AC Interference

AC InterferenceAC InterferenceAC InterferenceAC Interference

Columbia Gas of Ohio/Kentucky

Tim JenkinsCorrosion Front Line Leader

ObjectivesObjectivesObjectivesObjectives Develop a basic understanding of Develop a basic understanding of

the principles and components of ACthe principles and components of AC Develop an understanding of the Develop an understanding of the

different types and effects of AC different types and effects of AC Influence Influence

Develop methods of mitigation Develop methods of mitigation Understand safety protocols Understand safety protocols Cover AC calculationsCover AC calculations

Basic Electric - ACBasic Electric - ACBasic Electric - ACBasic Electric - AC

0

Half Cycle

Half Cycle

Peak of positive side of cycle.

Peak of negative side of cycle

00

AC - alternating current will reverse in polarity 120 times per second. A full cycle is considered one hertz. Typical AC has 60 hz per second.

SINE WAVESINE WAVE

Single PhaseSingle Phase

Basic Electric - ACBasic Electric - ACBasic Electric - ACBasic Electric - AC

0

Half Cycle

Half Cycle

Peak of positive side of cycle.

Peak of negative side of cycle

00

AC – Three Phase, each conductor has the same amount of current and are 120 degrees out of phase

SINE WAVESINE WAVE

Three PhaseThree Phase

If any of the AC waveforms are to get of frequency with each other greater than or less than 120°, then a possible fault current can occur.

Fault currents are large magnitude of current that can occur in brief amount of time (normally in milliseconds, typically .1 second) Normally electrical towers or structure has

grounding and protection devices for this situation that limits the fault current to a brief amount of time

Fault CurrentsFault CurrentsFault CurrentsFault Currents

It’s not possible to know whenn, how or how or wherewhere fault currents will occur, in which makes it difficult to predict the effects of the fault and the mitigation required to protect both the pipeline and personnel

Need to calculate locations of more acceptable for fault currents to occur, such as – Electrical storms, ice storms, & high winds Distance from the Power lines Information provided from the electric

company


Even though the fault current is brief, it still presents a danger to personnel and the pipeline Coating damage can occur Pipeline failure due to melting or

cracking of the pipeline wall Discuss more in Conductive

coupling


Three Phase –

Three conductorsShielded wiresCounterpoise Lines – Counterpoise Lines –

Used for the grounding Used for the grounding system, normally system, normally buried and above buried and above connected to each connected to each tower to provide tower to provide groundinggrounding

Counterpoise Lines – Counterpoise Lines – Used for the grounding Used for the grounding system, normally system, normally buried and above buried and above connected to each connected to each tower to provide tower to provide groundinggrounding

Method of measuring AC voltage on Structures Connect to ground with one lead and measure

the AC volts onto the structure with the other lead.

Use an accurate volt meter, set meter on AC volts

Use rubber gloves during measurement and/or Use a rubber mat for added insulation High dielectric boots are available as well

Common method, use a copper-copper sulfate half cell with the meter set at AC volts

Must have good soil contact with half cell

Effects of AC InfluenceEffects of AC InfluenceEffects of AC InfluenceEffects of AC Influence

Two key factors to consider with Two key factors to consider with AC InfluenceAC Influence Safety Safety CorrosionCorrosion



SafetySafetySafetySafety

Electrical ShocksElectrical Shocks Step voltagesStep voltages Touch voltagesTouch voltages

Arcing Arcing Ignition of volatile liquids Ignition of volatile liquids

SafetySafetySafetySafety Maximum allowable AC voltage = 15

Vac Based on a typical individual is at 1000

ohms body resistance And the individual can tolerate up to 15

mA Ohms law = 15 volts Anything above 15 Vac, could cause

muscular contractions Prevents the person from letting go

SafetySafetySafetySafety Electrical Shock, such as fault currents

Can occur by physical contact or standing in the vicinity of an energize structure in contact with earth

Short time frames of electrical shocks are a concern when currents are above 50mA or greater

