Email: wangjian31791@ncepu.edu.cn
Funding information: Beijing Natural Science Foundation (L201018)
Abstract: Low and medium speed magnetic levitation traffic with short
power supply distance and complex grounding network structure, prone to
power supply rail grounding faults. However, the existing fault location
methods do not accurately locate the fault point, which in turn makes it
difficult for the protection device to act to cut off the fault. To
address the above problems, this paper builds a dynamic simulation model
of the low and medium speed magnetic levitation power supply rails to
study the distribution characteristics of the fault traveling waves
after a ground fault occurs in the power supply rails, and analyses the
generation mechanism of the traveling wave spectrum through formula
calculation. The first step is to determine whether an earth fault has
occurred by analysing the difference in current between the positive and
negative busbars. Secondly, the direction of the current difference
between stations is compared to locate the faulty section. Finally, the
fault distance is calculated from the frequency difference of the fault
voltage at the double-ended station. Through simulation, the method is
validated to be unaffected by fault location, fault transition
resistance, noise interference, and is applicable to short circuit
faults caused by lightning strikes, and the ranging error always remains
within 20m. The method has strong robustness, can effectively solve the
problem of protection misoperation and accurately locate the fault
point. It is suitable for low and medium speed magnetic levitation
transportation power supply rail ground fault.
Introduction
Low and medium speed magnetic levitation is a new type of urban rail
transit operation, operating at speeds between 100-120km/h [1]. The
advantages of low operating costs, strong climbing ability and small
turning radius compared with high-speed magnetic levitation [2].
Compared with subway and light rail, it has the advantages of safety and
environmental protection, low noise, high speed and low construction
cost [3]. It is suitable for intercity traffic linkage and can
effectively relieve the traffic pressure in cities without affecting the
environment. At present, low and medium speed magnetic levitation
transportation has been vigorously developed at home and abroad [4].
The low and medium speed magnetic levitation uses the positive rail for
power supply and the negative rail for return, and the power supply rail
and return rail are installed on both sides of the track beam through
insulators, so the possibility of short circuit faults between the
positive and negative poles is low [5] - [6]. However, due to
aging insulators and track fouling, there is an increased possibility of
ground faults in the supply rails during train operation [7].
Therefore, when an earth short circuit fault occurs in the power supply
system, it is important to quickly determine the fault occurrence
section, activate the corresponding fault protection device to cut off
the fault, accurately locate the fault occurrence location and then
eliminate the fault to improve the train operation efficiency [8] -
[9].
In 2015, the Changsha Maglev Express occurred in a 5.12 blackout, which
was caused by a protection device malfunctioning due to a positive rail
grounding fault during train operation, and the overvoltage grounding
protection devices (hereinafter referred to as 64D) at all traction
power stations on the line detected the fault current and tripped, which
eventually led to the blackout of the entire line. Moreover, the short
power supply distance between low and medium speed magnetic levitation
traffic stations and the complex structure of the grounding network make
it difficult to locate the fault point accurately after a DC ground
fault occurs [10] - [11].
At present, the low and medium speed magnetic levitation traction power
supply protection system mainly includes DC equipment frame protection,
earth leakage protection and train frame protection [12]. The ground
leakage protection system mainly uses 64D protection devices. For the
problem of 64D misoperation, Woyang Li et al. [13] proposed to
change the overvoltage protection to high current protection, and the
protection action current value is set to 6000A, which can narrow the
accident range and achieve trip selectivity. However, at this point the
64D becomes a single conductor device, requiring a device with a very
high resistance to flow.
Fault location in DC systems is mainly based on the fault analysis
method and the travelling wave method [14]. The fault analysis
method, which is mainly used in traction power supply systems, is a
method of analysing and calculating the relevant parameters of the
system and the electrical quantities at the measurement points, and thus
finding the fault distance [15]. Carlos A. Platero et al. [16]
proposes an improved impedance ranging method based on the traditional
line current relationship between the two ends considering the effect of
high and low voltage at both ends. Abdelhamid Bendjabeura. et al.
[17] proposes a double-ended asynchronous ranging method based on
solving differential equations. Zhengqing Han et al. [18] presents
an improved single-end fault location algorithm solved by genetic
algorithm. Xincui Tian et al. [19] proposes a double-ended
steady-state fault ranging algorithm that takes into account the
distributed parameters of the rail-ground loop. The above method is
improved on the basis of the original method of solving differential
equations, which is less affected by the transition resistance of the
fault point and can significantly reduce the distance measurement error.
