Energy and Power Engineering, 2010, 2, 271-282
doi:10.4236/epe.2010.24039 Published Online November 2010 (http://www.SciRP.org/journal/epe)
Reassessment of Permissible Negative Sequence Current
for Power Plant Operation of Taipower
Ping-heng Ho1, Chi-jui Wu1, Chung-liang Chang2, Yuin-hong Liu2
1Department of Electrical Engineering, National Taiwan University of Science and Technology,
Taipei, Taiwan, China
2Department of System Planning, Taiwan Power Company, Taipei, Taiwan, China
Received July 21, 2010; revised September 2, 2010; accepted October 5, 2010
Abstract
The N3 power plant of Taipower is located in the southern tip of Taiwan and connected to the power pool by
four out-linking 345-kV overhead transmission circuits. There are two 951-MW generators. Each generator
occupied 11% of the system peak load in 1985 when the generator was in commercial operation. Since Tai-
power is an isolated system, at the N-2 conditions, those generators were reduced to 75% loading to protect
the power system. By the way, to avoid damage of negative sequence current (NSC), the limits of the N3
power plant are stricter than those in the IEEE Standard. However, in 2010, the capacity ratio of each gen-
erator in the plant to the system peak load has been reduced to 3% only. To increase the economic benefit of
those generators, it is required to reassess the operation limits of NSC. EMTP was used to calculate the lev-
els of NSC from the out-linking transmission circuits. From the results of this study, the effects of NSC
could be ignored when the four out-linking circuits are in N-0, N-1, and N-2 conditions. The generators can
be operated in full loading under these conditions. The modifications to the NSC limits of the N3 power
plant are also suggested.
Keywords: Negative Sequence Current, Transmission Line, Generator, EMTP
1. Introduction
The damages to generators by negative sequence currents
(NSC) from the asymmetrical transmission lines depend
on the levels of unbalanced conditions. The damages will
not only reduce generator lifetime, but also increase sys-
tem loss. The NSC may induce double-frequency cur-
rents in the rotor surface, retaining ring, slow wedge, and
field winding. Then, the short period dangerous tem-
perature in windings may be induced by abnormal rotor
current [1,2]. There are articles describing the three-
phase unbalanced systems [3]. The study about EHV
double-circuit untransposed transmission lines and field
measurements under different conditions were given [4].
The definitions of voltage unbalance to understand the
implications were reviewed [5]. A simple and approxi-
mated method for assessing the NSC in EHV lines was
described [6]. The three-phase unbalanced systems with
unbalanced loads supplied from a three-wire line were
dealt with, and the current was decomposed into three
different components [7]. A mitigation technique was
presented to reduce current unbalance in heavily loaded
multi-circuit power lines [8]. The method to detect the
NSC and to apply relays to protect machines under sys-
tem fault conditions was given [9]. A methodology using
software was used in the estimation of NSC injected into
utility generators [10]. A solution method was given to
formulate a multiphase power flow model and state es-
timation for distribution systems [11]. A new measure-
ment procedure based on neural networks was presented
for the estimation of current/voltage symmetrical com-
ponents [12]. The measurements and simulation studies
of NSC before and after outage of major transmission
systems were compared [13].
The N3 power plant of the Taiwan Power Company
(Taipower) is located at the southern of Taiwan. There
are two 951-MW generators in the plant. The plant is far
away from the load center and is connected to the power
pool by two long double-circuit overhead transmission
lines. Totally there are four out-linking 345-kV circuits
to transmit generation power of the plant. The capacity
of each circuit is 2,187-MVA. Each transmission line is

