AC Interference and Mitigation

Florida Energy Pipeline Association

Pipelines & HVAC Lines Collocated

• Collocated Utilities Pipelines and HVAC Power Lines

often share the same right of way

There are problems that must be addressed when HVAC and buried pipelines share the same right of way

Enhanced Regulatory Scrutiny

• Regulators are focusing more on this issue given recent risk findings by operators and enhanced pipeline safety regulations

• NACE Standard RP0177 (Latest Revision) - Recommended Practice on Mitigation of Alternating Current & Lightning Effects on Metallic Structure and Corrosion Control Systems. Also, ANSI/IEEE Standard 80 specifies safety design criteria for determining maximum acceptable touch and step voltages during fault conditions.

Three Distinct Issues

• Health and Safety of Personnel/Public/Livestock Well known and easily fixed

• AC Fault Currents Short duration occurring at a particular tower location

• AC Induced Corrosion Not well understood but affects well coated pipe Steady state condition Can be quite damaging and intense

How do Pipelines and AC Interact

• Electrostatic Coupling Capacitive nuisance effect

• Conductive Coupling Fault Currents at tower footings

• Inductive Coupling Steady State Induced AC voltage buildup

Pipe and Power line create a circuit of two capacitors in series. A capacitor is a passive electronic component consisting of a pair of conductors separated by an insulator (air)

Electrostatic Coupling

Can generate very high AC voltage levels – but there is not enough power to do much more than create a minor electrostatic shock. Generally a nuisance, however can be an issue and grounding may be required.

Electrostatic Coupling

How do Pipelines and AC Interact

• Electrostatic Coupling Capacitive nuisance effect

• Conductive Coupling Fault Currents at tower footings

• Inductive Coupling Steady State Induced AC voltage buildup

AC Fault Conditions• Relatively rare• Short duration• Generally due to weather

(lightning and high winds)• Can be structural failure

Causes intense stressing of pipeline coating and possibly the pipeline wall

Conductive Coupling

Conductive Couplings

• Rare occurrences that can result in significant current discharging through the ground

• Separation distance of the pipeline from the fault is critical• Soil resistivity is critical – note soil layering can affect current

path• Requires arcing through the soil for a current path – not

easy

How do Pipelines and AC Interact

• Electrostatic Coupling Capacitive nuisance effect

• Conductive Coupling Fault Currents at tower footings

• Inductive Coupling Steady State Induced AC voltage buildup

I1

φ

I2

• A function of Line Current not Voltage

• Power transferred is •Proportional to line current•Proportional to parallelism•Inversely proportional to separation distance

• Can result in high voltages on long sections of pipeline even if the pipeline is grounded

Electromagnetic Induction

• Current through the HVAC lines generate a Longitudinal Electric Field (LEF)

• The separation between the phase conductors has a significant effect on the LEF and increases with separation

• Bundled buried conductors have no separation and provide only a minimal effect on pipelines

Electromagnetic Induction

A CB

Electromagnetic Induction

• The arrangement of phases on multiple circuit HVAC lines can have a large impact on the LEF

ABC

ABC

ABC

CBA

Center Line Symmetric

Center Point Symmetric

Electromagnetic Induction

• If all characteristics are perfectly uniform along the pipeline/HVAC then there will be a zero voltage at the mid point and peaks where the HVAC and pipeline separate if the pipeline is electrically “short”

0 L

L/2

Electromagnetic Induction

• If the length is electrically “long” it would look more like this…

0 L

L/2

What are the effects of Coupling

• Electrostatic Coupling Capacitive nuisance effect

• Conductive Coupling Fault Currents at tower footings

Safety Concern Stress Voltage

• Inductive Coupling Steady State Induced AC voltage buildup

Safety Concern AC Induced Corrosion

AC Fault Conditions• Relatively rare• Short duration• Rapid localized increase in

voltage• Can cause significant coating

damage• Could result in a direct arc from

tower footing to pipeline heating the metal quickly (burning a hole)

