Energy and Power Engineering, 2013, 5, 1192-1197
doi:10.4236/epe.2013.54B226 Published Online July 2013 (
A Study on the Transmission Line Transient Protection
Containing Static Synchronous Compensator
Kezhen Liu1,2, Ting Liu2, Hongchun Shu2, Jilai Yu1
1Department of Electrical Engineering, Harbin Institute of Technology, Harbin, China
2School of Electrical Engineering, Kunming University of Science and Technology, Kunming, China
Received February, 2013
This paper focuses on the transient protection of transmission line with FACTS Devices; the principle and mathematical
model of static synchronous compensator (STATCOM) has been analyzed. And feasible control strategy has been pro-
posed considering the voltage source inverter (VSI) as a core part of the FACTS components. The simulation results
verify the rightness using electromagnetic transient simulation software PSCAD/EMTDC to set up electromagnetic
transient model.
Keywords: FACTS; Transient Protection; Direction Elements; Harmonic
1. Introduction
With the emergence of a large number of FACTS devices
in the power grid, transient process has great changes
after power system fault, which affect the reliability of
traditional distance protection; additional FACTS ele-
ment mutual coordination and control strategy is more
and more complex, while device protection configuration
can also affect the line relay protection reliability [1].
There are two ideas of contain STATCOM transmis-
sion line protection at domestic [2, 3]. One is upgrading
the original protection, some scholars conducting re-
search, its principle is based on the impact of STATCOM
on distance protection operating characteristics, to
achieve adaptive distance protection to contain comple-
ment components, considerable progress has been made.
In literature [4] stat COM installed in a different location,
protection measurements at the fault line impedance
changes were studied, indicating that STATCOM in-
stalled on measuring the impedance of the influence is
bigger, thereby affecting the reliability of distance pro-
tection action. As only for the improvement of traditional
protection can not overcome its inherent defects, poor
reliability and sensitivity, and therefore it is urgent need
for new principles and scheme that transmission lines
with and complement component protection [5, 6].
Another idea is to use line fault rich transient informa-
tion, and use effective mathematical tools to study with
and complement component transmission line protection.
The basic idea: after transmission line failure, we can
think in fault point additional excitation voltage source,
which will spread high frequency traveling wave to the
ends of line, a large number of fault information ex-
tracted by traveling wave which was detected , accord-
ingly to detect fault properties, compared to the tradi-
tional protection in a shorter time for detection failure
The second method is used in this paper, proposed
border protection which based on the power ratio of tran-
sient current high frequency /low frequency.
Transient Protection usually divided into two: travel-
ing wave protection and boundary protection. In litera-
ture[9] STATCOM installed in the line at different loca-
tions, its harmonic content in the case of steady-state and
fault analysis, and clearly pointed out that the harmonic
content of the protection installed in line middle at mi-
nimal impact. Mathematical tools developed in the late
1980s - the wavelet analysis brought hope to the protec-
tion and development of the traveling wave re malfunc-
tion signal can accurately extract all kinds of information,
but also to distinguish between varieties of white noise
interference, is considered the most appropriate analyti-
cal tools of the traveling wave signal [10, 11]. In litera-
ture [1] the criteria for the installation of transmission
lines of STATCOM, the use of the HHT method of spec-
trum analysis of the transient current line fault, which
come to the region and outside recognition, but its reli-
ability has yet to be tested.
In the 1990s, the University of Bath, UK A. T. Johns
and BO Zhiqian groundbreaking No channel Transient
protection principle to distinguish between protected
areas, both inside and outside the fault [12] by detecting
Copyright © 2013 SciRes. EPE
K. Z. LIU ET AL. 1193
the fault noise in the frequency domain characteristics.
As a result of the fault transient signal, compared with
the traditional line protection, its speed mobility greatly
improved [13]. Transient protection no channel of trans-
mission lines only single-ended electrical quantities can
accurately determine the fault occurs within or outside
having certain superiority.
Based on the above analysis, we propose the use of
fault transient information constitute adapt to the amount
of transmission line transient protection method with and
complement the device has a very broad prospects, will
be a hot research and breakthrough point of the line pro-
2. The Spectrum Characteristic Analysis of
Transmission Lines with STATCOM
2.1. Analysis of STATCOM Harmonic Spectrum
The study containing STATCOM transmission line pro-
tection, distance protection, due to STATCOM input, the
injected current change measured impedance, making the
distance protection operating characteristics change, fail-
ure protection. From a traveling wave perspective,
STATCOM can be equivalent to an inductor, assume that
the per-unit value of approximately 0.1Pu , Leq = 0.8H,
therefore the line wave within STATCOM reactive pow-
er compensation device equivalent impedance much can
be regarded as an open circuit, and therefore does not
affect the traveling wave in the its refractive coefficient,
shown in Figure 1.
