Optimum Coil Design for Inductive Energy Harvesting in Substations

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Optimum Coil Design for Inductive Energy
Harvesting in Substations
Dr Nina Roscoe, Dr Martin Judd Institute for
Energy and Environment University of Strathclyde
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Overview

• Background
• The role of condition monitoring sensors
• Supplying energy to condition monitoring sensors
• Inductive energy harvesting
• Coil design
• Core materials and dimensions
• Determining the number of turns
• Experimental test equipment
• Results
• Converting ac output voltage to regulated dc
  voltage
• Conclusions

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The role of condition monitoring sensors

• Reliability of electrical power supply
• Good asset management improves reliability of
  supply
• Knowledge of local environmental conditions
• Electrical power supply asset management
• Increased life expectancy
• Environmental stress, e.g.
• Temperature cycling or humidity
• Pollution (measured through leakage current)
• Degradation monitoring, e.g.
• Increasing conductor temperature
• Breaker operating mechanisms (accelerometer
  readings)
• Maintenance and replacement of assets only when
  required

• Cost reduction

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Supplying energy to condition monitoring sensors

• Two main conventional methods
• Batteries
• At HV potential, or on HV conductors, require a
  power outage to change batteries
• Mains power
• Only available in the safe areas
• Expensive to install in remote areas of the
  substation

Fit-and-forget self powered wireless sensors
enable low cost condition monitoring

• Many energy sources available for harvesting
• solar, wind, thermal, electromagnetic etc.
• All may have a have a role in a particular range
  of sensor applications
• Inductive electromagnetic harvesting

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Inductive Harvesting
Two inductive harvester approaches

1. Threaded harvester

1. Free-standing harvester

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Free-standing inductive harvesters
Harvesting coil
µr_eff Voc-iron_core Voc- air core Voc
open circuit coil voltage
D
L
Wireless sensor and transmitter from Invisible
Systems
Cast iron core
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Core materials and dimensions

• Aim
• Demonstrator to deliver 0.5 mW output power in 25
  µTrms (safe area)
• Invisible Systems wireless sensor
• Core Material
• 3 materials compared cast iron, laminated steel,
  ferrite
• Length to diameter ratios (L/D) lt 12 µr_eff not
  strongly linked to µr
• L/D gt 12 µr_eff of ferrite outperforms others
• Highest L/D realisable in cast iron
• Length to (effective) diameter explored
• High L/D for high Pout/Vol
• Limit to practical and safe L/D
• Compromise 0.5 m long, 50 mm diameter for
  demonstrator
• Less than optimal Pout/Vol
• Achieves adequate output power in suitable B

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Determining the number of turns

• Optimum impedance match
• Coil approximated by self inductance and series
  resistance
• Self inductance can be compensated with series
  capacitance
• Optimum load resistance equal to coil series
  resistance
• Optimum number of turns
• Output power is proportional to the number of
  turns only if
• Inductance is compensated
• No significant distributed effects
• Affected by inter-turn and inter-layer
  capacitance

Measured Pout vs number of turns (0.5 m long
cast iron cored coils)
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Converting ac output voltage to regulated dc
voltage

• ac to dc conversion
• Single stage Cockcroft-Walton multiplier
• Useful output voltage
• Low conduction losses in diodes (only one
  conducting at a time)
• Poor reverse leakage losses
• Problem for coils with many turns
• dc to dc conversion
• Commercial dc-dc converter chips
• Upconverters much less efficient than
  downconverters
• Upconverters need start up circuitry
• Downconverters preferred
• May be possible to achieve better efficiency with
  single stage switching ac to dc conversion

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Experimental Test Equipment
3 Current carrying coils
The blue arrows show the location and orientation
of the uniform magnetic field
Harvesting coil placed in uniform magnetic field
Maxwell coils
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Results

• Output power measurements for coil placed in 25
  µTrms

Cast iron core
1.3mW _at_ 6.5 Vrms, RL 33 kO
40,000 turns
1mW _at_ 10Vdc RL 100 kO
ac-dc converter
50 mm
500 mm
ac-dc converter
dc-dc converter
Rs 33 kO Ls 100 H Ccomp 100 nF
0.85mW _at_ 3.6Vdc RL 15 kO
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Conclusions

• Free-standing harvester shows promise for
  low-power condition monitoring applications
• Demonstrator has been built and tested
• Sufficient output power for a wireless sensor has
  been demonstrated
• low safe magnetic flux density deployment
• Design approach has been clearly established
• Future work
• Demonstrator to work at HV potential
• Better performance expected in higher B
• Higher Pout/Vol
• Fewer problems with distributed effects
• Corona shielding needs to be included for safe
  long-term operation
• Integration with wireless sensor
• Single stage a.c. to regulated d.c. output
  voltage conversion?