However, after a ground fault occurs in the supply rail, the fault
current flows into the grounding network, the impedance distribution of
the grounding network is very complex, and the idealised circuit model
is difficult to apply in engineering practice.
The traveling wave method is mainly applied to high-voltage DC
transmission line faults [20], where the fault distance is
calculated by identifying the fault traveling wave head through the
reflection of the fault traveling wave back and forth between the
measurement end and the fault point [21] - [22]. Zewen Li et al.
[23] proposes an improved single-ended traveling wave ranging method
based on current detection at the midpoint of the line, where a current
sensor is installed at the midpoint of the line to improve the
single-ended traveling wave ranging system. Hongchun Shu et al. [24]
proposes a method for fault ranging on high-voltage DC transmission
lines using a two-terminal traveling wave frequency difference ratio.
The traveling wave method is a hot research topic in the field of fault
ranging, with the advantages of high fault ranging accuracy,
independence from fault point transition resistance and low fault
ranging cost. However, the current traveling wave method of fault
ranging mostly studies ground faults in high-voltage DC transmission
systems and has rarely been applied in low and medium-speed magnetic
levitation traffic.
This paper proposes a method to protect the traction power supply system
of low and medium speed magnetic levitation traffic based on the fault
traveling wave characteristics. Firstly, a dynamic simulation model of
the low and medium speed magnetic levitation power supply rail is built
to simulate a DC earth fault on the power supply rail. Secondly, the
fault section is identified by the difference between the positive and
negative bus fault currents, and the corresponding station protection
device is tripped to cut off the power supply. Finally, the frequency
domain distribution of the line wave of the fault voltage variation at
the double-ended fault station is extracted and the fault distance is
calculated by the frequency difference. The proposed method is
unaffected by fault location, fault transition resistance and noise
interference and is suitable for short-circuit faults caused by
lightning surge voltage breakdown, with a range error always within 20m.
Compared to traditional fault location methods for DC traction power
supply systems, the measurement accuracy is higher and the problem of
protection malfunctions is effectively solved, making it suitable for
low and medium speed magnetic levitation traffic power supply rail earth
faults.
Simulation of low and medium speed
magnetic levitation traction power supply
system
2.1 Model of the traction power supply
system
The low and medium speed Maglev traction power supply system converts
three-phase AC high voltage power from the city grid to 1500V low
voltage DC power through transformers and rectifiers, and uses a contact
rail traction network to grant current. The traction network consists of
positive and negative rails, with the positive rail supplying power and
the negative rail returning current. The positive and negative rails
supply power to the train devices through the receiver installed on the
train, as shown in Fig. 1.
FIGURE. 1. Low and medium speed magnetic levitation traction
power supply system
2.2 Dynamic resistance of supply
rails
A centralised variable resistor is used to simulate the dynamic
resistance of the power supply rails during train movement, based on the
track changes during train operation. The external current bidirectional
function is implemented in the form of a rectifier bridge, as shown in
Fig. 2, and the module equivalent resistance is changed by controlling
the magnitude of the duty cycle of the switching tube S.
FIGURE. 2. Supply rail equivalent variable resistance
2.3 Negative rail dc protection
device
FIGURE. 3. Negative rail DC protection device
64D is provided between the negative DC bus and ground for each traction
substation of the medium and low speed maglev trains, as shown in Fig.
3. In the event of a positive earth fault, the fault current flows
through the fault point into the earth network and finally into 64D of
the station’s negative rail. The protection device trips when the
negative rail-to-ground voltage reaches the rectified value and the
power supply to the station is interrupted.
The station negative rail-to-ground voltage is calculated as:
As shown in (1), I is the fault current at the fault point of the
train, L is the total running distance of the train, x is the
distance of the train from the left traction substation, y is the
distance of the train from the right traction substation,Uxm and Uym is the
negative rail-to-ground potential of the station on the left and right
sides of the fault point.
The existing 64D rectification voltage is generally set to 200V, due to
the parallel operation of each traction substation. After a positive
ground fault occurred in a section, the negative voltage to ground in
each traction substation all rose simultaneously, with voltage values
exceeding 200V, resulting in 64D action on the entire line and
eventually leading to a loss of power on the entire supply track.
Method for determining the fault
area
3.1 Determining the occurrence of an
earth
fault
During normal train operation, the load current flows from the positive
terminal of the rectifier, through the positive busbar, the positive
supply rail, the train, the negative return rail and the negative
busbar, back to the negative terminal of the rectifier, as shown in Fig.