P.-H. HO ET AL. 272
in untransposed arrangement. The RST-T’S’R’ arrange-
ment is used in co-towered conductors to reduce the
Negative Sequence Current (NSC). Owing to serious salt
fog, shortage would make the out-linking circuits in N-1,
N-2, and N-3 conditions. The effects of NSC on the op-
eration of generators are important. Taipower has tried to
reduce NSC. The damage to the generator by NSC is
described in IEEE Standard C37.102. The NSC limits of
the N3 power plant (N3 Limit) are stricter than those of
the IEEE standard. The loading of each generator is re-
duced to 75% in the N-2 conditions to avoid the damage
of the NSC to generators. The economic benefit of the
power plant is lowered by the N3 Limit which is too
conservative.
In this paper, the EMTP was used to evaluate NSC
levels from the four out-linking untransposed overhead
345-kV transmission circuits [14-19]. The models of
EMTP were corrected by measurement data from the
wide area measurement systems (WAMS). The percent-
age permissible withstanding continuous NSC and the
ability of a generator to accommodate NSC were evalu-
ated in details considering the operation conditions of the
four circuits. The evaluation results according to the N3
Limits and the IEEE standard are compared. Important
suggestions are given to increase economic benefit of the
plant.
2. The Model of Non-Symmetrically
Three-Phase Transmission Lines
2.1. Asymmetrical Transmission Lines
As three-phase transmission line conductors are perfectly
symmetrically spaced in a triangular configuration, the
geometric distance between each conductor is the same
which makes the total flux linkage of each phase and the
three inductances are identical. The unbalanced current
in the transmission lines then reduced. For the considera-
tion of construction difficulty of the Taipower, the dou-
ble-circuit lines were arranged in co-towered and un-
transposed as non-perfectly symmetrical RST-R’S’T’
spaced to reduce the unbalanced current. However, in
some maintenance conditions, The RST-R’S’T’ arran-
gement may be used. The current direction of the RST
circuit may be same or different with that of the R’S’T’
circuit.
2.2. Sequence Network
Analyzing the unbalanced three-phase circuits, the
method of symmetrical components is usually used. A
three-phase system with self and mutual impedances, Zs
and Zm, is shown in Figure 1. Through an impedance Zn,
the load neutral terminal is grounded.
According to Kirchhoff’s current law, we have
nabc
I
III

 (1)
According to the Kirchhoff’s voltage law, the line-to-
ground voltages are

asmma n
bm s mbnnnn
cmmscn
VZZZI I
VZZZIZZZI
VZZZI I

 

 


 

 

 
(2)
Substituting for In from (1) into (2) and the voltage
vectors and current vectors could be changed into zero-,
positive- and negative-sequence component vectors.
Then
00
11
22
sn mnmn
mn sn mn
mnmn sn
VZZZZZZI
A
V ZZZZZZAI
VZZZZZZI







 








 
(3)
where
2
2
11 1
1
1
A
aa
aa











(4)
00
1
11
22
sn mnmn
mn sn mn
mnmn sn
VZZZZZZI
V AZZZZZZAI
VZZZZZZI

















 
(5)
Substitute (4) into (5), we have
00
11
22
00
11
22
32 00
00
00
00
00
00
snm
sm
sm
VZZZI
VZZI
VZZI
ZI
ZI
ZI
















 





(6)
The sequence impedance Z012, sequence voltage
012
a
V, and sequence current 012
a
I
could then be com-
posed into three independent sequence networks as
shown in Figure 2.
3. Problem Formulation
3.1. N3 Power Plant and Taipower
The Taipower network is a medium-size system with
longitudinal structure, covering 400-km distance from
north to south. The highest voltage level of transmission

