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Magnetic Field Measurement to Detect and Locate Underground Power Cable
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P. Wang1, K. F. Goddard2, P. L. Lewin3, S. Swingler4, P. Atkins5 and K. Y. Foo6
1 The Tony Davies High Voltage Laboratory, University of Southampton,
Southampton, SO17 1BJ, UK; PH (44) 23 80595352; FAX (44) 23 8059 3709; email:
[email protected]
2 The Tony Davies High Voltage Laboratory, University of Southampton,
Southampton, SO17 1BJ, UK; PH (44) 23 80595352; FAX (44) 23 8059 3709; email:
[email protected]
3 The Tony Davies High Voltage Laboratory, University of Southampton,
Southampton, SO17 1BJ, UK; PH (44) 23 80593586; FAX (44) 23 8059 3709; email:
[email protected]
4 The Tony Davies High Voltage Laboratory, University of Southampton,
Southampton, SO17 1BJ, UK; PH (44) 23 80593358; FAX (44) 23 8059 3709; email:
[email protected]
5 The School of Electronic, Electrical and Computer Engineering, University of
Birmingham, Birmingham, B15 2TT, UK; PH (44) 12 1414 4329; FAX (44) 12 1414
4291; email: [email protected]
6 The School of Electronic, Electrical and Computer Engineering, University of
Birmingham, Birmingham, B15 2TT, UK; PH (44) 12 1414 7313; FAX (44) 12 1414
4291; email: [email protected]
ABSTRACT
Accurate location of buried cables is crucial for future application of trenchless
technologies in urban areas. This paper describes an experimental study on detection
and location of underground power cables using magnetic field measurement. To do
this, a measurement system is designed to be able to measure the magnetic field of an
underground power cable at a number of points above ground. It was constructed
using battery powered data acquisition modules connected to a coil array, and
controlled by a laptop. The experimental investigation has been carried out in a
university car park, where the university’s utility map shows an isolated power cable.
The results shows that the measurement system and cable location method predict a
reasonable position of target cable.
INTRODUCTION
There are around 4 million holes dug by utility companies annually involving
construction projects and works in the street across the UK (Costello et al., 2007).
Hitting unknown underground obstructions has the potential to cause property
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damage, injuries and, even deaths. Over the past 10 years there have been over 1000
recorded injuries in the UK as a result of contact with underground electricity cables.
Thus, before commencing excavation or other work where power or other cables
may be buried, it is important to determine the location of the cables to ensure that
they are not damaged during the work. In fact, conventional cable detectors such as a
Cable Avoidance Tool (CAT) are effective when used to locate buried power cables
where only minimal interference exists from nearby cables and metallic utilities.
However, in a high density urban environment where the situation is more
complicated, it is necessary to employ multiple magnetic sensing devices coupled
with appropriate algorithms that can distinguish between the field of the target cable
and that of other interfering sources so that power cables can be accurately located
and mapped.
In addition, in order to save the search cost and time, the further requirements are to
scan the ground surface at the same time and try to obtain the position of all buried
utility service cables and pipes rather than detect certain type target by using
different technologies separately. One of Mapping the Underworld projects
(www.mappingtheunderworld.co.uk) wants to meet these requirements. The second
phase of Mapping the Underworld (MTU2) project in the UK is a multi-university,
multi-disciplinary project with support from over 40 partner companies and
organisations. It aims to create and develop a multi-sensor device, which using
different kinds of sensors but sharing a single platform, to locate and map (in 3
dimensions) buried utility service pipes and cables without excavation. Here, we only
consider the magnetic field technologies part of this project so that it can be
integrated in the multi-sensor device.
In this paper the principle of cable detection and location using magnetic field
method are introduced. A measurement system with magnetic field sensors is
described and some experimental test results are presented.
PRINCIPLE OF CABLE DETECTION AND LOCATION
The magnetic field produced by the normal working current flowing in a power cable
can be used to detect the presence of the cable and estimate its position. In addition,
earth currents and currents induced in networks of metal pipes by nearby power
cables may be detected, as these currents also flow at power frequencies and
harmonics thereof. Figure 1 shows a schematic diagram of the cable detection and
location method.
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Figure 1. Schematic diagram of the cable detection and location
The cable location programs work by comparing the measured magnetic field with
that predicted by a simple numerical model of one or more cables and adjusting the
parameters of the model to minimize the total square error. The parameters of the
model are divided into two categories: geometric parameters that do not change, but
are related to the field by non-linear functions; and unknown currents that can change
between one set of measurements and the next, but are related to the field by linear
functions.
The least squares fitting of the linear functions to estimate the unknown currents and
the residual errors can be achieved by matrix manipulation; hence the computational
costs are relatively low. However, since the coefficients of these functions are
non-linear functions of the geometric parameters, the calculations need to be
repeated for each set of measurements whenever any of the geometric parameters are
changed. Consequently, the computational cost of the non-linear minimization
procedure required to estimate the geometric parameters can be very large.
