Analysis on influence of long vertical grounding electrodes

The relationship between the grounding resistance decreasing rate and the length of vertical grounding rods under different rod locations are shown in Fig.3.. Obviously, the resistance d

Analysis on Influence of Long Vertical Grounding Electrodes

on Grounding System for Substation

Dept of Electrical Engineering, Tsinghua University Shandong Electric Power Bureau Dept of Electr and Computer Sys Eng Beijing 100084, China

Abstract: Three dimensional grounding system is introduced to

decrease the grounding resistance, step and touch voltages of

grounding system in areas with high soil resistivity or with limited

areas for grounding system The relationship between the number of

vertical grounding rods and the grounding resistance is discussed The

new means to add grounding rods to the grounding system with

explosion grounding technique, have efficiently decreased the

grounding resistance in the actual grounding engineering Reasons

why vertical grounding rods can efficiently decrease the maximum

touch and step voltages of the earth surface above the grounding

system are analyzed Calculated results shows the vertical grounding

rods can also effectively decrease the influence of seasonal factor on

safety of grounding system This paper provides the rule to choose the

vertical grounding rods in multi-layer soil based on the relationship

analysis between the number and the length of the vertical grounding

rods

Key words: Grounding system, vertical grounding electrode, step

voltage, touch voltage, seasonal factor

I INTRODUCITION

With the rapid increase of electrical load in recent years in

China, in order to promise the safety of the power system, it is

urgent to decrease grounding resistance of the grounding

system, especially when power generating stations or

substations are built in the soil with high resistivity The

limitations of the land requisition or the landform make

researchers turn their sight from the horizontal grid to the

vertical rod But so far most of the discussions on the vertical

rods are only samples of application or conclusion of project

experiences[ 13 Several researchers analyzed the role and the

design rule of rods, but they focused on the uniform soil The

detailed discussion of three-dimensional grounding system

design in the different soil structures almost cannot be

found[2,3] This paper will discuss how to design three-

dimension grounding system scientifically and economically

Jinan 250001, shandong, China Monash University

Clayton, Victoria 3168, Australia

11 NUMERICAL ALGORITHM RESEARCH OF

GROUNDING SYSTEM Since 1972, scholars have introduced various numerical

algorithm into the calculation of grounding parameters In 90’s

liner filter method and complex image method were used Numerical algorithm solves many problems emerged with the calculation in experiential formula Comprehensively considering the actual grounding system structure and the non- even dispersion of the fault current in the different parts of the grounding system, numerical algorithm helps to get exact result

of any complex grounding system and makes it possible to compare the economic efficiency of the projects scheme on the basis of technical feasibility

Dawalibi has improved the theoretical model and optimized the numerical algorithm over 20 years His multi-layer soil model numerical algorithm based on the base image method has come

into mature CDEGS (grounding system parameters calculating software developed by him and his collaborators) has been widely used in the world This software can not only get comparatively accurate parameters of various grounding system and also get grounding system parameters in the layered soil This paper makes simulative calculation by the means of CDEGS

111 DESIGNING RULE ANALYSIS OF VERTICAL

GROUNDING RODS The discussion is based on a llOkV substation as shown in Fig 1 The horizontal grounding grid is 150x1 50m2 in area The even grounding conductor span of the grounding grid is 15m

and horizontal conductor radius is 0.01 Im (r,=0.01 lm) The

uniform soil resistivity is 200R.m

A Number of Vertical Grounding Rods Vertical rods are added to the existed horizontal grounding grid The length of the rods is 50m The relationship between the

number N of the rods and the grounding resistance R is

illustrated in Fig.2 Here the rod radiuses are r2=3.5m and r2=0.025m used in calculation, Radius of 3.5m is the result considering the effect of explosion technique From Fig.2, we observed that while other conditions are fixed, grounding

resistance decreases with the increase of the number of rods

However, the decrease of R reaches saturation when N reaches

a certain number because the shielding effects increase with the

decrease of the interval among rods Besides rods can restrain

the dispersed current on the grid, i e the total grounding

resistance is not the simple parallel connection of grounding

resistances of the grounding grid and vertical grounding rods

There is a shielding coefficient of horizontal grounding grid to

the vertical grounding rods The shielding coefficient increases

with the number of the grounding rods

same as the upper Now a rod is arranged in the center of the grid and another in the comer, to check which rod has better resistance decreasing effects The grounding resistance decreasing rate is defined as:

here, R, is the grounding resistance after rods are added Ro is the grounding resistance of the horizontal grounding grid The equivalent radius of the horizontal grounding grid and is defined as:

Fig 1 The arrangement of grounding grid

-

0 5 10 15 20 25 30 35

Grounding Rod Number

Fig.2 The relationship between the number N of the rods and the grounding

resistance R

When the rod radius is 3Sm, their effects to decrease grounding

resistance reach 35%, which is better than that when the rod

radius is 0.025m

B Location of Vertical Grounding Rods

Usually rods are well distributed in the grid and the rod density

in the center of the grounding grid is often higher than that on

the periphery of the grounding grid Contrast analysis illustrates

that such a design is improper Horizontal grounding rid is the

here, S is the area of the horizontal grounding grid

The relationship between the grounding resistance decreasing rate and the length of vertical grounding rods under different rod locations are shown in Fig.3 Obviously, the resistance decreasing effect of the rod arranged on the periphery of the grid is better than that of the rod arranged in the center due to the shielding effect between the horizontal conductor and the vertical rods increases when the rod is arranged in the center of

Fig.3 The relationship between the grounding resistance decreasing rate and the length of vertical grounding rods under different rod locations

Analyzing the results in Fig.3, the dispersed current by vertical grounding rods on the periphery of the grids are more than twice of the current dispersed by the rod of the same length in the center of the grid As a result rods should be arranged on the periphery of the grid if rods are added to decrease the grounding resistance In this way the shielding effects between the grid and rods can reach the minimum With the number of the rods increase, the advantage of this arrangement becomes apparent

To increase the resistance decreasing effect and to decrease the shielding coefficient of horizontal grounding grid to the vertical grounding rods, the best way (if possible) is that rods are

arranged far away from the grid and connected with the grid

through the horizontal grounding conductors This way can

benefit to fault current dispersing, efficiently decrease the

grounding resistance and can make the touch voltage

distribution rational

C Length of Vertical Grounding RODS

Since parameters that influence the grounding resistance are too

much, all the discussion of this part is based on the horizontal

grounding grid with a fixed area Analysis indicates that these

conclusions are still in effect when the area is changed

Take a square horizontal grounding grid as an example, the

ratio of the grounding resistance decreasing rate to the vertical

grounding rod length, and another ratio of grounding resistance

decreasing rate to grounding grid radius are obtained In the

analysis, the grounding resistivity is 200R-m One used

horizontal grounding grid area is isox isom', while the other is

lOOx 1 OOm' The span between the horizontal conductors of the

grounding grid is 10m and the buried depth of the horizontal

grounding grid is 0.6m The number of vertical grounding rods

are 4 The influence of the vertical grounding rod length on the

grounding resistance decreasing rate with different grid areas is

illustrated in Fig.4

40

4 s

(a) uniform soil

S:150m*150m S:lOOm*l OOm

(b) two-layer soil Fig4 Influences of rod length on grounding resistance decreasing rate

The calculation conditions of two-layer soil are the same as that

of uniform soil The soil resistivity of the upper layer, p , equals

200R.m, and the soil resistivity of the lower layer, f i equals 600Q-m., the upper soil layer depth is 20m The analysis results

is shown in Fig.4 (b)

We observed in Fig.4 that the two curves are almost in superposition, i.e grounding resistance decreasing rates of horizontal grounding grids with different areas are almost the same when the ratio of equivalent radius and the rod-length is the same Another analysis result from Fig.4 is that the two curves coincide with each other when they are far from the interface of the two layers; while they differ greatly when they are near the interface That is also the rule of current intensity pass through the interface (discussed in Part 111)

The calculation illustrates that in the similar soil conditions and the same number of rods, the grounding resistance decreasing

rates do not change if the ratios of grounding-grid radius to

rod-length is fixed

IV ARRANGEMENT OF VERTICAL GROUNDING RODS

IN THE NONUNIFORM SOIL The arrangement of long rods in the nonuniform soil from the point of dispersed current distribution on the rods are discussed here

A Current Distribution on A Single Rod The reflective coefficient of two-layer soil is defined as

k = ( p , A , p, are soil resistivities Fig.5 is the

distribution curves of the current intensity J dispersed into earth along rod length x under different reflective coefficient Here,

the length L equals 20m; radius r, equals 0.02m; upper-soil

resistivity p, equals 100SZ.m; the depth h is 7.5m

I

225

v

$150

?