P.-H. HO ET AL. 273
Figure 1. A three-phase balanced system model.
Figure 2. The sequence network of grid network.
lines is 345-kV. Usually it can be divided into four areas
as northern, central, southern, and eastern region. To
serve the system base load, two 951-MW generators in
the N3 power plant were in commercial operation in
1985 in the southern region. Each generator occupied
11% of the system peak load in 1985, which was 8,716-
MW. In comparison to other plants of Taipower, the four
out-linking circuits of the N3 power plant are the longest
and also in untransposed arrangement. The serious salt
fog in the southern region makes the operation limits of
the N3 power plant more conservative than others.
Figure 3 shows the 345-kV transmission system of the
southern region in 1985. The N3 power plant at BUS1 is
located at the southern tip of Taiwan. The generator un-
balanced currents are influenced by the geographic ar-
rangement of conductors of the long transmission lines.
BUS4 was connected to BUS1 through circuits A and
B on a 92-km co-towered double-circuit line. BUS3
was connected to BUS1 through circuits C and D
on a 128-km co-towered double-circuit line. To reduce
the NSC, the non-perfectly symmetrical RST-T’S’R’
phase arrangement was used.
The power data of Taipower in 2010 is listed in Table
1. The maximal generation capacity and the system peak
load were 38,082-MW and 31,011-MW, respectively.
The generation capacity ratio of the northern, central,
southern, and eastern region was 31%, 32.4%, 36.4%,
and 0.2%, respectively. The northern, central, southern,
and eastern region occupied, respectively, 42.5%,
25.89%, 28.65%, and 1.2% of the total system peak load.
Since the unbalance between generation and load, elec-
tric power should be delivered from the southern region
to the northern region. Each 951-MW generator in the
N3 power plant only occupied 3% of the system peak
load in 2010.
Due to maintenance requirement, the double-circuit
lines were opened into single-circuit conditions occa-
sionally. The single-circuit conditions make the NSC
more serious than that of double-circuit conditions. To
serve the regional load center, the 345-kV transmission
lines in the southern Taipower network in 2010 is shown
in Figure 4. BUS2 is placed between BUS1 and BUS4.
Figure 3. The 345-kV transmission lines of the southern
Taipower network in 1985.
Figure 4. The 345-kV transmission lines of the southern
Taipower network in 2010.
Table 1. Power data (MW) of Taipower in 2010.
Area GenerationLoad Balance Line loss
Northern11,565 13,180 −1,812 158
Central 9,682 8,201 1,368 155
Southern10,144 9,131 876 142
Eastern 80 499 −432 13
Total 31,471 31,011 0 469

P.-H. HO ET AL. 274
There are five 345-kV overhead transmission circuits
under consideration, where each transmission capacity is
2,187-MVA. A shorter 60-km co-towered double-circuit
line connects BUS2 and BUS1 through circuits A and B.
The circuit C on the other line also connects BUS2 and
BUS1. BUS 3 is connected to BUS 1 and BUS2 through
circuits D and E, respectively. So C and D, C and E, and
D and E are partially co-towered, respectively. The long-
est circuit is the 128-km circuit D.
3.2. Conductor Arrangement in Transmission
Lines
Figure 5 shows the one-line diagram of the study system,
where four circuits, A, B, C, and D, out-link from BUS1.
Two 951-MW generators are connected to BUS1. Circuit
E connects BUS2 and BUS3. The non-perfectly symmet-
rical RST-T’S’R’ arrangement, as shown in Figure 6(a),
is used to reduce the NSC. However, in some conditions,
the asymmetrical RST-R’S’T’ arrangement, as shown in
Figure 6(b), may be used. The current direction of the
RST circuit may be same or different with that of the
R’S’T’ or T’S’R’ circuit. Considering the difficulty in
the construction of the Taipower transmission network,
the double-circuit lines are untransposed. Figure 7
shows the conductor arrangement of the five circuits.
Circuits A and B are co-towered and have the same cur-
rent direction. The conductors of circuits A and B are in
RST-T’S’R’ arrangement. Circuits C, D, and E are par-
tially co-towered. The co-tower conductors of C and D,
C and E, and D and E are, respectively, in RST-T’S’R’,
RST-R’S’T’, and T’S’R’-RST arrangement. In normal
condition, the current direction of C and E are different,
which reduces the NSC for RST-R’S’T’ arrangement.
3.3. N3 Limit and NSC Relay Setting
The ability of a generator to withstand NSC is specified
in [20] and [21]. According to [20], the percentage per-
missible withstanding continuous NSC of generators
from 351-MVA to 1,250-MVA is given by

28350300 %IMVA  (7)
The ability of a generator to accommodate NSC is de-
scribed by