4. During normal operation, the currents in the positive and negative
buses in the traction substation are approximately equal in magnitude,
with the currents shown as the solid blue line in Fig. 4. When a
positive earth fault happens, the positive earth fault current is shown
as the solid red line in Fig. 4. Under fault conditions, the fault
current does not pass through the negative busbar, resulting in an
unequal current amplitude between the positive and negative busbars when
a positive earth fault occurs and producing a large current difference.
Based on this, this paper proposes to use the current difference between
positive and negative busbars to determine the positive earth fault,
calculate the current difference between positive and negative busbars,
and determine the occurrence of an earth fault when the current
difference is not 0A.
FIGURE. 4. Positive train ground fault current
3.2 Determining the fault
area
When a positive supply rail earth fault occurs on a train, each power
point supplies a short circuit current to the fault point. For faulty
areas, the fault current supplied by the rectifier in the traction
station at both ends of the area flows through the positive busbar in
the station to the fault point. For non-faulty areas, the fault current
supplied by the rectifier at the traction station within the area flows
across the positive busbar at both ends of the non-faulty area into the
adjacent area.
To this end, this paper proposes a method for identifying fault segments
using the positive bus current direction, with current flowing from the
positive bus into the positive contact rail as the positive direction
and current flowing from the positive contact rail into the positive bus
as the negative direction. When the protection element detects that the
difference between the positive and negative bus currents is greater
than the set threshold for determining a positive earth fault, the
protection unit initiates a determination of the positive bus current
direction. If the protection unit detects the positive direction of the
positive bus current and the same direction as the positive feeder
current at its opposite end, the zone is determined to be a fault area.
If the protection unit detects that the positive feeder current is in
the opposite direction to the positive feeder current at its opposite
end, the zone is determined to be a non-faulty area.
3.3 Algorithm flow for
determining the fault
area
When a positive earth fault occurs, the fault point resistance and the
fault voltage can be different from the fault current generated by the
fault resistance under the type of fault resistance. Current measuring
devices are set up on the positive and negative supply rails to detect
changes in current. When the difference in current between the positive
and negative buses ΔI≠0, it is judged that a supply rail earth fault has
occurred. Then, the current direction determination is activated to
collect information on the current direction of the positive supply
rails at each station and, based on the current direction at each
station, the faulty station zone is determined.
FIGURE. 5. Algorithm process 1
At the same time the positive earth fault protection operates, cutting
off the power supply to the faulty station and issuing an alarm signal
to inform the staff. The fault ranging device is further activated to
accurately locate the fault point. The algorithm flow is shown in Fig. 5
Fault location
methods
4.1 Time domain and frequency domain
methods
(a) Time domain waveforms
(b) Frequency domain waveforms
FIGURE. 6. Fault voltage time domain and frequency domain
waveforms
When a DC earth fault occurs on the supply rail, the fault traveling
wave is reflected between the fault point and the stations on the two
sides, and the fault voltage traveling wave measured at the station
fluctuates in a certain time sequence, as shown in Fig. 6(a).
The fault voltage traveling waves obtained at both stations have the
same time-domain attenuation characteristics. The fft transform is
performed on the station fault voltage traveling wave at both ends and
the frequency distribution is shown in Fig. 6(b). Where Δf=1/2τand τ is the time required for the fault traveling wave to propagate
from the fault point to the station.
Due to the short power supply range of low and medium speed magnetic
levitation traffic, when a positive ground fault occurs, it is not easy
for the measuring devices set up at both sides of the traction power
supply station to capture the wave head of the fault traveling wave, and
there are difficulties in using the traveling wave method for fault
location. At the same time, low and medium speed magnetic levitation
transportation has the advantages of low noise and smooth operation
compared with other rail transportation. Therefore, the frequency
amplitude is more easily identified in the event of a fault grounding.
Based on this, this paper proposes a fault location method based on the
frequency variation characteristics of double-ended stations.
4.2 Mechanisms for the generation of
faulty traveling wave
spectra
In the case of an earth fault on the supply rail between stations A and
B, the fault line wave will be reflected several times between the two
stations and the fault point. In the frequency domain this can be
expressed as the sum of an infinite number of harmonics of a particular
frequency, which is the spectral distribution of the fault wave.
