T40,

1995 (in Japanese).

[10] K. Berger, R. B. Anderson, and H. Kroeninger, “Parameters of Lightning

Flashes”, CIGRE, Electra, No. 41, pp. 23-27, 1975.

[11] R. B. Anderson, and A. J. Eriksson, “Lightning Parameters for Engineering

Application”, CIGRE, Electra, No. 69, pp. 65-102, 1980.

[12] A. J. Eriksson, H. J. Geldenhuys, and G. W. Bourn, “Fifteen Years Data of

Lightning Measurements on a 60 Meter Mast”, Trans. S. African Inst. Elec.

Engrs., Vol. 80, No. 1, pp. 98-103, 1989.

[13] Alternative Transients Program (ATP) Rule Book. OR: Canadian / American

EMTP User Group, 1987.

[14] N. Nagaoka, “A Flashover Model Using a Nonlinear Inductance”, Trans. IEE

Japan, Vol. 111-B, No. 5, pp. 529-534, 1991 (in Japanese).

[15] A. Ametani, and T. Kawamura, “A Method of a Lightning Surge Analysis

Recommended in Japan Using EMTP”, IEEE Trans. Power Delivery, Vol. 20,

pp. 867-875, 2005.

[16] T. Watanabe, Y. Yamagata, and E. Zaima, “Insulation Coordination for UHV

System”, CIGRE Paper, No. 33-101, 1998 Session.

[17] S.Okabe, S.Yuasa, M.Koto, and E.Zaima, “Evaluation of lightning surge

waveform for LIWV reduction of substation equipment”, 13th Intern. Symps.

High Voltage Eng. (ISH), P.05.66, 2003.[18] J. Takami, S. Okabe, and E. Zaima, “Study of Lightning Surge Overvoltages at

Substations Due to Direct Lightning Strokes to Phase Conductors”, IEEE Trans.

Power Delivery, Vol. 25, pp. 425-433, 2010.

[19] S. Taniguchi, T. Tsuboi, S. Okabe, Y. Nagaraki, J. Takami, and H. Ota,

“Improved Method of Calculating Lightning Stroke Rate to Large-sized

Transmission Lines Based on an Electric geometry model”, IEEE Trans.

Dielectr. Electr. Insul., Vol. 17, pp. 53-62, 2010.

Shigemitsu Okabe (M’98) received the B.Eng., M.Eng. and

Dr. degrees in electrical engineering from the University of

Tokyo in 1981, 1983 and 1986, respectively. He has been with

Tokyo Electric Power Company since 1986, and presently is a

group manager of the High Voltage & Insulation Group at the

R & D center. He was a visiting scientist at the Technical

University of Munich in 1992. He has been a guest professorat the Doshisha University since 2005, at the Nagoya

University since 2006, and a visiting lecturer at the Tokyo University. He works as a

secretary/member at several WG/MT in CIGRE and IEC. He is an Associate Editor of

the IEEE Transactions on Dielectrics and Electrical Insulation.

Jun Takami (M’06) received the B.Eng., M.Eng. and Dr.

degrees in electrical engineering from Doshisha

University, Kyoto in 1995, 1997 and 2010, respectively.

He joined Tokyo Electric Power Company in 1997 and at

present is a member of the High Voltage & Insulation

Group at R & D center in Tokyo Electric Power

Company. His main research interest is the insulation

design of power systems. He is a member of CIGRE.

isolation failure

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