10 0.00625800KMVA (8)
The longest permissible withstanding operation time is
obtained by
2
2
tKI (9)
The N3 power plant limits of NSC (N3 Limit) and that
Figure 5. The single-line diagram of N3 power plant BUS1
and the 345-kV out-linking lines (arrows indicate flow di-
rections in the N-0 condition).
Figure 6. The conductor space arrangement (a) non-per-
fectly symmetrical RST-T’S’R’. (b) asymmetrical RST-
R’S’T’.
R
S
TR'
S'
T' R
S
TT'
S'
R'
R
S
TR'
S'
T'
R
S
TR'
S'
T' R
S
TR'
S'
T'
R
S
T
T'
S'
R'
R
S
T
T'
S'
R'
R S TT' S' R'R S TT S R
R
S
T
T'
S'
R'
A
B
C
D
E
D
ABC E
E D
CD
A B
ABC E
BUS1
BUS3
BUS2
Figure 7. Conductors arrangement of the 345-kV out-link-
ing transmission lines of N3 power plant BUS1.d
of the IEEE standard are listed in TABLE II. In this
study, each generator rating is 1,057.5-MVA. Then, by
the IEEE standard, the permissible withstanding con-
tinuous NSC is 5.64%, and the K value of the generators
to accommodate NSC is 8.39. However, it is shown in
TABLE II that the N3 Limit is stricter than the IEEE
standard. By the way, by the N3 Limit, when the four
345-kV out-linking circuits are in N-2 conditions, the
generator should be reduced to 75% loading for the op-
eration safety of the power grid, ignoring the values of
NSC.

P.-H. HO ET AL. 275
Table 2. The NSC limits of generator in N3 Limit and IEEE
Standard.
N3 Limit IEEE Standard
percentage permissible withstand-
ing continuous NSC 5% 5.64%
ability of a generator to accom-
modate NSC (K) 7 8.39
generator loading in N-2 condition
of four out-linking circuit 75% －
4. Assessment Method
4.1. NSC Expression
The expression of NSC is based on [20] and [21]. The
normalized NSC of transmission circuits is defined as
2,
100%
L
Negativesequencecurrent
IPositivesequencecurrent  (10)
For the generators, it is defined as
2,
100%
G
Negative sequence current
IRated stator current 
(11)
4.2. EMTP Models
The analysis models were established in EMTP. The
LINE CONSTANTS supporting routines of EMTP could
be used to solve the steady-state problems, considering
complicated coupling effects under the power frequency
[22]. According to the data about tower types, geometri-
cal space, line material, conductor specification, and
length, the transmission circuit matrices [R], [L], and [C]
are calculated.
The standard type A, 345-kV steel towers are used.
The cross section of the power line corridor is shown in
Figure 8. The resistance of the tower to ground is 10-20
ohms. The characteristics of conductors are listed in Ta-
ble 3. The conductor type and circuit length are listed in
Table 4. The 795-MCM (26/7) ACSR/AW conductors
are used. The radius of the conductor is 2.8143 cm. The
DC resistance is 0.064907 ohm/km in 20 degrees Celsius.
The bundling and skin effects are considered. The radius
of the overhead grounded wire is 1.632 cm. Its DC resis-
tance is 0.275 ohm/km in 20 degrees Celsius.
Each 23.75/345-kV, △-Y connected main transformer
of generators consists of three single-phase units. The
rating of each single-phase unit is 336-MVA. The equi-
valent impedance, Zps, is 0.2289 + j17.0256 ohm/phase.
The exciting admittance is 273.43-j825.64 micro-siemens
/phase. In the open-circuit test, there are 100% voltage,
0.075% current, and 170-kW loss. In the short-circuit test,
there are 14.4% impedance and 660-kW loss.
4.3. Adjustment of EMTP Model by WAMS
The data in the EMTP model were adjusted by the wide
area measurement systems (WAMS). In order to monitor
the dynamic and transient behaviors of the 345-kV net-
work, Taipower had installed 10 phasor measurement
units (PMU). Figure 9 shows the schematic arrangement
of the WAMS hardware. The functions include power
system real-time monitoring, analysis of dynamical be-
haviors, fault recording, and steady state analysis of
phasor recording.
The block diagram of the PMU in WAMS is shown in
Figure 10. The PMU is composed of the signal condi-
Figure 8. Cross section of the 345kV power line corridor.
Figure 9. Schematic arrangement of the WAMS hardware.
Figure 10. Block diagram of synchronous phasor measure-
ment unit.

