Power and Data Cable Monitoring

The Problem

Underground Infrastructure; once installed needs to stand the test of time. In some cases, data cables, wires, and power transmission lines can be expected to have an operating life from a few decades up to almost a century without the possibility of a visual inspection.
Monitoring for wear, damage, or corrosion of the cable is extremely difficult and often power failure or data outage is the first sign of a problem.
Often, these assets are installed in areas that have multiple uses by various stakeholders and in the event of a failure or damage can be logistically difficult and expensive to access for repair.
Due to their accessibility when compared to overhead lines, it should also be noted that third-party intrusion (both accidental and nefarious) is a much greater threat to buried power transmission lines. Subsequently the risk of exposure to Arc Flash from live conductors and the potential for electric shock or electrocution are much greater. As well as the possibility for optical cables to be damaged leading to extended telecommunication outages.
Additionally performance of these cables is constantly degrading over time due to material and environmental factors. This effect although slow and unpredictable is inevitable and ultimately causes failure of the conductor on the optic. The location of these faults is a major challenge and a significant contributor to downtime.
Ultimately to manage these risks a method of around-the-clock monitoring is required that can be fitted during construction or retrofitted to existing infrastructure. Due to the long runs transmission cables need to cover, traditional instrumentation must be installed at regular intervals along the cable run and require supporting infrastructure (power and communications) to each device, an entirely impractical proposal.

The Solution

To solve this problem a solution would need to do the following:

  • Detect issues as a distributed sensor rather than a point sensor
  • Require no additional field infrastructure such as power or communications
  • Operate in real time
  • Low cost per meter
  • Autonomous detection with low false alarm rate
  • Autonomous detection with low false alarm rate

Principle of Operation

The Praetorian system interrogator unit is connected to one end of a fiber optic cable which is attached to or buried with the cable or infrastructure being monitored. The interrogator produces rapidly pulsed laser light set at a precise frequency that excites the fiber and causes it to be responsive to physical changes around it. Some of this light is reflected back (backscattered) to the light source where the interrogator records and analyses looking for changes to its color relating to physical effects in the application.

Functionality of Power and Data Cable Monitoring

Praetorian Fiber Optic Sensing (FOS) uses a combination of Distributed Acoustic Sensing (DAS) and Distributed Temperature Sensing (DTS) to protect underground buried assets. By exciting a fiber optic core within a cable the Praetorian Interrogator is able to utilize the fibers as a distributed network equivalent to up to 1.6 Million individual vibration, temperature, and strain sensors.
Because a praetorian utilizes a number of different sensing methods it is possible to observe events in a number of physically independent ways, consequently Praetorian is inherently resistant to taking a given reading and giving a false alarm due to the requirement for multiple physical effects to simultaneously occur at the same location to signify an event and trigger an alarm.

Through a combination of distributed vibration, temperature and strain monitoring it is possible to determine multitudes of different physical events along a cable, including but not limited to:

  • Detection of partial discharge
  • Detection of hot spots
  • Early alert of third party intrusion (accidental or nefarious)
  • Conductor break detection
  • Ground condition assessment
  • Prevention arc flash events from conductor contact
  • Detection of optical loss
  • Detection of fiber break
  • Detection of pit or trench lid being opened
  • Determination of network operational status (thermal loading)

The key inputs to the RTTR cable rating modeling are required as follows:

  • Cable size and type, installation configuration (cable laying formation)
  • Soil ambient temperature
  • Real time loading
  • Soil thermal resistivity and cable backfill material thermal resistivity if used

The outputs from a RTTR used in a buried cable environment include:

  • Real time conductor temperature along the power cable
  • Transient calculations for Time/ Current/Temperature
  • Emergency ratings – this can be for a range of times from typically 30 minutes to 48 hours+

Heat Image Under The Thermal Detection

Temperature Monitoring: Temperature Profiling

The use of distributed temperature monitoring for ground temperature monitoring is a common supplementary use of fiber optic sensing DTS systems. Benefits include:

  • Monitoring of annual and seasonal changes in ground condition
  • Ground freezing and flood monitoring
  • Ground water table interaction
  • Ground temperature monitoring for material degradation monitoring.

Live Optical Condition Monitoring

By analyzing the resultant return signal from Praetorians Distributed Acoustic Sensing (DAS) light pulse a number of optical fault conditions can be detected due to the presence of a loss of return light. Time of flight is used to determine the location of that loss where it occurs. Different optical loss conditions can be detected including (but not limited to):

  • Micro-bends
  • Macro-bends
  • Connector losses
  • Fusion splice losses
  • Impurities
  • Fiber cut
  • Time based degradation.
  • Moisture and hydrogen infusion loss.

Advantages of Power and Data Cable Monitoring

  • Praetorian can function where the cable cannot be visually inspected (due to burial)
  • Fiber Optic Sensing detects not only the presence of the fault of failure but its specific location
  • Praetorian is extremely sensitive, detecting sounds well below frequency human hearing can manage
  • Due to the use of “long haul” single mode fiber Praetorian is able to detect faults over long hauls
  • Existing fiber optic data infrastructure may be utilized
  • System is passive, no electricity is required in the field
  • No maintenance or calibration require after commissioning
  • Self diagnostics monitor the unit’s condition and maintain optimum performance
  • Not effected by electromagnetic fields (EMF), lightning or weather events
  • Easy, low cost installation with cable fingertips
  • Low cost per meter

Technical Specifications of Power and Data Cable Monitoring

Category Parameter Description
Sensing element Fiber Optic Sensing cable
Number of channels 1 or 2
Interrogator operating temperature 0 to -40ºC (32 to 104ºF)
Interrogator storage temperature -20 to -60ºC (-4 to 140ºF)
Interrogator operating humidity 10-85% @ 40°C (104°F) non-condensing
Interrogator storage humidity 10-95% @ 40°C (104°F) non-condensing
Dimensions (rack enclosure) 4RU, 19” Rack, 600mm minimum depth
Dimensions (physical) 430 x 177 x 479mm (16.93 x 5.9 x 18.85 inches)
Weight 25kg (55 lbs)
Power supply 110-240VAC (50-60Hz), 24VDC
Power consumption < 200W
Sensing range (DAS) Up to 40kms (25 miles) per channel
Sensing range (DTS) Up to 80kms (50 miles) loop per channel
Spatial resolution 250 or 500mm
Frequency response 1Hz-120kHz (range dependant)
Dynamic range Dynamic range
Temperature sensing range (cable) -30°C to 200°C (-22°F to 392°F) (special options for temps up to 800°C (1472°F) and down to -200°C (-328°F) available)
DTS Performance
Accuracy ±0.25°C (±32.45°F)
Resolution 0.01°C (32.018°F)
Scan time 1-2 Minutes (depending on temperature parameters)
Temperature sensing range -250°C (-418°F) to 700°C (1292°F)
Light source Laser (infra red) class 1M
Laser wave length 1550.12nm (nanometers)
Laser stability ±5pm (picometers)
Acquisition rate 400MHz
Processor transfer rate 64Bit (ultra high speed)
Operating system Linux
Output Modbus ethernet TCP/IP (standard), relay, USB, SCADA or user specified
Remote interfacing Ethernet, Wi-Fi and 3G/4G enabled
Processer architecture Field Programmable Gate Array (FPGA)
Data storage (removable) 2x 2TB HDD (removable)
Data storage (internal) 128GB solid state drive