2013 2nd International Conference on Electric Power Equipment - Matsue, Japan

Research on Interference Characteristics of Secondary Cables Due to Lightning Strol<e in

Grounding Grid of Substation LI Bingl,2 and XU Jianyuan 1,2

lSchool of Electrical Engineering, Shenyang University of Technology, Shenyang, 110870, Liaoning Province, China 2Liaoning Province Key Laboratory of Safe Operation and Monitoring of Power Grid, Shenyang, 1 10870, Liaoning

Province, China

Abstract- The lightning current might inject grounding

grid through the grounding lead when the arrester

suffered a lightning wave in substation. Then the ground

potential difference(the difference of ground potential

can/may) generated interference voltage on secondary

cables due to earth potential rising. The interference

voltage might lead secondary equipments misoperation

and even damage. To analysis the problems mentioned

above, a equivalent circuit model was established. The

transient voltage caused by ground potential difference to

secondary cables could be (can be or is) calculated by

EMTP-ATP. As a validation. the impulse voltage

generator was used as voltage source to simulate that

substation encountered with the lightning, and to measure

the interference voltage at cable terminal as well. Both

experiment and computer simulation had been taken to

analysis the influencing factors of the interference voltage

on secondary cables and the ports of secondary

equipments. The factors included the lightning current

injection point and the soil resistivity rate. The analysis

would provide a reasonable reference for wiring cables to

improve anti-jamming capability of secondary equipments

in substation.

Key words-lightning strike, ground potential difference,

secondary cable, interference voltage

1. INTRODUCTION

The ground potential of grounding resistance would

rise when the substation were struck by lightning and

lightning current injected into the grounding grid

through grounding lead. In addition, the area of

grounding grid was generally larger, this would cause

ground potential to distribute unevenly. These factors

leaded to ground potential difference[11. The ground

potential of two ground points was different when the

cable shield was grounded at both ends. The current that

flowed through cable shield would generate interference

on cable core through transfer impedance or transfer

admittance between cable shield and core [21.

During this paper, the experiment was set up with the

impulse voltage generator as the source of interference.

The experiment simulated that ground potential

difference interfered with secondary cable when the

lightning current injected into grounding grid. At the

same time, equivalent circuit was set up by ATP, and

compared the results of simulation and actual

measurement. Exhaustive papers analyzed the

interference voltage on cable core in some factors such

as the different lightning current injection points and

different soil resistivity rate. We could forecast the

interference voltage on secondary cable through the

combination of computer simulation and experiment,

and it also helped us to fmd out weak points of

secondary system. The analysis could be as a reference

to design secondary system of substation.

II. EQUIVALENT MODEL OF GROUNDING GRID

A. Model of Grounding Unit

The impedance characteristic of grounding electrode

under the impulse current was different from the power

current's. It was mainly due to the amplitude and

frequency of impulse current, which were both higher

than power current's. The grounding electrode were

resistive (conductive), inductive, capacitive when the

impulse current flowed through grounding electrode.

The length of grounding electrode and wave head of

lightning impulse was about same (the length of wave

head was calculated under the speed of light), so we

should consider wave to propagate along with

grounding electrode and the equivalent parameters of

grounding grid were distributed.

We should dispose grounding unit as loss of long

wire, and the figure 1 was equivalent circuit.

Rdx Ldx

---1 Cdx �Y""-1 I

I I

I

I Fig. I. The equivalent circuit diagram of distributed parameter

of grounding unit

Among the figure 1, the R , L , C and G respectively

represented resistance, inductance, both capacitance and

conductance to zero potential. Their formulas were as

following [5, 61.

L = jJl [In 21 -1] 27r r

C=cpG The grounding electrode was horizontal:

G _

27r 1-

12 p[ln - - 0.61]

2hr

The grounding electrode was vertical:

27r GJ =

41 p[ln--l] 2r

(Him) ( 1)

(F/m) (2)

(S/m) (3)

(S/m) (4)

Among the formulas, I , r and h respectively

represented length, radius and buried depth of

grounding unit, the unit was meter; E was the

permittivity of soil, the value was 9*8.86* 1 0-12; J.1 was

the permeability of soil, the value was 47r x 1 0-7, P was the resistivity of soil.

