Lightning Surge Simulations for Outdoor 69kV Cable Termination Steel Frames

Wei-Min Wang#1, Jiann-Fuh Chen#2, Chiang Cheng*3, Jen-Li Fan*3 #1Department of Electrical Engineering, Kun-Shan University

No. 949, Da-Wan Road., Yung-Kang Area, Tainan City, Chinese Taipei. #2Department of Electrical Engineering, Cheng-Kung University

No. 1, University Road., Tainan City, Chinese Taipei. 1w5210@mail.ksu.edu.tw

2chengjf@mail.ncku.edu.tw *3Taiwan Power Research Institute, Taiwan Power Company No.198, Sec. 4, Roosevelt Road., Taipei City, Chinese Taipei.

3u275281@taipower.com.tw

Abstract—This paper aims at simulating and observing the transient behaviours on the outdoor steel frame if lightning surges cause arresters at the transition points to discharge. The inductance effects of grounding leads and different grounding connection schemes are considered. The EMTP-ATP package is employed for simulations. A brief introduction of lightning characteristics of Taiwan is included in this paper.

I. INTRODUCTION The underground-type transmission/distribution systems

are widely adopted by Taiwan Power Company (TPC) in recent years. At the transition points which connecting overhead lines and underground cables, lightning arresters are installed aside to prevent damage from lightning surge and transient overvoltage. For open-frame outdoor substations, arresters are mounted on the supporting steel frame of cable termination assembly. The grounding leads are run from arresters along the steel frame to the common bonding points which then connected to ground grid.

When the effect of the lead length is considered, an inductive voltage drop (L di/dt) will be developed along the grounding lead wire when surge current diverted into the ground. This inductive voltage will be added to the protective device voltage. The values of di/dt range over wide limits, a value of 10 kA/us is representative[1]. A maximum value of 24.3 kA/us (first stroke) is obtained from CIGRE statistical report [2].

On the steel frame, the metallic sheathes of power cable are also connected to the bonding point. For a common structure, the bonding points are usually constructed by copper bars at the base of framework, such installations tend to have very long ground leads which may increase surge voltage stress on cable insulation.

Figure 1 shows a typical outdoor steel framework structure of TPC’s 69-kV cable termination assembly. The arresters are mounted directly to the metallic structure without insulation. Compared to the 161-kV and 345-kV transmission systems, the arresters are installed with additional insulation supporter. In this paper, the transient voltages on the 69-kV framework

for installing arresters with and without insulation are investigated and simulated with the EMTP-ATP program.

Figure 1 Steel framework of cable termination assembly and arrester

II. LIGHTNING CHARACTERISTICS OF TAIWAN

0%

2%

4%

6%

8%

10%

12%

14%

8 13 18 23 28 33 38 43 48 53 58 63 68 73 78 83kA

0%10%20%30%40%50%60%70%80%90%100%

probability density cumulative probability

Fig. 2 Average stroke current probability density and cumulative probability

of Taiwan .The TPC’s lightning location system (LLS) was installed

in 1989. This system consists of 7 combined direction finders, one position analyzer (NPA), and one central processing system. About two million lightning flash data have been

Arrester

Grounding Lead

Bonding Point

Cable Termination Assembly

Three-Stages Steel Frame

7th Asia-Pacific International Conference on Lightning, November 1-4, 2011, Chengdu, China

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recorded since the system installed. Based on the ground flash data collected, several research projects have carried out to investigate impact of the voltage sag on the power system caused by the lightning.

From the lightning data collected, the average probability

density and cumulative probability of stroke current are shown in Figure 2.

Following the format of equation as Eq. (1) proposed by

Anderson-Erikson[3], the cumulative probability of stroke current I0 exceeding i0 in Taiwan is derived as Eq. (2)

Anderson-Erikson 0 0 2.60

1( )1 ( / 31)

P I ii

� ��

(1)

Taiwan 0 0 3.150

1( )1 ( / 29.5)

P I ii

� ��

(2)

The difference between these two models is that in Taiwan, about 10 % of lightning current has magnitude greater than 59kA which is lower than that of Anderson (72kA). From the lightning data recorded, the statistical results can be summarized as follows:

(1) Most of the lightning occurred in the summer, especially during the period of months from June to October.

(2) The average magnitude of the lightning current was 32.48 kA. The rate of negative lightning current was about 94% of the total strokes.

(3) The probability of lightning currents whose magnitude larger than 59kA was about 10 %. There was a rate of 47% of the total lightning currents with magnitude ranged from 15kA to 35kA.

