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Article
A Novel High-Frequency Voltage Standing-Wave
Ratio-Based Grounding Electrode Line Fault
Supervision in Ultra-High Voltage DC
Transmission Systems
Yufei Teng 1 , Xiaopeng Li 1 , Qi Huang 2, *, Yifei Wang 3 , Shi Jing 2 , Zhenchao Jiang 1
and Wei Zhen 1
1
2
3
*
State Grid Sichuan Electric Power Research Institute, Chengdu 610072, China; [email protected] (Y.T.);
[email protected] (X.L.); [email protected] (Z.J.); [email protected] (W.Z.)
School of Energy Science and Engineering, University of Electronic Science and Technology of China,
Chengdu 611731, China; [email protected]
Department of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, China; [email protected]
Correspondence: [email protected]
Academic Editor: Silvio Simani
Received: 28 December 2016; Accepted: 28 February 2017; Published: 5 March 2017
Abstract: In order to improve the fault monitoring performance of grounding electrode lines in
ultra-high voltage DC (UHVDC) transmission systems, a novel fault monitoring approach based on
the high-frequency voltage standing-wave ratio (VSWR) is proposed in this paper. The VSWR
is defined considering a lossless transmission line, and the characteristics of the VSWR under
different conditions are analyzed. It is shown that the VSWR equals 1 when the terminal resistance
completely matches the characteristic impedance of the line, and when a short circuit fault occurs
on the grounding electrode line, the VSWR will be greater than 1. The VSWR will approach
positive infinity under metallic earth fault conditions, whereas the VSWR in non-metallic earth
faults will be smaller. Based on these analytical results, a fault supervision criterion is formulated.
The effectiveness of the proposed VSWR-based fault supervision technique is verified with a typical
UHVDC project established in Power Systems Computer Aided Design/Electromagnetic Transients
including DC(PSCAD/EMTDC). Simulation results indicate that the proposed strategy can reliably
identify the grounding electrode line fault and has strong anti-fault resistance capability.
Keywords: UHVDC transmission system; grounding electrode line; fault supervision; voltage
standing-wave ratio; injected current source
1. Introduction
UHVDC projects play an important role in modern long-distance electrical power delivery [1,2]
because of their capability of delivering massive electricity transmission capacity over long distances
in a controllable way [3,4]. The grounding electrode is an essential part in the UHVDC project in that
it provides the current loop from the pole to the earth and acts as an unbalance reference between
two poles. In practice, the grounding electrode is sited far from the converter station via two parallel
grounding wires, to avoid the impact of the DC magnetic bias at the converter stations [5,6]. Therefore,
the protection strategies and supervision devices for grounding electrode line faults are very important
in practical UHVDC projects.
A typical UHVDC earth electrode design can be found in [7]. Normally, the grounding electrode
line comprises two parallel lines without separate switches, therefore, the two parallel lines always
function as one line in practice. The unbalanced current protection is generally equipped for the earth
Energies 2017, 10, 309; doi:10.3390/en10030309
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Energies 2017, 10, 309
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electrode line, and improvement measures were proposed to enhance the reliability of the existing
unbalanced current protection strategy [8]. The authors in [9] proposed a fast tripping scheme by
inserting DC breakers into both grounding electrode lines. The control and protection strategies were
revised, in order to avoid the blockage of the other pole if one pole was blocked and failed to restart.
In [10], by considering the characteristics of grounding electrode line faults, it was suggested that
the unbalanced current protection system should adopt fault location and the inverse-time function.
The authors in [11] proposed to integrate differential current protections with the existing unbalanced
current protection to realize fast and accurate location of electrode grounding line faults. However, the
proposed current-based protection strategy can only be used in monopolar earth return mode and
cannot work under conditions where the UHVDC system operates in bipolar earth return mode or
monopolar metallic return mode, because there is no current in the grounding electrode line under
these operation modes. It is also noted that the monopolar earth return configuration is not commonly
used because the return current via earth may endanger the surrounding human beings or animals [12].
Therefore, current-based protection techniques are not applicable for grounding electrode lines in
practical UHVDC projects.
In order to solve the grounding electrode line fault problem in bipolar balanced operation mode,
a high-frequency impedance measurement technique for grounding electrode line fault supervision,
in which the impedance was detected by injecting a high-frequency current source, was proposed
in [13]. In such methods, the voltage variation can also be used as the indication of fault or not [14].
This approach is widely adopted in UHVDC projects because it is applicable for all the operation
modes. However, in practice it is found that such schemes may fail to operate under conditions of
earth faults via transition resistance. The incorrect operation of grounding electrode lines, due to
defects in the protection system, has caused many faults in practical UHVDC projects in recent years,
leading to great economic losses, so it is of great importance to develop an electrode grounding line
fault detection system with better performance.
In this paper, a novel fault monitoring algorithm based on the high-frequency voltage
standing-wave ratio (VSWR) principle is proposed, by combining the high-frequency signal injection
and traveling wave fault detection technique. The traveling wave based fault detection is widely
utilized because it has the ability to operate fast and overcomes the challenge from the earth fault via
high transition resistance. The characteristics of the VSWR on the long-distance lossless transmission
line are investigated, and the performance of the proposed approach is studied by comparing with
existing grounding electrode line fault detection systems under various operation conditions.
The rest of this paper is organized as follows: Section 2 briefly introduces the principle and
evaluates the performance of the existing high-frequency impedance detection method. Then Section 3
mainly presents the definition and calculation of the high-frequency VSWR. The characteristics of
the high-frequency VSWR is analyzed, and then a novel grounding electrode line fault supervision
methodology based on the high-frequency VSWR is proposed. The proposed method is verified by
conducting simulations on PSCAD/EMTDC platform in Section 4. Finally, conclusions are presented
in Section 5.
2. Overview of High-Frequency Impedance Supervision Technique for Grounding Electrode
Line Fault
2.1. The Operation Principle
Figure 1 illustrates a typical grounding electrode line impedance supervision (ELIS) system
configured within a UHVDC system. ELIS is used to continuously monitor the status of the electrode
line. A high-frequency AC current is injected into the electrode line and the voltage to ground at
the injection point is measured. Following the practice in all HVDC projects of the Chinese state
grid, the frequency of the injected signal is chosen as 13.95 kHz [15]. The impedance is calculated
from the current and voltage and is used as a measure of the conditions of the electrode line. At the
two terminals of the grounding electrode line, a blocking filter is installed to block the injected
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high-frequency current. At the electrode station side terminal the blocking filter is also provided
high-frequency current. At
terminal
thethe
blocking
filter
is also
provided
with
high-frequency
At the
theelectrode
electrodestation
stationside
side
terminal
blocking
filter
is also
provided
with a resistor, whose resistance corresponds to the characteristic impedance of the electrode line.
a
resistor,
whose
resistance
corresponds
to
the
characteristic
impedance
of
the
electrode
line.