Because The STATCOM components as a core com-
ponent of its turn-on and turn-off of high power elec-
tronic devices based on voltage source inverter will pro-
duce harmonics, when the pulse voltage source inverter
model will produce 6k ± 1 this characteristic harmonics
and other non-characteristic harmonic the SPWM control
using carrier than 33, also in the future bring 31, 35 infe-
rior odd harmonics [14].
Whether STATCOM to install the line, will produce
31, 35 sub-harmonic, but different installations brought on
Figure 1. STATCOM equivalent circuit of the range of the
wave of a line.
the harmonic content was significantly different. When
installed in the line of the head and midpoint, 31 times
harmonic content is highest. When installed at the end of
the line, the 35th harmonic content is highest. STAT-
COM installed in line midpoint, its harmonic compo-
nents is the smallest.
2.2. Transmission Line Boundary Frequency
Traps for EHV transmission lines are used to achieve a
power line carrier communication, which is characterized
by a short circuit presents a high impedance, high fre-
quency and low frequency presents a low impedance,
power frequency, can be regarded as short circuit. The
wave trap installed at both ends on the line spread only in
the line so that the high-frequency signal, which is not
spread through lines boundary to peer line. Therefore,
according to the traps for blocking high frequency and
the low frequency characteristics constitute a boundary
element to distinguish the region, outside the fault, but
using single-ended electrical quantities they can protect
the line length, clear physical concept without communica-
tion channel, low investment, and high reliability [15].
The impedance value of wave trapper is small in the
low frequency band (0-10kHz); rather large value in the
band of line trap (58-126kHz), the impedance presented
to the band signal difficult wave trap. The wave trap on
different frequencies exhibit different impedance values,
lead to different frequency components of the electrical
quantity with different degree of attenuation.
Shown in Figure 2, the CT installed in the M bus side,
due to the influence of traps and stray capacitance that
between boundary bus and ground, during external faults,
the generated high-frequency components of the transient
current through the bus and trapper have a large number
of attenuation, while during internal faults, the current
high-frequency components almost no attenuation, can
not through lines boundary and this difference may con-
stitute a protection criterion. And the use of high-fre-
quency components of the transient current transient
protection can take full advantage of the current trans-
former has a good broadband Transfer Characteristics
effective pass variable current fault traveling wave wi-
deband signal.
For transmission lines, which do not install trapper in
the bus terminal, have a greater relationship of the reflec-
tive and refractive coefficient of line boundary with the
stray capacitance of bus to ground, When the stray ca-
pacitance the greater the smaller refractive index, the
greater the reflection coefficient, and thus the greater
through the boundary of the high-frequency attenuation.
In contrast, when the bus to ground stray capacitance is
small, the boundary of the high-frequency attenuation is
also smaller.
Copyright © 2013 SciRes. EPE
Copyright © 2013 SciRes. EPE
Figure 2. Transmission line model.
In 500kV transmission line, both ends of the line often
equipped with trapper, the high frequency signal through
the boundary having serious attenuation in the band of
line trap, when the internal fault occurs, the high-fre-
quency signal is locked up in the region, in external faults,
the high-frequency signal can not through the boundary
and reach the protected areas. Figure 2, MN for the pro-
tection of research line, when the external signals enter
into the protected line, it will through the equivalent ca-
pacitance of bus to ground and the two resistance filter
subject rule, according to Peterson principle, the equiva-
lent circuit diagram shown in Figure 1, and the reflective
and refractive coefficient of line boundary is:
() ()
Hs zz zz
 (1)
()()(1/ )(1)1
KsHsz zzz z
 
11 2
//( )//()
Z'= Z+ZT+(Z+ZT)//ZC (4)
Where, z isthe line the wave impedance, ZC is the equiv-
alent impedance of the bus capacitance to ground, ZT is
the equivalent impedance of the trapper.
The transmission line after retrofitting trapper, the line
boundary of the high-frequency (50kHz ~ 100kHz) has
the larger reflection coefficient, the smaller refractive
index, thus making the high-frequency signal attenuation
seriously through the boundary and can be found that it
has the same frequency band (58kHz ~ 126kHz)with
trapper; for the low frequency band (0 ~ 10kHz), the
boundary has little influence.
