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 Transformer “Free-standing” harvester Magnetic flux

“Free-standing” inductive harvesters

Harvesting coil

µ r_eff = V oc-iron_core V oc- air core V oc

= 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 µT

rms

(safe area) – Invisible Systems wireless sensor Core Material – 3 materials compared: cast iron, laminated steel, ferrite – Length to diameter ratios (

L/D

) < 12;

µ r_eff

not strongly linked to

µ r

–

L/D

> 12;

µ r_eff

of ferrite outperforms others – Highest

L/D

realisable in cast iron Length to (effective) diameter explored – High

L/D

for high

P out /Vol

– Limit to practical and safe

L/D

– Compromise: 0.5 m long, 50 mm diameter for demonstrator • Less than optimal

P out /Vol

• Achieves adequate output power in suitable

B

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 14 12 10 8 6 4 2 0 0 Measured

P out

vs number of turns (0.5 m long cast iron cored coils) Maximum output power in 65 uTrms flux density 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 µT

rms

Cast iron core 40,000 turns

1.3mW @ 6.5 V

rms ,

RL= 33 kΩ

50 mm

R s L s = = C comp

33 kΩ 100 H

=

100 nF

500 mm

ac-dc converter ac-dc converter

1mW @ 10V

dc

RL= 100 kΩ

dc-dc converter

0.85mW @ 3.6V

dc

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

P out /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?