Can cause ventricular fibrillation Certainly occurs at body currents of

greater than 100 mA Death will occur unless De-

fibrillation is given

SafetySafetySafetySafety Electrical ShockElectrical Shock

Fault currents - passes from the structure Fault currents - passes from the structure to ground creating a voltage gradientto ground creating a voltage gradient

Step Voltage – Step Voltage – Is the potential difference between two points Is the potential difference between two points

on earth’s surface separation by a distance of on earth’s surface separation by a distance of 1 pace (approx. 1 meter) in the direction of 1 pace (approx. 1 meter) in the direction of max. potential gradientmax. potential gradient

Touch Voltage – Touch Voltage – Potential difference between the grounded Potential difference between the grounded

metallic structure and the point of earth’s metallic structure and the point of earth’s surface separated by a distance equal to the surface separated by a distance equal to the normal maximum horizontal reach (approx. 1 normal maximum horizontal reach (approx. 1 meter)meter)

SafetySafetySafetySafety

9 kV8 kV

7 kV

Potential Touch voltage = 2kV

Potential Touch voltage = 2kV

Ouch!!!

I (Fault Current)

10 kV

f

SafetySafetySafetySafety I (Fault Current)

9 kV 8 kV 7 kV

10 kV

Potential Step voltage = 1kV

Potential Step voltage = 1kV

Ouch!!!f

Safety – Safety – (Maximum Current (Maximum Current Calculation)Calculation)

Safety – Safety – (Maximum Current (Maximum Current Calculation)Calculation)

Maximum current IMaximum current IB B a human body a human body can tolerate depends on shock can tolerate depends on shock duration tduration ts s (seconds) and body weight (seconds) and body weight

calculated as follows:calculated as follows: IIBB = = 0.157/ 0.157/ ts ts ( for a 70 kg body)( for a 70 kg body) IIBB = = 0.116/ 0.116/ tsts ( for a 50 kg body) ( for a 50 kg body)

Safety – Safety – Step and Touch Voltage Step and Touch Voltage CalculationCalculation


Maximum voltage that human body Maximum voltage that human body can tolerate by touch or step – can tolerate by touch or step –

Step formula - Step formula - VVStepStep = (1000 + 6 = (1000 + 6) 0.157/ 0.157/ ts ts ( for a 70 kg body)( for a 70 kg body) VVSSteptep = (1000 + 6 = (1000 + 6) 0.116/ 0.116/ tsts ( for a 50 kg body) ( for a 50 kg body)

Touch formula - Touch formula - VVTouchTouch = (1000 + 1.5 = (1000 + 1.5) 0.157/ 0.157/ tsts ( for a 70 kg body)( for a 70 kg body) VVTouchTouch = (1000 + 1.5 = (1000 + 1.5) 0.116/ 0.116/ tsts ( for a 50 kg body)( for a 50 kg body)

Pipe line running parallel to a power line may exhibit 500 volts for a duration of ½ second during line to ground fault

What is the tolerable touch voltage for a 50 kg individual with a soil resistivity of 50 ohms m touching the structure during the fault?



VVTouchTouch = (1000 + 1.5 = (1000 + 1.5) 0.116/ 0.116/ tsts - - ( for a 50 kg body)( for a 50 kg body) VVTouchTouch = (1000 + 1.5 = (1000 + 1.5 • • 50) 0.116/ 0.116/ (.5) (.5) = 176 VAC= 176 VAC

Since the possible fault voltage is 500 V then we need to raise the soil resistance

Try 3000 Ω-m of crush stone added to the site

Now the calculation equals to 902 VAC902 VAC Which exceeds the maximum pipe to earth Which exceeds the maximum pipe to earth

voltage of voltage of 500 VAC500 VAC, the pipe is now safe, the pipe is now safe

Voltage gradient matsVoltage gradient mats could provide a higher could provide a higher earth voltage to decrease the potential difference earth voltage to decrease the potential difference between the hand or feet touching the pipebetween the hand or feet touching the pipe