75

O ' " ' 5 " 10 " 15 . 20 '

x (m)

Fig3 Current distribution on the rods in the hvo-layer soil

From Fig.5, we observed that the current is well distributed on the rods except that current intensity increases quickly at the

bottom of the rods in the uniform soil It also shows that current

intensity increases a little with the increase of depth However,

the high current intensity area occupies a little percentage As a

result, the current distribution is considered as well-distributed,

this would not cause apparent error

The dispersed current distributions differ with each other when

rods are in the two-layer soil The current intensity in the low-

resistivity soil layer is higher than that in the high-resistivity

soil layer In each layer current distribution almost has no

change but there is a sharp shift along the interface

The difference of current distributions in different layers rises

when reflective coefficient increases For example, when k

equals 0.8, the current intensity is 300" in the upper layer,

but it is only 25A/m in the lower layer Hence the proper choice

of rod length can not only decrease grounding resistance

efficiently but also achieve better economic effect

B Arrangement of Rods in Two-layer Soil

Here is the discussion how vertical grounding rods affect the

electrical behaviors of the three-dimensional grounding grid in

the two-layer soil The relationship between the rod length and

the grounding resistance decreasing rate with different

reflective coefficient is shown in Fig.6 Here the horizontal

grounding grid area is lOOx loom*, the horizontal conductor

span is 10m; the upper layer soil resistivity is 200R.m Four

steel rods are arranged on the comers of the grid, the upper-

layer soil thickness h is 40m

2- -0.9

3- -0.7

4- -0.5

Therefore, the method of long rods does not fit the situation that the soil resistivity of bottom layer is very high On the other hand, when the soil resistivity of the bottom layer is low, adding

of long rod can achieve good results

From current distribution curves in Fig.5, it is understandable that a tuming point emerges on the grounding resistance

decreasing rate curve in Fig.6 when k<O, and the rod length

approaches the upper layer depth, i.e the grounding resistance decreasing rate increases remarkably when rod length is longer

than the upper layer depth; but when the rod length is 3.5 times

of the depth of the upper layer, the grounding resistance decreasing rate approaches saturation Thus the effective rod length is determined by reflective Coefficient k, and as a result, long rods can not achieve good result when the resistivity of the bottom layer is apparently lower than that of the upper layer

C Arrangement of Vertical Grounding Rods in Three-layer Soil

In some situations, three-layer soil models are put forward to avoid the remarkable error caused by simplification of soil structure Since the actual situations are too complex, the analysis is based on the typical soil structure But the result is universal The soil structures of three-layer soil according to soil resistivity can be divided into three types: high resistivity layer (H), medium resistivity layer (M) and low resistivity layer (L) High resistivity is assumed to be 1000R.m, the low resistivity is 100R-m and the medium is 500R.m The upper layer depth is 10m, the middle layer thickness is 50m The horizontal grid and rods are all the same as those in the two- layer soil The influence of added vertical grounding rods on the grounding resistance decreasing rate is shown in Fig.7

L",

Fig.6 Relationship between the grounding resistance decreasing role of rods in

the two-layer soil Fig.7 The influence of added vertical grounding rods on the grounding

resistance decreasing rate

As illustrated in Fig.6, when reflective coefficient k is lower

than 0.5, i.e the soil resistivity of the bottom layer is high, the

way of adding rods can not decrease the grounding resistance

efficiently Even if rod length approaches the radius of grid, the

grounding resistance decreasing rate is still lower than 10%

From Fig.7, we observed that just as in the two-layer soil, the vertical rods can not decrease the grounding resistance effectively when the upper layer resistivity is low So in such situation, long rod is not suitable In the situation of strictly limited area and high resistivity layer in the middle layer, rods

should reach the bottom layer with low resistivity

2

4

6

8

12

16

20

32

Comparing the two curves of “M,H,L” and “M,L,H in Fig.7,

we observed that before rods touch the bottom layer, the

grounding resistance decreasing rate of the former is larger than

that of the latter with the role of low resistivity middle layer

When the soil structure is “M,L,H’, it is better that rods do not

reach the bottom layer But in contrast, rods should reach the

bottom layer when the soil structure is “M,H,L”

