Energy and Power Engineering, 2013, 5, 897-901
doi:10.4236/epe.2013.54B172 Published Online July 2013 (http://www.scirp.org/journal/epe)
The Research on Neutral Grounding Scheme of Fengxian
35 kV and 10 kV Power Grid
Siming Hua1, Hua Zhang1, Feng Qian1, Chunjie Chen2, Meixia Zhang2
1Dispatch Control Center of the Fengxian Supply Power, Shanghai, China
2Shanghai University of Electric Power, Shanghai, china
Received March, 2013
ABSTRACT
Combined with actual situation of Fengxian power Supply Company, the neutral grounding modes of Fengxian 35 kV
and 10 kV power grid are studied in the paper. The different frequencies injected method is used to measure the capaci-
tive current of Fengxian 28 substations, and the neutral grounding modes of the 28 substations are determined based on
the measured values of capacitive current.
Keywords: Neutral Grounding Mode; Different Frequency Injected Method; Capacitive Current; Arc Suppression Coil;
Low Resistance
1. Introduction
Shanghai fengxian power grid has eight 35 kV trans-
former substations and forty 10 kV transformer substa-
tions. The neutral grounding modes of these substations
include: ungrounded, grounded via arc-suppression coil
and grounded via low resistance. Among them, 21 sub-
stations such as Qixian station are use ungrounded, 7
substations such as Taidong station are use grounded via
arc-suppression coil, and 20 substations such as Zugang
station are use grounded via low resistance.
With the increasing of cable line in fengxian 35 kV
and 10 kV distribution systems, the capacitive current of
single-phase grounding fault is growing, which brings
new topic to the choice of neutral grounding mode in
Shanghai fengxian distribution network. In this paper,
according to the measured value of capacitive current,
the 28 substations which use ungrounded and grounded
via arc-suppression coil were studied.
2. The Analysis of Neutral Grounding Mode
in Distribution Network
2.1. Common Neutral Grounding Modes
There have four common ground modes in power system:
the neutral point directly grounded, grounded via arc-
suppression coil, grounded via low resistance and un-
grounded. Among them, ungrounded and grounded via l
high resistance are called as non-effectively grounded
system, the neutral point directly grounded and grounded
via low resistance are called as effective grounded sys-
tem [1].
1) Neutral point directly grounded system: the required
levels of over-voltage and power transmission equipment
insulation are low in neutral-point solid ground system,
thereby it can reduce the cost of equipment and line, but
the current of single-phase grounding is high at the fault
of single-phase grounding, which inevitably lead to the
tripping of the circuit breaker and reduce supply continu-
ity, thereby, it reduce supply reliability.
2) Neutral point ungrounded system: the current of
single-phase grounding fault is very small in ungrounded
system. It can operate with fault for some time in order to
find the fault line, thus the reliability of power supply can
greatly improve. And the interference with communica-
tion lines is low. But it will emerge arc restricting over-
voltage, thus it requires a higher level of insulation. If the
line is long, grounding capacitive current will be very
high that easy to form intermittent arc grounding or sta-
ble arc grounding. It requires other neutral grounding
modes to avoid the impact on the safe operation of power
grid.
3) Neutral point grounded via arc-suppression coil: arc
suppression coil is adjustable inductor coil which installs
in the neutral point of grid. The inductive current be
formed when single-phase ground fault occurs, which
close to but direction opposite to grounded capacitor
current, so that the current at fault point becomes small
or close to zero. According to compensation rate, it can
be divided into: under compensation, full compensation
and over compensation. In the grid, the over compensa-
tion widely use. When single-phase ground fault occurs,
this grounding mode can improve the reliability of power

S. M. HUA ET AL.
898
supply, and significantly reduce the current of single-
phase grounding fault. So arc at fault point can quickly
extinct and prevent over-voltage which due to the inter-
mittent arc.
4) Neutral point grounded via resistance: neutral point
grounded via resistance divided into high resistance
grounded and low resistance grounded. High resistance
grounded is adopted when the current of single phase
grounding fault is less than 10 A. This mode can elimi-
nate most of the resonance over-voltage and it are certain
restrictions on single-phase intermittent arc grounding
overvoltage, but it requires a higher level of insulation;
Low resistance grounded is adopted when the current of
single phase grounding fault 100 - 1000 A. This mode
can remove fault quickly and overvoltage is low, so cable
and equipment which insulation level is lower can be
used.
