Fault location on power cables
Pre location
Impulse-Current-Method (ICE)
Table of contents
1.
ICE (Impulse current method three phased ......................................................... 3
1.1. Ionisation delay time..................................................................................... 3
1.2. DIRECT MODE ............................................................................................ 3
1.3. Output impedance of the generator .............................................................. 4
Surge generator as impulse source ..................................................................... 4
High voltage test set as impulse source .............................................................. 4
1.4. The current coupler ...................................................................................... 5
1.5. Typical waveforms........................................................................................ 5
1.6. Evaluation of the measurement .................................................................... 7
1.7. Improving the measurement accuracy ......................................................... 7
1.8. Influence of a connection cable .................................................................... 7
2. Comparison mode I ............................................................................................. 8
2.1. Straigt networks............................................................................................ 8
2.2. T’eed Networks .......................................................................................... 11
The fault lies on the main conductor between generator and loop. ................... 11
Very long T’eed networks .................................................................................. 14
2.3. Influence of a connection cable .................................................................. 15
3. Comparison mode II .......................................................................................... 16
3.1. UnT’eed Networks ...................................................................................... 17
3.2. T’eed Networks .......................................................................................... 18
The fault is located on a main conductor between generator and loop. ............ 18
The fault is located in a tee................................................................................ 18
3.3. Influence of a connection cable .................................................................. 20
4. Differential comparison mode ............................................................................ 21
4.1. The differential coupler ............................................................................... 21
4.2. Straight networks........................................................................................ 22
4.3. T’eed Networks .......................................................................................... 24
The fault is located on the main conductor between generator and loop........... 24
The fault is located in a tee................................................................................ 24
4.4. Practical Application ................................................................................... 25
4.5. Influence of a connection cable .................................................................. 26
5. Loop On - Loop Off Method ............................................................................... 27
6. Measuring system Centrix 3 .............................................................................. 28
ICE 3PH.................................................................................................................... 28
2
1.
ICE (Impulse current method) three phased
Four impulse current circuit methods have proven to be successful in prelocating high
resistance and intermittent faults in power cables:




direct mode
comparison mode
differential-comparison mode
loop-on – loop-off method
1.1.
Ionisation delay time
The ionisation delay time “t” is the period of time prior to the flashover at the fault
during which the charge carriers are ionized. This can take up to some milliseconds.
Many times a flashover only takes place after the voltage wave has passed the point
of fault and has been reflected at the open cable end with the same polarity, which
leads to a voltage doubling. When the voltage wave passes the fault again, usually a
flashover occurs.
In order to reduce the ionisation delay time “t”, it is recommended to use the
maximum output energy of the generator without however exceeding the permissible
voltage of the cable.
Since the ionisation delay time can be some milliseconds, the test range to be set on
the transient recorder always has to exceed the length of the faulty cable many times
over.
1.2.
DIRECT MODE
A surge generator or a DC test set serve as an impulse source. Any tee between the
start end of the cable and the fault will inevitably cause multiple reflections which
disturb the reflection process to be evaluated. Hence, the direct mode is only applied
in cables without tees.
Cable with fault
G
Generator
M
Fig 1: Principle of operation „Direct Mode“
The reflections in the faulty cable are recorded via a current coupler which is
connected to the faulty conductor and to a pulse reflection instrument with built-in
transient recorder.
3
1.3.
Output impedance of the surge generator
In the direct mode, the current coupler is always positioned in direct vicinity of the
generator (e.g. surge or shock wave generator - SWG). Each time a transient current
wave travelling towards the start end of the cable passes the current coupler, it is
almost simultaneously reflected with the same polarity at the low resistance output
impedance of the generator so that in this moment the outgoing and reflected waves
superimpose (add up).
In order to achieve a current doubling, the following must apply:
Zgenerator << Zcable
Zgenerator (output impedance of the generator)
Zcable (characteristic impedance of the cable)
Surge generator as impulse source
As far as the transient phenomenon is concerned, the output impedance of the
surge generator is zero. This enables a current doubling at the current coupler.
High voltage test set as impulse source
G
A current doubling is only possible by connecting a capacitor of some
microfarad to the high voltage output. A surge generator, too, can serve this purpose,
provided the output voltage is set to DC mode, while increasing the voltage. In this
operation mode, the surge generator functions similar to a high voltage test set.