Such
with a resistor, whose resistance corresponds to the characteristic impedance of the electrode line.
Such a configuration ensures a reflection free termination. With less reflection at the line
a configuration
ensuresensures
a reflection
free termination.
With less With
reflection
the line terminations,
Such
a configuration
a reflection
free termination.
less atreflection
at the line
terminations, impedance shifts caused by a fault can be better defined, making fault detection more
impedance
shifts
caused
by
a
fault
can
be
better
defined,
making
fault
detection
more
accurate,
terminations, impedance shifts caused by a fault can be better defined, making fault detection
more
accurate, especially for faults close to the electrode.
especially
for
faults
close
to
the
electrode.
accurate, especially for faults close to the electrode.
DC Line
DC Line
Converter
Converter
Signal
Signal TA
injection
injection TA
Neutral
Neutral
Bus
AC Side Bus
AC Side
Blocking
Blocking
Filter
Filter
resistor
resistor
Electrode line
Electrode line
Blocking
Blocking
Filter
Filter
TV
TV
Figure 1. The ELIS device in a typical bipolar UHVDC system.
Figure
Figure1.1.The
TheELIS
ELISdevice
devicein
inaatypical
typicalbipolar
bipolarUHVDC
UHVDCsystem.
system.
A sudden change of the measured impedance value is the criterion for an electrode line fault
A sudden change of the measured impedance value is the criterion for an electrode line fault
(openAcircuit
orchange
grounding
fault).
ELIS may
alarm ifvalue
the change
greaterfor
than
certain value.
The
sudden
of the
measured
impedance
is the is
criterion
anaelectrode
line fault
(open circuit or grounding fault). ELIS may alarm if the change is greater than a certain value. The
voltagecircuit
and current
at the converter
end of
thealarm
grounding
are measured
to calculate
(open
or grounding
fault). ELIS
may
if the electrode
change isline
greater
than a certain
value.
voltage and current at the converter end of the grounding electrode line are measured to calculate
the voltage
high-frequency
impedance.
It is found
the measured
high-frequency
impedance
is a
The
and current
at the converter
end of that
the grounding
electrode
line are measured
to calculate
the high-frequency impedance. It is found that the measured high-frequency impedance is a
periodic
function
of
the
fault
distance
[12].
Therefore,
the
operation
of
ELIS
is
different
from
the
the high-frequency impedance. It is found that the measured high-frequency impedance is a periodic
periodic function of the fault distance [12]. Therefore, the operation of ELIS is different from the
distance
(1) defines
the operation
conditionofofELIS
ELIS,
is illustrated
in Figure
2:
function relay.
of the Equation
fault distance
[12]. Therefore,
the operation
is which
different
from the distance
relay.
distance relay. Equation (1) defines the operation condition of ELIS, which is illustrated in Figure 2:
Equation (1) defines the operation condition
of ELIS, which is illustrated in Figure 2:
Z  Z ref  Z set ,
(1)
Z. mm  Z. ref
 Z set ,
(1)
(1)
Z m − Zre f ≥ Zset ,
 represents the calculated high-frequency
where Z
impedance based on the voltage and current at
m

where Z m represents the calculated high-frequency impedance based on the voltage and current at
.
Zthe
where
Z
ref calculated high-frequency impedance based on the voltage and current at the
m represents
the converter
end.
is the reference impedance calculated with electrode line parameters under
Z ref
.
the
converter
the reference
impedance
calculated
with electrode
line parameters
under
converter
end.end.
impedance
calculated
with electrode
line parameters
under normal
Zre f is theisreference
Z set
normal
operation.
is
the
threshold
setting,
usually
30
Ω
[15].
Z
operation.
Zset is the set
threshold
setting, usually
Ω [15].30 Ω [15].
normal
operation.
is the threshold
setting,30
usually
jX
jX
0
0
R
R
Figure 2.
2. Typical
Typical operation
operation characteristics
characteristics of
of ELIS
Figure
ELIS in
in UHVDC
UHVDC system.
system.
Figure 2. Typical operation characteristics of ELIS in UHVDC system.
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2.2. Performance Evaluation of Conventional ELIS Technique
.
It can be seen that Zre f is related to the electrode line parameters, such as the resistance, inductance
and capacitance. In reality, the values of resistance, inductance, and capacitance of the grounding
electrode line are measured at fundamental frequency, while the injected high-frequency current is
13.95 kHz. Furthermore, the parameters change with temperature, aging processes, humidity and
other factors. Thus the measured grounding electrode line parameters are not guaranteed to be equal
to the actual values, so that the high-frequency impedance based system may operate erroneously.
·
·
Assume Zre f −real and Zre f −meas are, respectively, the reference impedance calculated with the
actual electrode line parameters and the measured electrode line parameters. Obviously, the reference
·
impedance is a function of line inductance L and line capacitance C. The difference between Zre f −real
·
and Zre f −meas caused by the variation of L and C can be represented as follows after neglecting the
third and higher order terms:
·
·
·
·
Zre f −meas − Zre f −real =
∂ Zre f −real ( Lr ,Cr )
( Lm
∂L
∂2 Zre f −real ( Lr ,Cr )
( Lm
∂L2
∂ Zre f −real ( Lr ,Cr )
(Cm
∂C
− Cr )
·
·
+ 12
− Lr ) +
− Lr )2 +
∂2 Zre f −real ( Lr ,Cr )
( Lm
∂L∂C
− Lr )(Cm − Cr )
(2)
·
∂2 Z
real ( Lr ,Cr )
+ 12 re f −∂C
(Cm
2
− Cr )2
where, Lr and Cr are the real line parameter values. Lm and Cm are the measured line parameter values.
According to (2), we can obtain the effect of line parameter variations on the reference impedance,
as shown in Figure 3. It can be seen in Figure 3 that a 5% deviation of inductance or capacitance will
result in more than 15% deviation of the reference impedance Zref . That is to say, the reference
impedance Zref is sensitive to the line parameters. The traditional ELIS technique subject to
mal-operation due to the variations of line parameters.
The measured values listed in Table 1 are the per-unit length values of resistance, inductance
and capacitance of the electrode line for Yibin-Jinhua ±800 kV DC transmission project in China.
The measured values are obtained using the measuring methods described in [16]. The real values
of the line parameters are unknown. In order to illustrate the performance of the conventional ELIS,
the real capacitance value is considered to be 1% deviation from the measured one, as listed in Table 1.