P.-H. HO ET AL. 276
Table 3. Parameters of 345-kv conductors.
Phase Radius (cm) DC esistance (ohm/km) Horiz (m)Vtower (m)Vmid (m)Separ (cm) Per Phase Conductor Number
R 1.408 0.064907 −7.05 34.35 30.35 40 4
S 1.408 0.064907 −7.4 25.45 21.45 40 4
T 1.408 0.064907 −7.8 16.5 12.5 40 4
R’ 1.408 0.064907 7.8 16.5 12.5 40 4
S’ 1.408 0.064907 7.4 25.45 21.45 40 4
T’ 1.408 0.064907 7.05 34.35 30.35 40 4
0 0.816 0.275 −8 45.35 41.35 0 1
0 0.816 0.275 8 45.35 41.35 0 1
Table 4. Conductor type and Length of 345-kv circuit.
Circuit Conductor Type Length (km)
A ACSR795Q/26 60
B ACSR795Q/26 60
C ACSR795Q 60
D ACSR795Q 128
E ACSR795Q 80
tioning unit (SCU), the measurement unit (MU), and the
satellite signal synchronizing unit (SSU). To acquire the
phase angles and bus frequency data among substations,
the time stamp of the PMU is the same as the global po-
sitioning system (GPS). The data are then computed by
using the discrete Fourier transform algorithm on a
common time base. The symmetrical components of
voltages and currents are computed from the instantane-
ous values.
In this study, the data from WAMS of the four
out-linking circuits at BUS1 and the inter-linking circuit
between BUS2 and BUS3, as shown in Figure 5, were
adopted. The measurement values from WAMS are used
to correct the models in EMTP. Table 5 gives six meas-
urement cases to adjust the EMTP models. The models
are modified until the simulation results and the meas-
urement values are approximated.
5. Assessment Results with Two Generators
Connected
Two generators are connected to BUS1. The output
power of each generator is 951-MW. Considering the
four out-linking circuits from BUS1, the N-0 condition
means that the four circuits are used normally. The N-1,
N-2, N-3, and N-4 conditions mean that 1, 2, 3, and 4
circuits, respectively, have been opened. Then, there are
32 study cases, as listed in Table 6. The I2,G and I2,L were
calculated by EMTP. The results of the longest permissi-
ble withstanding operation time were calculated by the
N3 Limit and IEEE Standard.
5.1. N-0 and N-1 Conditions
In N-0 conditions, such as Cases 0-A and 0-B, I2,G are
1.665% and 2.016%, respectively. In N-1 conditions,
there are 8 cases, i.e., Case 1-1A to Case 1-4B, and I2,G
lies in 1.952% to 3.297%. Then, the generators would
not be affected by NSC.
5.2. N-2 Conditions
In N-2 conditions, there are four cases with co-towered
arrangement. They are Case 2-1A, Case 2-1B, Case 2-6A,
and Case 2-6B. In these cases, I2,G lies in 1.915% to
3.532%. The generators would not be affected by NSC.
Table 5. Measurement cases of circuits for EMTP model
adjustment.
Circuit WAMS
Case
ABCDE Date Time
0-A XXXXX 2007.08.02 16:50:00
0-B XXXX－ 2007.03.27 04:20:00
1-1A －XXXX 2007.05.21 02:48:00
1-2AX －XXX 2007.02.12 08:00:00
1-3AXX －XX 2007.05.18 06:24:00
1-4AXX X －X 2006.10.11 07:52:00
Note: “X” circuits closed, “－” circuits opened

