Safety Distance Determination for 500 kV AC Transmission Line’s Helicopter Inspection

doi:10.4236/epe.2013.54B219

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Energy and Power Engineering, 2013, 5, 1153-1158

doi:10.4236/epe.2013.54B219 Published Online July 2013 (http://www.scirp.org/journal/epe)

Safety Distance Determination for 500 kV AC

Transmission Line’s Helicopter Inspection

Shuwei Wan, Xingming Bian, Lan Chen, Liming Wang, Zhicheng Guan

Graduate School at Shenzhen, Tsinghua University, Beijing, China

Email: [email protected]

Received February, 2013

ABSTRACT

As an efficient and advanced line inspection method, helicopter line patrol is gradually more and more used in trans-

mission lines inspection, promoting the elaborate operation of transmission lines and reducing the management cost.

However, as a 'floating-potential conductor' near to a high-voltage transmission line, the helicopter would be at a high

electric field region; and bring security risk to equipment and operating personnel. In this paper, the electric field

strength near the cabin at locations of different distance from transmission lines is investigated by calculation, and the

field in the helicopter cabin is also calculated with finite element method (FEM). The result indicates that the potential

difference becomes higher with the decrease of the distance between the helicopter and transmission line. Considering

the discharge energy and the guarantee of the persons’ safety, the safety distance is determined as d≥15 m.

Keywords: Transmission Lines; Electric Field; Helicopter Inspection; Safety Distance; Limit-value; Transient Electric

Shock

1. Introduction

Power transmission lines are a key part of the power sys-

tem and are exposed all the year round to the prevailing

weather conditions. It follows that th ey are more prone to

faults than other parts of the system. As an efficient and

advanced line inspection method, helicopter line patrol is

used in inspection of transmission line gradually, pro-

moting the elaborate operation and management. Heli-

copter has the ability to ascend and descend vertically

and to hover motionless at any height, making them ide-

ally suited for monitoring overhead transmission lines

[1-5]. Helicopter enables close observation of the con-

ductors, insulators, fittings and towers, while with infra-

red and UV inspection systems, operators can clearly see

where corona and other discharges are occurring [6-10].

However, when a helicopter is close to a transmission

line, it will, as an isolated conductor, cau se a distortion of

the electric field distribution. This will be particularly

important around those parts of the helicopter having a

small radius of curvature where the field will be high,

perhaps causing localized discharges and the possibility

of a breakdown between line, helicopter and ground,

with the consequent dangers to helicopter, personnel and

equipment.

Calculations concerning floating conductors have been

considered by several investigators [11-16]: Tadashi used

a charge simulation method to determine the field near a

floating conductor touching two dielectric media [11];

Lucian employed a boundary-element method to deter-

mine the field in the region between a charged body and

a conducting disc at a floating potential [12]. Yu dis-

cussed the effects of a floating-potential conductor, near

to a transmission line; the electric field was studied both

by experiment and finite-element computation. An iso-

lated metal sphere and a model of a helicopter were used

as the floating-potential conductors and were placed near

the overhead transmission line [13].

Several authors have considered the fields near or be-

low 3-phase and DC transmission lines with reference to

sag and ground characteristics. Many researchers used

2D-model to calculate the field distribution near trans-

mission lines [17-19], i.e. sag, span and pylons are ne-

glected. The error of result is large. Trlep analyzed a case

more realistic than the ideal case that assumes a flat sur-

face under transmission lines. The result reveals a strong

impact of the configuration of the ground surface and the

presence of conducting bodies under transmission lines

on the electric field distribution [18]. Some researchers

used 3D-model to analyze it [20-23]. Dein took the in-

fluence of tower height, sag, span and the angle of two

spans into consideration [20], calculating the earth’s sur-

face field distribution below 500 kV transmission lines.