The reference impedance Zre f −real and Zre f −meas are [14]:
Zre f −real = 247.326 + j7.399 Ω
(3)
Zre f −meas = 251.482 + j17.2853 Ω
Table 1. The real values and measured values on grounding electrode line.
Values
L (mH/km)
R (ohm/km)
C (uF/km)
Real value
Measured value
2.3709
2.3709
0.2626
0.2626
0.007638
0.007714
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Deviation of reference impedance|Zref| / %
30
25
20
15
10
5
0
-5
-4
-3
-2
-1
0
1
2
3
4
2
3
4
5
Deviation of inductance L/ %
(a)
Deviation of reference impedance|Zref| / %
25
20
15
10
5
0
-5
-4
-3
-2
-1
0
1
5
Deviation of capacitance C/ %
(b)
Figure 3. The effect of line parameters variation on the reference impedance. (a) Effect of inductance
Figure 3. The effect of line parameters variation on the reference impedance. (a) Effect of inductance
variation on VSWR; (b) Effect of capacitance variation on VSWR.
variation on VSWR; (b) Effect of capacitance variation on VSWR.
Suppose that a metallic short circuit fault occurs on the grounding electrode line at 4 km away
Suppose
that a metallic
circuit fault
occurs on the
grounding
electrode
at 4 km
from the converter
end. Theshort
measured
high-frequency
impedance
under
fault line
condition
is
away from the converter end. The measured high-frequency impedance under fault condition is
Z  249.234  j 42.4819 . Thus the operation conditions of ELIS with actual reference
Zmm = 249.234 + j42.4819Ω. Thus the operation conditions of ELIS with actual reference impedance
impedance
and thereference
measured
reference are:
impedance are:
and
the measured
impedance
Z m  Z ref real  35.14

Zm − Zre f −real = 35.14 Ω
(4)
(4)
Z m Z ref meas  25.30

Zm − Zre f −meas = 25.30 Ω
This shows that the difference between the calculated high-frequency impedance Z m under
shows and
that the
the actual
difference
between
the calculated
high-frequency
impedance
Zm under
fault
fault This
conditions
reference
impedance
is greater than
the threshold.
The ELIS
Z ref real
conditions and the actual reference impedance Zre f −real is greater than the threshold. The ELIS should
should
sendinformation
alarm information
under
such conditions.
the difference
between
the
send
alarm
under such
conditions.
However,However,
the difference
between the
calculated
calculated high-frequency
reference impedance
lessthreshold.
than the
m and theimpedance
ref  meas
high-frequency
impedanceimpedance
Zm and theZ reference
Zre f −meas isZless
thanisthe
The
ELIS will
take
any
actions.
threshold.
Thenot
ELIS
will
not
take any actions.
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3. The 2017,
VSWR
Technique
6 of 17
for Grounding Electrode Line Fault
3.1. VSWR on a Lossless Transmission Line
3. The VSWR Technique for Grounding Electrode Line Fault
Generally, standing waves are formed by the overlap of two waves with the same frequency
3.1.
VSWR
on directions.
a Lossless Transmission
Lineamplitude appears at the position termed as the wave peak
but opposite
The maximum
whereas
the minimum
positionby
is the
called
waveofvalley.
Under
normal
operation,
the
Generally,
standing amplitude
waves are formed
overlap
two waves
with
the same
frequency
standing
wave
effect
can
be
neglected
because
the
wavelength
of
the
fundamental
frequency
wave
is
but opposite directions. The maximum amplitude appears at the position termed as the wave peak
far greater
the length
of the
transmission
the Under
injected
high operation,
frequency the
current,
the
whereas
the than
minimum
amplitude
position
is calledlines.
waveFor
valley.
normal
standing
standing
waves
can
be
utilized
in
the
grounding
electrode
line
fault
supervision
techniques.
wave effect can be neglected because the wavelength of the fundamental frequency wave is far greater
Figure
4 isof
a the
typical
distributed
circuit
of a lossless
transmission
line.the
The
characteristic
than the
length
transmission
lines.
Formodel
the injected
high frequency
current,
standing
waves
equations
of
voltage
and
current,
which
will
be
used
for
later
derivation,
are
rewritten
below:
can be utilized in the grounding electrode line fault supervision techniques.
Figure 4 is a typical distributed circuit
modelof
transmission line. The characteristic
x
U 
A1e x a Alossless
2e
equations of voltage and current, which will be used
for later
derivation, are rewritten below:
(5)
 A1 x A2 x ,
.
I

e
e
U = AZ1 ec−γx + AZ2 ceγx
,
.
A1 −γx
A2 γx
I
=
e
−
e
transmission
propagation
constant,
Z
Zc
c
(5)
.
and
  j L0C0
p
√
the characteristic
impedance
of
the
transmission
Z c  γz1is/ ythe
(r1  jl1 ) /(propagation
g1  jc1 ) isconstant,
where
and
γ
=
jω
L
C
.
Z
=
z1 /y1line.
=
1  transmission
c
0
0
p
(r1the
+ jωl
characteristic
the Z
transmission line. For the lossless
1 ) / ( gtransmission
1 + jωc1 ) is the
For
lossless
line,
r1 and g1 areimpedance
set top
zero, of
thus
√ c  z1 / y1  l1 / c1 . A1 and A2
transmission line, r1 and g1 are set to zero, thus Zc = z1 /y1 = l1 /c1 . A1 and A2 are the coefficient
are the coefficient determined by the voltage and current at the two ends of the line.
determined by the voltage and current at the two ends of the line.
where
γ
is
the
Figure 4. Distributed parameter circuit
circuit of
of aa power
power transmission
transmission line.
line.
Assuming that
that the
the end
end point
point of
of the
the transmission
transmission line
line is
is xx == 0,
0, the
the voltage
voltage at
at the
the end
end of
of the
the
Assuming
transmission
line
is
U
0
,
and
the
load
impedance
is
Z
L
.
The
voltage
and
current
at
position
x
on
the
transmission line is U0 , and the load impedance is ZL . The voltage and current at position x on the
transmission line
line is:
is:
transmission
U ( x ) = U0 (eγx +
Γ L e−γx )
U ( x)  U 0 (ex  L e x ),
(6)
0
I (x) = U
(eγx − Γ L e−γx ) ,
ZcU
(6)
x
x
0
I ( x) 
(e  L e
where, Γ L is the reflection coefficient at the loadZside:
c
)
Z at
− the
Zc load
Z side:
/Zc − 1
Z −1
where,  L is the reflection coefficient
= L
= Ln
,
ΓL = L
ZL + Zc
ZL /Zc + 1
ZLn + 1
Z L  Zc
Z L / Zc 1
(7)
Z Ln  1
,
Limpedance.