2.2. Main Factors in the Selection of Neutral
Grounding Mode
The selection of neutral grounding mode is a comprehen-
sive question which involves many aspects of power
system. The following aspects are mainly considered
when select neutral grounding mode [2]:
1) The insulation levels of electrical equipment and
wiring;
2) The reliability of relay protection works;
3) Power supply reliability of distribution network;
4) The interference on communication and signaling
system;
5) The influence of over voltage;
6) Personal safety.
Combining the actual situation of the grid is the key to
selection of neutral grounding mode. For meeting the
need of running, it can make timely adjustments accord-
ing to the development trend of the grid.
2.3. Present Situation of the Selection on Neutral
Grounding Mode
In the United States grid, neutral point directly grounded
and via low resistance have been adopted [3]; In Japan
Grid, neutral point grounded via arc-suppression coil,
grounded via resistance and ungrounded are commonly
used [4, 5]; In Germany, grounded via arc-suppression
coil has been widely used before the 1970s, but grounded
via resistance is proposed recently. In former Soviet Un-
ion and Moscow city, grounded via arc-suppression coil
has been used [6, 7].
Most of the domestic provinces and regions remains
neutral point grounded via arc-suppression coil. But in
recent years, Shanghai Urban Power Supply Company,
Guangzhou, Shenzhen, Zhuhai, Suzhou have selected the
grounded via low resistance [8].
3. The Solution Configuration of Neutral
Grounding Modes in Fengxian Power
Grid
3.1. The Detection Method of Capacitor Current
The determination of neutral grounding mode is based on
the accurate detection of bus capacitor current. The tradi-
tional capacitive current measurement methods have two
kinds: direct and indirect methods. The two methods
need directly contact with the main circuit of grid, so the
operator and the distribution system are not safe, and the
operation is more complex. For safely measuring the
capacitor current of the neutral point, the measurement
method from the secondary side of the grid is proposed,
and different frequency injected method to measure the
capacitive current is widely used.
Different frequency injection method is injecting weak
different frequency test signal from the voltage trans-
former (PT) triangular openings, and then measuring the
voltage magnitude and phase of opening triangular side,
thus calculating the capacitive current of power grid.
This method reduces the risk of testing, and improves the
efficiency of testing [9]. The schematic is shown in Fig-
ure 1.
In the figure: Ea, Eb, Ec as the power supply side; AX,
BX, CX as the high voltage windings of bus PT; ax, bx,
cx as the secondary windings of bus PT; N, L as triangu-
lar openings windings; C0 as capacitance to ground.
3.2. The Solution Configuration of Neutral
Grounding Modes
In this paper, the determination of measured capacitive
current neutral grounding modes is based on the meas-
ured capacitive current of 28 substations. According to
the regulations: when the capacitor current of 10 kV grid
is less than 30 A, or the capacitor current of 35 kV grid is
less than 10 A, Neutral point ungrounded is adopted;
when the capacitance current of 10 kV grid is greater
Figure 1. The schematic of different frequency injection
method.

S. M. HUA ET AL.
899
than 30 A, or the capacitance current of 35 kV grid is
greater than 10A, grounded via arc-suppression coil is
adopted; when capacitor current is greater than 100 A,
grounded via low resistance is adopted.
In the Fengxian grid, the substations of neutral point
ungrounded are shown in Table 1.
As seeing from Table 1, the capacitor current of Non-
gchang station, Qixian station and Qingcun station is less
than 30 A, but they are very close to 30 A. When the
three substations change the overhead lines to the cable
line, or increase line, their capacitance current value may
be greater than 30 A, therefore these three substations
must be given great attention.
In the Fengxian grid, the substations of neutral point
grounded via arc-suppression coil are shown in Table 2.
The choice of arc suppression coil capacity is based on
present capacitive current of grid, and is taken the de-
velopment vision for 5-10 years into account.