FIGURE. 7. Supply rail short circuit diagram
In Fig. 7, ZL is the wave impedance of the supply rail,
ZA and ZB are the equivalent impedances
of the traction power stations of the stations on both sides
respectively, Rf is the transition resistance at the
point of failure, lf is the distance from the
point of failure to station A, v is the wave speed,e A(t ) ande B(t ) are the equivalent power supplies at
stations A and B, and Uf is the equivalent fault
voltage at fault point f.
The refraction and reflection of the fault travelling wave at station A
and fault point f is analysed. The equivalent fault resistanceZf is shown in (2), and the refraction and
reflection coefficients of the travelling wave at the fault point f are
shown in (3) and (4).
The refraction and reflection coefficients of the traveling wave at
station A are shown in (5) and (6).
By Davinan’s equivalence theorem, the supply system is equated when a DC
earth fault occurs in the supply rail. The supply rail is equated to a
wave impedance ZL in series with a controlled voltage
source f A(t ). Station A is equivalent to
the system impedance ZA with an independent voltage
source eA(t ). The fault is equated to the
controlled voltage source f f(t ) as shown
in Fig. 8.
FIGURE. 8. Davinan equivalent circuit for traction power supply
system after failure
The voltage at station A, u A(t ), and the
voltage at the fault point f, u f(t ), are
shown in (7) and (8).
The controlled power supplies f A(t ) andf f(t ) are shown in (9) and (10).
The Laplace transform of (9) and (10) leads to (11) and (12).
In (11) and (12), f A(s ) andf f(s ) are the controlled sourcesf A(t ) andf f(t ), β A(s )
and βf (s ) are the pull domain transforms
of the reflection coefficients β A andβf , P (s ) = exp(-sτ ) is the
pull-domain operator for the propagation delay of the faulted traveling
wave; E 1(s ) is the pull-domain transform
of the ideal DC source on the system side with frequency 0.
Combining (11) and (12), it can be concluded that the natural frequency
of the fault traveling wave obtained at the traction power supply
station A is the root of the characteristic equation as (13).
The natural frequency equation obtained by solving is (14).
In (14), the fn is the nth component of the
natural frequency; vn is the wave speed at that
frequency, is the angle of reflection of the traveling wave at the
traction power supply station A, is the angle of reflection of the
traveling wave at the fault point f.
4.3 Line wave frequency difference
ratio and fault
ranging
A traveling wave measurement device is set up at the location of the
traction power supply station to extract the fault traveling wave
frequency, and from (14) the nth traveling wave frequency can be
expressed as (15).
The (n+1)th frequency is shown in (16).
In (16), θ 1 and θ 2 denote
the reflection angles of traction power supply stations A and B
respectively; L is the full length of the line.
According to equations (15) and (16) can be derived from the double-end
frequency difference as shown in (17).
Because the reflexion coefficients are all less than 1 in absolute
value, when the fault traveling wave is refracted through the fault
point and reaches the opposite side, it is then reflected back to the
voltage through the station. The voltage reflected back from the fault
point directly is small compared to the fault voltage, so the frequency
difference can be determined by taking the extreme points of the change
in the frequency domain waveform, as shown in (18).
The fault distances calculated from the frequency differences of the
stations on both sides of A and B (both being the distance of the fault
point from the station on the A side) are shown in (19).
From the above formula can be learned, the frequency distribution of the
fault line wave is not affected by the traction power supply station
impedance, transition resistance, only with the location of the fault
point and the fault line wave arrives at both ends of the line related
to the time. That is to say, the fault line wave frequency difference
obtained at both ends of the line and the fault location has a clear
mathematical relationship. The location of the fault point can therefore
be calculated from the fault frequency difference and the speed of the
fault travel wave.
4.4 Fault location algorithm
flow
Each station is equipped with a travelling wave coupling measurement
device. After the fault zone has been determined by algorithm process 1,
the corresponding station protection device trips and cuts off the power
supply. At the same time, the fault traveling waveforms of the stations
at both ends are extracted and the fault distance is measured by the
fault traveling waveform frequency difference method.
Fast Fourier transform of the station fault voltage signal, extract the
double-ended station spectrum fA and fB,
take the more regular part of the spectrum distribution, find the
amplitude point, calculate the difference between the two ends of the
frequency, and take the plural to get ΔfA and
ΔfB. If
ΔfA<<ΔfB and
ΔfA>>ΔfBoccur, it proves that a near-station ground fault has occurred and the
station on the side with the larger frequency difference is closer to
the fault point; to reduce the error, the fault distance
lf is calculated by substituting Δfmaxinto (19). If the above situation does not occur, the
fault frequency difference ΔfA and ΔfBbetween the two stations is substituted into (19) to calculate the fault
distance lfA and lfB, and the average of
the two is taken as the final fault distance lf. The
flow of the algorithm is shown in Fig. 9.