(7)
where, ZLn is the normalized load
Z L  Z c Z L / Z c  1 Z Ln  1
The high-frequency VSWR is defined as the ratio of voltage amplitudes at the wave peak and the
adjacent
valley
on the transmission
line, as follows:
where, Zwave
Ln is the
normalized
load impedance.
The high-frequency VSWR is defined as the ratio of voltage amplitudes at the wave peak and
|U ( x )|max
1 + |Γ L |
the adjacent wave valley on the transmission
follows:
VSWR = line, as
=
,
(8)
1 − |Γ L |
|U ( x )|min
3.2. Calculation of VSWR
VSWR 
U ( x) max
U ( x) min

1  L
1  L
,
(8)
In practice, the VSWR on the grounding electrode line is easily obtained if the terminal voltage and
current are given. Assume that the voltage and current at the converter station end of the grounding
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.
.
electrode line, U 1 and I 1 , are measured, then the voltage and current at the point x can be calculated as
follows, from (5):
.
.
.
.
.
U ( x ) = 21 (U 1 + Zc I 1 )e−γx + 21 (U 1 − Zc I 1 )eγx
,
(9)
.
.
.
.
.
I ( x ) = 21 ( UZc1 + I 1 )e−γx − 21 ( UZc1 − I 1 )eγx
√
For a lossless transmission line, the transmission propagation constant γ = jω L0 C0 is a pure
imaginary number. Therefore, (9) can be rewritten as:
.
.
.
U ( x ) = U 1 cos βx − jZc I 1 sin βx
.
.
.
I ( x ) = I 1 cos βx − j UZc1 sin βx
,
(10)
√
where, β = ω L0 C0 is the phase constant.
Assuming that the phase angle of the current at the converter station end is zero, then the voltage
measured at this point is:
.
U 1 = Us (cos ϕu1 + j sin ϕu1 )
.
,
(11)
I 1 = Is
where, US and IS are the amplitudes of the measured voltage and current at the converter station end
.
.
on the grounding electrode line, respectively. ϕu1 is the phase difference between U 1 and I 1 .
Substituting into (10), one has:
.
U ( x ) = Us cos ϕu1 cos βx + j(Us sin ϕu1 cos βx − Zc Is sin βx ),
(12)
.
2
1
1
1
U ( x ) = (Us2 + Zc2 Is2 ) + ( Us2 − Zc2 Is2 ) cos 2βx − Us Zc Is sin ϕu1 sin 2βx
2
2
2
(13)
Let:
s
Ac =
2
1
1
( Us2 − Zc2 Is2 ) + Us2 Zc2 Is2 sin2 ϕu1 ,
2
2
(14)
The maximum and minimum of the voltage amplitude and the VSWR can be calculated:
.
2
= 21 (Us2 + Zc2 Is2 ) + Ac
U ( x ) max
,
.
2
= 12 (Us2 + Zc2 Is2 ) − Ac
U ( x ) (15)
v
u1
u (U 2 + Zc2 Is2 ) + Ac
VSWR = t 21 s
,
2
2 2
2 (Us + Zc Is ) − Ac
(16)
min
Therefore, once the transmission parameters of the grounding electrode line are known, the VSWR
can be easily calculated using the measured voltage amplitudes, current amplitudes, and the phase
angles. Equation (16) shows that the VSWR along the transmission line is independent of the distance x,
but only determined by voltage US and current IS , their phase angle ϕu1 , and the internal characteristics
of the grounding line, such as Zc .
3.3. The Characteristics of the VSWR under Various Fault Conditions
The VSWR may be subject to change, depending on the resistor installed at the end of the
grounding line, including the conditions of complete matching of the terminal resistor and incomplete
matching terminal resistor. If the resistor completely matches the characteristic impedance, there is no
reflection at the end of the grounding line and the VSWR equals 1. Otherwise the VSWR will be greater
3.3. The Characteristics of the VSWR under Various Fault Conditions
The VSWR may be subject to change, depending on the resistor installed at the end of the
grounding line, including the conditions of complete matching of the terminal resistor and
incomplete matching terminal resistor. If the resistor completely matches the characteristic
Energies
2017, 10,
309 is no reflection at the end of the grounding line and the VSWR equals 1. Otherwise
8 of 17
impedance,
there
the VSWR will be greater than 1. In terms of the earth fault on the grounding electrode line, the
equivalent resistor should be considered to determine whether the equivalent resistor matches the
than 1. In terms of the earth fault on the grounding electrode line, the equivalent resistor should be
characteristic impedance or not.
considered to determine whether the equivalent resistor matches the characteristic impedance or not.
(1) Normal operation with matching terminal resistor
(1) Normal operation with matching terminal resistor
If the
the end
equals to
to the
the characteristic
characteristic impedance
as illustrated
illustrated in
in Figure
ZLL equals
ZCC, , as
If
end resistor
resistor Z
impedance Z
Figure 5.
5.
The reflection
reflection coefficient
coefficientand
andthe
thevoltage
voltagestanding
standingwave
waveratio
ratiocan
canbe
becalculated
calculatedas:
as:
The
(
 0
L
ΓL =
 0
,,
VSWR
VSWR=11
(17)
(17)
It is
is shown
shown that
that that
that there
there is
is no
no reflection
reflection at
at the
the end
end of
of the
the grounding
grounding line
line and
and the
the voltage
voltage
It
amplitude
remains
constant
along
the
whole
grounding
line.
amplitude remains constant along the whole grounding line.
Figure 5.
5. Matching
Matching terminal
terminal resistor
resistor at
at the
the earth
earth end.
end.
Figure
(2) Normal operation with mismatching terminal resistor
(2) Normal operation with mismatching terminal resistor
If the end resistor Z L does not equal to the characteristic impedance Z C , i.e., the terminal
If the end resistor ZL does not equal to the characteristic impedance ZC , i.e., the terminal resistor
resistor mismatches the characteristic impedance, the reflection coefficient and voltage standing
mismatches the characteristic impedance, the reflection coefficient and voltage standing wave ratio
wave ratio can be calculated as:
can be calculated as:
(
ΓL6=
0
L  0
,
(18)
(18)

VSWR > 1 ,
VSWR  1
Let Vreal and Vmeas be the VSWRs calculated with the actual line parameters and the measured
V
meas be the
line parameters.
TheVdifference
between
Vmeas
and Vrealwith
caused
the variation
of L and Cand
can the
be
Let real and
VSWRs
calculated
thebyactual
line parameters
represented as follows after neglecting the third andVhigher order
terms.