The R was ignored because R « L when high

frequency characteristic of lightning was considered [71. Not only self-inductance bus also mutual inductance

should be considered when the two grounding

electrodes were parallel. Otherwise, the mutual

inductance was approximate to zero. The following was

the formula of mutual inductance between two parallel

grounding electrodes.

Among the formula, the d was the distance of two parallel grounding elelctrodes and other parameters were same to the formulas from (1) to (4). The value of d should be not less than S meters, and when the value of d was greater than or equal to 10 meters, the mutual inductance could be ignored when it's value was less than S% of self-inductance's.

B. The Parameters of Ground Grid

According to the laboratory's grounding grid, the

equivalent circuit was established by ATP. The

laboratory was established for experimenting on high

voltage equipments in my university. The figure 2 was

the laboratory's grounding grid.

Fig. 2. The diagram of grounding grid of laboratory

2

The material of horizontal grounding electrode was

hot dip galvanized flat steel and vertical grounding

electrode was hot dip galvanized steel pipe. The type of

former was 40 x S mm and the buried depth was 0.6Sm.

The diameter of later was SOmm and the length was 3m.

The material of grounding lead on the line was brass

and its type was 63 x 3.55 mm. The parameters of

grounding electrode unit were calculated according the

formulas from (1) to (4), which were used to establish

equivalent circuit in ATP.

III. EQUIVALENT MODEL OF CABLE

A. Interference Principle of Grounding Electrode to

Cable

The lightning current injected grounding grid through the grounding lead when the arrester operated under lightning. As shown in figure 3, the U AB was potential difference of two grounding points which connected grounding grid with both ends of the cable shield. ZAB was grounding impedance of two grounding points.

ground A UAI3

Fig. 3. The diagram of ground potential

The I was interference current, which flowed through

shield of cable by Zr. and then interfered cable core

by transfer impedance of Zt .If the amplitude of

interference voltage exceed its maximum withstand

voltage of secondary device port, it would influenced or

even destroyed secondary devices.

B. Equivalent Circuit of Shielded Cable

To make single-core shielded cable as an example,

equivalent circuit was shown in figure 4. The ground

potential difference would generate current excitation

source of iu in cable shield. Then the iu would

generate interference on cable core through transfer

impedance of Zt . In the end, we could calculate out the

voltage of cable core. The length of cable was unit

length. The ZI was impedance of cable core and Z2 ZI

r'···· ... .. I I I I I I

UG :e12 L12 : I I . . ._--- --- --

Ze12

Fig. 4. Equivalent circuit of single-core shielded cable

was impedance of cable shield. The Zt that included

L12 and C12 was transfer impedance between shield

and core of cable. The Zc was characteristic impedance

of shielded cable.

IV. EXPERIMENT AND SIMULATION

A. The Validation of Model

The experimental loop was established by impulse

voltage generator and its corollary equipments. The

highest amplitude of impulse voltage was 2400 kY. The experimental loop had simulated the interference of ground potential difference to secondary cable. During the experiment, the shield of cable was grounded at both ends, the grounding points were N33

and N37. The amplitude of impulse voltage was 400kV,

waveform parameter was 1.2/S0 IJS, and grounding

point of generator was N22. The oscilloscope was used

to measure the interference voltage of cable core at the

end of cable and its equivalent impedance was I MQ. At the same time, equivalent circuit of experiment was

established by ATP and then calculated out interference

voltage of cable core. The figure S showed the

compared result of actual measurement and simulation.

20�-�--�--------� -actual measurement

-simulation

10 20 30 40 50

Time (�s)

Fig. 5. The interference voltage curve of actual measurement

and simulation on cable core

According to actual measurement, the amplitude of

interference voltage on cable core was 17.2V,

peak-to-peak value was 33.2V The interference voltage

might cause secondary equipment that connected with

cable to misoperate, mistakenly measure and even be

damaged. The simulation value was slightly lower than

the actual measurement value. This was due to that

radiation interference from lightning current in the

grounding grid to secondary cable was not considered.

Meanwhile, the propagation process of electromagnetic

wave might be encountered wall or other devices, and

then the refractive or reflex electromagnetic wave

would be dispersive. This resulted in measurement

result attenuating faster than simulation result's. The

simulated interference voltage on cable core also

couldn't be ignored because the amplitude of

interference voltage was 13.43V and peak-to-peak value

was 23.64V The equivalent model could truly reflect

the actual situation. So it was convenient to further

study the influence of lightning current injection point

and soil resistivity on cable core.

