Analysis of Insulator Strings for 69 kV and 115 kV

Subtransmission Lines in MEA’s

Power Distribution System A. Phayomhom#1, T. Thasananutariya#2, S. Sirisumrannukul*3

#Power System Planning Department, Business Investment Department

Metropolitan Electricity Authority (MEA)

Bangkok, Thailand 1attp@mea.or.th

2tosakt@mea.or.th

*Department of Electrical Engineering

Faculty of Engineering, King Mongkut’s University of Technology North Bangkok

1518 Pibulsongkram, Bangsue 10800, Bangkok, Thailand 3spss@kmutnb.ac.th

Abstract — Presently, the design and installation of insulator strings for 69 kV and 115 kV

subtransmission lines in Metropolitan Electricity Authority (MEA) are based on American National

Standards Institute (ANSI). The applications of this standard may not be appropriate for insulation

coordination evaluation in MEA because of the different number of thunderstorm days. As a result, the

failure on insulator strings of subtransmission system in MEA was often found. This paper analyzes the

existing performance of insulator strings described in term of the total flash over rate (TFOR) for 69 kV

and 115 Kv subtransmission lines in a service area of MEA. The results show that the difference in

TFORs of the lines is significant for different thunderstorm days and probability peak current curves.

Keywords: Subtransmission lines, Insulator strings, Back flashover rate (BFOR), Shielding failure

flashover(SFFOR).

I. INTRODUCTION

Induced overvoltage on a transmission line can be caused by direct lightning strikes to overhead ground

wire (OHGW) or due to lightning strikes to nearby the line [1], [2]. A direct lightning strike to an OHGW can

cause back flashover on the insulators if the voltage difference between the phase conductor and the OHGW

exceeds the critical flashover (CFO) of the insulators. In the real practice of the 69 kV and 115 kV

subtransmission lines in MEA’s distribution system, back flashover can cause insulator failure and lead to power

interruption. The back flashover rate (BFOR) depends on the footing resistance, span, line configuration, tower

surge impedance, lightning current, CFO, and lightning current probability [3]. This paper analyzes the lightning

performance of subtransmission lines in MEA’s distribution system. A case study is performed by varying the

number of insulators of 69-and 115 kV subtransmission lines. The total flash over rate (TFOR) is calculated by a

TFlash program. It is expected that the obtained results could provide a good indicator for the design of

subtransmission lines in MEA.

II. DATA AND MODEL OF SUBTRANSMISSION LINES

A. Data of 69 kV and 115 kV Subtransmission Lines

The double circuits of 69 kV and 115 kV subtransmission lines used for analyzing the lightning

performance consist of 2 x 400 mm2 all-aluminum conductor (AAC) per phase and a 1 x 38.32 mm2 OHGW.

The OHGW is directly connected to a ground wire embedded in concrete pole and concrete pile. The ground

wire is connected to a 0.5-m-long ground rod with a diameter of 15.875 mm [4], [5]. Figure 1 shows 69 kV and

115 kV subtransmission line configurations and grounding system. The poles are 18 m and 20 m high for the 69

kV and 115 kV subtransmission lines respectively. A surge impedance of concrete pole can be calculated by [6]:

Zt = 30 ln ���

����

� +

2

22 )(2

r

rh

where ZT is the surge impedance of concrete pole (ohm), H is the pole height (m) and r is the 0,5 widht tower

(m). The surge impedance of 69 kV and 115 kV subtransmission lines in MEA’s distribution system are 445.18

� and 451.50�. The span of these lines is 40 m.

B. Insulator Model

The installation of a porcelain insulator type 52-3 (Figure 2) in the MEA network complies with Thai

Industrial Standard: TIS.354-1985. A string of 4 and 7 insulator units are installed associated with phase

conductor of 69 kV and 115 kV subtransmission lines respectively.

C. Probability Distribution of Stroke Current Magnitudes

The probability of stroke peak current exceeding a given current value can be approximated by the following

equation [2]:

where P(I) is the probability of the current exceeding the current I . The parameters a and b are 31 and 2.6 for

the IEEE standard 1243-1997[7], and 34.4 and 2.5 obtained from Thailand statistical data[3]. Comparison

between the two probability stroke peak current curves is shown in Figure 3.

D. Ground Flash Density

The lightning strokes in any area are defined by ground flash density (GFD) [7]. GFD is a function of

the number of thunderstorm days per year and can be estimated by [3]:

Ng = 0,04 . Td1,25 (3)

where Td is the number of thunderstorm days per year. From statistical record of the Thai Meteorological

Department, the number of thunderstorm days in Bangkok area averaged over 1993 to 1997 is 68 days per year

[3].

