Energy and Power Engineering, 2013, 5, 792-796
doi:10.4236/epe.2013.54B152 Published Online July 2013 (http://www.scirp.org/journal/epe)
Study on Earth Surface Potential and DC Current
Distribution around DC Grounding Electrode
Zhi-chao Ren, Chun-yan Ye, Hai-yan Wang
1Planning &evaluation center, Sichuan Power Economic Research Institute, Chengdu, China
2Equipment condition assessment center, Sichuan Electric Power Research Institute, Chengdu, China
Received April, 2013
ABSTRACT
DC magnetic biasing problem, caused by the DC grounding electrode, threatened the safe operation of AC power grid.
In this paper, the characteristics of the soil stratification near DC grounding electrode was researched. The AC-DC in-
terconnected large-scale system model under the monopole operation mode was established. The earth surface potential
and DC current distribution in various stations under the different surface thickness was calculated. Some useful con-
clusions are drawn from the analyzed results.
Keywords: DC Grounding Electrode; Magnetic Biasing; Soil Stratification; Earth Surface Potential; DC Current
Distribution
1. Introduction
From an economic point of view, at the first stage of
construction, domestic HVDC transmission project often
put into an electrode firstly, as shown in Figure 2. At this
point, DC flowed through the grounding electrode into
the earth, and its breadth and depth of the diffuser was
still an open. However, the negative effects of this opera-
tion mode had been a consensus[1-18], mainly the fol-
lowing three points: electromagnetic effect, thermal ef-
fect and eletrochemical effect. In this paper, the surface
potential distribution was a kind of electromagnetic ef-
fects. In this commissioning and operation of DC trans-
mission system, these problems really existed. Therefore,
the study of surface potential and DC current distribution
was of great significance.
2. Calculation Model
Take the Shanxi-Jiangsu ± 500 kV DC transmission line
project for example, which was from ± 500 kV HVDC
converter station (in Yangcheng county, shanxi prov-
ince)to another one(in Liyang, Jiangsu province).
Figure 1. The monopolar ground circuit operation mode
2.1. Location Coordinate
The relative position of each station in AC-DC intercon-
nected power system was demarcated in the form of rec-
tangular coordinate, as shown in Figure 3. Gaoping DC
grounding electrode was located in Jincheng city, Shanxi
province. The position of this electrode was considered
as the origin of coordinate, with the radius of 100km as
the research scope.
Figure 2. AC-DC interconnected system location coordinate
diagram.
Figure 3. The view of DC electrode.

Z.-C. REN ET AL. 793
2.2. Soil Stratification Structure
Soil surface with low resistivity was usually very shallow.
Below the surface was the salt soil layer, which the resis-
tivity was up to thousands of Ω•m. Then it reached to the
rock, with the resistance rate was up to 100000 Ω•m
above and the thickness was about hundred km. The re-
sistivity of the lower crust began to decrease, which was
only 200 Ω•m or less; Corresponding to 100-200 km
depth, the resistivity of low-velocity layer was 10 Ω•m.
In the depth of 400-650 km, the resistivity in turn drasti-
cally reduced to 1 Ω•m. The resistivity of the lower man-
tle was only 0.1 Ω•m in the depth of 1500 km[19-21] .
The loess layer in southeast of Shanxi was thinner,
mostly about 50 meters[22], and the thickness of various
regions was uneven , therefore, it should select different
thickness values when calculated, which were as fol-
lows:100 meters, 50 meters, 10 meters. Based on the
typical reference value, the loess resistivity was defined
as 200 Ω•m. The soil in Jindongnan area was divided
into five layers, as can be seen in Table 1.
2.3. DC Power System
The full-length of HVDC transmission lines was 865.45
km, of which the Shanxi segment was 45 km, and Jin-
cheng grounding electrode line was 80 km. The overlook
and view of DC double-ring grounding electrode were
shown in Figure 4.The large ring with a radius of 400m,
the small ring with a radius of 300 m. Grounding con-
ductor was made of round steel materials, whose radius
was 35 mm. With 8 cables, the current, from the current
injection points, was introduced into the grounding ring.