The DC voltage potential at the fault increases until the breakdown voltage is
reached. After a flashover appeared at the fault position, the whole energy stored in
the cable and SWG discharges abruptly, thus generating a transient wave.
G
Faulty cable
C=1uF
Generator
M
Fig 2: High voltage test set with capacitor
This test mode does not involve an ionisation delay time. In practice, the DC voltage
discharge method delivers very clear, reproducible reflection diagrams. However, it
will fail in the case of faults which do not allow a charging of the cable due to a shunt
resistance.
4
1.4.
The current coupler
The current coupler consists of an inductive pickup which is mounted to the high
voltage output of the surge generator. The high-pass effect of the current coupler
produces clear waveforms with steep rising edges. Since the current on the cable
shield and in the conductor is the same and only the direction changes, it is of no
consequence over which conductor the current coupler is positioned.
+
G
G
+
Generator
Generator
Fig 3: Position of the current coupler
1.5.
Typical waveforms
Waveform No. 1:
Parasitic Reflections
tx
tx
Fig 4: Reflections at an open cable end
Waveform No. 2:
tx
tx
tx
Parasitic Reflections
Fig 5: Reflection at a shorted cable end
5
Waveform No. 3:
Reflection in the event of a flashover at a discharge fault or an insulation resistance
fault. Here, an ionisation delay time t may occur.
tx+
t
tx
tx
Parasitic Reflections
Fig 6: Reflection at a flashover fault
Waveform No. 4:
Similar to waveform no. 3 but with a longer ionisation delay time t. The fault only
ignites after a reflection at the open cable end, whereby the transient voltage wave is
reflected with twice the amplitude.
tx+
t
A
tx
tx
Parasitic Reflections
tL
Parasitic Reflections
Fig 7: Flashover fault: long ionisation delay time
If the point “A” is difficult to determine, then the surge voltage has to be increased
without exceeding the permissible voltage of the cable. The ionisation delay time t
can thus be reduced. The resulting waveform corresponds to waveform no. 3.
6
1.6.
Evaluation of the measurement
The evaluation of the waveforms is done between the negative reflections at the fault
in consideration of the parasitic reflections. These parasitic reflections are caused by
the inductance of the test leads of the generator.
V
 tx
2
X
X: Distance between fault and current coupler
Equation 1: Calculation of the fault distance
1.7.
Improving the measurement accuracy
It is possible to eliminate the uncertainty of the factor v/2 and to obtain a more
accurate test result by applying a surge wave to a healthy conductor (typical
waveform no. 1). The total length “L” of the cable must be known. The parameter v/2
is changed until the cable length “L” is shown on the pulse reflection instrument. This
v/2 value is then retained for all further measurements on this cable.
Note: When using a high voltage connection cable:
L = faulty cable + connection cable.
1.8.
Influence of a connection cable
When using a high voltage connection cable between the generator and the faulty
cable, then the current coupler has to be placed as close to the generator as possible
(point of reflection) in order to obtain a current doubling.
High voltage test lead
Cable with
fault
G
l´
M
M
Fig 8: Position of the current coupler with test lead
In these measurements, do not forget to subtract the length of the test leads
(transmission time t' or fictitious length l'). The length of the coaxial cable which
connects the current coupler to the pulse reflection instrument has no effect on the
measurement.
7
2.
Comparison mode I
A high voltage test set or a surge generator (with the surge button pressed) is used
as an impulse source. This method is mainly used in T’eed networks.
Requirement: A healthy conductor must be available and it must be possible to
charge the cable (no parallel resistance).
Good conductor
M
TDR
Faulty conductor
G
Generator
Fig 9: Schematic circuit diagram Comparison Mode I
In this method, two traces recorded during a flashover at the fault are compared.
2.1.
Straigt (unbranched) networks
First measurement:
The generator charges the cable capacitance and an arc (flashover)develops at the
fault position. This flashover generates a travelling wave which propagates in both
directions from the fault position.
Good conductor
G
Faulty conductor
Generator
X
Y
Fig 10: Measurement without loop at the cable end
The reflection towards the cable end will travel at a velocity “V” and will be reflected
between this point and the fault. This reflection will not influence the trace.
