Optimum Coil Design for Inductive Energy Harvesting in Substations

Dr Nina Roscoe, Dr Martin Judd

Institute for Energy and EnvironmentUniversity 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 sensorsReliability 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

requiredCost reduction

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

Supplying energy to condition monitoring sensors

“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

High current conductor

Wire wound on toroidal core

Toroidal core is “threaded” onto conductor

1. “Threaded” harvester

2. “Free-standing” harvester

Transformer

Magnetic flux

“Free-standing” harvester

“Free-standing” inductive harvestersHarvesting coil

Wireless sensor and transmitter from

Invisible Systems

µr_eff = Voc-iron_core

Voc- air core

Voc = open circuit coil voltage

LD

Cast iron core

Core materials and dimensionsAim:

– 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

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

Determining the number of turns

Measured Pout vs number of turns (0.5 m long cast iron cored coils)

Converting ac output voltage to regulated dc voltageac 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

Maxwell coils

3 Current carrying

coils

Harvesting coil placed in uniform magnetic

field

The blue arrows show the location and orientation of

the uniform magnetic field

Results

Output power measurements for coil placed in 25 µTrms

Rs = 33 kΩ Ls = 100 HCcomp = 100 nF

ac-dc converter

1mW @ 10Vdc RL= 100 kΩ

ac-dc converter

dc-dc converter

0.85mW @ 3.6Vdc RL= 15 kΩ

1.3mW @ 6.5 Vrms, RL= 33 kΩ

500 mm

Cast iron core

50 mm

40,000 turns

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?