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DC MEASUREMENTS IN STRAY VOLTAGE INVESTIGATIONSMichael F. Stringfellow, Ph.D., P.E.

The Infamous “Bucket Test”

• Currents measured through water bucket as cows drink

• Currents present when AC power cut to farm

• Used as evidence of stray voltage produced by “earth currents”

• Spectrum of current identified as from utility

The Infamous “Bucket Test”

• Currents measured typically < 1 mA DC

• Indicates driving voltage ~ 0.5 V DC

Bucket Test Recordings

Bucket Test Waveform & Spectrum Allegations from Bucket Test Results

• Cow contact currents are caused by off-farm earth currents • Originally, direct currents were ascribed to rectified AC

• Presence of “frequencies” identifies the utility as the source

• Presence of electricity at waterers effects herd health and productivity

• The utility is responsible for the negative effects on the herd

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Sources of Direct Voltages and Currents

• Telluric (Earth) Currents• Geomagnetic origin

• Man-made origin• DC power transmission

• DC rail traction

• Galvanic Action (Natural)• Dissimilar metals (or alloys) in contact

• Direct contact or via electrolyte

• AC to DC power converter (man-made)• Motor drives

• Cathodic protection

• HVDC transmission

DC as a Stray Voltage Issue• First major issues raised in Utah with Intermountain

Power Project (IPP) HV DC system• Power plant located near Delta, Utah• Generates AC then converts to HV (± 600 kV) DC• HV transmission line routed to Los Angeles area • Full load current ~ 2500 A• Unbalance current injected into earth at custom electrode

• About 22 miles from power plant• DC currents routed away from AC transformers and their grounds• Typical unbalance ± 5A

• Plaintiffs allege earth currents from DC line are source of stray voltage on farms based on bucket test results

Location of IPP Facility and Farms The IPP Converter Station

IPP DC Transmission Line

• Two lines operating at

• ±600 kV

• Two overhead shield wires• Insulated for first 20 miles

and carry imbalance current to remote ground electrode

• Normal imbalance < ±5A

• Maximum on earth return ~ 2,800 A

IPP Ground Electrode

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Ground Electrode

• Consists of a ring array of 60 individual vertical electrodes

• Ten groups of six electrodes

• Arranged on circle of diameter 3,000 feet

• Each electrode consists of 17 cast-iron anodes

• Surrounded by coke breeze

• Each electrode assembly 14 inches in diameter and 285 feet deep

• Design resistance not to exceed 0.2 ohms

• Measured resistance between 3 and 19 milliohms

IPP Ground Electrode Current

IPP Direct Current SpectrumMeasuring Earth Voltages from Telluric Currents

• Using driven copper or copper-clad rods

• Distances from a few feet up to 200 feet

• Recorded using solar-powered automatic data logger

• Voltage difference due to:• Galvanic component from

rod/soil• Voltage drop from both

natural and man-made telluric currents

Geomagnetic Telluric Currents Earth Voltage Variations on Farm

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Earth Voltage & Injected Earth Current

• Changes of Direct Current during recording help to identify changes

Earth Voltages Near the Electrode

Earth Voltage and Electrode Current Earth Voltage from DC Converter Fault

• Well to reference rod ~ 300 feet

• N/S and E/W rods at 200 feet (60 m)

• Waterer to reference rod at 2 meters

Earth Voltage and Electrode CurrentCorrelation Between Earth Voltage and Electrode Current

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Step Voltage vs Distance from Electrode Conclusions on Electrode Current

• Earth voltages can be measured at distances of up to 30 miles (~ 50 km)• Large electrode current (~2,500 A)

• Large test electrode spacing (> 150 feet or 50 m)

• Step potentials (6.5 feet or 2 m) are small ~ mV

• Even close to electrode (~10 m), step voltage < 0.05 V

• Not measurable anywhere under normal operation of line• DC line imbalance current < ±5A

• Step potentials ~ microvolts

• On farm voltages result from local sources not utility

On Farm Direct Current Sources

• Galvanic currents from dissimilar metals• Essentially pure DC

• Stray currents from AC to DC converters• Rectifiers

• Motor drives

• DC likely contains AC harmonic signature of rectifier

Galvanic Action

• Galvanic action occurs when two electrochemically dissimilar metals are in contact and a conductive path occurs for electrons and ions to move from one metal to the other.