The reflection towards the start end of the cable will also travel at a velocity “V”.
At the contact points, a small part of the energy is reflected to the fault. The
remaining part travels to the generator.
When passing the current coupler, the transient current wave is recorded.
8
The trace might look as follows:
Memory 1
Fig 11: Measurement without loop at the cable end
Second Measurement
A loop is connected at the cable end between the faulty conductor and a healthy
conductor.
Good conductor
G
Faulty conductor
Y
X
Generator
Loop
Fig 12: Measurement with loop at the cable end
An increase in voltage will lead to another flashover at the fault. The reflection
towards the start end of the cable is also recorded. The start of the trace corresponds
to that of the first measurement.
In this case, however, the reflection towards the far end of the cable which did not
have any influence in the first measurement, will pass the loop and reach the start
end of the cable via the healthy conductor. Only at the point where this reflection
passes the current coupler, the trace will differ from the first measurement.
The trace obtained could be as follows:
Memory 2
Fig 13: Measurement with loop at the cable end
9
2.2.
Evaluation of the measurement
The distance covered by the transient current wave which passes the loop at the
cable end during the second measurement is calculated as follows:
2X + 2Y
The distance covered by the transient wave which is reflected at the fault in the first
measurement is: 2X.
The point of deviation of the two superimposed traces can be easily and accurately
determined. The point corresponds to:
2X + 2Y - 2X = 2Y.
ty
M2
M1
Memory 1 / 2
Fig 14: Comparison trace
Hence, the distance of the fault from the far end of the cable is:
Y
V
 tY
2
Equation 2: Calculation of the fault distance
10
2.3.
T’eed Networks
The fault is located on the main conductor between generator and loop.
A
B
G
Loop
Y
Generator
Fig 15: Fault on the main conductor
First measurement without loop at "B":
Despite the fact that the reflection in the direction of the start end of the cable has
passed through the different tees, it reaches the coupler and is recorded in the
transient recorder.
Second measurement with loop at "B":
The reflection in the direction of the far cable end which did not have any influence
on the first measurement passes the loop and returns to the start end of the cable via
the healthy conductor. At the point where this reflection passes the current coupler,
the trace will deviate from the previous one.
The tees do not have any effect on the principle. However, in each tee, only part of
the signal will propagate in forward direction. Hence, the signals at the current
coupler will be weakened and the deviations of the traces will be less distinct.
As for a cable without tees, the measurement gives the distance of the loop to the
fault.
The fault is located in a tee
A
B
D
G
Y
Generator
Loop
C
Fig 16: Fault in in a tee
The comparison of the traces recorded with and without loop at "B" is similar.
The measurement gives the distance between the loop and the point of origin of the
tee in which the fault lies.
11
Now there is one uncertainty factor, since the fault can be located at any point on the
tee "D-C". In order to eliminate this uncertainty, the loop has to be positioned at the
end of the tee "C". Now the section "A-C" is the main current circuit and "D-B" a tee.
Example: Given the following 3-phase 20 kV network with a discharge fault at the
phase R at a flashover voltage of 14 kV.
1. Measurement
G
A
420m
B
300m
D
330m
120m
330m 2. Measurement
120m
J
420m
180m I
G
480m
Generator
C
M
375m
E
L
240m
K
F
H
Fig 17: 20 kV Network
The generator is connected at the start end "A" of the cable between the phases R
(faulty conductor) and S (healthy conductor). The current coupler of the phase S (I2)
is used. The propagation velocity v/2 of 80 m/s is an approximate value.
First measurements:
Trace "a" without loop.
Trace "b" with loop at "E" between the phases R and S.
tyE= 720m
b
a
Fig 18: Comparison with and without loop at far cable end
12
The measurement shows the fault at a distance of 15 m from the tee
"C" in the direction of "B".
Considering the measuring error, especially in the propagation velocity v/2, the fault
could also be located directly at "C" and possibly also in the tee "C-H".
Second measurements:
Trace "a" without loop.
Trace "b" with loop at "H" between the phases R and S.
Ty H= 398m
b
a
Fig 19: Comparison with and without loop at the end of the tee
The measurement shows the fault at a distance of 172 m from "C" in the direction of
"G".