III. EMTP-ATP MODEL AND SIMULATION A. Model and Parameters

As shown in Figure 1, the steel framework is constructed as a three-stages structure with angle steels. Each stage is 93cm in height. The dimensions and relevant parameters are listed in Table 1. The angle steel bar is modelled as a series of resistance and inductance which calculated with Eq. (3) and (4)[4]. The resistance is

( )lRA

�� , �/m (3)

And inductance is 0 ( )

6r tL

w� �

� , H/m (4)

where t: thickness of angle steel bar w: is width of angle steel bar �r: relative permeability of steel material

The 100 mm2 PE cable is used as the arrester’s ground

leads. The lead is represented as a lumped parameter reactance with typical value of 1.3�H/m[5]. The distance from the frame bonding point (node “N” in Fig. 3) to ground grid (node “E” in Fig. 3) is about 1 meter.

The lightning arrester is modelled with the EMTP type 92 MOV component with V-I characteristics of 72-kV class.

TABLE 1 DIMENSION AND PARAMETERS OF 69-KV STEEL FRAME

Parameters Vertical Bar

Horizontal Bar

Cross Bar

thickness 7.5cm 5cm 5cm

width 0.7cm 0.4cm 0.4cm

height (single stage) 93cm 74cm 114 cm

resistance By Eq. (3) 0.18m� 0.37m� 0.57m�

inductance(H) By Eq. (4) 1.82e-5 1.24e-5 1.86e-5

Steel resistivity ( ) is selected as 10-7 �-m; relative permeability �r is 1000.

Fig. 3 Model of 69 kV cable termination steel framework

The lightning stroke is modelled using EMTP type 15

Heidler Source as 1.2/50�s current source. The overhead line (‘OH’ in Fig. 3) and underground power cable are modelled as distributed components with surge impedances of 400� and 50� respectively.

In Fig 3, four additional switches ‘SW1’~’SW4’ are added to illustrate the different connected configuration. The

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downward ground lead is divided into three segments (nodes G1~G3 in Fig. 3) corresponding to the position of each stage of framework (nodes F1~F3). The common bonding point on framework is denoted as node “N”, and ground grid is denoted as node “E”.

The impedance of ground grid is modelled as a single 5-� resistance[7], the node ‘O’ is regarded as the zero potential reference point. B. Simulation Cases Description

Referring to the EMTP model of Fig 3, the lightning stroke of 10 kA (1.2/50�s) is assumed to hit the overhead line which is 50 meters away from the connection point. The travelling surge originated by the stroke will propagate to the underground cable. The voltage stress produced will cause the arrester to discharge.

The simulations are performed by considering the different connection structures summarized in Table 2. Among these cases, 1(a) is the most common adopted type which the ground lead of arrester is firstly bonded to the angle steel bar on framework and then connected to ground grid by solid copper wires.

The simulations focus on observing: 1) Surge currents flow through the arrester, ground

lead and framework. 2) Voltage of steel framework (F1~F3) which is

regarded as the touch voltage which may be a threat to personnel safety [8].

SIMULATION CASES

Configurations Case #

Ground lead is connected to common bonding point on framework then grounded to grid (SW3 close; SW4 open)

1(a)Arrester mounted without insulation (SW1 close) (SW2 close)

Ground lead and framework are connected to ground grid independently (SW3 open; SW4 close)

1(b)

Ground lead is connected to common bonding point on framework then grounded to grid (SW3 close; SW4 open)

2(a)Arrester mounted with insulation (SW1 open) (SW2 close)

Ground lead and framework are connected to ground grid independently (SW3 open; SW4 close)

2(b)

Without arrester ground lead Steel frame used as grounded path (SW1 close) (SW2 open)

Supposed that the ground lead is cut (stolen). Then the steel frame act as the ground path

3

3)Voltage differences between arrester line lead and ground grid which will affect the insulation stress of power cable.

4)Voltage difference between ground lead and steel framework.

C. Simulation Results The simulation results for case 1(a) are shown as Fig.4 ~

Fig.7. Fig.4 shows the current waveforms of lightning stroke, currents dispersed through arrester, lead and framework. The peak values of waveforms are listed in Table 3.

Figure 5a shows the discharge voltage of arrester (A1-A2), voltage difference from arrester line lead to bonding point (A1-N) and voltage difference to ground grid (A1-E). The waveforms at first 3 �s (dashed line encircled area) are enlarged as Fig. 5b. The peak values of these waveforms are listed in Table 4.

Figure 6 shows the fast-rise part transient voltage waveforms of three positions of framework (‘F1~F3’ to node O). Figure 7 shows the voltage differences between ground lead and framework (‘G1~G3’ to ‘F1~F3’). The simulated results for all cases are tabulated in Table 3 and Table 4.