FIGURE. 9. Algorithm process 2
Simulation
verification
A simulation model of the ±1500V DC transmission system is built, the
supply rails are equivalent to Π-type conductors, the distance between
stations is set to 3km, four stations are set up along the entire line,
and each station is equipped with a traveling wave coupling box to
measure fault traveling waves. The speed of the travelling wave is
calculated as v =33.41km/ms, the unit resistance of the supply
rail R0=0.028Ω/km, the unit inductance
L0=2.6629mH/km, the unit capacitance
C0=0.0211μF/km, the sampling frequency is
108Hz, and the equivalent impedance of the station
traction power supply RQ=2.5685, go for simulation
verification.
5.1 Determining the fault
area
Fig. 10 shows the positive and negative bus currents and their
differences at stations A, B, C, and D after a positive earth fault on
the supply rail.
From Fig. 10, the train was running normally 0.01s ago and the positive
and negative bus currents at stations A, B, C, and D were equal in
amplitude and opposite in direction, ΔI=0. After 0.01s the positive bus
current is greater than the negative bus current and ΔI≠0. From this it
can be determined that an earth fault occurred on the supply rail at
0.01s. The train sends out an alarm signal to notify the crew of an
earth fault and simultaneously initiates a current differential
direction determination.
The direction of the current difference between stations A and B is
reversed and the A-B section is determined to be a non-faulty section.
If the current difference between C and D is in the same direction, the
C-D section is judged to be a faulty section and the protection unit on
C and D stations operates to remove the faulty section.
FIGURE. 10. Positive and negative buses currents and their
differences at each station after a fault
5.2 Fault point
location
After identifying the fault zone from the above steps, the station C and
D earthing protection device trips and cuts off the power supply, while
extracting the fault traveling waves in the 1ms time window before and
after the fault. Fig. 10 shows the time domain distribution of the
faulty traveling waves in ① and ②. As can be seen from the graphs, the
waveheads of the fault line waves are not easy to capture and there are
difficulties in using the line wave method for fault location.
(a) Station C
(b) Station D
FIGURE. 11. Station fault voltage frequency distribution
Therefore, this paper proposes a fault location method based on
frequency variation characteristics, where the extracted fault line wave
is subjected to fast Fourier analysis and the fault distance is
calculated by the fault line wave frequency difference method.
Fig. 11(a) and Fig. 11(b) shows the frequency distribution of fault
voltages at double-ended stations. Δf C=66.2kHz,
Δf D=33.2kHz,
Δf C>Δf D, so
the fault point is closer to station C. Calculated by bringing in (19):l fC=1007.63m, l fD=990.81m,l f=999.22m, the fault distance error is 0.78m.
The above analysis shows that the fault ranging method with frequency
distribution characteristics can be used to locate the fault point
accurately.
Analysis of methodological
adaptability
This paper discusses the adaptability of the simulation model and
algorithm in terms of four aspects: fault point location, fault point
transition resistance, noise disturbance, and lightning surge voltage
effects.
6.1 The effect of fault
location
When an earth fault happens on a train, the distance between the fault
point and the station measurement point affects the distribution of the
fault frequency and the difference in frequency distribution has an
impact on the measurement error.
As shown in Fig. 12, single-ended fault ranging at near-fault stations
yields a smaller measurement error for near-fault faults, and
double-ended fault ranging yields a smaller measurement error for more
distant faults. Furthermore, as the fault distance increases, the
measurement error increases accordingly.
FIGURE. 12. Error distribution at different fault locations
As can be seen from (18), when a near fault occurs, the frequency
difference of the fault line wave is large and the measurement error
caused by the frequency difference error is small; when a long distance
fault occurs, the frequency difference of the fault line wave is small
and the measurement error caused by the frequency difference error is
relatively more. Therefore, in the case of close faults, single-ended
fault ranging using the station voltage on the side closer to the fault
point can effectively reduce the fault ranging error. In the case of
longer distance faults, the average of the error distances measured at
both ends is chosen as the total error distance, which can effectively
reduce the measurement error.
6.2 Effect of transition resistance at
the point of
failure
6.2.1 Effect of transition resistance
on the measurement
error
FIGURE. 13. Error distribution at different transition
resistances at different fault points
During the operation of low and medium speed magnetic levitation
traffic, the types of faults are diverse and high resistance earth
faults may occur due to fouling and ageing of insulators. Keeping the
above line conditions constant, the transition resistance is set to
Rf=0.01Ω and Rf=500Ω respectively to
simulate the earth fault in low and high resistance cases.
