DC MEASUREMENTS IN STRAY VOLTAGE INVESTIGATIONS

Transcription

1 DC MEASUREMENTS IN STRAY VOLTAGE INVESTIGATIONS Michael 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 Bucket Test Recordings Currents measured typically < 1 ma DC Indicates driving voltage ~ 0.5 V DC 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 1

2 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 IPP Ground Electrode 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 2

3 Ground Electrode IPP Ground Electrode Current 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 Direct Current Spectrum Measuring Earth Voltages from Telluric Currents Using driven copper or copper-clad rods Distances from a few feet up to 200 feet Recorded using solarpowered 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 3

4 Earth Voltage & Injected Earth Current Earth Voltages Near the Electrode Changes of Direct Current during recording help to identify changes 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 Current Correlation Between Earth Voltage and Electrode Current 4

5 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 5

6 Measuring Direct Step Voltages Fence to Earth Direct Voltage Precautions needed against galvanic potentials from measuring electrodes Typically use a copper sulfate half-cell as a standard reference 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 6

7 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) Current Harmonic Graphical Display Top trace kVA UPS - Normal operation Bottom trace kVA UPS with problems Input SCRs failed -- even order distortion about 40% Stray Direct Currents & Rail Systems Stray Currents from DC Rail Traction 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 7

8 Siemens Light Rail Car Electric Light Rail Not New Traction DC Stray Current Measurements Los Angeles (1910) Earth Currents from Rail System Track-Pipeline-Earth Potentials Typical Severe Corrosion 8

9 Pipeline to Soil Potentials Some rules-of-thumb: < 0.05 volt typically not a concern volt could develop a problem increased monitoring required 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 9