Energy and Power Engineering, 2013, 5, 873-876
doi:10.4236/epe.2013.54B167 Published Online July 2013 (http://www.scirp.org/journal/epe)
The Research of Voltage Sag and Power Frequency
Overvoltage on 110kV Resistance Grounding System
Guo Zeng1, Guangliang Feng1, Yanping Lv2, Shan Sun2
1Huangshi Power Supply Company, Huangshi, China
2School of Electrical Engineering, Wuhan University, Wuhan, China
Received January, 2013
ABSTRACT
This paper researches the voltage transfer characteristics when one-phase ground fault occurred in the resistance
grounding system, by using the theory of the asymmetric variable characteristics and the sequence network analysis of
the -11 transformer, and concludes the scope of voltage sag and swell and the degree of power frequency overvoltage
and their influencing factors in the 110 kV resistance grounding system. Accordingly this paper puts forward the resis-
tance choosing principle: the resistance grounding coefficient must be equal to or greater than 10. So it can not only
wipe out the voltage sag and voltage swell but also make sure the overvoltage is limited to electrical equipment allow-
ing range. The method mentioned above is verified by simulation results of a 110 kV power system in ATP.
Keywords: Neutral Grounding System by Resistance; One-phase Grounding Fault; Power Frequency Overvoltage
1. Introduction
In China, Most of 110 kV distribution grid use neutral
solid ground[1], in this neutral grounding mode, single-
phase fault current is very large, sometimes even more
than the three-phase fault current. And the single-phase
fault will cause a serious voltage sag in the load side
which can not satisfy the sensitive users’ requirements
for high power quality.Therefore terminal substation of
110 kV side using neutral solid grounding method has
been difficult to meet the sensitivity to the requirements
of the quality of the electric power load.
According to this problem, this paper puts forward the
110 kV neutral grounding system by resistance, and re-
searches the relationship between the neutral grounding
resistance and the voltage sag degree, power frequency
overvoltage level, and tries to make the terminal substa-
tion of 110 kV neutral grounding by resistance. After this
change, the 110 kV side can eliminate the voltage sag
and limit the voltage dip rising and also can ensure the
power frequency overvoltage degree not more than the
existing 110 kV electrical equipment in the insulation
level when one-phase ground fault happened.
2. Voltage Sag and Voltage Swell and Their
Trans Ferred by Transformer
Voltage sag (also called the voltage drop) is transient
electric energy quality problems. It is point to the voltage
root-mean-square value to suddenly down or rise in a
short time, the typical duration is 0.5 - 30 cycles. IEEE
standard defined as: in the power supply system, a cer-
tain point’s power frequency voltage RMS suddenly
dropped to 10% - 90% of the rating called voltage sag [2].
And these phenomena are returned to normal after the
short duration of the next 10 ms – 1 min.
3. The Voltage Sag and Swell in the
Resistance Grounding System
If the transformer high voltage side happened the A
phase grounding fault, the line voltage relationship be-
tween high and low voltage side of Y/△ -11 transformer
can described by the following formula (1):
110
101 1
3101
ab AB
bc BC
ca CA
UU
UU
k
UU





























(1)
Where, k is the transformation ration.
In 110 kV resistance grounding system occurring gold
attribute ground fault for a phase, we can deduce 110 kV
phase voltages per-unit expression after the fault occur-
ring [3].

G. ZENG ET AL.
874
*
*
0
1
*
0
1
0
33 3
22 2
33 3
22 2
A
B
C
U
j
Uj
Zj
Xj
Uj
Zj
X





 




 



(2)
where, the 0
Z
is the zero sequence impedance in the
resistance grounding system. It can be shown in the
equivalent circuit in Figure 1
00 00
()//(1/)
Z
RjX jC

 (3)
where, Rn is the neutral grounding resistance; X0 is the
system equivalent zero sequence impedance which from
the fault point. If the XC in the zero sequence system have
the same order of magnitude with the neutral grounding
resistance R0, the XC cannot be ignored.
According the type (1) and (2), we deduced:
*
0
1
*
0
1
*
0
1
13
22 2
13
22 2
2
1
2
ab
bc
ca
j
Uj
Zj
X
j
Uj
Zj
X
j
UZj
X









 




 