• Magnitude of action depends on dissimilarity of metals and effectiveness of conducting path (electrolyte)

Galvanic Series Galvanic Voltages for Common Metals

• Typical Galvanic Voltages between common metals

• Copper/iron: 0.2 to 0.7 Volts

• Copper/zinc: 0.5 to 1.0

• Iron/zinc: 0.1 to 0.3

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Measuring Direct Step Voltages

• Precautions needed against galvanic potentials from measuring electrodes

• Typically use a copper sulfate half-cell as a standard reference

Fence to Earth Direct Voltage

Waterer to Earth Direct Voltage Bucket Test Science

• Direct voltages are of local galvanic origin• Typical magnitude 0.1 to 0.5 volts

• Direct currents through bucket up to 1 mA• Consistent with muzzle to feet resistance of dairy cow

• Readily measurable

• Alternating currents may be present at low levels• Induction in test leads

• Current flow in ungrounded metalwork from on-farm sources

• Spectrum analysis will show spurious “frequencies”

Are Bucket Test Currents Problematic?

• Cow response measurements show 22% to 34% higher required direct current than alternating current• Front to rear hooves response ~ 2 mA DC

• Mouth to all hooves ~ 4 mA DC

• Cow step responses found at 60 Hz alternating currents ~ 4 mA

• Currents to stop cows drinking measured experimentally at 5 mA to 130 mA AC rms and 38 mA to 80 mA DC

• Voltages (presumably galvanic) of 0.6 V produced no effects on pigs

• Currents of magnitude measured in bucket test unlikely to be perceived and too small to elicit response

Possible Direct Voltages from AC Power Converters

• Many types of commercial and industrial equipment uses AC to DC power conversion

• Large motor drives most common large converter on farms

• Readily identifiable from harmonic current or voltage spectrum

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6 Pulse Voltage Fed Load

• Each half cycle• Double pulsed current

• Symmetrical current

• Power Factor• 0.8 true power factor - typical

• No displacement power factor

• Harmonics• Odd harmonics

• Frequency doublets (6n +/- 1)• 5th, 7th; 11th, 13th...

6 Pulse Current Fed Load• Pulsed current each half cycle

• Asymmetrical current due to commutation differences

• Power Factor• 0.6 to 0.8 true power factor• Displacement power factor due to

motor load

• Harmonics• Odd harmonics

• Doublets (6n +/- 1)• 5th, 7th; 11th, 13th...

• Even harmonics - limited amounts

12 Pulse Current (UPS)

• Top trace -- 100kVA UPS -Normal operation

• Bottom trace -- 100kVA UPS with problems• Input SCRs failed -- even order

distortion about 40%

Current Harmonic Graphical Display

Stray Direct Currents & Rail Systems

• May occur with DC traction systems where return rail grounded• Similar problem to the utility multiply-grounded neutral

• Electrolytic corrosion from stray currents main hazard• Pipelines that parallel track major problem

• Building metalwork corrosion also encountered

• Major problems in urban areas with light rail systems

• May occur in rural areas near electrified lines

• No known animal problems

Stray Currents from DC Rail Traction

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Siemens Light Rail Car Electric Light Rail Not New

Traction DC Stray Current MeasurementsLos Angeles (1910) Earth Currents from Rail System

Track-Pipeline-Earth Potentials Typical Severe Corrosion

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Pipeline to Soil Potentials

Some rules-of-thumb:

• < 0.05 volt – typically not a concern

• 0.05 – 0.1 volt – could develop a problem• increased monitoring required

• 0.1 – 0.3 volt –evaluation warranted• determine corrosion significance

• > 0.3 volt – likely a problem• investigation and mitigation appropriate

Measured Stray Current in Downtown Phoenix

Conclusions

• On farm direct voltages and currents almost always arise from galvanic action

• Cow-contact currents are measurable but likely below perception threshold

• Earth currents from external sources, including DC transmission lines and electric rail lines are likely to be negligible

• The “bucket test” cannot be used to identify the origin of measured direct currents