P.-H. HO ET AL. 277
Table 6. I2,G, I2,L, and the longest permissible withstanding operation time of generators.
I2,L (%)
Out-linking circuit Inter-linking
circuit
Longest permissible withstanding
operation time (Min)
I
2
Condition Case
I2,G (%)
A B C D E N3 Limit IEEE Standard
0-A 1.665 0.76 1.31 1.97 3.39 2.79 ∞ ∞
N-0
0-B 2.016 0.42 0.83 0.82 6.27 － ∞ ∞
1-1A 3.071 － 4.78 1.42 2.94 2.16 ∞ ∞
1-1B 3.297 － 4.44 0.54 4.93 － ∞ ∞
1-2A 3.013 4.55 － 1.58 2.73 2.16 ∞ ∞
1-2B 3.233 4.29 － 0.65 4.96 － ∞ ∞
1-3A 2.339 1.04 1.42 － 6.32 0.66 ∞ ∞
1-3B 2.733 0.71 0.73 － 7.94 － ∞ ∞
1-4A 2.461 1.01 1.08 5.91 － 9.04 ∞ ∞
N-1
1-4B 1.952 0.3 0.65 5.38 － － ∞ ∞
2-1A 3.532 － － 3.64 3.33 5.44 ∞ ∞
2-1B 2.284 － － 3.42 4.32 － ∞ ∞
2-2A 5.383 － 4.94 － 6 3.19 40.26 ∞
2-2B 5.545 － 4.8 － 6.63 － 37.98 ∞
2-3A 4.776 － 4.76 4.32 － 10.42 ∞ ∞
2-3B 3.986 － 4.14 3.7 － － ∞ ∞
2-4A 5.188 4.76 － － 5.89 1.64 43.38 ∞
2-4B 5.352 4.56
－ － 6.67 － 40.74 ∞
2-5A 4.715 4.5 － 4.34 － 10.54 ∞ ∞
2-5B 3.974 3.91
－ 3.88 － － ∞ ∞
2-6A 2.492 3.45 2.61 － － 9.35 ∞ ∞
N-2
2-6B 1.915 1.81 2.05 － － － ∞ ∞
3-1A 9.564 － － － 9 29.76 12.78 15.29
3-1B 8.661 － － － 8.2 － 15.54 18.64
3-2A 6.246 － － 6.3 － 11.34 29.88 35.84
3-2B 5.888 － － 5.82 － － 33.72 40.33
3-3A 6.514 － 6.36 － － 12.47 27.48 32.95
3-3B 5.95 － 5.8 － － － 32.94 39.50
3-4A 6.167 6.23 － － － 12.31 30.66 36.77
N-3
3-4B 5.776 5.69
－ － － － 34.98 41.91
4-1A － － － － － － － －
N-4
4-1B － － － － － － － －
Note: “－” circuits opened

P.-H. HO ET AL. 278
Also in some N-2 conditions at different towers and with
circuit D being opened, such as Case 2-3A, Case 2-3B,
Case 2-5A, and Case 2-5B, I2,G lies in 3.974% to
4.776%. The generators also would not be affected by
NSC.
In the other N-2 conditions at different towers and
with the short length circuits being opened, the 60-km
long circuit A or B and the 128-km long circuit D are
closed. There are Case 2-2A, Case 2-2B, Case 2-4A, and
Case 2-4B, as shown in Figure 11. The value of I2,G lies
(a)
(b)
(c)
(d)
Figure 11. N-2 conditions at different towers with short
length circuits being opened (a) Case 2-2A (circuits A and C
being opened), (b) Case 2-2B (circuits A, C, and E being
opened), (c) Case 2-4A (circuits B and C being opened), (d)
Case 2-4B (circuits B, C, and E being opened).
in 5.188% to 5.545%. According to the N3 Limit, the
generators would be affected by NSC in these four cases.
The longest permissible withstanding operation time of
the generators is finite and lies in 37.98 minutes to 43.38
minutes. However, according to the IEEE Standard, the
generators would not be affected by NSC.
5.3. N-3 Conditions
Figure 12 shows Case 3-1A and Case 3-1B for the N-3
conditions with short length circuits being opened. In
these conditions, only one circuit is connected to BUS1.
The value of I2,G is 9.564% and 8.661%, respectively.
Six cases are given in Figure 13 for the N-3 conditions
with long circuit D being opened. The value of I2,G lies in
5.776% to 6.246%. According to the N3 Limit or the
IEEE Standard, the generators would be affected by NSC.
However, according to the N3 Limit, the longest permis-
sible withstanding operation time of the generators is still
smaller. By the N3 Limit, the shortest operation time is
12.78 minutes. But it is 15.29 minutes by the IEEE
Standard.
5.4. N-4 Conditions
When the four out-linking circuits are opened, such as
Case 4-1A and Case 4-1B, it is impossible to transmit the
power of generators.
(a)
(b)
Figure 12. N-3 conditions with short length circuits being
opened (a) Case 3-1A (circuits A, B, and C being opened),
(b) Case 3-1B (circuits A, B, C, and E being opened).

