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Optimum Coil Design for Inductive Energy Harvesting in Substations

Optimum Coil Design for Inductive Energy Harvesting in Substations. Dr Nina Roscoe, Dr Martin Judd Institute for Energy and Environment University of Strathclyde. Overview. Background The role of condition monitoring sensors Supplying energy to condition monitoring sensors

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Optimum Coil Design for Inductive Energy Harvesting in Substations

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

  2. 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

  3. 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

  4. 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

  5. Toroidal core is “threaded” onto conductor High current conductor Wire wound on toroidal core “Free-standing” harvester Transformer Magnetic flux Inductive Harvesting: Two inductive harvester approaches “Threaded” harvester “Free-standing” harvester

  6. “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

  7. 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) < 12; µr_effnot strongly linked to µr • L/D > 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

  8. Determining the number of turns • Measured Pout vs number of turns (0.5 m long cast iron cored coils) 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

  9. 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

  10. 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

  11. Results • Output power measurements for coil placed in 25 µTrms Cast iron core 1.3mW @ 6.5 Vrms, RL= 33 kΩ 40,000 turns 1mW @ 10Vdc RL= 100 kΩ ac-dc converter 50 mm 500 mm ac-dc converter dc-dc converter • Rs = 33 kΩ • Ls = 100 H • Ccomp = 100 nF 0.85mW @ 3.6Vdc RL= 15 kΩ

  12. 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?

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