The uncertainty factor is now eliminated, since it is known that there is no other tee
between these points.
The main sources of error in these measurements are to be traced back to
- an estimated propagation velocity v/2
- inaccurate network plans.
13
Very long T’eed networks
In order to minimize the time spent for the location of faults in long cable networks, it
is possible to pre-locate the fault roughly through a survey measurement so that the
loop can be positioned as close to the start end of the cable as possible.
Conducting a survey measurement
First measurement: Surge wave on a healthy conductor.
Memory 1
Fig 20: Measurement at a good conductor
Second measurement: Surge wave on a faulty conductor
Memory 2
Fig 21: Measurement on a faulty conductor
Evaluation of the measurement
The superposition of the two traces shows a deviation.
tx + t
1
2
Memory 1/ 2
Fig 22: Comparison reflectogram
14
Example:
If tx + t = 32,5 µs, then it is known that the fault is located at a distance of less than
32.5 µs * 80 m/µs = 2600 m from the start end of the cable.
For accurate measurements the ionisation delay time t has to be reduced and the
propagation velocity on a cable with tees has to be reduced as follows :
v/2  80 - N (N: number of tees between the start end of the cable and the fault).
In the subsequent accurate pre-location, the loop is positioned at the end of a tee
whose origin is at least 2600 m from the start end of the cable. In this example, the
loop is positioned at "B" or "C".
A
420m
1900m
1100m
700m
1600m
3300m
2600m
3000m
2300m
2800m
C
3000m
3100m
B
Fig 23: Positioning the loop after the survey measurement
2.4.
Influence of a connection cable (test lead)
If a high voltage test leads is used between the generator and the faulty cable, then
the current coupler can be positioned either at the output of the generator, or at the
connection point of the faulty cable.
Fig 24: Positioning the current coupler
In both cases, the propagation velocities in the high voltage test leads or in the
coaxial cable do not influence the measurement.
15
3.
Comparison mode II
The impulse source is a high voltage test set or a surge generator
(with the impulse button pressed). This method is mainly used in T’eed networks.
Prerequisite : At least one healthy conductor must be available and it
must be possible to charge the cable (no parallel resistance).
Two traces generated through a rise in the cable voltage and a resulting flashover at
the fault, are compared.
The method differs from the previous one only in the position of the current coupler
and in the results obtained in T’eed networks.
Good conductor
G
Faulty
conductor
Generator
Fig 25: Basic circuit diagram Comparison Mode II
The current coupler is connected directly at the output of the generator. Since the
current in both conductors is the same, the coupler can be positioned over either of
the two conductors.
G
G
Generator
Generator
Fig 26: Positioning the current coupler
16
3.1.
Straight Networks
First measurement:
The voltage in the cable is increased until a flashover is obtained. The trace could be
as follows:
Memory 1
Fig 27: Recorded reflectogram 1
Second measurement:
A loop is connected at the far end of the cable between the faulty conductor and the
healthy conductor.
Good conductor
Loop
G
Faulty conductor
Generator
Fig 28: Measurement with loop at the far cable end.
An increase in voltage will cause another flashover at the fault.
The trace could be as follows:
Memory 2
Fig 29: Recorded reflectogram 2
17
Y
A comparison between the two traces allows a determination of the distance of the
loop at the far cable end to the fault.
ty
1
2
Fig 30: Comparison of the two recorded reflectograms.
3.2.
T’eed Networks
The fault is located on a main conductor between generator and loop.
X´
A
Y
B
G
Loop
X
Y
Generator
Fig 31: Fault in the main conductor
Note: This method always indicates the distance between the loop and the fault.
The result of this measurement indicates a fault either at the point "X" or "X´". Hence,
there is an uncertainty.
It is to be noted that with the method described under 7.2.1., the fault would be
prelocated directly at the point "X".
The fault is located in a tee
As in the preceding case, there is an uncertainty.
A
D
B
Y
G
X´
Y
X
Generator
Fig 32: Fault on a tee
18
Loop
Note: When using the comparison mode I for fault location on tees, the fault distance
indicated is the distance between the far cable end "B" and the point of origin of the
tee "D".