P.-H. HO ET AL. 279
(a) (b)
(c) (d)
(e) (f)
Figure 13. N-3 conditions with long length circuits being opened (a) Case 3-2A (circuits A, B, and D being opened), (b) Case
3-2B (circuits A, B, D, and E being opened), (c) Case 3-3A (circuits A, C, and D being opened), (d) Case 3-3B (circuits A, C, D,
and E being opened), (e) Case 3-4A (circuits B, C, and D being opened), (f) Case 3-4B (circuits B, C, D, and E being opened).
6. Reducing Generator Loading in N-3
Conditions
In the N-3 conditions, there is only one out-linking cir-
cuit. A 2,187-MVA circuit is still enough to transmit the
electric power of two 951-MW generators. However,
either by the N3 Limit or the IEEE Standard, the genera-
tors would be affected by NSC. To protect the generators,
the loading of each generator could be reduced. Table 7
gives the maximal generator loading in N-3 conditions to
avoid the effect of NSC according to the IEEE Standard.
For example, the loading of each generator should be
reduced to 59% in case 3-1A.
7. Assessment Results with One Generator
Connected
If one of the two generators is under maintenance, only
one generator is connected to BUS1. Table 8 gives the
assessment results with only one 951-MW generator. In
all of the 32 cases, the generator would not be affected
by NSC.
Table 7. Maximal generator loading in N-3 conditions to
avoid effect of NSC.
Case
Output power of
each generator
(MW)
Loading
(%)
I2,G
(%)
Longest permissible
withstanding
operation time (Min)
3-1A561 59 5.63 ∞
3-1B619 65 5.63 ∞
3-2A859 90 5.63 ∞
3-2B911 96 5.63 ∞
3-3A823 87 5.63 ∞
3-3B901 95 5.63 ∞
3-4A870 91 5.63 ∞
3-4B929 98 5.63 ∞