B. The Influence of Different Lightning Current

3

Injection Points on Voltage of Cable Core

The simulated lightning impulse voltage injected at

different grounding points by ATP. They were NIl, N24

and N3S. The amplitude of impulse voltage was 400kV

and waveform parameter was 1.2/S0 IJS .Under the soil

resistivity was 100 n· m, we got the response cure of

voltage on cable core, as shown in figure 6.

From the figure 6, when the injection point of

lightning impulse voltage was N3S, the interference

voltage on cable core was lower 63.43% than the NIl's.

15 f� "n24

0

1 1 1 'I ' c35

5 1[ �l 'iI, o ll l l l'III .lIt,I ,II( '�" i ' :7;',,",'" {i./l C " . I r [ 1 11 JI I;rirj':t ,ji,

I I ·,

'III ilil ii; 1'1 I r

0

II 5'

10 15 20 25 30 35 40 45 50

Time (�s)

Fig, 6, The voltage curve on cable core under the different

injection points

Therefore, it was better to choose the center of

grounding grid as the grounding point of arrester so that

it could reduce the interference on secondary cable and

its attached equipments.

C The Influence of Different Soil Resistivities on Voltage of Cable Core

The injection point of lightning impulse voltage was

NIl, the amplitude of voltage was 400kV and

waveform parameter was l.2/S0 IJS . The soil resistivity

were lOOn· ill 500n· ill and 10000· ill , the

corresponding curves of voltage were shown in figure 7.

50

'I ' -sol' reslstl"'y100 , f -soil resistivity500

I' u: -soil resistivity1000

III i[11 , : ', UI, I." Irl, )1';' JI :1 '\ ',2'. to. .':

40

30

20

10 �

" if rill'] h' [';1<: ;1 l' 'f 'f \' r: "I E 0

� -10

·20

·30

'"'0

·50 o

, '1 11 11 11 , I il t ',I I

..

10 15 20 25

Time (�s)

Fig, 7, The voltage curve on cable core under the different soil

resistivity

The interference voltage on cable core was

significantly increased with increasing soil resistivity.

When the soil resistivity was 10000· ill , the amplitude

of interference voltage was 46.97V It was 3.S times to

the voltage under the resistivity was 1000· ill .

Therefore, the design of grounding grid should reduce

soil resistivity reasonably, thereby reducing the

grounding resistance to reduce the interference of

secondary cable.

V. CONCLUSION

According to analysis in this article, such conclusions

can be obtained as following:

(l)Under the lightning voltage with amplitude of

400kV and waveform parameter was 1.2/50 f1S , the

amplitude of interference voltage measured on cable

core was 17.2V and the peak-to-peak value was 33.2V,

when the grounding point of impulse voltage generator

was set to be N22 and both ends of the cable shield are

grounded.

(2) The equivalent circuit was established according

to the experiment by ATP. Through the simulation, the

amplitude of interference voltage on cable core was

13.43V and peak-to-peak value was 23.6y' The result

was close to the actual measurement. The equivalent

model could be used to analyze the influencing factors

on secondary cable. The influencing factors included the

injection point of lightning voltage, soil resistivity and

so on.

(3) The lowest amplitude of interference voltage on

cable core was 4.97V when the injection point of

lightning impulse voltage was N35. With the increasing

of soil resistivity, the interference voltage on cable core

was significantly increased. When the soil resistivity

was 10000· ill , the amplitude of interference voltage

on cable core was 46.97Y.

Through the experiment and simulation, the lightning

impulse voltage would lead to ground potential

difference and then interfered on secondary cable when

the shield of secondary cable was grounded at both ends.

The cable might introduce interference voltage into

secondary system and effect on the sampling circuit of

computer monitoring equipment, the control circuit, the

communication circuit and son on. It would cause the

logic confuse, computer freeze, chip damage and so on.

The grounding point of arrester, reactor and other

equipments should be selected near the center of

grounding grid. When we lay cables, we should try to

avoid the grounding point of these devices. In order to

reduce the interference on secondary cables, the laying

of grounding grid should be reasonable to reduce soil

resistivity of grounding grid.

ACKNOLEDGMENT

Department of Science and Technology of Liaoning

Program(20 1 12200 1 l);supported by NSFC(51 177104,).

REFERENCE

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4

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E-mailofauthors:356399182@qq.com