1. AC Interference and Mitigation
Florida Energy Pipeline Association

2. 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

3. 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.

4. 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

5. 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

6. Electrostatic Coupling
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)

7. 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.

8. 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

9. Conductive Coupling
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

10. 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

11. 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

12. Electromagnetic Induction
•A function of Line Current not
Voltage
•Power transferred is
I1 •Proportional to line
current
•Proportional to
parallelism
φ
•Inversely proportional to
separation distance
•Can result in high voltages on
I2 long sections of pipeline even
if the pipeline is grounded

13. Electromagnetic Induction
• Current through the HVAC lines generate a
Longitudinal Electric Field (LEF)
A B C • 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

14. Electromagnetic Induction
• The arrangement of A
B
A
B
phases on multiple C C
circuit HVAC lines can
Center Line Symmetric
have a large impact on
the LEF A C
B B
C A
Center Point Symmetric

15. Electromagnetic Induction
• If all characteristics are
perfectly uniform along
the pipeline/HVAC then
there will be a zero
voltage at the mid point 0 L
and peaks where the L/2
HVAC and pipeline
separate if the pipeline is
electrically “short”

16. Electromagnetic Induction
• If the length is electrically
“long” it would look more
like this…
0 L
L/2

17. 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

18. Stress Voltage
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)

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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.

25. 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

26. 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

27. Step and Touch Potential
During a fault condition or even steady
state AC Voltage presence on the 10 kV
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 9 kV
8 kV
the man standing is exposed to 1 kV in 7 kV
this diagram

28. Gradient Mats
Creates
equipotential
environment for
personnel

29. 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.

30. Key Issues
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

31. 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…

32. 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

33. 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

34. 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

35. 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

36. 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)

37. Typical Modeling

38. 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

39. Modeling of Mitigation

40. Modeling Results

41. Conductive Coupling Modeling
• Dabkowski – Corrosion 2003 presented the following:

42. 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

43. 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

44. 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

45. AC Mitigation Project

46. 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

47. PCR Installation

48. Copper Ground Wire Detail

49. 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

50. MATCOR’s MITIGATOR™
What is the MITIGATOR™?
Looks like the SPL™ Linear
Anode.
Not an anode but a copper
grounding cable
Special backfill

51. Installation
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.

52. 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.

53. 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.

54. 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

55. Areas for more investigation
• Sophisticated modeling of Mitigator™ vs. Zinc
• Investigation of “propagation constant” and the spacing
of decouplers for zinc vs. Mitigator™

56. 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

57. 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

58. Questions
Questions?