3. Protection Criterion of Transmission Line
Containing the STATCOM Elements
Through the analysis of the boundary conditions, the
compensation device when the line is not installed, when
the external fault, which produces a transient high-fre-
quency component will attenuate plenty at the boundary,
the higher the frequency, the greater the attenuation, the
low frequency terms, almost did not happen attenuation,
all entered the protected circuit. Since the role of the
boundary, the protection at the high and low frequency
energy ratio detected smaller. Region fails, due to line
border will produce high-frequency component of the
lock in the region, not to peer transmission lines in the
area, the high and low frequency energy ratio is large,
and far greater than the ratio of external fault. Signal
wavelet coefficients energy defined J scale wavelet co-
efficients energy expression as follows:
 (5)
where: Wj is j scale wavelet coefficients, the N is sam-
pling points within the window. By analysis of the char-
acteristics of the region, external fault can constitute dis-
tinguish between lines, external failure criterion based on
the high and low frequency energy than the transient
 (6)
min2/, 2/
tlcl c (7)
wherein: EH and EL, respectively, for the low scales and
high scales of the wavelet coefficients energy, if λ>λ
set,it is judged that the fault occurred in the line internal,
on the contrary, it can be determined for the external
fault. This criterion there are several issues need to be
emphasized: First, the data time window, lPM, lNQ respec-
tively, for the protection of the installation at the dorsal
line, and the forward lower line length, dorsal or subor-
dinates do not line connection, it will as unlimited long.
General, EHV transmission lines the Δt is taken as 1ms;
Second, the high and low frequency band range determi-
nation, corresponding frequency band in the first dimen-
sion of the high-frequency band selected wavelet trans-
form [62.5kHz, 125kHz] signal, namely, the wave trap
blocking band. Boundary for the band 10kHz or less
transient protection of existing single-ended general the
select band signal as its low-frequency part; Third, the
protection given value of the setting, select, in which the
energy of the high and low frequency end of the line bus
K. Z. LIU ET AL. 1195
fault ratio, k is the reliability coefficient, whose value is
1.2. It needs to be emphasized is that due to the end of
the protected line bus fault to tuning the threshold value,
if the first end of bus failure (reverse zone external fault)
to tuning, the boundary element protection scope than
failure at terminal bus bar tuning the threshold value of
the scope of protection is greatly reduced, this is because
the transient high-frequency component is generated
when the first end of the bus fault to reach the protected
measured directly via line boundary at the high-fre-
quency component when the terminal bus fault to go
through the line boundary, and transmission the attenua-
tion of the line in order to arrive at the protective meas-
ure, tuning by the end of the first bus fault threshold will
be greater than the setting value when the end of the bus
fault, narrowing the scope of protection.
For installation of STATCOM Transmission Line has
been known through the analysis, since in the case of
steady-state and the fault transient will be generated
harmonics, its high-frequency component, but is located
in the above-said low-frequency harmonics generated
part, this will be inevitable criterion of its high and low
frequency impact. Since the selected time window length
is 1ms, within this time window, STATCOM control
system does not occur in response, does not impact the
current waveform of the protected premises measured.
Therefore, the line has STATCOM installed no channel
Transient protection is mainly due to the impact of the
harmonics generated, the high-frequency part of the cri-
terion in the selection based on high and low frequency
energy ratio is 62.5kHz ~ 125kHz, in this band, the pow-
er electronic components and harmonics generated by the
SPWM carrier than has been completely attenuation, will
not affect the high-frequency part of the chosen. This
will inevitably due to the low-frequency part of the gen-
eral 10kHz carrier than harmonics generated mainly be-
tween 39 to 78 times, harmonic power electronic devices
mainly 5,7,9 sub harmonic the impact to low frequency
energy, resulting in the calculation of the criterion value
decreased, making the region the fault may misjudge
external fault criterion of failure.
For the installation of the STATCOM single-ended
transmission lines mal function transient protection. One
idea is keeping the low frequency band unchanged, re-set
the setting value, but the relationship between different
fault harmonics and fault conditions exist distance and
transition resistance case, the harmonic component is
changed greatly, so that protection action threshold tun-
ing becomes difficult; lines in different positions failure
occurs, the protection of the harmonic content of the
premises detected there will be greater difference. Situa-
tion is different in the steady state, in case of failure, 7,9
The harmonic large, followed by the harmonics gener-
ated by the carrier ratio. In addition, the harmonic com-
ponents produced by different fault distance are quite
different line near the end of the fault harmonic compo-
nent. Therefore, the high and low frequency energy than
the band selection influenced also larger, making protec-
tion criterion threshold value is not easy tuning. Can be
found in the figure, the harmonic component in 10 to 20
times between the harmonics less affected by the fault
location, and its amplitude is small, the impact to the
criterion is also the smallest and which is located in the
wavelet transform section 8 layer band, as the high and
low frequency energy than the low-frequency part, i.e.
between 488.28Hz ~ 976.56Hz band, this band are less
affected by harmonics, is conducive to the tuning of the
threshold value.
