Before commencing excavation or other work where power or other cables may be buried, it is important to determine the location of cables to ensure that they are not damaged. This paper describes a method of power-cable detection and location that uses measurements of the magnetic field produced by the currents in the cable, and presents the results of tests performed to evaluate the method. The cable detection and location program works by comparing the measured magnetic field signal with values predicted using a simple numerical model of the cable. Search coils are used as magnetic field sensors, and a measurement system is setup to measure the magnetic field of an underground power cable at a number of points above the ground so that it can detect the presence of an underground power cable and estimate its position. Experimental investigations were carried out using a model and under real site test conditions. The results show that the measurement system and cable location method give a reasonable prediction for the position of the target cable

Goddard, K F

Lewin, P L

Swingler, S G

Wang, P

English

Southampton (e-Prints Soton)

ELECTROMAGNETIC FIELD APPLICATION TO UNDERGROUND POWER CABLE DETECTION P Wang*, K Goddard, P Lewin and S Swingler University of Southampton, Southampton, SO17 1BJ, UK *Email: pw3@ecs.soton.ac.uk  Abstract: Before commencing excavation or other  work where power or other cables may be buried, it is important to determine the location of cables to ensure that they are not damaged. This paper describes a method of power-cable detection and location that uses  measurements of the magnetic field produced by the currents in the  cable, and presents the results of tests performed to evaluate the method. The cable detection and location  program works by comparing the measured magnetic field signal with values predicted using a simple numerical model of the cable. Search coils are used as magnetic field sensors, and a measurement system is setup to measure the magnetic field of an underground power cable at a number of points above the ground so that it can detect the presence  of  an  underground  power  cable  and  estimate  its  position.  Experimental investigations were carried out using a model and under real site test conditions. The results show that the measurement system and cable location method give a reasonable prediction for the position of the target cable.   1  INTRODUCTION The location of buried utility service infrastructure is becoming a major engineering and social issue worldwide  for  their  successful  operation  and maintenance.  The  lack  of  accurate  positioning records of existing services such as water, sewer, power  and  telecommunications  can  create engineering  and  construction  challengers  and safety hazards when new construction, repairs, or upgrades  are  necessary.  Two  main  reasons  can cause  the  lack  of  records.  One  is  that  these infrastructures  can  exist  underground  for  many years, largely forgotten; other one is that records almost always refer to positions relative to ground level physical features that may no longer exist or that  may  have  been  moved  or  altered.  It  is estimated that up to 4 million holes dug by utility companies annually in order to repair, or replace, or install buried assets in the street across the UK. Every time a hole is dug, there is a risk of hitting and  damaging  buried  pipelines  and  cables,  with the consequent risk of service disruption and threat to safety. In fact, over the past 10 years there have been over 1000 recorded injuries in the UK as a result  of  contact  with  underground  electricity cables. Hence, 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.  To  address  these  issues,  a  consortium  of  UK universities and industrial partners are undertaking a  large  research  project,  entitled  Mapping  the Underworld  (MTU)  [1].  MTU  aims  to  assess  the feasibility of potential techniques that can be used in a combined manner to determine the accurate location  of  underground  utilities,  as  well  as  to assess  the  condition  of  buried  assets  without breaking the ground’s surface. The second phase of  MTU  focuses  on  creating  and  developing  a prototype multi-sensor device, a platform that will use  four  different kinds  of sensors  to  locate and map  buried  utility  service  infrastructure  without excavation.  The  latest  progress  and  outcome  of this project can be found in [2, 3]. Here, we only consider  the  passive  magnetic  field  technologies part  of  this  project,  in  which  the  magnetic  field produced  by  the  working  currents  in  the  power cables is used to locate underground cables and metal pipes. 