(4)
We can concluded that when one-phase grounding
fault occurred in the resistance grounding system, the
voltage of the load side are closely related to the
01
Z
X.
Making the 01
X
Xm, 1 /
C
X
X
n, 01 RX k

,
and calling k is the grounding resistance coefficient:
*
*
*
[3(2)][3(2)
2(2)2( 2)
[3(2)][3(2)
2(2)2( 2)
(2) (2)
ab
bc
ca
mnn kjmnnk
Umnjnk
mnn kjmnnk
Umnjnk
mn jnk
Umnjnk



 

 

 

 






]
]
(5)
For the existing 110 – 220 kV system, generally 0.23 ≤
m = 01
X
X≤ 3; And for the neutral non-grounding sys-
tem, 1
/
C
X
X
=26～∞[4]. Beacause when n > 100, the
value of n has little affection to the value of voltage, so
this paper mainly studies the value area of n is 26≤ n ≤
100.
The boundary value combination of the m, n go into
the type (5), seeking the rule of load line voltages
changing with the coefficient k. we find the voltage drop
seriously is Uab as shown in Figure 2. Between the m
and n range, other combination have the impaction on
voltage amplitude should be between them.
From the Figure 2 can be seen ，when choosing ap-
propriate resistance(such as k ≥ 10), voltage sag of three
line voltages in the load side will never more than 0.1p.u,
that can ensure the load side line voltages in line with the
requirements of power quality of voltage fluctuation
range.
For a practical system, we can seek the biggest X1 ac-
cording to the change of the mode of operation and the
structure of the system when one-phase fault occurred in
the system, going into the 01
RX k≥ 10 to seek the
resistance range were able to eliminate the voltage sag.
0
10R1 X (6)
4. The Relationship between the Power
Frequency and the Grounding Resistance
From the above discussion, when 110 kV using neutral
grounding resistance method, if the resistance is appro-
priate, it can eliminate voltage sag. But it can lead to the
power frequency overvoltage, also conversing the type (2)
to the functions on m, n, k such as type (7) showing:
*
*
*
0
[33(2)][3(2)3]
2(2)2( 2)
[ 33(2) ][3(2)3]
2(2)2( 2)
A
B
C
U
mnnkjmnnk
Umnjnk
mnnkjmnnk
Umnjnk






 

 

 
 


(7)
0
X 0
3
n
R
R

0
1/
C
j
XjC


Figure 1. Equivalent circuit of zero sequence impedance Z0.
Figure 2. Load line voltage Uab with R0/X1 changes.

G. ZENG ET AL. 875
when A phase grounding fault occurred, C phase power
frequency overvoltage is the most serious. The boundary
value combination of the m, n go into the type (7), seek-
ing the rule of UC* changing with the coefficient k, as
shown in Figure 3. In the m and n range, other combina-
tion have the impaction on voltage amplitude should be
between them.
It shows that: The power frequency overvoltage ex-
treme maximum value will be not more than 2.0p.u,
When choosing the resistance meeting constraint condi-
tions that k ≥10.
Literature [5] regulated that the 1min withstanding
voltage value of 110kV transformer and switch equip-
ment is 200 kV (i.e. not less than 3.0p.u). Because of 110
kV system adopting the neutral grounding resistance
method, relay protection is still in action trip, which can
quickly resection fault, so the power frequency over-
voltage would be for a very short time, and can be con-
trolled in within 0.5 s[6]. So the power frequency over-
voltage to the electric equipment insulation will not
cause damage.
5. Based on the Simulation Results for ATP
A simulation model is established in the ATP-EMTP for
a 220kV terminal substation, as shown in Figure 4.
Where, power supply side equivalent impedance: Xsmax.1*
= 0.01583, Xsmax.0* = 0.03594, Xsmin.1* =0.04934, Xsmin.0* =
0.12811(S B = 100MVA，UB=Uav); 1# and 2# main
transformer are three winding step-down transformer,
YNynd11, Se = 180MVA, Ud1-2 = 14.10%, Ud1-3 = 23.10%,
Ud2-3 = 7.9%. For the neutral point of 220 kV side, 1#
directly grounded and 2# not grounded. The neutral point
of 110 kV side, both of the transformers are grounded by
resistance. Line model is LGJ-240, their length shown in
Figure 4, , ;
0
0
10.1181 /rk
/,rkm
4 /.Fkm
m
11.2450 /LmH
2.4900 /LmHkm km
00.2881
0.005C,