When using the comparison mode II, the fault distance indicated is the distance
between the far cable end "B" and the fault position "X".
A combination of the two comparison methods (I and II) enables a reduction of the
number of measurements (shifting of the loop).
Example:
Given the following intentionally simple network:
1
A
D
B
Y1
2
G
X´
Y2
Loop
X
Generator
C
Fig 33: Network with fault on a tee
A first measurement is carried out using the Comparison Mode I (coupler 1) with the
loop at the end position "B".
Y1 = BD (fault at the point of origin of the tee "DC").
A second measurement is conducted using the Comparison Mode II (coupler 2) :
Y2 = BX = BX' (fault at X or X').
Conclusion: the fault is at X
19
3.3.
Influence of a connection cable
The generator can be connected to the start end of the cable by means of one or two
high voltage test leads. To simplify matters, the current coupler is mostly positioned
at the level of the generator.
HV test lead
G
Faulty conductor
Generator
HV test lead
Good conductor
G
Faulty conductor
Generator
Fig 34: Connection via high voltage connection cables
The propagation velocity in the high voltage connection cables has no influence on
the measurement.
20
4.
Differential comparison mode
The impulse source is a surge generator. This method is mainly used in T’eed
networks.
Prerequisite: a healthy conductor must be available.
Good conductor
Differential coupler
Faulty conductor
Generator
Fig 35: Basic circuit diagram of the differential comparison mode
In this method, two traces which have been generated by a surge wave, are
compared. The surge generator is connected simultaneously to the faulty and healthy
conductors.
4.1.
The differential coupler
The differential coupler consists of two identical couplers which are mounted over the
conductors. The difference formation is effected by the software.
Conductor 1
I1
Conductor 2
I2
Fig 36: the differential coupler
When a pulse or a reflection passes the two conductors simultaneously, then the
difference of the two signals I1 and I2 will be Zero: A signal will not be coupled out.
(Example: reflection at a tee).
If however the reflection returns on one conductor only, then the same signal as with
a single current coupler is obtained. (Example: Reflection at the flashover at the
fault).
21
4.2.
Straight networks
First measurement
Good conductor
Differential coupler
Faulty conductor
Generator
Fig 37: Connection for the first measurement
If a surge wave passes at the level of the differential coupler, then it will trigger the
recorder, notwithstanding the differential measurement.
All identical reflections at the two conductors and at the tees will neutralize because
of the differential formation. However, the reflection which has been caused by the
flashover at the fault and travels to the start end of the cable will generate a
differential signal.
The trace could be as follows:
tx +
t1
M1
Surge wave
Memory 1
Fig 38: Recorded reflectogram after the first measurement
Second Measurement
A loop is connected at the far cable end between the faulty and healthy conductors.
Good conductor
Differential coupler
Faulty conductor
X
Generator
Fig 39: Second measurement with loop
22
Loop
Y
A second surge wave generates another flashover at the fault and triggers the
transient recorder again. The reflection travels in the direction of the start end of the
cable and leads to a trace which is identical to the preceding one.
The trace could be as follows:
tx + ∆t2
M2
Surge wave
Memory 2
Fig 40: Recorded reflectogram after the second measurement
Evaluation of the measurement
The difference in the distance covered by the reflection which passes the loop
(2X+2Y) and the reflection without loop (2X) is twice as large as the distance of the
loop to the fault (2Y). The corresponding time is "ty". Since the ionisation delay time
t can vary between two measurements, the points M1 and M2 have to be
superimposed for a comparison of the traces.
ty
Memory 1 / 2
Fig 41: Superposition of the two traces
Hence, the distance between the loop and the fault is :
V
Y   ty
2
Equation 3: Calculation of the fault distance
23
4.3.
T’eed Networks
The fault is located on the main conductor between generator and loop
A
B
X
Loop
Y
Generator
Fig 42: Network with T’ees
As in the comparison mode I, this measurement gives the distance between the loop
and the fault, provided the fault lies on the main conductor between the generator
and the loop.
The fault is located in a tee
A
D
B
Y
Generator
Loop
X
C
Fig 43: Fault indicated at the point of origin of a tee
If the fault lies on a tee, then the measurement will give the distance of the loop to
the base of this tee.