P.-H. HO ET AL. 280
Table 8. I2,G, I2,L, and the longest permissible withstanding operation time with only one generator connected.
I2,L (%)
Out-linking circuit Inter
linking circuit
Longest permissible withstanding
operation time (Min) I2
Condition Case
I2,G
(%)
A B C D E N3 Limit IEEE Standard
0-A 0.749 0.342 0.590 0.887 1.526 1.256 ∞ ∞
N-0
0-B 0.907 0.189 0.374 0.369 2.822 － ∞ ∞
1-1A 1.382 － 2.151 0.639 1.323 0.972 ∞ ∞
1-1B 1.484 － 1.998 0.243 2.219 － ∞ ∞
1-2A 1.356 2.048 － 0.711 1.229 0.972 ∞ ∞
1-2B 1.455 1.931 － 0.293 2.232 － ∞ ∞
1-3A 1.053 0.468 0.639 － 2.844 0.297 ∞ ∞
1-3B 1.230 0.320 0.329 － 3.573 － ∞ ∞
1-4A 1.107 0.455 0.486 2.660 － 4.068 ∞ ∞
N-1
1-4B 0.878 0.135 0.293 2.421 － － ∞ ∞
2-1A 1.589 － － 1.638 1.499 2.448 ∞ ∞
2-1B 1.028 － － 1.539 1.944 － ∞ ∞
2-2A 2.422 － 2.223 － 2.700 1.436 ∞ ∞
2-2B 2.495 － 2.160 － 2.984 － ∞ ∞
2-3A 2.149 － 2.142 1.944 － 4.689 ∞ ∞
2-3B 1.794 － 1.863 1.665 － － ∞ ∞
2-4A 2.335 2.142 － － 2.651 0.738 ∞ ∞
2-4B 2.408 2.052 － － 3.002 － ∞ ∞
2-5A 2.122 2.025 － 1.953 － 4.743 ∞ ∞
2-5B 1.788 1.760 － 1.746 － － ∞ ∞
2-6A 1.121 1.553 1.175 － － 4.208 ∞ ∞
N-2
2-6B 0.862 0.815 0.923 － － － ∞ ∞
3-1A 4.304 － － － 4.050 13.392 ∞ ∞
3-1B 3.897 － － － 3.690 － ∞ ∞
3-2A 2.811 － － 2.835 － 5.103 ∞ ∞
3-2B 2.650 － － 2.619 － － ∞ ∞
3-3A 2.931 － 2.862 － － 5.612 ∞ ∞
3-3B 2.678 － 2.610 － － － ∞ ∞
3-4A 2.775 2.804 － － － 5.540 ∞ ∞
N-3
3-4B 2.599 2.561 － － － － ∞ ∞
4-1A － － － － － － － －
N-4
4-1B － － － － － － － －
Note: “－” circuits opened

P.-H. HO ET AL. 281
8. Discussion
1) Since the percentage of each generator in the N3
power plant occupying the system peak load has reduced
from 11% to 3%, Taipower already has enough spinning
reserve to sustain the outage of the N3 power plant. By
the way, the two out-linking 345-kV circuits in N-2 con-
ditions still have over 100% redundant capacity to
transmit the full loading power of the plant. The re-
quirement in the N3 Limit of reducing generator loading
to 75% in N-2 conditions is suggested to be deleted.
2) By the IEEE Standard, the generators of the N3
power plant in the N-2 conditions would not affected by
NSC. It is still suggested that the percentage permissible
withstanding continuous NSC and the ability of a gen-
erator to accommodate NSC can follow the IEEE stan-
dard. Then the two generators can work in full loading in
N-2 conditions.
3) In N-3 conditions, there is only one out-linking cir-
cuit. It still has enough capacity to transmit the full load-
ing power of the two generation in the plant. However,
either by the N3 Limit or the IEEE Standard, the effects
of NSC can not be ignored if the two generators are op-
erated in full loading.
4) Since the N3 is a nuclear power plant, special safety
operation rules are required. By the transmission system
planning rules of Taipower, a nuclear power plant should
have more then two out-linking circuits. It is suggested
that if the total system generation capacity is enough, the
N3 power plant had better avoid being operated in N-3
conditions. If the total system generation capacity is not
enough, the generator loading should be reduced. An-
other feasible approach is to shut down one of the gen-
erators.
9. Conclusions
The levels of NSC from the four asymmetrical
out-linking circuits have been examined in details. The
effects of NSC on the operation of generators have been
investigated by using the N3 Limit and the IEEE Stan-
dard. It is revealed that the N3 Limit is stricter than the
IEEE standard. It is found from the simulation results
that, by the IEEE Standard, the effects of NSC in N-0,
N-1, and N-2 conditions could be ignored. If the two
generators are operated in full loading, the effects on
NSC in N-3 conditions should be paid attention. Since
the Taipower already has enough spinning to sustain the
outage of the N3 power plant, it is suggested that the
limits of NSC in the IEEE Standard could be adopt. The
generators could work normally in N-2 conditions. Since
the N3 is a nuclear power plant, special safety operation
rules are still required. The suggestions on the plant op-
eration in N-3 conditions are given, since there is only
one out-linking circuits. The simulation results and sug-
gestions in this study can increase the economic benefit
of the power plant.
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