4. Digital Simulation and Analysis Based on
Transient Cu rrent Single- ended
Simulation system is shown in Figure 2, bus bar stray
capacitance value 0.1μF, When line A phase to ground
fault occurs, the fault initial angle of 90˚, the transition
resistance of 10Ω, a sampling rate of 250kHz, when
window length of 1ms, measured transient current pro-
tection at high and low frequency energy than the fault
distance relationship as shown in Fig ure 4.
Figure 4 shows the installation of STATCOM trans-
mission lines, less affected by harmonic scale 8 band as
the low-frequency, by using the Figure 5. Criterion area,
outside the characteristic differences is significant, can
Figure 3. Equivalent circuit.
050 100 150 200250 300 350 400450500
x 10-3
Figure 4. λchange with the change of the fault position.
Copyright © 2013 SciRes. EPE
1020 3040 506070 8090
Figure 5. k change with the initial angle of the fault.
effectively reduce harmonic protection criterion. When
Forward outside or reverse zone external fault, because
the transient current wave to go through the line bound-
ary, and its high-frequency component attenuation severe,
causing the ratio of the low and high frequency energy
smaller, while the internal fault, due to the not via line
boundary, the high-frequency component of the basic
attenuation of high and low frequency energy ratio, this
may constitute a unit transient protection criterion. When
region fails, high and low frequency energy ratio will
change with the location of fault. The fault near the end
of the busbar, the ratio of high and low frequency energy
is minimum.
External faults, which may occur when the high and
low frequency energy than the maximum occurs at the
end bus N fault, this paper occurred bus N Phase A me-
tallic ground fault for tuning, fault initial angle 90˚ and
the calculated maximum ratio λ = 8.4e-5, setting value
λset = 1.2,λ = 1.005e-4. In accordance with the setting
value, normalization process, which defines, when k > 1
determines the fault region and vice versa for the purpose
of external faults.
Still shown in Figure 2, simulation System, the analy-
sis of the various factors that affect the discrimination
index value k. A phase to ground short circuit fault, the
fault occurs to select high and low frequency energy ratio
is smaller at the location of the fault 130km and transi-
tion resistance 20Ω, the time window length is 1ms, and
the sampling rate is 250kHz, fault initial angle varies
between 5˚ ~ 90˚ indicators discriminate value k with
fault initial angle changes as shown in Figure 6. The
figure shows, at the end of the line fault, and fault small
initial angle, index discriminating value k is still greater
than the threshold value 1, and protection can be accurate
Select high and low frequency energy ratio is rela-
tively small fault distance location 130km at fault initial
angle of 30˚, the fault transition resistance between 0 ~
300Ω 30Ω incremental change in its traversal results
are shown in Figure 7, as the figure shows, with the in-
crease of the transition resistance the discriminate index
value k decreases gradually ,and reduced modest, even if
the transitional resistance is 300Ω, still reliable identifi-
cation, can effectively overcome the traditional protec-
tion of vulnerable transition resistance, high impedance
fault can not be reliably action defects.
Based on the high-frequency component of the tran-
sient current no channel protection principle is based on
bus-to-ground equivalent capacitance Cs of the role of
high-frequency current signal, that is, the use of external
fault current of high frequency components due to the
link shunt lot of attenuation region failure when the cur-
rent high-frequency noise is almost no attenuation,
therefore, the performance of the merits of the protection
depends directly on the size of the Cs .
The actual transmission system Cs is usually between
2000pF ~ 0.1μF ,using Figure 2 for the simulation sys-
tem, line A phase to ground short-circuit fault occurs, the
transition resistance of 20Ω, the failure initial angle of
30°, 0.1 μF, 0.05μF 0.005μF when discriminate
index value k with the fault distance changes as shown in
Figures 3-4, the bus ground stray capacitance index val-
ue k, especially at the end of the line fails when the bus
ground the stray capacitance is small, the calculated in-
dex value k <1, the criterion of occurrence of false posi-
tives, failure protection.
050100 150 200 250300
Figure 6. k change with the change of the transition resis-
050100 150
Figure 7. k change with the change of the fault position.
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Copyright © 2013 SciRes. EPE
5. Conclusions
This paper focuses on analysis of the basic principles
containing the STATCOM transmission line border pro-
tection, proposed for different electricity gas, based on
high and low frequency energy than the transient current
border protection, and analyzed the of containing the
STATCOM transmission line boundary frequency char-
acteristics well STATCOM brought harmonic effects,
traps for the voltage transfer function of the amplitude-
frequency characteristics, the theoretical basis for the
protection criterion. Protection criterion proposed a large
number of digital simulations, simulation results show
that the criterion of tolerance transition resistance, energy
smaller by the location of the fault and the fault type, but
on the case of glitches angle, the end of the line fault
criterion may result in a miscarriage of justice.
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