V
measured line parameters. The difference between meas and real caused by the variation of L and
∂Vreal
( Lr ,Cr ) order terms.
real ( Lr ,Cr )
C can be representedVas
follows after
third and
higher
= ∂Vneglecting
( Lthe
(Cm − Cr )
meas − V
m − Lr ) +
real
∂L
∂C
V ( Lr , Cr2) ∂2 Vreal ( Lr,CVrreal
) ( Lr , Cr ) (C  C )
( Lm  Lr ) 
(19)
( Lmreal− L
) +
( Lm − Lr )(m Cm r− Cr )
∂L∂C
L r
C
2
( Lreal
r ,C
 2V
(rL)r(, C
Cr ) − C )2 2  2Vreal ( Lr , Cr )
+ 12 ∂ 1Vreal
m ( Lm r Lr ) 
( Lm  Lr )(Cm  Cr )
(19)
∂C2
2
2
L
LC
( Lr , Cr )the effect2 of line parameters variation on VSWR, as shown in
1  2V obtain
According to (19), we
 canreal
(Cm  Cr )
2
2
C
Figure 6. It can be seen in Figure
6 that a 5% deviation of inductance or capacitance will result in less
+ 21
∂2 Vreal(V
Lr ,Cr)
V
meas
∂L2 real
than 2.6% deviation of VSWR. That is to say, VSWR is insensitive to the line parameters. The VSWR
based electrode line fault supervision technique is more robust.
Energies 2017, 10, 309
9 of 17
According to (19), we can obtain the effect of line parameters variation on VSWR, as shown in
Figure 6. It can be seen in Figure 6 that a 5% deviation of inductance or capacitance will result in less
than 2.6%
of VSWR. That is to say, VSWR is insensitive to the line parameters. The VSWR
Energies
2017,deviation
10, 309
9 of 17
based electrode line fault supervision technique is more robust.
3
Deviation of VSWR / %
2.5
2
1.5
1
0.5
0
-5
-4
-3
-2
-1
0
1
2
3
4
5
3
4
5
Deviation of inductance L/ %
(a)
Deviation of VSWR / %
2.5
2
1.5
1
0.5
0
-5
-4
-3
-2
-1
0
1
2
Deviation of capacitance C/ %
(b)
Figure
6. The
Theeffect
effectofof
line
parameters
variation
on VSWR.
(a) Effect
of inductance
variation
on
Figure 6.
line
parameters
variation
on VSWR.
(a) Effect
of inductance
variation
on VSWR;
VSWR;
(b)ofEffect
of capacitance
on VSWR.
(b)
Effect
capacitance
variationvariation
on VSWR.
Assuming that the maximum deviation between the terminal resistor and the characteristic
Assuming that the maximum deviation between the terminal resistor and the characteristic
impedance is less than 10%, the reflection coefficient and voltage standing wave ratio are within the
impedance is less than 10%, the reflection coefficient and voltage standing wave ratio are within the
following range:
following range:
(
 0.0526,
0.05260.047
,0.047
ΓLL ∈(−
) 
,,
(20)
VSWR ∈ [1 , 1.11)
VSWR  1,1.11


It is
there is
It
is shown
shown that
that when
when there
is aa reflection
reflection at
at the
the end
end terminal
terminal of
of the
the grounding
grounding line,
line, and
and the
the
mismatching
degree
is
less
than
10%,
the
maximum
VSWR
is
only
1.11.
mismatching degree is less than 10%, the maximum VSWR is only 1.11.
(3)
short
circuit
condition
(3) Metallic
Metallic
short
circuit
condition
When aametallic
circuit
occurs
at any
of the grounding
electrode electrode
line, the equivalent
When
metallicshort
short
circuit
occurs
atposition
any position
of the grounding
line, the
resistor
is
zero.
According
to
Equations
(7)
and
(8),
the
reflection
coefficient
and
VSWR
(approaching
equivalent resistor is zero. According to Equations (7) and (8), the reflection coefficient
and VSWR
infinity) are: infinity) are:
(
(approaching
Γ L = −1
,
(21)
VSWR → ∞
Energies 2017, 10, 309
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10 of 17
L  1
,

(4) Non-metallic short circuit with matching terminal
resistor
VSWR 


(21)
Under this condition, the ground fault via transition resistance occurs with the matching terminal
(4) Non-metallic
circuit
with matching
terminalload
resistor
resistor,
as shown inshort
Figure
7. Because
the terminal
is equal to the characteristic impedance,
the equivalent
impedance
seenthe
from
the fault
to the end
terminaloccurs
Zeq can
be calculated.
Under this
condition,
ground
faultposition
via transition
resistance
with
the matching
terminal resistor, as shown in Figure 7. Because the terminal load is equal to the characteristic
Zc Z f
impedance, the equivalent impedance seen
Zeqfrom
= the fault
, position to the end terminal Z eq can be
(22)
Zc + Z f
calculated.
Figure
matchingterminal
terminalresistor.
resistor.
Figure7.7.Non-metallic
Non-metallicshort
short circuit
circuit with matching
Normally, the characteristic impedance ZC is about
Z c Z f 250 Ω, and if the fault transition resistance
,
(22)the
is in the range of 0–250 Ω, the range of theZreflection
coefficient
and the VSWR would be in
eq 
Zc  Z f
following range:
Zeq Z
∈ [0,is0.5Z
c ] 250 Ω, and if the fault transition resistance
Normally, the characteristic impedance
about
C
,
(23)
ZL 6= Zc
is in the range of 0–250 Ω, the range of the reflection coefficient and the VSWR would be in the
(
following range:
Γ L ∈ (1, 0.3333)
(24)
VSWR
)
Z eq ∈ 0∈,0[.25,Z∞
c
(23)
,
Z L ≠ Z c resistance is equal to the characteristic impedance,
Equation (24) indicates that if the fault transition
the equivalent impedance is half of the characteristic impedance, and the VSWR under this condition
is 2, whereas if a metallic earth fault occursat
the
( end of the)grounding line, the VSWR will approach
L ∈ 1, 0.3333
(24)
infinity, which is similar to the above condition.
[ )
[
]
VSWR ∈ 2,∞
(5) Non-metallic short circuit with mismatching terminal resistor
Equation (24) indicates that if the fault transition resistance is equal to the characteristic
In case that
earth fault
via transition
occurs atimpedance,
the middleand
of the
say,
x = lf
impedance,
thean
equivalent
impedance
is half resistance
of the characteristic
the line,
VSWR
under
(thethis
total
length of
the
grounding
line is l),
as shown
in Figure
8. end
Under
such
condition,
thethe
equivalent
condition
is 2,
whereas
if a metallic
earth
fault occurs
at the
of the
grounding
line,
VSWR
load
impedance
from
the fault
position
toabove
the end
terminal ZLeq is:
will
approach seen
infinity,
which
is similar
to the
condition.