 The line terminal transformer are
110/10.5 kV, -11.The 1# and 2# transformers
parameters are Se = 50 MVA, Ud1-2=10.56%；3# and 4#
transformers parameters are Se= 40 MVA，Ud1-2=10.74%
/Y
Analysis system the most serious condition under the
possible operation mode, and making the simulation
schemes are as follows:
1) The system operate in the minimum way, the
one-phase grounding fault respectively happening in the
head end of line(d1) and the tail end of the longest
line(d2).
2) The system is running in the biggest way, and the
one-phase grounding fault respectively happening in the
d1 and d2.
We can calculated that when the system is running in
the minimum way, and the fault happened at d2, the X1
will be the biggest 26.49 Ω. According to the type(8), we
can find out the resistance which eliminate voltage sag
and swell.
0 1
R ≥ 10
X
= 10 × 26.49 = 260.49
Making R0=300

, and the resistance of each main
transformer is RN=2Rn=200

.
When the RN=200

, the simulation results shown in
Table 1.
Seen from the Table 1, when neutral grounding resis-
tance is 200

, the highest power frequency overvoltage
is 1.78p.u,the maximum line voltage drop is 9%,So the
line voltage fluctuation have not more than system al-
lows range, and the power frequency overvoltage under
2p.u.Since the model in ATP considering the resistance
of transformer and line, the simulation results have a
certain error compared to the theoretical calculation val-
ue, and the simulation results may be more close to the
practical situation.
6. Conclusions
This paper researches the voltage sag and power fre-
quency overvoltage for the 110 kV neutral grounding
system by the resistance, and concludes :
1) When the selected resistance meet the requirement
that 01
≥ 10, the voltage sag degree less than an
average 10%. It can eliminate voltage sag when most
frequent one-phase fault happened in high voltage side.
/RX
Figure 3. Phase voltage UC* with R0/X1 changes.
1
d
2
d
N
R
N
R
Figure 4. Substation simulation syste m diagr am.

G. ZENG ET AL.
876
Table 1. Simulation results.
110 kV bus voltage 1#transformer Y side phase voltage/△ side
line voltage
3#transformer Y side phase voltage/△ side
line voltage
Simula-
tion
scheme
Running
status
B
U* C U*
B
U*C U*ab U*bc U*ca U*
B
U*C U* ab U* bc U*ca U*
normal 1.0 1.0 0.99 0.99 1.0 1.0 1.0 0.96 0.96 0.99 0.99 0.99
(1)
1A
d 1.64 1.78 1.64 1.76 0.96 1.04 0.99 1.63 1.73 0.94 1.02 0.98
schenme
1
(1)
2A
d 1.37 1.76 1.36 1.75 0.96 1.03 0.97 1.47 1.65 0.91 1.02 0.91
normal 1.0 1.0 0.99 0.99 1.0 1.0 1.0 0.96 0.96 0.99 0.99 0.99
(1)
1A
d 1.67 1.77 1.67 1.75 0.98 1.03 1.0 1.66 1.72 0.96 1.01 0.98
scheme
2
(1)
2A
d 1.40 1.77 1.40 1.75 0.98 1.02 0.98 1.50 1.66 0.93 1.01 0.93
[3] D. Tao and X. N. Xiao, “Voltage Sags Types under Dif-
ferent Grouding Modes of Neutral and Their Propagation:
partⅡ, Transactions of China Electrotechnical Society,
Vol. 22, No. 10, 2007，pp. 156-159.
2) The power frequency overvoltage will increase in
the neutral grounding resistance system, but the over-
voltage will be not more than 2.0p.u.Since the relay pro-
tection can fast action to react the fault, so it will be not
harmful to the electrical equipment. [4] G. R. Xie, “Power System Overvoltage,” Wuhan Water
Electrical Institute Press, China, 1985.
[5] The People’s Republic of Electrical Power Industry.
Overvoltage Protection and Insulation Coordination for
AC Electrical Installations, Beijing, China: China Electric
Power Press, 2010.
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[2] IEEE Recommended Practice for Evaluating Electric
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