Transcript Slide 1
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 – 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 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 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 Inductive Harvesting: Two inductive harvester approaches 1. “Threaded” harvester Toroidal core is “threaded” onto conductor High current conductor Wire wound on toroidal core 2. “Free-standing” harvester “Free-standing” harvester Transformer Magnetic flux “Free-standing” inductive harvesters Harvesting coil µr_eff = Voc-iron_core Voc- air core Voc = open circuit coil voltage D L Cast iron core Wireless sensor and transmitter from Invisible Systems 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_eff not 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 Determining the number of turns Optimum impedance match 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) 14 Output power (mW) – Coil approximated by self inductance and series resistance – Self inductance can be compensated with series capacitance – Optimum load resistance equal to coil series resistance 12 10 8 6 4 Maximum output power in 65 uTrms flux density 2 0 0 10000 20000 30000 Number of turns 40000 50000 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 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 Results Output power measurements for coil placed in 25 µTrms Cast iron core 40,000 turns 50 mm 500 mm Rs = 33 kΩ Ls = 100 H Ccomp = 100 nF 1.3mW @ 6.5 Vrms, RL= 33 kΩ ac-dc converter 1mW @ 10Vdc RL= 100 kΩ ac-dc converter dc-dc converter 0.85mW @ 3.6Vdc RL= 15 kΩ 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: 1. 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 2. Integration with wireless sensor 3. Single stage a.c. to regulated d.c. output voltage conversion?