2  CABLE DETECTION AND LOCATION METHOD Generally,  cable  detection  using  magnetic  field technology is based on two basic principles: firstly, that  an  electric  current  can  produce  a  magnetic field  (electromagnetism)  and  secondly  that  a changing  magnetic  field  within  a  coil  of  wire induces  a  voltage  across  the  ends  of  the  coil (electromagnetic induction). There are a variety of types of magnetic field sensors that can be used to detect  the  presence  of  magnetic  fields  or  to measure  their  strength  and  direction.  Induction coils (also called search coils), which are based on Faraday’s induction law, are chosen. It is easy to design  and  manufacture  such  coils  with  suitable parameters. Our cable  location method works by comparing  the  measured  flux-density  data  with values predicted using a simple numerical model of one or more cables. In the simple version used to produce the presented results, it is assumed that the  cable  configuration  that  minimizes  the  total square error in fitting the measured field data is the most likely to represent the true configuration.  It should be noted that this approach is vulnerable to  problems  with  over-fitting.    To  control  this XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011problem,  it  is  important  that  the  number  of unknown currents being considered is significantly lower than the number of simultaneous magnetic field measurements.  The latter number is limited by how closely we can place the sensors while still obtaining independent measurements, and by the number  of  sensors  that  we  can  reasonably consider  using.  Consequently,  this  requirement places a fundamental limit on the complexity of the cable configurations that can be considered.  To simplify the task of optimizing the parameters of the  model,  they  are  divided  into  two  categories: unknown  currents  (and  possibly  other  similar parameters) that are related to the field by linear equations,  but  can  change  between  one measurement  and  the  next;  and  geometric parameters that are assumed to be constant, but are related to the field by non-linear equations.  The set of unknown current values that minimizes the  total  square  error  can  be  found  by  matrix division; however, as the coefficients of the matrix equations are non-linear functions of the geometric parameters,  these  calculations  must  be  repeated each time the geometric parameters are changed.  Consequently, the matrix division must be included in  the  objective  function  used  by  the  non-linear optimization  program  used  to  find  the  geometric parameters.  3  MEASUREMENT SYSTEM AND SEARCH PROCEDURE A  measurement  system  consists  of  an  array  of search coils, a NI compact DAQ chassis with two NI data acquisition modules powered by a battery, and a laptop, as shown in Figure 1. Seven search coils are mounted on a support frame to measure the magnetic field driven by an underground power cable. Figure 2 is a schematic diagram of the coil array, while the coil position parameters are given in Table 1. All the coils have the same geometric parameters  (2000  turns  and  100  mm  internal diameter), which were chosen to give appropriate sensitivity and spatial resolution. More information on coil parameters and laboratory tests have been reported in [4]. The detailed description about    Figure 1: A measurement system on a footpath  Figure 2: Schematic diagram (3D view) of the coil array  Table 1: Coil position parameters Coil No.  X  Y  Z  Nx  Ny  Nz 1  0  620  0  1  0  0 2  0  220  0  1  0  0 3  -75  27  0  0  -1  0 4  0  170  113  0  0  -1 5  -75  120  -113  0  0  -1 6  75  120  -113  0  0  -1 7  75  27  0  0  -1  0 (X, Y and Z define the centre of each coil (in mm), while Nx, Ny and Nz define the component of the field measured by each coil. The coordinate system is shown in Figure 2.)  experimental setup also can be found in our other paper [5].  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. 4  EXPERIMENTAL RESULTS AND DISCUSSIONS Experimental tests were carried out in a street with real  operational  power  cable  and  also  using  a simulation cable, respectively.  4.1  Street testing  This test was carried out on a footpath (shown in Figure 1) close to a power transformer. We guess x z XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011there  is  an  underground  power  cable  nearby.  