. 0 10 20 30 40 50 60[us]0

2

4

6

8

10[kA]

Fig. 4 Simulation surge currents of case 1(a)

TABLE 3 CURRENT PEAK VALUES OF SIMULATION RESULTS

CaseCurrentthrough 1(a) 1(b) 2(a) 2(b) 3

Arrester 6.28 kA 6.33 kA 6.30kA 6.31kA 6.21kA

Lead 4.51 kA 4.33 kA 6.30kA 6.31kA Xframe 1.77 kA 2.00 kA X X 6.21kA

0.0 0.1 0.2 0.3 0.4 0.5 0.6[ms]0

50

100

150

200

250

300

350[kV]

Fig. 5a Simulation results of voltages for case 1(a)

Lightning

Arrester

Lead

Frame

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5[us]0

50

100

150

200

250

300

350[kV]

Fig. 5b Enlargement of encircled part of Fig. 5a

(file 69-1a-2.pl4; x-var t) v:F1 v:F2 v:F3 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8[us]

0

30

60

90

120

150[kV]

Fig. 6 Transient voltages on framework of case 1(a)

0.0 0.3 0.6 0.9 1.2 1.5[us]-12

-8

-4

0

4

8

12[kV]

Fig. 7 Transient voltages on framework of case 1(a)

TABLE 4 SIMULATION RESULTS OF VOLTAGES

Case Voltage (kV) 1(a) 1(b) 2(a) 2(b) 3

A1-A2 169 (discharge voltage of arrester) A1-N 266 286 286 325 367A1-E 311 301 326 325 344F1-O 93 72 41 6 106F2-O 118 103 41 6 153F3-O 135 123 41 6 184G1-F1 -12 -2 40 80 XG2-F2 -5 0.5 80 119 XG2-F3 11 14 119 160 X

By examining the voltages of ‘A1-A2’, ‘A1-N’ and ‘A1-E’

in Table 4. If the metallic sheath of cable is connected to node ‘N’ or ‘E’, then the insulation stress on the cable varies from 266kV to 367kV. If the sheath is connected directly to the ground side (node A2) of arrester, the insulation stress equals to the discharge voltage (169kV) without any additional inductive voltage.

. For the voltage of node F1 on the framework, the voltage of Case 2(b) is the lowest (about 6 kV), which is least threatening to the safety of personnel. It is seen that for cases that arresters mounted with isolate base, the touch voltages are lower than other cases. Nevertheless, the voltage difference

between ground wire and framework (G1~G3 to F1~F3), the voltages varies from 80kV to 160kV which may result in flashover if the metallic parts of ground leads is very close to the framework.

By examining the discharged currents, with modelling the lightning stroke as a current source, the amounts of currents diverted into ground grid for all cases are of little difference, about 63% (6.3 kA) is diverted into ground grid. Case 3 shows that the touch voltages are higher than those of other cases which is the most dangerous to personnel.

IV. CONCLUSIONS

We simulated the lightning surge responses on outdoor 69-kV steel frameworks of cable terminations for different grounding connection schemes. The inductance effects of grounding leads caused by the fast-rise surge currents are considered. EMTP-ATP package is used to establish the models and perform the simulations.

The simulation results show that for a lightning current source model, the portions of lightning currents diverted into ground grid for all cases are almost the same.

From the personnel safety point of view, the simulation case 2(b) which arrester bases were isolated from the steel framework, besides, the ground lead wire and framework were separately connected to the ground grid, then the touch voltage is lowest which is least threatening to personnels. But the voltage difference between ground wire and framework is the highest which may lead to transient flashover.

For insulation stress of power cables, the voltage stress is the lowest when sheath is connected directly to the ground base of arrester. For the cases which the sheath is connected to ground with separate ground wire, the inductive voltage is added to the insulation stress. For common practice of TPC 69-kV system, the sheath of cable is usually separately connected to the ground grid.

ACKNOWLEDGEMENT The authors are grateful to Taiwan Power Company for the

valuable data and helpful discussions.

REFERENCES [1] IEEE Recommended Practice for Grounding of Industrial and

Commercial Power Systems, IEEE Std 142-1982. [2] IEEE Guide for Improving the Lightning Performance of Electric

Power Overhead Distribution Lines, IEEE Std 1410-1997 [3] Eriksson, A. J., “The Incidence of Lightning Strikes to Power Lines,”

IEEE Transactions on Power Delivery, vol. 2, pp. 859-870, July 1987 [4] W. H. Hayt, J. A. Buck, Engineering Electromagnetics, sixth ed., Mc

Graw Hill, 2001. [5] IEEE Guide for the Connection of Surge Arresters to Protect

Insulated, Shielded Electric Power Cable Systems, IEEE Std 1299/C62,22. 1-1996.

[6] Alternative Transients Program Rule Book, Canadian American ATP User Group, 1995.

[7] A. Mansoor, F. Martzloff, “The Effect of Neutral Earthing Practices on Lightning Current Dispersion in a Low-Voltage Installation,” IEEE Transactions on Power Delivery, Vol. 13, No. 3, pp. 783-790, July, 1998.

[8] IEEE Guide for Safety in AC Substation Grounding. IEEE Std 80-2000.

: A1-E �: A1-N �: A1-A2

�: F1-O �: F2-O

:F3-O

�: G3-F3 �: G2-F2

:G1-F1

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2011