SafetySafetySafetySafety I (Fault Current)

10 kV

10 kV

7 kV

10 kV

Potential Step voltage = 0 kV

Potential Step voltage = 0 kV

Cool !!!f

Voltage gradient Mat = 10 kV

Voltage gradient Mat = 10 kV

Gradient control Gradient control mats – Placed at mats – Placed at all test station all test station locations in the locations in the AC CorridorAC Corridor

Gradient control Gradient control mats – Placed at mats – Placed at all test station all test station locations in the locations in the AC CorridorAC Corridor

Zinc Grounding Zinc Grounding MatMatZinc Grounding Zinc Grounding MatMat

12” crushed gravel

6” Low resistance material – Coke breeze or benonite

6” Low resistance material – Coke breeze or benonite

Note : You can use the native soil, providing soil has good moisture content

Cut hole for Test station

Dimensions = 4’x4’

Zinc ribbon

Wire connected to the zinc ribbon

Zinc Grounding Zinc Grounding MatMatZinc Grounding Zinc Grounding MatMat Wire

connected to the zinc ribbon

Connected Connected to pipeline to pipeline in Test in Test station boxstation box

Connected Connected to pipeline to pipeline in Test in Test station boxstation box

Copper Copper rods rods installed installed to get to get low low resistancresistance with e with groundingrounding matg mat

Copper Copper rods rods installed installed to get to get low low resistancresistance with e with groundingrounding matg mat

One of the greatest concern in dealing with fault currents between a power line structure and the pipeline is whether or not there is enough energy available to create an electric arc through the soil.

Could result in pipeline damage

Safety Safety (Calculation for Arcing)(Calculation for Arcing)Safety Safety (Calculation for Arcing)(Calculation for Arcing)

Greatest prevention of Arcing with fault currents is to maintain safe distance between power lines and the pipeline

One must obtain information from the electric company or producer such as fault currents maximum measurements

Need to find soil resistivity in area Perform sufficient amount of testing

samples in order to accurate obtain average

Safety Safety (Calculation for Arcing)(Calculation for Arcing)Safety Safety (Calculation for Arcing)(Calculation for Arcing)

One Safe distance calculation by Sunde - for prevention of arcing

Distance r (m) over which an arc could occur, based on soil resistivity in (Ω-m) and fault magnitude If (kA).

Safety -Safety -(Calculation for Arcing)(Calculation for Arcing)Safety -Safety -(Calculation for Arcing)(Calculation for Arcing)

R(m) = 0.08 Use this formula with lower resistivity

R(m) = 0.047 If • ( = > 1000-m) Use this formula with extremely high

resistivity R(m) = Distance measured in meters If = Magnitude of fault current = Soil resistivity measured in

meters


If • ( = < 100-m)

For an example, Soil = 6700 ohms-cm = 67 ohms

-m Fault Current If =17.9 kA Use formula R(m) = 0.08 R(m) = 87.6 meters


If • ( = < 100-m)

Pipeline

Fault Currents

Lightening

90 Meters

PipelineFault Currents

Lightening

90 Meters

Zinc Ribbon

If safe distance can not be obtain, Screening electrodes between the

pipeline and towers maybe used to intercept the fault currents

Such as zinc ribbon, or banks of sacrificial anodes


Pipeline

Fault Currents

Lightening

Pipeline

Fault Currents

Lightening

Zinc Ribbon

Zinc



AC Corrosion on AC Corrosion on PipelinesPipelinesAC Corrosion on AC Corrosion on PipelinesPipelines

AC influence can cause corrosion to take AC influence can cause corrosion to take place on coated steel pipe lineplace on coated steel pipe line

Study performed in Germany, recently in Study performed in Germany, recently in the 1990’s, had determined that corrosion the 1990’s, had determined that corrosion occurs at specific occurs at specific AC current densityAC current density - - (>100 A/m(>100 A/m²²) = ) = Corrosion will resultCorrosion will result (20 A/m(20 A/m²² - 100 A/m - 100 A/m²²) = ) = Corrosion is unpredictableCorrosion is unpredictable (< 20 A/m(< 20 A/m²²)) = = Corrosion will not resultCorrosion will not result

