Safety Considerations for AC Mitigation Designs

Mike Tachick Dairyland Electrical Industries Inc.

Western Regional Gas Conf – Aug 2012

Protection Concepts

• Minimize voltage difference between points of concern: – At worker contact points – Across insulated joints – From exposed pipelines to ground – Between grounding systems – Across a grounding system – Pipe to other nearby structures: casing, tower, substation

Over-Voltage Sources

Temporary: – Lightning – AC power system faults Steady-state: – Induced AC voltage

Touch Voltage

Step Voltage

Key Parameters of Lightning Waveform

Slope = di/dt (Rate of rise, Amps/µsec) 1.0

Crest Amperes 1/2 Crest Value 0 8 20 Time in microseconds

Lightning has very high di/dt (rate of change of current)

AC and Lightning Compared

Time (milliseconds) Alternating Current Time (microseconds) Lightning

Lightning standards

• Typical waveforms for testing replicate lightning characteristics – 8 x 20 microsecond waveform – 4 x 10 microsecond waveform • Current levels for testing are usually 50kA, 75kA or 100kA peak • Actual: 1kA to 200kA

AC Faults

• • • • Events last several cycles typically (60Hz = 60 cycles per second) Current levels are hundreds or thousands of amps AC on pipeline. Can be higher if transmission involved.

Usually relates to electrical insulation breakdown or reduction of some type Breaker or fuse clears the fault

AC Faults

AC Faults

• • • Conductors, connectors, safety products must withstand AC fault current magnitude and duration Product ratings should exceed available fault current Conductors – compare to ampacity charts

AC Faults

Source: NACE SP0177-2007 Cable sizing chart

AC Faults

Manufacturer’s fault data

Arc Distance – AC Faults

40 30

138 kV 230 kV 500 kV

20 10 0 0 6500 15000 30000

Soil Resistivity (ohm-cm)

45000 Courtesy of Bob Gummow – Corrosion Service

Arc Distance

• • • Distance is your friend: Locate pipeline farther from transmission tower For a given distance, lower voltage systems present less risk Higher soil resistivity reduces fault exposure

Lightning Over-Voltage

• • • Protective products (decouplers, arresters) have voltage drop across them Conductors that attach products have even higher voltage drop during lightning surge Use very short conductors or bus bars for product attachment

Lightning Over-Voltage

• Voltage determined by the inductance of the current path and the rate-of-rise of current V = L  di/dt • • Inductance relates to conductor length or multiple conduction paths Rate-of-change of current is a high value for lightning

Lightning Over-Voltage

• • Given typical wire, 0.2μH/foot Given waveform with 10,000A/μs Result is: V = L di/dt = 0.2μH/ft  10,000A/μs = 2,000V/ft This is the voltage per foot of conductor length

Conductor length example

Conductor length example

Lightning data

• • Products and components tested for lightning capability Typical ratings: – 8x20 µs waveform – 4x10 µs waveform – Magnitude: 75kA to 100kA peak

Lightning data – lab tests

Lightning - summary

• • • Conductor length kept short to limit over voltage, where possible Bus bars for mounting across flanges reduces over-voltage Conductor diameter not a major concern

Step and Touch Voltage

• • • • Grounding mats used to equalize voltage across earth near pipeline structures Brings earth and pipeline voltage near each other, locally Not a significant AC mitigation ground Purpose: limiting step and touch voltage

Step and Touch Voltage

• • • Use grid-type grounding mat designs for lowest step and touch voltage Install within 4 ft of ungrounded pipeline segments Connect mats to pipe with short conductors (important for lightning)

Step and Touch Voltage

• • • • Use mats at test stations in utility right-of-way Even with dead-front TS construction, step voltage exists Apply mat from test station to 4’ away Decouple to remove mat from CP system

Gradient Control Mats

Grounding mat design

• • • • Best performance comes from a gridded mat Worst performance from single conductor system (spiral, zig-zag) Difference can be 1000:1 in performance Connect any mat to pipe with short bonds, otherwise touch voltage is raised for lightning conditions