The maximum field strength is about 10 kV/m, and the

main component is perpendicular to the surface. Hameyer

used semi-numerical method and finite-element method

S. W. WAN ET AL.

1154

(FEM) to calculate the field. The result of 157 kV trans-

mission lines shows that the maximum field strength (1m

above the ground level) is about 0.5 kV/m [21]. Zhang

discussed the effect of altitude height to 500kV-HVDC

with calculation and analog experiment. Under the same

conditions, the surface field under DC transmission lines

increases as the altitude height rises, and the rising rate is

2~4 kV/m per 1km [22]. Amiri used fi ni t e -element method

(FEM) and charge simulation method (CSM) to discuss

the effect of sag on field distribution under 500 kV

transmission lines. The maximum surface field strength

is about 8.5 kV/m at mid-span while it is about 5 kV/m

near the pylon [23].

At present, the safety distance of 500 kV transmission

lines helicopter line patrol is mainly estimated by ex-

perience. It is twice the length of helicopter wing plus

power operations safety distance. According to the esti-

mation, the distance from helicopter to lines – both hori-

zontal direction and vertical direction – should be over

20 m [24], and it should be over 25 m while helicopter is

hovering motionlessly near pylon [25].

As the helicopter flies close to power line, transient

electric shock phenomena might occur, resulting in the

uncomfortable feeling generated by the persons in the

helicopter. Therefore, the clarification of transient elec-

tric shock is of obvious significance to power corporation,

and also it has been gradually becoming an interesting

object of research work by both physicists and electrical

engineers [26-28]. However, most of the early research-

ers carried out the investigation about transient electric

field on the ground or underwater, none of them noticed

or studied such phenomenon in the helicopter. The effect

of small helicopter model on the electric field near over-

head transmission lines was studied in our previous work

[13].

Many researchers have considered the field distribu-

tion near or below transmission lines, but none appear to

have considered the field distortion due to helicopter near

the transmission lines. In this paper the effect on the

electric field around helicopter n ear to transmission lines

is considered by numerical calculation. The transient

electric shock was analyzed, and then, the safety distance

of helicopter inspection was suggested.

2. Numerical Calculation

The main material of pylon is angle iron. Because the

ratio of maximum and minimum component size is large;

it is hard to finite-element meshing. Taking the network

structure of pylon into account (Figure 1(a)), and ignor-

ing the inner field distribution, th e pylon is modeled as a

solid tower in the model used for the computation, as

shown in Figure 1(b).

The 3D electric field calculation model and the dimen-

sions of BELL206 helicopter are present in Figures 2(a)

and (b). There are two persons in the cabin while inspec-

tion, which is shown in Figure 2(c). The sectional plane

of the model in the helicopter cabin is shown in Figure

2(d). As shown in Figure 2(c), it is clear that the cabin is

not a pure Faraday cage because of the windows; there-

fore the electric field will certainly exist in the cabin. The

line’s voltage is symmetrical three-phase and the line-

to-line voltage is 500 kV (Effective value).

3. Results for Field Calculation and

Experiment

As the mid-span of transmission lines is the closest part

to the ground, the field strength is larger than other plac-

es. The electric field at mid-span along the horizontal

line or 45°-line (As shown in Figure 3) normal to the

bundled conductor and linking the conductor and the

helicopter was calculated and measured.

(a) The parameter of 500 kV pylon

(b) FEM computing model

Figure 1. 500 kV pylon.

S. W. WAN ET AL. 1155

(a) 3D Calculation model

(b) The dimensions of helicopter BELL206

(d) Diagram of Calculation Model in Cabin

Figure 2. Calculation Model.

The calculated and measured results are as shown in

Figure 4 for horizon tal line an d 45°-line, the agreement

is good.

Consideration of Figure 4 shows that the calculated

field values are consistently higher than the experimental

ones by some 15%. The greater discrepancies are perhaps

because any small positioning errors of the helicopter

and meter up or down, or sideways, or away from the

transmission lines, especially the positioning errors of

helicopter, will cause a decrease in the measured field.

Possibly the effect of the finite size of the metal parts of

the electric field meter itself may lower the reading in a

non-linear and rapidly-changing field.

Figure 3. The diagram of helicopter at different location

from transmission lines.