/3
Cn
QkIU (1)
where,
Q—The compensation capacity of arc suppression
coil,KVA;
k— Coefficient;
C
I
— The capacitor current of grid circuit, A.
n
U—The nominal voltage of the circuit, kV.
The capacity of arc suppression coil is calculated ac-
cording to equation (1), and the model of arc suppression
coil is selected according to the calculated capacity, as
shown in Table 3.
For describing the compensation degree of arc sup-
pression coil, it is need to calculate out-of-resonance de-
gree of arc suppression coil. Out-of-resonance degree v is
calculated as following:
() /*100%
CLC
vIII (2)
Table 1. The substation of neutral point ungrounded.
No. Station Name Capacitive Current(A)
1 Nongchang station(10 kV) 29.68
2 Qixian station(10 kV) 28.12
3 Xiaotang station(10 kV) 17.21
4 Xiaotang station(10 kV) 13.17
5 Qianqiao station(10 kV) 19.56
6 Qingcun station(10 kV) 25.62
7 Liaoyuan station(10 kV) 17.09
Table 2. The substation of neutral point grounded via arc-
suppression coil.
No. Station Name Capacitive Current(A)
1 Pingan station(10 kV) 39.44
2 Mingcheng station(10 kV) 39.68
3 Baishi station(10 kV) 47.19
4 Taidong station(10 kV) 41.49
5 Fengpu station(10 kV) 38.22
6 Fengcheng station(10 kV) 48.44
7 Pengcheng station(10 kV) 54.89
8 Zhangwenmiao station(10 kV) 63.18
9 Yaojiaxiang station(10 kV) 71.27
10 Jingxing station(10 kV) 47.06
11 Wuqiao station(10 kV) 32.66
12 Hongmiao station(10 kV) 44.78
13 Caijian station(10 kV) 35.67
14 Youyi station(10 kV) 74.33
Table 3. The capacity and the model of arc suppression coil.
No. Station Name Neutral grounding mode
in current
The capacity of arc suppression coil
(kVA)
The model of arc
suppression coil
1 Pingan station ungrounded 307.4382 2*XDJI-150/10
2 Mingcheng station ungrounded 309.2958 2*XDJI-150/10
3 Baishi station ungrounded 367.8045 XDJI-150/10+ XDJI-300/10
4 Taidong station via arc-suppression coil 323.3696 XDJI-150/10+ XDJI-300/10
5 Fengpu station via arc-suppression coil 297.9188 2*XDJI-150/10
6 Fengcheng station ungrounded 377.5265 XDJI-150/10+ XDJI-300/10
7 Pengcheng station ungrounded 427.8252 XDJI-150/10+ XDJI-300/10
8 Zhangwenmiao station via arc-suppression coil 492.4212 2*XDJI-300/10
9 Yaojiaxiang station via arc-suppression coil 555.4687 2*XDJI-300/10
10 Jingxing station ungrounded 366.7704 XDJI-150/10+ XDJI-300/10
11 Wuqiao station ungrounded 254.5855 2*XDJI-150/10
12 Hongmiao station ungrounded 348.9996 XDJI-150/10+ XDJI-300/10
13 Caijian station Neutral point ungrounded 278.0461 2*XDJI-150/10
14 Youyi station Neutral point ungrounded 579.345 2*XDJI-300/10

S. M. HUA ET AL.
900
where: C
I
-the capacitive current, A;
L
I
-the inductor current of arc suppression coil A.
Under normal circumstances, the neutral point voltage
displacement should not exceed 15% of the system phase
voltage for a long time in the gird which takes grounded
via arc-suppression coil. Therefore, it is required to ver-
ify the neutral point displacement voltage.
Neutral point displacement voltage be verified ac-
cording to the following formula:
22
/
Obd
UU dv (3)
where, bd is the asymmetric voltage of neutral point
before arc suppression coil into the grid operation kV,
the value normally takes 0.8% of system phase voltage;
is damping rate, it takes 5% for 35 kV overhead lines
and below, and it takes 2% to 4% for cable lines.