The protruding curve MLH in Fig.7 of low resistivity middle

layer illustrates that resistance almost stops decreasing when

vertical grounding rod passes the middle layer Therefore the

economic rod length is the s u m of the depth of the upper and

middle layers

As the situation of high-resistivity upper layer, rods can reach

the best results Just as in the two-layer soil, the rod length is

determined by reflective coefficient k and layer structure The

effect to decrease the grounding resistance are below the

interface with smaller reflective coefficient (see Fig.5 and its

analysis), especially when the resistivity of the lower layer is

lower than that of the middle layer

VERTICAL GROUNDING ROD

Soil resistivity changes greatly in winter For example, the

measure results of Muliduo in Qinghai, China show that the

resistivity range of frozen and unfrozen soil is 500 to

15000 n * m In the north-east of China, the depth of frozen soil

can reach 1.6m The relationship between rod length and the

touch voltage in different seasonal factors is shown in Fig.8, the

seasonal factor is presented by different high resistivity soil

layers The touch voltage is a ratio between the calculated

values with the calculated ones without considering the

seasonal factor When the depth of the soil affected by the

season is smaller than the burial depth of the horizontal

grounding grid and the rod length is fixed, the surface touch

voltage almost does not change with the increase of the

resistivity of the upper high resistivity soil layer; while with the

increase of rod length, the surface touch voltage decrease

becomes saturation When the depth of the seasonal soil layer

affected by the season is higher than the depth of the grounding

grid, the surface touch voltage have a linear increase with the

increase of the soil resistivity But the increase velocity

decreases with the increase of the rod length

Rods buried on the periphery of the horizontal grounding grid

can not only decrease the grounding resistance efficiently, but

also can decrease the touch voltage and step voltage by

improving the potential distribution Table 1 records the

influence of rod buried around the grounding grid on the touch

and step voltages Here the horizontal grounding grid is buried

in the uniform soil without the seasonal soil layer

From Table 1, we can conclude that the vertical rods play a great role in decreasing the touch voltage When the number of rods is 12, the maximum touch voltage decreases to 40% When the number reaches 32, touch voltage decreases to 63.49% The reasons are: first, the vertical grounding rods decrease GPR (grounding potential rise) which directly influences the touch and step voltages; second, when vertical grounding rods are added to the grounding system, the dispersed current by horizontal conductors of grounding grid decreases due to a large amount of fault current flowing into the soil through vertical grounding rods, and as a result, the electrical field intensity of earth surface above the grounding system decreases greatly For example, when the number of rods is 12, the dispersed current on the horizontal grid is 25%, while the current on the rods is 75% When it is in the nonuniform soil, especially the resistivity of the upper layer is apparently larger than that of the lower layer, the percentage of the current on the rods are larger, It can reach 90% As a result, the touch and step

voltages on the surface decrease greatly

Table1 the influence of number of the rods on the maximum touch and step voltages

1

10 11 3,4: i i t h vertical rod1 j /

grounding grid burial depth

J

Depth (m) Fig8 The relationship between rod length and the touch voltage in different

seasonal factors

VI.CONCLUSI0N

1 The proper number of the vertical grounding rods can be

obtained from analyzing the number and the grounding

resistance decreasing rate The resistivity-decreasing

effects differ greatly with different radius when the rod

length is fixed To reach best results, rods should be

arranged on periphery of the grid instead of in the center

2 The discussion of rod length in various nonuniform soil

shows it is wrong that long rods can always reach better

results The choice of rod length, location and the number

are determined by the soil structure

3 The influence of seasonal factor on the grounding and

touch voltages is discussed The vertical grounding rods

can effectively decrease the influence of seasonal factor on

safety of grounding system

[ 11 H.G Sarmiento, R.J Fortin, D Mukhekar, “Substation ground impedance:

comparative field measurements with high and low current injection

methods,” IEEE Transactions on Power Apparatus and System, V01.103,

no.7, July 1984, pp 1677-1683

[2] H.G Sarmiento, R Velazquez, “Survey of low ground electrode

impedance measurements ,” IEEE Transactions on Power Apparatus and

System, V01.102, no.9, Sept 1983, pp 2842-2849

[3] DUT621-1997 Grounding for AC electrical installations Beijing: China

Electrical Power Industry Ministry, China, 1997

[4] F.P Dawalibi, D Mukhedkar, “Resistance measurement of large

grounding systems,” IEEE Transactions on Power Apparatus and System,

Vo1.98, n0.6, June 1979, pp 2348-2354

[ 5 ] S.T Sobral, S.J Horta, D k Mukhedkar, “A proposal for ground

measurement techniques in substations fed exclusively by power cables,”