In Fig. 13, the maximum range error is 10m when a low resistance earth
fault occurs, with relatively little change in the range error as the
fault location changes. When a high resistance earth fault occurs, the
maximum range error is 16.94m, which is greater than the range error
caused by a direct earth fault. And when a close high resistance earth
fault occurs, the fault error is so large that it is even impossible to
calculate the fault point.
6.2.2 Discussion of fault resolution
under high resistance earth faults
b
Under a high resistance earth fault, the shunt effect at the fault point
is small and the current flowing back to the station negative rail
through the earth network is small, so the voltage detected by the
station 64D is small and may not reach the protection action voltage,
resulting in the circuit breaker not being able to act in time to cut
off the power supply to the faulty station. This paper uses the positive
and negative bus current difference to determine the occurrence of
grounding faults. When a high resistance ground fault occurs, the
current difference between the positive and negative buses is small but
not 0, which can determine the occurrence of grounding faults and avoid
the problem that the circuit breaker cannot operate in time.
As can be seen from (6), under the high resistance ground fault, the
refraction coefficient of the fault point is larger, the fault line wave
is refracted through the fault point to the opposite end of the station,
and then reflected back to the local side of the voltage is larger, the
frequency domain shows a small difference in the amplitude of the
different frequencies, the frequency difference resolution is lower,
interfering with the acquisition of the frequency difference, resulting
in a larger measurement error.
6.3 Effect of noise
interference
(a) Time domain waveforms
(a) Frequency domain waveforms
FIGURE. 14. Time-domain and frequency-domain distribution of
faulty traveling waves under noise interference
This paper uses the difference between positive and negative bus
currents to determine the occurrence of earth faults. The noise mainly
interferes with the time domain distribution of the fault waveform and
has less effect on the amplitude of the fault current, so it has less
influence on determining whether a fault has occurred.
Keeping the above supply rail fault conditions constant, simulate a
ground fault occurring in a train with large Gaussian random noise, and
observe the change in the waveform of the fault line wave after adding
noise, as shown in Fig. 14(a) and Fig. (b). After adding noise, the
time-domain waveform of the faulty line wave is almost always masked by
noise and the wavehead signal cannot be distinguished, as shown in ① in
Fig. 14(a). However, the frequency domain waveform is highly resistant
to noise, with small fluctuations in frequency distribution, and overall
the spectrum distribution is relatively stable and does not affect fault
location.
6.4 Lightning surge voltage
effects
For low and medium speed magnetic levitation traffic, the power supply
and return rails are located on both sides of the track girders and
there are no shielding protection facilities such as contact nets above
the car body, which makes the car body vulnerable to lightning. The
duration of the lightning surge voltage is very short, a non-periodic
voltage variation of only a few microseconds to a few tens of
microseconds, but the voltage generated is so high that it can seriously
damage the insulation of the train’s power supply rails and track
support devices, or even break through and cause grounding faults in the
power supply rails.
Keeping the above line conditions constant, simulate the ground fault
caused by lightning surge voltage, where the station C traveling wave
measurement device to obtain the fault traveling wave time domain,
frequency domain distribution as shown in Fig. 15(a) and Fig. 15(b).
As can be seen from Fig. 15(a), the lightning surge voltage only affects
the wavehead steepness and amplitude of the fault line wave, not the
overall time-domain distribution of the line wave, which in turn does
not affect the distribution of the fault voltage line wave frequency
domain, meaning that the frequency difference remains constant. After
the supply rail is disturbed by lightning, the station protection device
is controlled by the Algorithm 1 process to cut off the power supply to
the faulty station. The measurement device extracts the fault travelling
wave spectrum, the frequency distribution is consistent with the ground
fault except for the amplitude, and the measurement error calculation
follows the Algorithm 2 process.
(a) Time domain waveforms
(b) Frequency domain waveforms
FIGURE. 15. Time-domain and frequency-domain distribution of
faulted traveling waves under lightning faults
Therefore, lightning strikes resulting in a supply rail fault are
consistent with the short-circuit fault ranging algorithm process.
However, the high inrush current of lightning for a short period of time
requires an increase in the current resistance of the current detection
device.
The distance measurement results for different fault conditions are
shown in Table Ⅰ.
TABLE I Units for Magnetic Properties