(5) Non-metallic short circuit with mismatching
terminal resistor
eγ(l −l f ) + Γ L e−γ(l −l f )
ZLeq = Zc γ(l −l )
,
(25)
In case that an earth fault via transitione resistance
f − Γ occurs
e−γ(l −latf ) the middle of the line, say, x = l f
Energies 2017, 10, 309
11 of 17
L
(the total length of the grounding line is l), as shown in Figure 8. Under such condition, the
equivalent load impedance seen from the fault position to the end terminal Z Leq is:
 ( l l )
Z Leq
 ( l l )
f
e f  L e
,
 Z c  (l l f )
 ( l l f )
e
 L e
(25)
Figure 8. Non-metallic short circuit with mismatching terminal resistor.
Figure 8. Non-metallic short circuit with mismatching terminal resistor.
Taking the fault transition impedance Z f into consideration, the equivalent impedance Z eq
is:
(1  L2 )  2 jL sin 2 (l  l f )
Energies 2017, 10, 309
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Taking the fault transition impedance Z f into consideration, the equivalent impedance Zeq is:
Z f Zc
Zeq =
(1−Γ2L )−2jΓ L sin 2β(l −l f )
1+Γ2L −2Γ L cos 2β(l −l f )
Z f + Zc
(1−Γ2L )−2jΓ L sin 2β(l −l f )
1+Γ2L −2Γ L cos 2β(l −l f )
,
(26)
Thus the reflection coefficient at the converter station is:
ΓL =
( Z f − Zc )
(1−Γ2L )−2jΓ L sin 2β(l −l f )
1+Γ2L −2Γ L cos 2β(l −l f )
− Zf
( Z f + Zc )
(1−Γ2L )−2jΓ L sin 2β(l −l f )
1+Γ2L −2Γ L cos 2β(l −l f )
+ Zf
,
(27)
It shows that when the fault position is not at the end terminal of the grounding electrode line,
the equivalent impedance is no longer pure resistance. Hence, the reflection coefficient presents
complex number characteristics. The amplitude of the reflection coefficient varies periodically with
the fault position distance l f . Therefore, the VSWR varies periodically with the fault position distance
l f as well.
3.4. High Frequency VSWR-Based Grounding Electrode Line Fault Supervision
The VSWR-based technique for grounding electrode line fault monitoring systems has a similar
structure as shown in Figure 1, and can be implemented on an existing ELIS system. The devices,
including the injected current source, the blocking filter on both ends of the grounding line, the resistor
equal to the characteristic impedance of the electrode line, can be used. The voltage and current at the
converter station end of the grounding electrode line are measured by installed PT and CT.
From the above analysis, it can be safely concluded that the VSWR is close to 1 when the electrode
line is under normal operation, whereas the VSWR will increase if there are earth faults on the
grounding electrode line. Based on the different values under normal and faulty condition, the criteria
for fault judgement can be developed:
v
u1
u (U 2 + Zc2 Is2 ) + Ac
> VSWRset ,
VSWR = t 21 s
2 2
2
2 (Us + Zc Is ) − Ac
(28)
where VSWRset is the setting value for fault alarm in the proposed fault supervision scheme.
Since the end terminal resistor may not match completely with the characteristics impedance,
the VSWR may be greater than 1 even under normal operation. Thus the setting value of VSWR
should be greater than 1. Assume that the maximum deviation between the terminal resistor and
the characteristic impedance is less than 10%, the maximum value of the VSWR is 1.1. The following
setting value of VSWR is suggested:
VSWRset = k I × VSWRmax_normal ,
(29)
where, VSWRmax_normal is the maximum value of VSWR under normal operation, and k I is the reliable
coefficient, taking 1.11. Therefore, the setting value of VSWR equals to 1.22.
4. Case Study
4.1. Simulation Model
The verification of the proposed scheme is conducted on a ±800 kV UHVDC project in south-west
China with PSCAD/EMTDC simulation. The transmission capacity of this UHVDC project is 8000 MW.
The length of the transmission line is 1652 km. The grounding electrode line of this UHVDC
Energies 2017, 10, 309
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project adopts parallel line and the length of the grounding line is 100 km. The parameters of the
grounding electrode line are listed in Table 2. The high frequency current source with a frequency of
13.95 kHz is configured at the converter station end to inject current into the grounding electrode line.
The blocking filter is installed on both ends of the grounding line to reduce the standing wave effect in
high-frequency inflection.
Table 2. Parameters of single grounding electrode line in case study.
Parameters
L (mH/km)
R (ohm/km)
C (uF/km)
Values
2.3709
0.2626
0.0077
q
The characteristic impedance of the double electrode lines is Zc = 12 CL = 277.4478 Ω. The setting
value of the VSWR follows (29). To verify the characteristics of the VSWR under various operation
conditions, the case study is performed considering two scenarios: (a) the end terminal resistor
completely matches the characteristic impedance; and (b) the terminal resistor does not match the
characteristic impedance. Each scenario includes three kinds of operation conditions: normal operation,
double-line earth fault, and single-line earth fault. For the earth fault, three different values of fault
impedance are considered, which are 0.1 Ω, 100 Ω, and 200 Ω. Table 3 lists the simulation scenarios.
Table 3. Simulation scenarios in case study.
Simulation Scenarios
Operation Conditions
With Matched terminal resistor
Normal
operation
Double-line earth fault
Single-line earth fault
0.1 Ω/100 Ω/200 Ω
0.1 Ω/100 Ω/200 Ω
With mismatched terminal resistor
Normal
operation
Double-line earth fault
Single-line earth fault
0.1 Ω/100 Ω/200 Ω
0.1 Ω/100 Ω/200 Ω
4.2. With Matched Terminal Resistor
When the terminal resistance is 277.48 Ω, the end resistor matches with the characteristic
impedance of the double grounding electrode line.
(1) Normal operation condition
Under normal operation condition, the voltage and current at the converter station end are
measured to calculate the VSWR:
VSWR = 1.02 < VSWRset ,
(30)
The VSWR keeps around 1.0, which is less than the VSWR thresholds, thus the proposed VSWR
based scheme works correctly.
(2) Double-line earth fault condition
Figure 9 shows the simulated VSWR results under double-line earth fault condition with a
matched terminal resistor. The double-line earth fault with metallic earth fault (0.1 Ω), via 100 Ω
transition impedance, and via 200 Ω transition impedance are all plotted. The horizontal axis represents
the distance from the converter station to the fault position, on the grounding electrode line.
Figure 9 shows the simulated VSWR results under double-line earth fault condition with a
matched terminal resistor. The double-line earth fault with metallic earth fault (0.1Ω), via 100 Ω
transition impedance, and via 200 Ω transition impedance are all plotted. The horizontal axis
represents
distance from the converter station to the fault position, on the grounding electrode
Energies 2017, the
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line.