In fact,  very  strong  magnetic  field  signals  were previously traced from the transformer compound to the  test  area  using  a  handheld magnetic  field meter. Using the cable search program mentioned in section 3, this test was realized for a 2.2 × 3.5 m test area as shown in Figure 3.  Figure  4a  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 also can be confirmed by the frequency analysis of the signal shown in Figure 4b. 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 1). Using  the  existing  matlab  programs,  which  were used in our previous test [5], maps of the minimum (rms) fitting error were plotted for a single cable as a  function  of  its  position  in  a  specified  vertical plane. Figure 5 shows a typical fitting error for this testing at z=0.5 m when we fixed the cable twist rate = -1.5 rad/m. This indicates that the possible cable position is 1.8 m in horizontal distance and 0.6  m  in  depth.  Similar  results  are  obtained  for twist rates of -1.8 and -1.2 rad/m. This test result is believed  to be reasonable  for  the  following  three reasons.  First,  a  switch  label  in  the  substation indicates that an underground power cable goes in this general direction. Secondly, a handheld meter shows  a  maximum  signal  strength  of  3.94  µT  at X=1.8 m position. Thirdly, the minimum fitting error of  2%  to  3%  is  much  lower  than  that  for  other locations. 00.511.522.533.50 0.5 1 1.5 2 2.5X (m)Z (m) Figure  3:  Test  points  and  route  (here  z  and  x-coordinate value as defined in Figure 1) 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-0.02-0.015-0.01-0.00500.0050.010.0150.02 (a) 0 100 200 300 400 500 600 700 800 900 100000.0010.0020.0030.0040.0050.0060.0070.0080.0090.01 (b) Figure 4: Measured 50 Hz signal for a street test, showing  large  3rd  and  9th  harmonic  components.  (a) time domain, (b) frequency domain  Horizontal distance (m) Figure 5: Fitting error for a street test at Z=0.5 m (possible  cable  position  is X=1.8 m and Depth = 0.6 m) 4.2  Simulation cable testing  This  experiment  used  a  single-phase  simulation cable,  placed  on  the  floor.  Measurements  were taken with the coil array at a number of positions, all the same height above the cable. This cable is not twisted and carried a 6A return 50 Hz current. Using  our  search  program  and  cable  location method mentioned above, some test results were obtained.  Figure  6a  shows  the  waveform  of  the   0  0.2  0.4  0.6  0.8  1  1.2  1.4  1.6  1.8  2 2.2  1.4 1.2 1 0.8 0.6 0.4 0.20.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 Start end Voltage (V) Time (s) Amplitude (V) Frequency (Hz) Fundamental f = 50 Hz Depth (m) XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011measured voltage signal obtained from one of the coils. It is clearly a 50 Hz signal with quite small harmonic  components.  This  is  confirmed  by  the frequency analysis of the signal shown in Figure 6b. Figure  7  shows  the  map  of  the  minimum  fitting error for one such test; the red circle indicates the real  position  of  the  cable.  It  is  clear  that  the predicted position is very close to the real position.  0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-5-4-3-2-1012345x 10-3 0 100 200 300 400 500 600 700 800 900 100000.511.522.533.54x 10-3  Figure 6: Measured 50 Hz signal for the simulation test,  showing  the  low  harmonic  content.  (a)  time domain, (b) frequency domain.    0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8-1.4-1.2-1-0.8-0.6-0.4-0.20.040.050.060.070.080.090.10.110.120.130.14  0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8-1.4-1.2-1-0.8-0.6-0.4-0.20.040.050.060.070.080.090.10.110.120.130.14 Figure 7: fitting error for the simulation test  4.3  Discussion From two test results above, it can be seen that the measurement system and cable location method is suitable  to  complete  such  cable  detection  and location. Table 2 lists a brief summary on above two tests. It can be seen that the minimum fitting error  for  the  street  test  is  less  than  that  for  the simulation test. It is possible that this could indicate that the signal-to-noise ratio is better in the street test; we know that the signals from this street test are  stronger  than  those  from  the  simulation test, but we have no way to measure the noise levels in the street tests. However, in street tests, the cable location  program  searches  for  a  twisted  cable; hence, it has an additional parameter that can be used to reduce the fitting error.  Another difference for these two tests is that the signal  propagation  medium  for  magnetic  fields  is different. However, soils have no significant effect on low-frequency magnetic fields. Hence, magnetic field  measurements will  only  yield  information on metal  structures,  primarily  electricity  cables.  