The cable was made of copper, and its cross- sectional
area was 240 mm2, which was 0.1 m high from the
ground. The grounding electrode was 3 m below the
ground. The center of which was located in the Gaoping
area of Jincheng. Current injection segment was made of
copper wire, and the cross-sectional area: 240 mm2×
8=1920 mm2.
2.4. AC Power Grid
AC system mainly includes substations, AC grounding
nets and HVAC transmission lines. According to the
Table 1. The soil stratification.
layers Resistivity(Ω•m) thickness(m)
surface 200 10~100
layer 2 1500 2000
layer 3 6000 10000
layer 4 100000 100000
layer 5 200 ∞
Figure 4. The view of earth surface potential.
importance of substations,1000 kV, 500 kV and 220 kV
substations, which are located in the scope of 100 km
radius from Gaoping DC grounding electrode, were stu-
died. The equivalent DC resistance of transmission line
was decided by wire type, length, division number and
loop number.
3. Results Analysis
In this paper, the station with symbol “■” was ±500kV
converter station. The station with symbol “▲” was
1000kV substation. The stations with symbol “●” were
500kV substations. Others without symbol were 220kV
substations.
3.1. Earth Surface Potential
As shown in Table 2, Table 3 and Figure 5, with the
different thickness of loess, the surface potential was
difference. the value of Dc grounding electrode was
-1489.2 V, -1894.3 V and -2529.6 V. The value of con-
verter station was -173.73 V;-181.94 V and -189.18 V.
The value of 1000 kV UHV AC station was -226.15 V;
-237.88 V and-248.29 V.
The value of 500 kV and 220 kV was about from -550
V to -100 V.
Take the surface soil thickness 10m for example, the
closer to the distance from DC grounding electrode, the
larger of the step voltage. It ranges from 0 to 1 km. The
potential decreased to 1.251 V of the average per pa-
rameter, and the largest step voltage was about 1.6 V,
which was lower than the largest safety limit value 2.5 V
prescribed by the Chinese government.
3.2. DC Current Distribution
DC current distribution at station in the city Jincheng and
Changzhi were as shown in Table 4 and Table 5. The
“+”represented inflow, and “-” represented outflow.

Z.-C. REN ET AL.
794
Table 2. Earth surface potential AT STATION in Jincheng
city.
Earth suface potential/V
Station name 100m 50m 10m
DC electrode -1489.2 -1894.3 -2529.6
SN -485.23 -517.93 -547.50
JC● -468.51 -499.47 -527.26
DH -443.30 -471.79 -497.12
BYC -336.39 -356.01 -373.49
LC -312.57 -330.47 -346.43
DG -236.05 -248.46 -259.48
JS -232.79 -245.00 -255.83
QD -216.35 -227.39 -237.13
FC -188.64 -197.85 -205.98
YCB● -176.68 -185.07 -192.50
YC■ -173.73 -181.94 -189.18
QC -169.62 -177.54 -184.50
Table 3. Earth surface potential AT STATION in Changzhi
city.
Earth suface potential/V
Station name 10 0 m 50 m 10 m
DPT -251.16 -264.62 -276.60
JDN▲ -226.15 -237.88 -248.29
SD -221.19 -232.58 -242.65
XZ -195.48 -205.14 -213.68
CZ -193.65 -203.18 -211.59
DM -180.07 -188.68 -196.27
KZ -171.13 -179.17 -186.25
JA● -167.21 -174.66 -181.46
RH -160.76 -168.12 -174.60
HJG -153.53 -160.44 -166.54
HB -146.57 -153.03 -158.70
PC -133.32 -139.05 -144.19
ZC -129.81 -135.25 -140.01
QZ -111.07 -115.32 -118.99
WJY -105.74 -109.70 -113.14
When the surface soil thickness was considered as 100
m，50 m，10 m respectively, the inflow DC current could
respectively reach 1.2 A、1.4 A、1.4 A at the Yangcheng
converter transformer side, and the outflow DC current
of the Jindongnan 1000 kV substation could respectively
reach 2.2 A、2.3 A、2.8 A.The largest influence subtations
of DC grounding current were Shengnong, Jincheng,
Danhe and Beiyicheng.
Table 4. DC current distribution AT STATION in Jincheng
city.