Transactions on Power Delivery, Vo1.3, no 4, Sept 1983, pp 2842-2849

VIII BIOGRAPHIES

Dr Rong ZENG was born in Shaami, P R China in 1971 He received his B

Sc Degree, M Eng., and Ph.D Degree from the Department of Electrical

Engineering, Tsinghua University, respectively in July 1995, July 1997, July

1999 And now, he is a lecturer in the same department, Tsinghua University

His research interests include high voltage technology, grounding technology,

power electronics and computer application

Dr Jinliang H E was bom in Changsha, P R China, in 1966 He received the

Ph.D degree from Tsinghua University in Electrical Engineering, in March

1994 In July 1996, he became an associate professor in the same department

From 1994 to Jan 1997, he was the head of high voltage laboratory inTsinghua

University During April 1997 to April 1998, he was a visiting scientist in Korea Electro-technology Research Institute He is a SM of CES Now he is vice chief of High Voltage Research Institute in Tsinghua University His research interests include overvoltages and EMC in power systems and electronic systems, groundingtechnology, power apparatus, dielectric material, and power distribution automation He is the author of three books and 80 technical papers His E-mail address is hejl@tsinghuaedu.cn

Dr Wang Zanji was bom in Fuqing Fujian, China, in 1946 Hereceived his B

S Degree, M S Degree, and Ph D Degree from Dept of Electrical

Engineering Tsinghua University, all in electrical engineering, respecitvely in

1970, 1985, and 1990 He became a lecturer in the Department of Electrical Engineering, Tsinghua University, in 1983 And in 1990, He became an associate professor In 1993, he was promoted as a professor Since may 1995,

he have been being the Dean of the Department of Electrical Engineering, Tsinghua University And at present, he is a SM of CES, member of CES Standing Council, Chairman of Editorial Working Committee of CES, Vice Chairman of Education Working Committee of CES; SM of CSEE, member of Theoretical Electrotechnics committee of CSEE, member of Transmission

Commitbe of CSEE; and chairman of Organizing Committee, IPEMC’2000, Aug 2000, Beijing His research fields include circuit theory and application, numerical computation of electromagnetic field, electromagnetic transient, fault location and protective relay, static electrification in EHV power

transformer, soft computing and multi-objective optimization, communication

in power system, and chaos He is the author ofmore than 40 technique papers

Mr Yanqing GAO received his B Sc From the Department of Electrical Engineering, Tsinghua University in July, 1999 And now he is a Ph.D candidate in the Department ofElectrical Engineering Tsinghua University, his research fields include overvoltage in power system, grounding technology

Mr Weimin SUN obtained his B Sc Degree from the Department of Electrical Engineering, Shondong Industry University in July, 1984, M Eng Degree from EPRI, China, in 1989 And now he is a senior engineer in Shandong Electrical Power Bureau, his research fields include high voltage technology and power distribution

Dr Qi SU received his MEng in 81 (WUHEE, China) and PhD in 90 (UNSW, Australia) He was a tests and operations engineer from 71-78, an Honorary

Research Associate with the University of Western Australia in 1985 and a

Lecturer in the University of New South Wales in Sydney from 90-91 Since

1992, he has been with Monash University in Melboume and is now a Senior Lecturer and the Head of High Voltage and Insulation Condition Monitoring Group in the Department of Electrical and Computer Systems Engineering He also took short term positions as a Senior Fellow in Singapore National

University in 1998 and a Guest Professor at the Technical University of Denmark in 1999 Dr Su’s main research interests include insulation condition monitoring reliability-centred maintenance, fuzzy diagnosis of electrical plant, high voltage tests, transformer and generator high frequency models, system disturbance location, pattern recognition using artificial intelligence and web- based engineering education He holds two Australian patents and has

published over 100 journal and conference papers He is a Charlered Professional Engineer, a member of CIGRE Australian Panel 11 (Generators) &

21 (Power cables) and a Senior Member of IEEE since 1991