Stress Voltage

Steady State AC Corrosion

• Until recently not a concern for pipeline operators• Published studies pre-1990s discounted AC corrosion as

a possibility• Regulators not focused on this as a risk until quite

recently

German Experiments

• Pipeline failures in Germany in the early 90s on well protected new pipelines puzzled investigators

• Previously, AC corrosion rates were not considered a threat• Testing on coupons with 1 cm2 holidays in low resistivity

soils found corrosion rates of 210 mpy on steel polarized to 1800-2000 mV cse

Morphology of AC Corrosion

Round crater like corrosion with deep pits typical of very active corrosion

May have some false indications of Microbiologically Induced Corrosion

Occurs in the presence of AC Transmission and in some cases Distribution lines

Likely in lower soil resistivities

Optimum Coating Holiday

• Testing has found that the optimum coating holiday size for high AC Corrosion rates is between 1-3 cm2 coating holiday.

• AC Current density is the key consideration 0-20 A/m2 no corrosion 20-100 A/m2 corrosion risk unpredictable Above 100 A/m2 corrosion can be expected

Sample Calculation

Sample calculation for a 1 cm2 holiday in 10 ohm-m soil

Even at very low AC voltage levels this could yield corrosion rates in excess of 20 mpy even with good CP applied

AC Voltage vs. Soil Resistivity

This graph shows the holiday size and AC Voltage required to exceed the 100 A/m2 “corrosion can be expected” threshold at varying soil resistivities.

AC Induced Corrosion

• The higher the quality of the coating the greater the risk of AC induced corrosion

• AC Induced corrosion with well coated pipelines can create significant and rapid corrosion even at low levels of induced AC Voltage even with good CP levels on pipeline

• Corrosion mechanism still being researched but evidence is clear that it occurs

Safety Concerns

• High voltage levels either from induction (steady state) or from fault conditions (rare and short duration) present a danger to personnel 15 VAC threshold is well established by NACE

Based on release threshold calculations Gradient Mats are well established for a long time in the

industry

Step and Touch Potential

10 kV

9 kV8 kV7 kV

During a fault condition or even steady state AC Voltage presence on the pipeline can create a safety condition at above ground structures (test stations, valves, etc…)

The person touching the structure is exposed to 2 kV touch potential while the man standing is exposed to 1 kV in this diagram

Gradient Mats

Creates equipotentialenvironment for personnel

AC Mitigation

AC Mitigation typically involves installation of one or more grounding devices to allow AC current to readily discharge off of the pipeline thus minimizing coating stress during fault conditions and reducing the inductive voltage levels to well below any threshold for personnel safety or AC induced corrosion.

1. Step and Touch potentials at above ground appurtances (15 VAC NACE criteria)

2. Conductive coupling dumping excessive Fault Current onto pipeline causing damage

3. Induced Voltage discharging through smaller holidays on well coated pipelines causing AC induced corrosion

Key Issues

AC Modeling

• Very complex mathematically to model Numerous variables Some very difficult to quantify Requires input from utility on the operating conditions Requires field data gathering Different modeling software

PRCI, SES/CDEGS, ARC Engineering, Dabkowski, others…

Goals for AC Modeling

• Calculating Fault Condition Stress Values • Calculating Induced Voltage at various points along the

model• Evaluating Impact of Mitigation Measures

Where How much How effective

Shortcomings of Modeling

• Modeling is only as good as the data being used• Modeling is only as good as the assumptions being

made• Modeling has to focus on worst case conditions

What is being modeled

• The power line Peak loads, winter and summer Max fault current (line to ground) Shield wire data – type and geometry (mostly for fault analysis –

only minor impact on steady state) Phase wire data Phase imbalance data Tower data

What is being modeled

• The pipeline Pipe diameter Wall thickness Depth of cover Coating resistance and thickness (generally a guess since it is

not practical to measure this) Centerline distance from towers

What is being modeled

• The environment Soil resistivity along the colocation Soil resistivity at various depths (used in some of the more

sophisticated modeling) Foreign structures of note (multiple pipelines and multiple