MAGNETIC FIELD SENSOR AND MEASUREMENT SYSTEM
The most basic magnetic field sensor is a search coil, which consists of a coil of wire
with an air core. Its working principle is based on Faraday’s induction law, which
states that any change in the magnetic flux linking a coil of wire will cause an
induced voltage (electromotive force) in the coil. Using search coils has a number of
significant advantages. They can be designed to give appropriate sensitivity and
spatial resolution, and then produced in small numbers at reasonable cost. Obtaining
matched coils to an adequate standard is easily achieved by ensuring that the
geometric parameters of the coils are closely matched; hence there is no need to
calibrate the coils. In addition, since search coils detect only changes in magnetic
field or movement of the coil through the field, holding the coils stationary ensures
that the dominant signals are those produced by the power-frequency currents in
buried cables and metal pipes. Figure 2 shows a search coil designed and
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manufactured for this experimental testing as a magnetic field sensor. The coil
parameters and relevant tests under laboratory conditions can be found in our
previous published paper (Wang et al., 2010).
Figure 2. A search coil designed for this experiment
A measurement system was constructed as shown in Figure 3 that uses a battery
powered data acquisition system with two NI 9239 modules connected to a coil array,
and controlled by a laptop. The array consists of seven identical coils mounted on a
X
Z
Figure 3. Measurement system put on one of university car parks
support frame. The system is designed to measure the magnetic field of an
underground power cable simultaneously at a number of points above the ground.
NUMERICAL MODEL OF THE CABLE
In the absence of nearby magnetic or conducting structures, the external field of an
infinitely long straight twisted multi-core cable can be represented by the field of a
central current filament and a Bessel function series in scalar magnetic potential.
∑
∞
n =1
(an cos(n (θ +α z )) + bn sin(n (θ +α z ))) K n (nα r )
Only the first pair of terms is required to represent the field, as the higher order terms
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CABLE SEARCH PROCEDURE
The cable search program mainly consists of measurement and signal processing and
analysis. First of all, the support frame with its seven search coils is placed at a
number of positions above the search area, and its position is recorded. The voltages
induced in the coils are measured, and then Fourier analysis is used to extract the 50
Hz and harmonic signal components. Next, a least square error algorithm is applied
to the resulting data in order to estimate the cable currents and residual errors for
various assumed cable positions. Finally, the rms amplitude of the residual errors is
plotted against the assumed cable position to give an indication of the likely locations
of a buried cable.
TEST RESULTS
The results presented below were obtained from measurements taken with the coil
array in 16 different test points in a 3 m by 3 m test area in one of our campus car
parks (Figure 3). This area was chosen because the university’s utility map shows an
isolated power cable there. Figure 4 shows the waveform of the voltage signal
obtained from one of the coils. It is clear that it is a 50 Hz signal with lots of odd
harmonics. This is confirmed by the spectral analysis shown in Figure 5.
Voltage (V)
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decay much more rapidly and make negligible contributions to the field above the
ground. In a cable that carries only a single-phase load the two coefficients a and b
have a fixed ratio, whereas in a cable with a 3-phase load the ratio varies over the AC
cycle. Although the 3-phase cable has an additional unknown current, the
single-phase case requires an additional geometric parameter. The computational cost
of searching for a 3-phase cable is therefore lower.
Time (s)
Figure 4. 50 Hz signal with lots of harmonics
Amplitude (V)
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Frequency (Hz)
Figure 5. Frequency analysis (Fundamental f = 50 Hz)
By comparing the obtained voltage signals with the values obtained by modelling a
long straight horizontal cable at various positions, a fitting error map can be plotted
for any Z-coordinate value (as defined in Figure 3). Two such maps are shown in
Figure 6; these show the minimum fitting error for each position in two different
planes, and are produced from the same measurement data. The low error values of
4-5% give a high degree of confidence that most of the measured signal is due to a
cable near to these positions, i.e. the possible cable position is 1.4 m in horizontal
distance and 0.6 m in depth. This view is supported by the fact that the university’s
utility map shows the cable at X = 1.4 m, and by amplitude measurements taken with
a hand-held magnetic field meter.
0.14
-0.2
0.12
-0.4
Depth (m)
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-0.6
0.1
-0.8
0.08
-1
-1.2
0.06
-1.4
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1
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Horizontal distance (m)
(a)
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0.04
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0.14
-0.2
0.12
Depth (m)
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-0.4
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Horizontal distance (m)
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0.04
(b)
Figure 6. Typical results for car park tests at different Z-coordinate. (a) Z= 1 m;
(b) Z= 3 m. (possible cable position is X=1.4 m and Depth = 0.6 m)
CONCLUSIONS
The principles of the cable locating programs using magnetic field measurement
have been introduced. The design of sensors and construction of measurement
system are described. Experimental testing has been conducted to test the proposed
cable location method with an operational power cable in a university campus car
park. The results show the measurement system and cable location method give a
high degree of confidence for a position of target cable.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support for this work provided by
the UK’s EPSRC under grant reference EP/F0659731/1. This work forms part of the
Mapping the Underworld project.
REFERENCES
Costello, S. B., Chapman, D. N. and et al. (2007). Underground asset location and
condition assessment technologies. Tunnelling and Underground Space Technology,
Trenchless Technology, 22, 524-542.
Wang, P., Lewin, P. and et al. (2010). Design and testing of an induction coil for
measuring the magnetic fields of underground power cables. Proceedings of the
IEEE International Symposium on Electrical Insulation (ISEI 2010), San Diego, CA,
1-5.