VSWR
1400
1200
100
1000
50
800
0
VSWRset
20
600
40
60
80
Partial enlarged view
400
200
0
0
20
40
60
Fault distance / km
(a)
80
100
4
3.5
VSWR
Fault resistance 100 ohm
3
Fault resistance 200 ohm
2.5
2
VSWRset
1.5
1
0
20
40
60
Fault distance / km
(b)
80
100
Figure 9. The VSWR for different fault position of double-line earth fault with matched terminal
Figure 9. The VSWR for different fault position of double-line earth fault with matched terminal resistor.
resistor. (a) the transition resistance is 0.1 Ω; (b) the transition resistance are 100 Ω and 200 Ω,
(a) the transition resistance is 0.1 Ω; (b) the transition resistance are 100 Ω and 200 Ω, respectively.
respectively.
It can
can be
be observed
observed from
from Figure
Figure 99 that
earth fault
fault occurs
occurs on
on the
the grounding
grounding
It
that when
when non-metallic
non-metallic earth
electrode
line,
the
VSWR
decreases
with
the
increase
of
the
transition
resistance.
The
VSWR
will also
also
electrode line, the VSWR decreases with the increase of the transition resistance. The VSWR will
reduce when
when the
the fault
fault distance
rises. Therefore,
Therefore, the
the double-line
double-line earth
earth fault
fault via
via aa 200
200 Ω
Ω transition
reduce
distance rises.
transition
resistance
on
the
end
terminal
of
the
grounding
line
results
in
the
lowest
VSWR.
However,
the VSWR
resistance on the end terminal of the grounding line results in the lowest VSWR. However,
the
under this
worst
is still greater
the than
threshold,
as shownas
inshown
Figure in
9b.Figure
Hence9b.
theHence
proposed
VSWR
under
thiscondition
worst condition
is stillthan
greater
the threshold,
the
VSWR
based
technique
can
perform
correctly
and
reliably
under
this
condition.
proposed VSWR based technique can perform correctly and reliably under this condition.
(3) Single-line
(3)
Single-lineearth
earthfault
faultcondition
condition
Figure 10 shows the VSWR results under single-line earth fault condition with matched terminal
Figure 10 shows the VSWR results under single-line earth fault condition with matched terminal
resistor. The single-line earth fault also includes the metallic earth fault (0.1 Ω), earth fault via 100 Ω
resistor.
The
single-line
earth
faultΩalso
includes
the metallic earth fault (0.1 Ω), earth fault via 100 Ω
transition
resistance,
and
via 200
transition
resistance.
transition
Ω transition
resistance.
Figureresistance,
10 shows and
that via
the 200
VSWR
in single-line
earth fault with matched terminal resistor condition
is a periodical function of the fault distance because the currents on each grounding electrode line are
no longer equal to each other. However, the current and voltage are measured at the station end of the
grounding electrode line, and the equivalent VSWR can be calculated following (16). The equivalent
VSWRs are still greater than the thresholds, so that the proposed VSWR based technique can operate
correctly under such case.
Energies 2017, 10, 309
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14 of 17
14 of 17
16
Fault
resistance
100 ohm
14
12
Fault
resistance
200ohm
VSWR
10
2.2
2
1.8
1.6
1.4
1.2
VSWRset
Partial enlarged view 0
8
20
40
60
10
0
80
6
4
2
0
0
10
20
30
40
50
60
70
80
90
100
Fault distance / km
(a)
160
2.5
140
2
120
1.5
VSWRset
VSWR
100
0
80
20
40
60
60
80
Partial
enlarged view
40
20
0
0
20
40
60
Fault distance / km
(b)
80
100
Figure
Figure 10.
10. The
The VSWR
VSWR for
for different
different fault
fault position
position of
of single-line
single-line earth
earth fault
fault with
with matched
matched terminal
terminal
resistor.
(a)
the
transition
resistance
are
100
Ω
and
200
Ω,
respectively;
(b)
the
transition
resistor. (a) the transition resistance are 100 Ω and 200 Ω, respectively; (b) the transition resistance
resistance is
is
0.1
0.1 Ω.
Ω.
Figure 10 shows that the VSWR in single-line earth fault with matched terminal resistor
4.3. With Mismatched Terminal
condition is a periodical function of the fault distance because the currents on each grounding
In this
thelonger
terminal
resistance
is assumed
to be
Ω, which
is about
of the
electrode
linecase,
are no
equal
to each other.
However,
the250
current
and voltage
are90.25%
measured
at
characteristic
impedance.
the station end of the grounding electrode line, and the equivalent VSWR can be calculated
following
The equivalent
(1) Normal(16).
operation
condition VSWRs are still greater than the thresholds, so that the proposed
VSWR based technique can operate correctly under such case.
Under normal condition, the voltage and current at the station end are measured to calculate
the
VSWR:
4.3. With Mismatched Terminal
VSWR = 1.108 < VSWRset
(31)
In this case, the terminal resistance is assumed to be 250 Ω, which is about 90.25% of the
It is concluded
that the VSWR is slightly greater than 1.0 because the end terminal resistor is not
characteristic
impedance.
completely equal to the characteristic impedance. This result conforms with the analysis in Section 3.
(1) Normal operation condition
However, the VSWR is still less than the setting threshold 1.22. Therefore, the VSWR-based fault
Under normal
condition,
the voltage
and current at the station end are measured to calculate
supervision
device will
be operating
correctly.
the VSWR:
(2) Double-line earth fault condition
VSWR  1.108  VSWR ,
(31)
Figure 11 shows the VSWR under double-line earth faultsetcondition with mismatched terminal
resistor.
to the
double-line
with matched
terminal
resistor, the
the end
double-line
faultiswith
It is Similar
concluded
that
the VSWRfault
is slightly
greater than
1.0 because
terminalearth
resistor
not
mismatched
terminal
resistor
also
includes
the
metallic
earth
fault
(0.1
Ω),
the
earth
fault
via
100 3.
Ω
completely equal to the characteristic impedance. This result conforms with the analysis in Section
transition resistance,
via less
200 Ω
transition
resistance.
However,
the VSWR and
is still
than
the setting
threshold 1.22. Therefore, the VSWR-based fault
supervision device will be operating correctly.
(2) Double-line earth fault condition
Figure 11 shows the VSWR under double-line earth fault condition with mismatched terminal
resistor. Similar to the double-line fault with matched terminal resistor, the double-line earth fault
with
mismatched terminal resistor also includes the metallic earth fault (0.1 Ω), the earth fault
via
Energies 2017, 10, 309
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100 Ω transition resistance, and via 200 Ω transition resistance.