AC Corrosion on AC Corrosion on PipelinesPipelinesAC Corrosion on AC Corrosion on PipelinesPipelines There has been documented cases of pipe

to soil potentials being above -1.170VCSE with pH samples at 11, indicating pipe being cathodically protected, but corrosion was found due to AC current density in the range of 800 A/m²

Pipe must be mitigated by dropping the AC voltage with the use of grounding devices such as zinc ribbon, copper wire, etc..

AC Calculation for AC Calculation for Current DensityCurrent DensityAC Calculation for AC Calculation for Current DensityCurrent Density

Calculation to determine AC current density - Iac = 8◦Vac/ ••d

Iac = Current density = soil resistivity in meters d = holiday area in cm’s

AC Calculation for AC Calculation for Current DensityCurrent DensityAC Calculation for AC Calculation for Current DensityCurrent Density Calculation to determine AC current density

- Iac = 8Vac/ ••d

Resistance and area of holiday will be the key factors in determining the AC current density

For an example – 1cm² holiday found with 5 Vac in a soil resistivity of 10 Ohms m (1000 ohms CM)

= 127 amp/m² ((( Corrosive))) But below the 15 Vac

Documented cases of Documented cases of AC Corrosion Found -AC Corrosion Found -Documented cases of Documented cases of AC Corrosion Found -AC Corrosion Found -

Pipe to soil potential readings were above –1.0v CSE DC

Pipe to soil potential readings were above –1.0v CSE DC

Pipe met DOT criteria for CP – above .850- V CSE

Documented cases of Documented cases of AC Corrosion Found -AC Corrosion Found -Documented cases of Documented cases of AC Corrosion Found -AC Corrosion Found -

AC Stray Current – AC Stray Current – Interference MethodsInterference MethodsAC Stray Current – AC Stray Current – Interference MethodsInterference Methods

Electromagnetic Coupling – Inductive

Electrostatic Coupling – Capacitive

Conductive Coupling - Direct path


Electromagnetic Coupling –Electromagnetic Coupling – InductiveInductive


Conductive Coupling Direct path

Works in the same capacity of a inductive pipeline locator – Induces an audio signal onto the

buried pipeline Or in the same capacity of a

transformer Primary coils inducing current by a

electromagnetic field to the secondary windings

Electromagnetic Electromagnetic Coupling – InductiveCoupling – Inductive

Primary characteristics include: Medium to High Voltages High induced current levels

Electromagnetic Electromagnetic Coupling - InductiveCoupling - Inductive

The level of interference decreases with increasing separation of conductors

The strength of the magnetic flux is in direct proportional to the current magnitude and inversely proportional to the distance of the conductor

Induction effects experienced during power line faults can be a hazard to personnel

Normally peaks at the point of entry of AC corridor and at the point of exit

Electromagnetic Electromagnetic Coupling - InductiveCoupling - Inductive

Installation of a low resistance grounding systems to reduce current and voltage levels Grounding mats for test stations (safety) Zinc ribbon Copper wire with the use of PCR or ISP

Achieve at least a 25 ohm impedance system Ideally one ohm system Normally deeper is better

Electromagnetic Electromagnetic Coupling - Inductive Coupling - Inductive (Remediation)(Remediation)



Electrostatic Coupling –Electrostatic Coupling – CapacitiveCapacitive


Electrostatic Coupling – Electrostatic Coupling – CapacitiveCapacitive

Any two conductors separated by a dielectric material (insulator) is considered a capacitor

Electrical Field Gradient between the transmission line and conductor takes place, builds up a electrical charge on the structure Such as a capacitor function

Primary characteristics include : Very High Voltage peaks on power lines


Conductors acceptable to capacitive coupling Pipelines suspended above ground on

skids Any above ground equipment isolated such

as vehicles or backhoes with rubber tires Electrostatic Coupling does not penetrate

the earth Long parallel exposure of buried metallic

structures to power lines

((

VAC

VAC

Ground

The voltage builds up until it has path to ground to discharge

Electromagnetic charge

Voltage Gradient – electrom-agnetic field



((

VAC

VAC

Ground

Circuit is open, the voltage charge will build to high voltage static capacity, until a ground source is provided.