Grounding Mat Design

Spiral Mat Grid Mat Single current path, high inductance Multiple current paths, very low inductance

AC Induction

• • • • Effect from current flow on power line nearby Magnetic field from current interacts with pipe Raises voltage on coated pipelines Worse effects on well coated lines

Induction Variables

Variable

Soil resistivity Coating resistance Load current Dist from tower Change in distance

Change

Increase Increase Increase Increase Any change

Induction result

Increase Increase Increase Decrease Increase

AC Mitigation

• • • • Establishes low resistance pipe to ground bond Collapses induced AC voltage, allows AC current to flow No effect upon CP if decoupled Must be rated for steady-state and fault conditions

What is a decoupler?

• • • • • • DC blocking/AC conducting devices Block DC up to threshold, then conduct to provide over-voltage protection Commonly solid-state construction Some certified as “ fail-safe ” Fail-safe = always fail shorted if current ratings exceeded Need to be rated for hazardous locations

Decouplers – Typical Ratings

• • • • • • • Voltage threshold: 2 or 3V DC leakage: <1mA at typ. voltage AC steady-state: 45A rms AC fault: 3kA to 10kA rms Lightning: 100kA peak Environmental: Rain-tight or submersible Hazardous: Div 2

Decouplers

• • • Purpose: to limit induced AC voltage on pipeline continuously… To provide over-voltage protection during AC faults, lightning strikes… But otherwise block DC from the CP system

Protection guidance

• NACE SP0177-2007, limits voltage to 15V • 15V based on human health issues, and is a steady-state limitation • Does not guarantee protection for AC faults and lightning, doesn ’ t address AC corrosion

AC Mitigation Practices

• • Use mats at all facilities and test stations near HVAC corridors and for those pipelines that come from corridors (basically good practice everywhere) Mats are for step and touch voltage but usually don’t provide AC mitigation (other specific AC mitigation grounding system needed)

AC Mitigation Practices

• • • Final mitigation design creates low resistance ground without affecting CP Can test by bonding pipeline temporarily to well grounded object (fence, culvert, station ground) Measure resulting AC voltage on pipe, and AC current drain

Field Measurements

Open Circuit Voltage

Field Measurements

Short Circuit Current

Final Installation

Same AC result, but decoupled

Final AC Design

• • • • Grounding system is one acceptable for safety (not transmission towers) Usually is separate system (copper mitigation wire, zinc ribbon, anodes as grounds, deep well) Decouplers used to provide CP isolation from ground, with AC continuity Other objects addressed: insulated joints, casings, proximity to substations and towers

AC Mitigation Results

• • • • • Voltage is limited on pipe Current path is established for lightning, AC fault current, steady-state AC current Safety problem has been addressed Decoupling prevents CP from being bonded to grounding system Didn ’ t create new hazards: not bonding to inappropriate grounded objects

Other Points with Voltage Difference (with respect to pipeline) • • • • • • • • Insulated joints - arc, shock Insulated measurement lines – arc, shock Station ground, fence – arc, shock Step voltage across soil surface - shock Vault contact points - shock Casings - arc Wells - arc Towers, substations - arc

Cautions

• • • • While testing, be aware of open circuit induced voltage (before mitigation) After mitigation, beware of open-circuiting any connections for test: voltage will rise to unmitigated levels Unmitigated steady-state voltage will rise fantastically during a fault or lightning event Operate safely in the field

Resources

• • • • • NACE SP0177-2007 (wire charts, safety considerations) NACE doc 35110, state of the art paper on AC corrosion Search nace.org for papers dairyland.com for tech articles/data Ask DEI for guidance on any of these topics

Appendix

• AC corrosion discussion

100 i ac vs. Soil Resistivity and Vac 15 10 5 1 Iac at 100A/m 2 Iac at 50A/m 2 Iac at 20A/m 2 0.1

100 1,000

Resistivity (Ohm-cm)

Courtesy of Bob Gummow – Corrosion Service 10,000 20,000