4
3.5
Fault resistance 100 ohm
VSWR
3
2.5
2
Fault resistance 200 ohm
1.5
VSWRset
1
0
10
20
30
40
50
60
70
80
90
100
Fault distance / km
(a)
1400
100
1200
50
VSWR
1000
VSWRset
0
800
50
600
60
70
Fault resistance 0.1 ohm
80
90
Partial enlarged view
400
200
0
0
10
20
30
40
50
60
Fault distance / km
70
80
90
100
(b)
Figure 11. The VSWR for different fault position of double-line earth fault with mismatched terminal
Figure 11. The VSWR for different fault position of double-line earth fault with mismatched terminal
resistor.
thetransition
transition
resistance
areΩ100
and
Ω, respectively;
(b) theresistance
transition
resistor. (a)
(a) the
resistance
are 100
andΩ200
Ω, 200
respectively;
(b) the transition
is
resistance
is 0.1 Ω.
0.1 Ω.
Figure 11 indicates that with mismatched terminal resistance, the VSWR decreases with the
Figure 11 indicates that with mismatched terminal resistance, the VSWR decreases with the raise
raise of the transition resistance, and with the raise of the fault distance. When non-metallic short
of the transition resistance, and with the raise of the fault distance. When non-metallic short circuit fault
circuit fault occurs, the VSWR varies periodically with the fault distance, which is in accordance
occurs, the VSWR varies periodically with the fault distance, which is in accordance with the above
with the above analysis in Section III. The VSWR under non-metallic earth fault with 200 Ω
analysis in Section 3. The VSWR under non-metallic earth fault with 200 Ω transition resistance at the
transition resistance at the end terminal of the grounding electrode line is still greater than the
end terminal of the grounding electrode line is still greater than the threshold value 1.22. The proposed
threshold value 1.22. The proposed VSWR technique will operate correctly.
VSWR technique will operate correctly.
(3) Single-line earth fault condition
(3) Single-line earth fault condition
Figure 12 presents the simulated VSWR for different fault positions under single-line earth fault
Figure 12 presents the simulated VSWR for different fault positions under single-line earth fault
condition with mismatched terminal resistance. Similarly, the single-line earth fault includes
condition with mismatched terminal resistance. Similarly, the single-line earth fault includes metallic
metallic earth fault (0.1 Ω), the earth fault via 100 Ω transition resistance, and via 200 Ω transition
earth fault (0.1 Ω), the earth fault via 100 Ω transition resistance, and via 200 Ω transition resistance.
resistance.
It can be seen from Figure 12 that the VSWR also varies periodically with the fault distance l f .
The VSWRs in different conditions are still greater than the setting thresholds therefore the proposed
scheme works correctly.
Energies 2017, 10, 309
16 of 17
It can be seen from Figure 12 that the VSWR also varies periodically with the fault distance l f .
The VSWRs in different conditions are still greater than the setting thresholds therefore the
16 of 17
proposed scheme works correctly.
Energies 2017, 10, 309
12
Fault resistance 100 ohm
Partial enlarged 1.8
1.6
view
10
VSWR
Fault resistance 200 ohm
1.4
1.2
1
8
VSWRset
20
40
60
80
6
4
2
0
20
40
60
Fault distance / km
80
100
(a)
160
2
140
1.5
VSWRset
VSWR
120
0
100
20
40
60
80
Partial enlarged view
80
60
40
20
0
0
20
40
60
Fault distance / km
80
100
(b)
Figure 12. The VSWR for different fault position of single-line earth fault with mismatched terminal
Figure 12. The VSWR for different fault position of single-line earth fault with mismatched terminal
resistor.
the transition
transition resistance
resistance are
are 100
100 Ω
Ω and
and 200
200 Ω,
Ω, respectively;
respectively; (b)
(b) the
the transition
transition resistance
resistance is
is
resistor. (a)
(a) the
0.1
0.1 Ω.
Ω.
4.4. Summary
4.4. Summary
From the simulations above, it can be concluded that the proposed VSWR-based technique for
From the simulations above, it can be concluded that the proposed VSWR-based technique for
grounding electrode line fault supervision will not act under normal operation condition no matter
grounding electrode line fault supervision will not act under normal operation condition no matter
whether the terminal resistor is matched or not. The proposed technique can reliably identify both
whether the terminal resistor is matched or not. The proposed technique can reliably identify both the
the metallic short circuit faults and non-metallic short circuit faults. The protection range covers the
metallic short circuit faults and non-metallic short circuit faults. The protection range covers the total
total length of the grounding electrode line.
length of the grounding electrode line.
5.
5. Conclusions
Conclusions
This
paper proposes
proposes aa novel
novel electrode
electrode line
line fault
fault supervision
supervision strategy
strategy based
based on
on high-frequency
high-frequency
This paper
VSWR for
for UHVDC
UHVDC transmission
transmission projects.
projects. Similar
Similar to
to existing
existing high-frequency
high-frequency impedance
impedance technique
technique
VSWR
devices,
the
high
frequency
current
is
injected
to
the
electrode
line
at
the
converter
station
end.end.
By
devices, the high frequency current is injected to the electrode line at the converter station
measuring
the the
voltage
andand
current,
the the
VSWR
can can
be calculated.
Theoretical
analysis
indicates
that
By measuring
voltage
current,
VSWR
be calculated.
Theoretical
analysis
indicates
the
VSWR
of
the
grounding
line
equals
to
1
under
normal
operation
with
the
matching
terminal
that the VSWR of the grounding line equals to 1 under normal operation with the matching terminal
resistance.
resistance. When
When aa short
short circuit
circuit occurs
occurs on
on the
the grounding
grounding line,
line, the
the VSWR
VSWR will
will be
be greater
greater than
than 1.
1.
Simulation
results
indicate
that
the
proposed
technique
can
reliably
identify
the
grounding
electrode
Simulation results indicate that the proposed technique can reliably identify the grounding electrode
line faults and has great anti-fault impedance capability. Meanwhile, under normal operation, the
proposed strategy will work reliably, even with mismatching terminal resistors.
Energies 2017, 10, 309
17 of 17
Acknowledgments: This work presented in this paper was supported by China Postdoctoral Science Foundation:
No. 2015M582543 and by China Postdoctoral Science Foundation: No. 2016M592659.
Author Contributions: All authors have contributed equally to this manuscript. More specifically, Yufei Teng
designed the methodology and wrote part of the paper. Xiaopeng Li and Qi Huang performed the simulations
and revised the paper. Yifei Wang and Shi Jing contributed specifically to the introduction and conclusion sections
of the paper. Zhenchao Jiang and Wei Zhen provided important comments on the paper’s structure.
Conflicts of Interest: The authors declare no conflict of interest.
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