DC current amount/A
Station name
100 m 50 m 10 m
SN -19.7 -21.1 -22.0
JC● -18.3 -19.6 -20.4
DH -17.0 -18.3 -19.0
BYC -10.0 -10.6 -11.0
LC -8.2 -8.7 -9.0
DG -2.9 -3.1 -3.2
JS -2.8 -2.9 -3.0
QD -1.7 -1.8 -1.8
FC +0.2 +0.27 +0.31
YCB● +1.0 +1.1 +1.2
YC■ +1.2 +1.4 +1.4
QC +1.6 +1.7 +1.8
Table 5. DC current distribution AT STATION in Changzhi
city
DC current amount/A
Station name
100 m 50 m 10 m
DPT -4.1 -4.3 -4.4
JDN▲ -2.2 -2.3 -2.8
SD -2.0 -2.1 -2.2
XZ -0.17 -0.15 -0.13
CZ -0.23 -0.16 -0.18
DM +0.72 +0.85 +0.86
KZ +1.4 +1.6 +1.6
JA● +1.7 +1.9 +2.0
RH +2.0 +2.2 +2.2
HJG +2.5 +2.7 +2.8
HB +3.1 +3.3 +3.4
PC +4.0 +4.2 +4.4
ZC +4.1 +4.4 +4.6
QZ +5.4 +5.8 +6.0
WJY +5.8 +6.2 +6.4
The DC grounding current could propagete to adjacent
other substations along the AC transmisson lines, when it
inflowed one substation. The Figure 6 and Figure 7 de-
scribed the DC current distribution in AC transmisssion
lines of Jincheng and Changzhi. When the soil thickness
was considered as 10 m, the maximum DC current was
on the 500 KV AC transmission lines from the substation
of Jindongnan to Jincheng.

Z.-C. REN ET AL. 795
Figure 6. DC current distribution in AC transmission lines
of Jincheng city.
Figure 7. DC current distribution in AC transmission lines of
Changzhi city.
4. Conclusions
The smaller the thickness of the surface soil is, the
greater the absolute value of earth surface potential. The
farther away from the grounding electrode, the smaller
the absolute value of the surface potential, which formed
the exponential decay function and tended to zero gradu-
ally.
The DC current amount of the distal substation was
not always less than the recent substaion. The DC current
would select the pathway of which the DC resistance was
the minimum. The DC current, flowing through the sub-
station, of which the topological structure was more com-
licated, was relatively large. The soil sructure had some
influence on the DC current distribution: the smaller of
the surface soil thickness, the larger of DC grounding
current influence on the AC system.
5. Acknowledgements
The authors would like to acknowledge Shanxi Electric
Power Corporation for the facilities provided during this
research.
REFERENCES
[1] J. E. Villas and C. M. Portela, “Soil Heating around the
Ground Electrode of an HVDC System by Interaction of
Electrica, Thermal,and Electroosmotic Phenomena,”
IEEE Transactions on Power Delivery, 2003,Vol. 18, No.
3, pp. 874-881.
[2] B. Zhang, J. Zhao, R. Zeng et al., “Estimation of DC
current Distribution in AC Power System Caused by
HVDC Transmission System in Ground Return Status,”
Proceedings of the CSEE, 2006, Vol. 26, No. 13, pp.
84-88.
[3] Z. C. Ren, G. N. Wu, W. Zhen, C. Wu and Y. Zhang,
“Model Simplication and Calculation Method Analysis
About the Shunt of DC Grounding Current Via AC Grid,”
High Voltage Engineering, 2011, Vol. 37, No. 4, pp.
1008-1014.
[4] Z. H. Re, J. G. Xu, Y. K. Zhang, W. Zhen and G. N. Wu,
“Study of the Simple Formula of DC Surface Potential in
AC-DC Interconnected Large Power System," Transac-
tions of China Electrotechnical Socitey,2011, Vol. 26, No.
7, pp. 256-263.
[5] W. Jiang, Z. Huang, C. Hu, Zhu, K. Wu, L. R. Zhou and
Z. C. Ren, “Optimized Network Configuration of Small
Resistances to Limit DC Bias Current of Transformers,”
Proceedings of the CSEE, 2009, Vol. 29, No. 16, pp.
89-94.
[6] X. H. Chi and Y. J. Zhang, “Protective Distance between
HVDC Electrode and Underground Metal Pipeline,"
Power System Technology, 2008, Vol. 32, No. 2, pp.