U
d
In order to improve the success rate of arc suppression
coil and reduce the operational burden of operating per-
sonnel, it should be preferred arc suppression coil which
has a good automatic tracking compensation effect. Au-
tomatic tracking arc suppression coil device can auto-
matically and timely track the changes of grid operation
mode, and can quickly adjust the inductance value of arc
suppression coil. According to the different methods of
changing the inductance, automatic Tracking Compensa-
tion Arc can be broadly divided into multitap, adjust the
air gap, tuning capacitors and tune DC biasing type.
Among them, tuning capacitors is widely used, mainly
because of its fast response, good linearity of the
volt-ampere characteristics and continuously adjustable.
In the Fengxian grid, the substations of neutral point
grounded via low resistance and resistance value are
shown in Table 4. The value of neutral point resistor is
calculated as the following:
/(2~3)
N
X
RU IC (4)
where,
N
R is the resistance of neutral point, X is the
phase voltage of distribution network, C
U
I
is the
grounded capacitor current of distribution network.
Table 4. The substations of neutral point grounded via low
resistance and resistance value.
No. Station Name Capacitive
Current(A)
resistance
Value Ω
1 Gangxiao station(35 kV) 135.02 50
2 Xinghuo station(35 kV) 231.33 29
3 Tairi station(35 kV) 138.41 49
4 Xinhai station(10 kV) 122.23 16
5 zhuangxing station(10 kV) 116.77 16
6 Yuxiu station(10 kV) 123.48 16
7 Laonanqiao station(35 kV) 120.38 56
Whether transforms small current grounding system
into low resistance grounding system is considering a
number of factors, the first is the factors of grid operation.
Changing substation does not mean that all the small
current grounding system transformed into large current
grounding system; Secondly, the small current grounding
system changing into low resistance grounding system is
relatively difficult, the related substation need to install a
small resistance, in addition, it need to transform the
feeder protection.
4. Conclusions
The correct detection of system capacitive current is the
basis of determination the neutral point operation mode.
There is a certain risk or personnel and equipment
whether takes direct method or the indirect method to
measure the capacitance current. This paper suggests that
takes different frequency injection method, because it is
measuring the capacitive current directly from the sec-
ondary side of the PT, thereby, it reduces the risk of test.
In this paper, the neutral grounding modes of Fengxian
grid is determined based on the measured value of ca-
pacitive current: due to the current value is less than 30 A,
the 7 substations, such as Qixian station, Xiao Tang Sta-
tion, is taken ungrounded. The 14 substations, such as
Taidong station, Pingan station, is taken grounded via
arc-suppression coil. The 7 substations, such as Xinghuo
station, Tairi station, is taken grounded via low resis-
tance.
REFERENCES
[1] Y. H. Ren, K. H. Bian and J. W. Yao, “The Design Man-
ual of Industrial and Ccivil Distribution,” Beijing: China
Electric Power Press, 2005.
[2] S. Li, “Analysis and Selection of Neutral Grounding
Scheme of Distribution Networks,” Journal of Shaanxi
Institute of Technology, 2003, Vol. 19, No. 1, pp. 29-31.
[3] Z. C. Chen and D. H. Tang, “The Planning and Trans-
formation of Urban Power Grid,” China Electric Power
Press, Beijing, 1998.
[4] Q. Z. Zhang, “Electric Power Industry in Japan(Ⅰ),”
International Electric Power for China, Vol. 17, No. 5,
2003, pp. 4-9.
[5] Q. Z. Zhang, “Electric Power Industry in Japan (Ⅱ),”
International Electric Power for China, 2003, Vol. 17,
No. 6, pp. 6-11.
[6] Y. Y. Miao, “Research into the Neutral Point Grounding
Mode of Distribution Power Grid and Its Decision Algo-
rithm,” Guangdong: Guangdong University of Technol-
ogy, 2007.
[7] M. G. Liu, “Brief Introduction of Electric Power Industry
of Former USSR,” Sichuan Electric Power Technology,
Vol. 1, 1995, pp. 20-27.

S. M. HUA ET AL. 901
[8] B. P. Wang, “Research of Neutral Point Grounded Mode
for Medium Voltage Network,” Shandong: China Uni-
versity of Petroleum (east China), 2009.
[9] Y. Yang, “The Capacitive Current Test Methods of Neu-
tral not Grounding,” Yunnan Electric Power, Vol. 39, No.
3, 2011, pp. 20-22.