048 121620

Cabin-surface field (kV/m)

Distance from helicopterto the nearest transmission line(m)

(a) Cabin-surface field while helicopter is at different location along

horizontal line at mid-span

048121620

Cabi n-su rfac e fiel d ( kV/ m )

Distance from helicopterto the nearest transmission line(m)

(b) Cabin-surface field while helicopter is at different location along

45°-line at mid-span

Figure 4. The cabin-surface electric field of helicopter at

mid-span.

Through the result in Figure 4, the filed near helicop-

ter is small and the persons is safe, while the distance

from helicopter to line is over 10 m.

S. W. WAN ET AL.

1156

4. Analysis for Transient Electric Shock

When the distance between the helicopter and transmis-

sion line (d) is in the range 4m to 20 m, the potential and

electric field around the helicopter cabin is calculated by

finite element method, the results (d=4 m, 10m and 20m)

are present in Figure 5. Case d=10 m is taken as an ex-

ample to explain the results.

As shown in Figure 5(a), the potential at middle cab in

changes slightly. The electric field near the cabin win-

dow is not shielded by the helicopter airframe, but de-

creases very fast at the middle cabin. The persons’ poten-

tial (U2, U3) induced is 103.53 kV and 103.11 kV respec-

tively. The maximum potential difference between the

person and helicopter is 1.4 kV (U12).

The case of the helicopter at different positions is also

calculated and the results are displayed in Figure 6.

Transient electric shock is mainly determined by the

discharge energy between two objects, which largely

depends on the capacitance of the objects and the poten-

tial difference between them. It is 50~400 pF while the

person is standing and 400~800 pF while the person is

sitting [29]. The capacitance between the helicopter and

person (sitting) was tested for at least 50 times, and it

-2-10123

100

120

140

160

139.85

d=20m

d=10m

104.93

69.0 1

70.78

135.31

Persons

102.12

d=4m

Potential (kV)

Distance From Center of Helicopter (m)

Cabi n

(a) Potential distribution along the calculation path

-2-10123

4.94

7.45

11.63

5.40

8.83

d=10m

14.92

d=20m

d=4m

Persons

Electric Field (kV/m)

Distance From Center of Helicopter (m)

Cabin

(b) Electric field distribution along the calcu lation path

Figure 5. Calculation result of d=4 m, 10 m and 20 m.

5101520

0.5

1.0

1.5

2.0

Potential Difference(kV)

Distance From T ransm ission Line to Helicopter(m)

Case d=10m

Figure 6. Potential difference of the helicopter at different

positions.

5 101520

0.0

0.5

1.0

1.5

2.0

Discharge Energy(mJ )

Distance From Tr ansmission Line to Helicopter(m)

Critical distance

Fiugre 7. Discharge energy between personnel and air-

frame.

was in the range 400 pF to 800 pF. In order to ensure the

inspection personnel’s persons safety, the severest condi-

tion is taken into consideration. Therefore, the capaci-

tance between person and helicopter airframe is set as

800 pF, then the discharge energy is acquired (Once the

inspection person touches the helicopter airframe), the

result is present in Figure 7.

While the discharge energy is over 0.5 mJ, the person

would feel uncomfortable [29], so the critical discharge

energy is set as 0.5 mJ. The safety distance could be de-

termined as d≥15 m through the curve in Figure 7. Ac-

cording to this suggestion, transient electric shock has

not happened to the persons in the helicopter during their

inspection anymore. Consequently, this analysis is of

great significance to the helicopter inspection in power

grid.

5. Conclusions

The surface electric field of helicopter was studied both

by experiment and finite-element computation near to

transmission lines. And the filed near helicopter is small

and the persons is safe, while the distance from helicop-

ter to line is over 15 m. Transient electric shock exits

S. W. WAN ET AL. 1157

while helicopter is in a high field region, and it is harm-

ful to inspection persons. Th e electric field near the cabin

window is not completely shielded by the helicopter air-

frame, but decreases very fast at the middle cabin.

Through the critical discharge energy for person, the

safety distance is determined as d≥15m, and it has been

well adopted in power grid already.

6. Acknowledgements

The authors acknowledge financial support from the key

Project of National Basic Research Program of China

(2009CB724503)

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