The voltage builds up until it has path to ground to discharge





VAC

Ground

Direct Path to ground –

You……

By touching the structure and ground at the same time.

The voltage discharges


((

VAC




Pipe suspended

500 VAC

Pipe suspended

500 VAC

Electrostatic Coupling – Electrostatic Coupling – Capacitive Capacitive (Remediation)(Remediation)

Temporary repair – Ground vehicles and equipment

Use temporary grounding rods (copper rods) normally in 3 meters in length

Use #2 Cable Use ½ in diameter rods in normal soils

Refuel away of influence area to prevent accidental ignition, bond to refueling tanks

Due to high resistance soil, you need to place multiple rods, space about 6 feet apart

Metal chains dragging from the vehicle’s bumper in High AC voltage corridors are commonly used

Pipe is Pipe is being being grounded by grounded by making making contact to contact to the soilthe soil

Pipe is Pipe is being being grounded by grounded by making making contact to contact to the soilthe soil

Electrostatic Electrostatic Interference – Interference – Capacitive Capacitive (Remediation)(Remediation)

Permanent repair – Above ground pipelines or valves

Install Zinc ribbon Install Zinc Grounding or voltage gradient Mats Grounding Rods

3 Meters in length Design cable size based on potential fault

currents Set depth until achieved a minimum of 25 ohms

impedance Lower the impedance as low as possible One ohm is desirable

Electrostatic Electrostatic Interference – Interference – Capacitive Capacitive (Remediation)(Remediation)

In most cases, the Electrostatic charge can not generate enough body current to create a shock hazard, more of a nuisance shock similar to static electricity.





Conductive Coupling Occurs when line to ground shorts or faults

take place On High Voltage power lines faults normally

occur during lighting strikes Fault currents can occur by accidental

contact with other structures Such as construction equipment or cranes

Fault currents is conducted to the pipeline through its coating Higher the coating dielectric strength, the less

amount of the transfer current on the pipeline

Conductive Coupling

Occurs in milliseconds Voltage and current is higher than

steady state but happens very briefly .1 or a tenth of a second is the normal

time frame that voltage is present due to the fault protection system

Conductive Coupling

Failure to the pipeline Coating damage Cracking and melting of the pipe wall

Materials Used for AC Materials Used for AC MitigationMitigationMaterials Used for AC Materials Used for AC MitigationMitigation Two reasons for mitigation of AC

influence Prevent corrosion on the pipeline Prevent of hazardous shock from

contacting the pipeline Materials commonly used

Zinc grounding and/or voltage gradient mats

Zinc ribbon or heavy gauge copper wiring Blind face test stations Galvanic anodes PRC or Inductive capacitive coupling

Materials Used for AC Materials Used for AC MitigationMitigationMaterials Used for AC Materials Used for AC MitigationMitigation Materials –

Zinc Ribbon – to mitigate the AC currents from the pipeline to the soil

Zinc is used in some low resistivity areas as a galvanic anode to protect structures

The AC currents will take the path of less resistance to the ground

Zinc provides this path Depending of soil resistance, distance to the

tower, the location of structure to the towers and the amount of magnitude influence of the towers must be calculated in the design of the amount of Zinc ribbon needed and the location

Materials Used for AC Materials Used for AC MitigationMitigationMaterials Used for AC Materials Used for AC MitigationMitigation

Programs available to profile the pipeline for AC mitigation PRCI

Materials Used for AC Materials Used for AC MitigationMitigationMaterials Used for AC Materials Used for AC MitigationMitigation Materials –

Zinc Ribbon – Installation –

Placed below the pipeline Depending on soil resistance Place in the lowest resistance area Minimum Two feet away from the pipeline