In this case, there is an uncertainty factor, since the fault could lie anywhere on the
tee "DC".
In order to eliminate the uncertainty, the loop has to be mounted at the end of the tee
"C". Now, "AC" is the main current circuit and "DB" a branch.
24
4.4.
Practical Application
Prerequisite:
- Three linear couplers (I1, I2, I3)
- The cable has two healthy conductors
1. Connect the surge generator and the three current couplers (I1, I2, I3)
to the cable conductors (L1, L2, L3)
2. Connect a loop between the faulty and healthy conductor at the far cable end.
To TDR
L3
L2
L1
To TDR
Y
Generator
Fig 44: Use of three couplers
3. Transmission of a surge wave and use of the differential coupler "I1-I2"
(Corresponds to trace without loop).
M1
Memory 1
Fig 45: First measurement “without loop“
4. Transmission of a surge wave and use of the differential coupler "I1-I3"
(Corresponds to a trace with loop).
M2
Memory 2
Fig 46: Second measurement “with loop“
25
5. Comparison of the content of the memory after the points M1 and M2 have been
shifted to superimpose.
ty
M
Memory 1/2
Fig 47: Both traces superimposed
Conclusion:
The advantage of this circuit is that if required, the traces with and without loop can
be repeated several times in order to select the comparison that provides the best
interpretation without having to connect and disconnect the loop. The safety factor
cannot be denied (no additional contact with the cable required).
4.5.
Influence of a connection cable
If a high voltage test leads is used between the generator and the start end of the
cable, then the differential coupler can be positioned either at the level of the
generator, or at the start end of the cable.
HV-Test lead
Good conductor
Faulty conductor
Generator
TDR
Coaxcable
Good conductor
Generator
Faulty conductor
HV-test lead
Fig 48: Position des Differential couplers
26
The propagation velocities in the high voltage test leads and in the coaxial cable do
not have any influence on the measurement, since this is always carried out from the
far end of the cable to the point of fault.
5.
Loop On - Loop Off Method
The impulse source is a high voltage test set or a surge generator. This method is
used in unT’eed networks.
Prerequisite: the high voltage test leads has to be sufficiently long (l' >> 50 m).
In this method, one compares to traces, first with loop and then without (loop on /
loop off), whereby the loop is connected at the end of the high voltage test leads.
Loop
HV-test lead
G
l´>50m
Generator
X
Fig 49: Connection with high voltage test lead
tx
M
Memory 1 / 2
Fig 50: Test result of the loop on / loop off method
If the measurement is carried out with surge waves, then the points M have to be
shifted to superimpose.
If a voltage generator (e.g. surge generator with the impulse key pressed) is used for
the measurement, then the left part of the waveform up to point "M" is not displayed.
The measurement always gives the distance of the fault to the start end of the faulty
cable.
27
The fault distance is calculated as follows:
X
V
 tX
2
Equation 4: Calculation of the fault
6.
6.1.
Measuring system Centrix 3 ICE 3PH
Phase selection
For three-phase ICE measurement, the phase selection results from the following
curcuit diagram:
In the phase selection menu a maximum of two phases can be activated for
simultaneous measurement. If the operator selects three phases, the measurement
cannot be started and an appropriate message is displayed.
6.2.
Phase selection menu
Connection of the test cable
Simplified block diagram showing the currently activated
phases (closed switch) and their polarity corresponding to
the active measuring coil selection
(in this case L1 and –L2).
Menu to select the phases applicable for measurement
(in this case L1 and L2).
Menu to select the measurement coil applicable for
measurement (in this case P1).
The phases measured by the respective coil and their
polarity are shown in parentheses
(corresponding to the active phase selection).
28
For a typical three-phased ICE measurements, two measurements have to be
performed. For the first measurement, the test cable has to be connected as follows:
Prior to the second measurement, the two selected phases (in this case L1 and L2)
have to be looped at the far end of the cable as shown in the picture below:
Operation mode
The operator can choose between two operation modes using the
Operation mode
Surge
Charge
menu item:
Description
The fault-flash over is forced by a capacitive discharge of
an impuls current.
The fault-flash over is forced by charging the cable with
HV (with a surge capacitor connected in parallel).
29