HVAC lines)

Typical Modeling

Stress Currents

• The concern is elevated short duration coating stress. Different coatings have different coating stress limits

• Computer modeling is very complex and requires numerous assumptions Geometry Soil Resistivity and layering Transmission Fault data

Modeling of Mitigation

Modeling Results

Conductive Coupling Modeling

• Dabkowski – Corrosion 2003 presented the following:

Risk Assessment w/o Modeling

• Look for changes that will cause voltage spikes Changes in the pipeline to HVAC distance from each other Changes in the HVAC line (phase transpositions) Changes in soil resistivity

• Identify what your concerns might be Stress voltages during fault conditions Steady state AC induced corrosion

Risk Assessment w/o Modeling

• Corrosion risk - Zero in on areas where voltage spikes can be anticipated and there is low soil resistivity.

• Fault current risk – Zero in on areas with the least separation between tower footings and pipeline

• Modeling may not be required

Field testing of LEF

• It is possible to take AC voltage readings and to measure the induced longitudinal electrical field (LEF) by placing a calibrated insulated cable on the ground parallel to the pipeline, grounding it, and using an high impedance voltmeter to measure the open circuit potential

• The value received reflects the operating conditions at the time

AC Mitigation Project

AC Test Coupons

• Designed to replicate a 1 cm2 holiday

• Can be used to determine the actual current density being picked up at the pipeline before applying mitigation and after installing mitigation

• Uses the same coating and geometry as the pipe

PCR Installation

Copper Ground Wire Detail

Optimum AC Mitigation

• Modeling is only as good as the model, the assumptions and the data being input

• Gradient control line(s) parallel to the pipeline for new well coated pipelines are recommended to minimize AC Corrosion risk

• Short lines at the tower footings are best for fault condition mitigation and can be used in conjunction with long gradient control lines

What is the MITIGATOR™?

Looks like the SPL™ Linear Anode.

Not an anode but a copper grounding cable

Special backfill

MATCOR’s MITIGATOR™

Installation of the MITIGATOR™ along a Williams (Transco) Gas pipeline in Northern New Jersey.

The pipeline is actually to the left of the MITIGATOR™ trench.

The MITGATOR™ provides for easy installation, a much larger surface area for discharging copper, and the copper conductor is housed in a special backfill with corrosion inhibitors.

Installation

From the Plattline™ Website

Life expectancy of Plattline in this application would be quite long and would generally be determined by Plattline as a projected cathodic protection system. The most common sizes of Plattline for AC mitigation are plus and standard.

Zinc Ribbon

SIZE SURFACE AREA COST

Standard 54.0 mm2 $2.50/ft.

Plus 76.2 mm2 $5.00/ft.

Super 114.3 mm2 $9.50/ft.

MITIGATOR™ 119.6 mm2 $5.25/ft.

Concerns with Zinc

• Zinc can passivate and should have a special backfill when used for AC Mitigation

• Zinc is much more difficult to handle and install relative to the Mitigator™

• Must use a torch to make connections• Requires more frequent use of decouplers• Will consume over time – not as long a life as copper

Areas for more investigation

• Sophisticated modeling of Mitigator™ vs. Zinc

• Investigation of “propagation constant” and the spacing of decouplers for zinc vs. Mitigator™

Summary of AC Interference

• There are three key threats Safety (15 V AC Threshold) Fault Conditions (rare but potentially damaging) AC Corrosion – for new well coated pipelines this can easily be

the most challenging and difficult threat to control and can cause damage even at lower levels of AC

Summary of AC Interference

• Modeling may not be fully effective – especially for AC Corrosion

• AC Coupons give information based on current operating conditions – changes in electrical flow affect the AC Induced Voltage

• Risk Assessment can often be performed without expensive modeling

Questions?

Questions