In addition, there are some differences between the signal waveforms in these two tests, even though both  have  a  fundamental  frequency  of  50  Hz. Significant  3rd  and  9th  harmonics  appear  in  the street test, while all the harmonics are small in the simulation test.    Table 2: A brief summary on these two testing Test site  Street test  Simulation test Test target three-phase real twisted cable, only know approximate   position. don’t know current single-phase simulation no-twisted cable, know exact position and current Signal strength (Maximum value) 3.94 µT  0.57 µT Signal propagation medium Soil  Air Signal waveform and frequency 50 Hz signal with lots of harmonics 50 Hz signal with quite small harmonics Minimum fitting error 2%  4%  5  CONCLUSIONS AND FUTURE WORK A  measurement  system  has  been  constructed, which uses an array of search coils to measure the distribution  of  power-frequency  magnetic  fields.  Voltage (V) Time (s) Amplitude (V) Frequency (Hz) Fundamental f = 50 Hz  (a) (b) Horizontal distance (m) Depth (m) XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011Using this system experimental investigations have been carried out in a street footpath and using a simulated  cable.  Results  show  that  the measurement  system  and  cable  location  method can  give  reasonably  precise  and  believable estimates of the cable’s location.  However, these tests  were  restricted  to  areas  that  are  each believed  to  contain  only  one  power  cable.    This was necessary since, with only 7 search coils, the search programs should not be used to search for more than one cable at once.  This restriction is required to avoid problems with over-fitting.  To  allow  the  system  to  consider  more  than  one cable in the search area and improve search range and precision, the measurement system has been expanded  and  a  larger  frame  manufactured  to support  27  coils.  This  will  provide  the  search programs with the larger number of simultaneous independent  flux-density  measurements  that  are required to search for more than one cable. Further testing  will  be  done  with  the  revised  system  to evaluate its performance. 6  ACKONWLEDGEMENTS The  authors  gratefully  acknowledge  the  financial support for this work provided by the UK’s EPSRC under reference EP/F0659731/1. This work forms part of the Mapping the Underworld project. 7  REFERENCES [1]  "Mapping  the  Underworld,  url: http://79.170.40.172/mappingtheunderworld.ac.uk/index.php., last accessed: May 12, 2011." [2]  A.  Royal,  P.  R.  Atkins,  M.  Brennan,  D. Chapman,  H.  Chen,  A.  Cohn,  K.  Foo,  K. Goddard,  R.  Hayes,  T.  Hao,  P.  L.  Lewin,  N. Metje,  J.  Muggleton,  A.  Naji,  G.  Orlando,  S. Pennock, M. Redfern, A. Saul, S. Swingler, P. Wang, and C. D. F. Rogers, "Site Assessment of Multiple Sensor Approaches for Buried Utility Detection," International Journal of Geophysics (Articles in Press), 2011. [3]  A. C. D. Royal, Rogers, C. D. F., Atkins, P. R., Brennan, M. J., Chapman, D. N., Cohn, A. G., Curioni, G., Foo, K., Goddard, K. F., Lewin, P. L.,  Metje,  N.,  Muggleton,  J.  M.,  Naji,  A., Pennock,  S.  R.,  Redfern,  M.  A.,  Saul,  A.  J., Swingler,  S.  G.  and  Wang,  P,  "Mapping  the Underworld:  Location  Phase  II  -  Latest Developments," presented at International No-Dig  2010;  28th  International  Conference  and Exhibition, Singapore, 2010. [4]  P.  Wang,  P.  Lewin,  K.  Goddard,  and  S. Swingler, "Design and testing of an induction coil  for  measuring  the  magnetic  fields  of underground  power  cables,"  presented at  the 2010  IEEE  International  Symposium  on Electrical Insulation (ISEI), 2010. [5]  P.  Wang,  K.  F.  Goddard,  P.  L.  Lewin,  S. Swingler, P. Atkins, and K. Y. Foo, "Magnetic field  Measurement  to  Detect  and  Locate Underground  Power  Cable,"  presented  at International  Conference  of  Pipelines  and Trenchless  Technology,  Beijing,  China,  2011 (accepted).  XVII International Symposium on High Voltage Engineering, Hannover, Germany, August 22-26, 2011

accessed: May 12, 2011.&quot; [2]

Mapping the Underworld: Location Phase II - Latest Developments,&quot; presented at International NoDig 2010;

Site Assessment of Multiple Sensor Approaches for Buried Utility Detection,&quot;

Electromagnetic field application to underground power cable detection

Electromagnetic field application to underground power cable detection

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