Make connection to the pipeline in a junction box or test station

Commonly used, a minimum of a no. 4 gauge wire connected to the pipeline and zinc ribbon

May need to increase size of cable due to greater magnitude of fault currents

Materials Used for AC Materials Used for AC MitigationMitigationMaterials Used for AC Materials Used for AC MitigationMitigation

Materials – Zinc Ribbon –

Installation – Placed between the pipeline and tower

To mitigate fault currents and prevent coating damage

Splice zinc ribbon by striping the zinc off the wire and crimp the connections together

Make crimp repair with epoxy resin kits, heat shrink sleeve, or electrical rubber tape

Chart for Zinc RibbonChart for Zinc RibbonChart for Zinc RibbonChart for Zinc Ribbon

Standard – ½ inch comes Standard – ½ inch comes in wooden spoolsin wooden spools

Ribbon is bonded to the Ribbon is bonded to the main in the test station main in the test station to be able to test AC to be able to test AC mitigation such as AC mitigation such as AC current density & current density & grounding system grounding system resistance resistance

Standard – ½ inch comes Standard – ½ inch comes in wooden spoolsin wooden spools

Ribbon is bonded to the Ribbon is bonded to the main in the test station main in the test station to be able to test AC to be able to test AC mitigation such as AC mitigation such as AC current density & current density & grounding system grounding system resistance resistance

Zinc ribbon is placed Zinc ribbon is placed below the pipeline below the pipeline and at least two feet and at least two feet away away

Zinc ribbon is placed Zinc ribbon is placed below the pipeline below the pipeline and at least two feet and at least two feet away away

Installed at test Installed at test station facility –station facility –

Coupon for AC Coupon for AC measurementsmeasurements

Grounding mat Grounding mat or voltage or voltage gradient mat for gradient mat for test point reader test point reader safetysafety

Zinc ribbon Zinc ribbon connectionconnection

Structure Structure connectionsconnections

Zinc Ribbon

Pipeline

AWG #4 w

hite

Zinc Ribbon

Coupon

AWG #4 w

hite

2 -AWG #

4 white

AWG # 12 Green 2-AWG #12 Blackswitch

AC Mitigation Test Station(recommendation - installationof voltage gradient mat)

Coupon

Used for AC currentdensity measurements

What's wrong with What's wrong with the next slide?the next slide?What's wrong with What's wrong with the next slide?the next slide?

Tower

Zinc ribbon

Pipe line

Zinc ribbon

Slide “A”

Slide “B”

Zinc ribbon is above Zinc ribbon is above the pipelinethe pipelineZinc ribbon is above Zinc ribbon is above the pipelinethe pipeline

Zinc Ribbon is on the Zinc Ribbon is on the wrong side of the wrong side of the pipelinepipeline

Zinc Ribbon is on the Zinc Ribbon is on the wrong side of the wrong side of the pipelinepipeline

Tower

Zinc ribbon

Pipe line

Polarization cells or Polarization cells or Insulated surge Insulated surge protections are great for protections are great for grounding the pipeline or grounding the pipeline or structure with out structure with out shorting out the DC shorting out the DC cathodic protection cathodic protection currents. It will block the currents. It will block the DC and allow the AC DC and allow the AC currents flow to ground.currents flow to ground.

Polarization cells or Polarization cells or Insulated surge Insulated surge protections are great for protections are great for grounding the pipeline or grounding the pipeline or structure with out structure with out shorting out the DC shorting out the DC cathodic protection cathodic protection currents. It will block the currents. It will block the DC and allow the AC DC and allow the AC currents flow to ground.currents flow to ground.

Dead Front Test Dead Front Test StationsStationsDead Front Test Dead Front Test StationsStations

To prevent To prevent electrical shock electrical shock in making in making contact with contact with wire connection wire connection to mainlineto mainline

To prevent To prevent electrical shock electrical shock in making in making contact with contact with wire connection wire connection to mainlineto mainline

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AC Interference