E. Number of Lightning Strikes to Lines

The number of lightning strikes to an OHGW or phase conductors installed on the top of pole is given

by the following equation [7], [11]:

NL = Ng (10

.28 6,0dh +

) (4)

where NL is the number of lightning strikes (flashes/100km/yr), Ng is the GFD (flashes/km2/yr), h is the average

conductor height (m) and b is the separation distance of phase conductor in each circuit (m).

III. CRITERIA OF LIGHTNING PERFORMANCE EVALUATION

A. Back Flashover Rate (BFOR)

Lightning Strike to Tower. Tower can be represented as surge impedance or inductance as shown in

figure 1. The inductance of tower can be calculated by (5)

L = ���

����

�

−���

����

� +

2)1(

..2

'

)'2'

ψ

tZw

gZ

oRgZ (5)

- Z’g = ZtZg

ZtZg

2

..2

+ - Zw = �

�

���

�

+

−

���

����

�

+ RZt

RZt

ZtZg

ZtZg2

2

)2(

.2 - t =

v

TowerHeight

- R’0 = RZt

ZtR

−

. - � = �

�

���

�

+

−

���

����

�

+

−

RZt

RZt

ZgZt

ZgZt

2

.2

where R’0 is represented by grounding resistance, Z’g is represented by surge impedance of ground wire, Zw is

surge impedance, � is tower damping factor, and �t is lightning wave travel time. Overvoltage occurred at the

insulators on the top of the tower is proportional to surge impedance and lightning current.

VM = I. R + L3

2sV

dt

di+ (6)

where I is lightning peak current (kA), R is tower footing resistance, L is tower inductance, di/dt is lightning

current steepness (kA), VS is system voltage (kV).

A back flashover occurs when a lightning strikes an OHGW or pole of a transmission line, resulting in

the voltage across the insulators exceeding CFO. BFOR is a product of the number of lightning strikes to the

OHGW and a probability of stroke critical current [7], [9], [10]. The critical current is defined as lightning stroke

current when injected into the conductor causing flashover.

BFOR = 0,6.NL.P(If) (5)

If = R

dt

diLkCFO −)/(5,1

CFO = n.s ��

���

�+

75,0

71,04,0

t

- n is sum dish, s is spacing, t is time flashover

- R is resistance tower, CFO (Critical flashover) and k is factor kopling ground wire and conductor phase

B. Shielding Failure Flashover Rate (SFFOR)

SFFOR is the annual number of flashovers on insulators due to shielding failure and can be estimated by

the following equation [7], [8], [11]:

SFFOR = 0,5 x � x P(Imax) x XS x 0,1 x Ng (7)

C. Total Flashover Rate (TFOR)

TFOR is a sum of BFOR and SFFOR and is used as a criterion for evaluating the lightning performance

in this paper.

TFOR = (BFOR+SFFOR) x 100

L (8)

where LP is Lightning performance in flash/100km/year, L is line length (km).

IV. CASE STUDY

This case study analyses lightning strikes to the OHGW of double circuits of 69 kV and 115 kV

subtransmission lines. Two cases are considered. In the first case, a probability peak current curve is based on a

= 31, b = 2.6 and 30 thunderstorm days. Note that these data follow the IEEE standard 1243-1997 and Ref [12].

The number of insulators is varied from 3 to 6 units for the 69 kV line and from 6 to 9 units for the 115 kV line.

The input parameters for the TFlash program are given in Section II. The second case is the same as case 1

except that the probability peak current curve is based on a = 34.4, b = 2.5 with 68 thunderstorm days. The

results of lightning performance analysis for the 69 kV and 115 kV subtransmission lines are shown in Tables I

to IV. The TFORs of both voltage levels for the two cases are graphically shown in Figures 4 and 5. As can be

seen, the calculated TFORs of the two cases are very much different. For example, the TFOR of 4 insulators,

existing configuration.

in the 69 kV line and 7 insulators, existing configuration in the 115 kV line, are about 70 % difference.

Therefore, design of lightning protection should be used with care for application of the IEEE standard. The

results also indicate that the TFORs decrease (i.e., lightning performance is improved) as the number of

insulators increases. Adding more insulators, of course, come at a cost and apparently, an incremental TFOR

tends to decrease when the number of insulators goes from 5 to 6 in the 69 kV line and from 8 to 9 in the 115 kV

line. For this reason, a cost-benefit analysis for reliability improvement should be carried out to determine the

optimum balance between the utility and customer sides.

VI. CONCLUSION

Lightning performance of subtransmission lines can be evaluated by the total flashover rate, which is a

sum of the back flashover rate and the shielding failure flashover rate. The results from a case study of 69 kV

and 115 kV subtransmission lines in a service area of MEA emphasize that the total flashover rate is dependent

on thunderstorm days and a probability peak current curve. In the other word, the total flashover rate is area-

specific and therefore it should be used with care for design of lightning protection. In addition, a financial study

should be done in order to confirm a decision of lightning performance improvement.

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