71-74.
[7] M. X. Wang and Q. Zhang,“Analysis on Influence of
Ground Electrode Current in HVDC on AC Power Net-
work,” Power System Technology, 2005, Vol. 29, No. 3,
pp. 9-14.
[8] J. Guo, J. Zou, J. L. He, et al., “Recur-sive Method to
obtain Analytic Expressions of Green’s Functions in
Multi-layer Soil by Computer,” Proceedings of the CSEE,
2004, Vol. 24, No. 7, pp. 101-105.
[9] E. F. Fuchs, Y. You and D. J. Roesler, “Modeling and
Simulation,and Their Validation of Three-phase Trans-
formers with Three Legs under DC Bias,” IEEE Transac-
tion on Power Delivery, 1999, Vol. 14, No. 2, pp.
443-449. doi:10.1109/61.754087
[10] L. S. Zeng, “Impact of HVDC Ground-ing Electrode
Current on the Adjacent Power Transformers,” High Volt-
age Engineering,2005, Vol. 31, No. 4, pp. 57-59.
[11] L. H. Zhong, P. J. Lu, Z. C. Qiu, et al., “The Influence of
Current of DC Earthing Electrode on Directly Grounded
Transformer,” High Voltage Engineering, 2003, Vol. 29,
No. 8, pp. 12-13.
[12] C. Shang, “Measure to Decrease the Neutral Current of
the ac Transformer in HVDC Ground-return System,”
High Voltage Engineering, 2004, Vol. 30, No. 11, pp.

Z.-C. REN ET AL.
796
52-54.
[13] D. Z. Kuai, D. Wan and Y. Zhou, “Analysis and Handling
of the Impact of Geomagnetically Induced Current upon
Electric Network Equipment in DC Transmission,” Auto-
mation of Electric Power Systems, 2005, Vol. 29, No. 2,
pp. 81-82.
[14] X. P. Li, S. X. Wen, L. Lan, Y. Zhang, Y. D. Fan, Z. X.
Liu and L. Guo, “Test and Simulation for Single phase-
Transformer Under DC Bias,” Proceedings of the
CSEE,2007, Vol. 27, No. 9, pp. 33-40.
[15] J. Z. Sun and L. Liu, “Derivation of Greens Function by
Equivalent Complex Image Method in a Horizontal and
Vertical Combined-layer Soil Structure,” Proceedings of
the CSEE, 2003, Vol. 23, No. 9, pp. 143-151.
[16] J. M. Lu, D. Xiao, C. X. Mao and G. H. Mei, “Analysis of
Effects of DC Earthed Pole on Earth Surface Potential
Distributions,” High Voltage Engineering, Vol. 32, No. 9,
2006, pp. 55-58.
[17] D. Kovarsky, L. J. Pinto, C. E. Caroli, et al., “Soil Sur-
face Potentials Induced by ITAIPU HVDC Ground Re-
turn Current Part I-theoretical Evaluation,” IEEE Trans-
actions on Power Delivery, 1988, Vol. 3, No. 3, pp.
1204-1210. doi:10.1109/61.193904
[18] J. E. T. Villas and C. M. Portela, “Calculation of Electric
Field and Potential Distributions into Soil and Air Media
for a Ground Electrode of a HVDC System,” IEEE
Trans.on Power Delivery, 2003, Vol. 18, No. 3, pp.
867-873. doi:10.1109/TPWRD.2003.809741
[19] W. Ruan, J. Ma, J. liu and F. P. Dawalibi, “Performance
of HVDC Ground Electrode in Various Soil Structures,”
Power System Technology, Proceedings, International
Conference, Vol. 2, 2002, pp. 962-968.
[20] J. W. Teng, “Solid Geophysics Conspectus,” Beijing,
Earthquake Publishing House, 2003, Vol. 2, No. 1.
[21] H. Xu, X. S. Wen, X. Shu, Q. S. Zhou and W. Deng,
“New Method of Computing Soil Parameters,” High-
VoltageEngineering,2004, Vol. 30, No. 8, pp. 17-19.
[22] X. Y. Lei, “Geological Disasters and Human Activities in
the Loess Plateau Area,” Beijing, Earthquake publishing
house,2001, 10