Lightning, AC Faults, and Over-Voltage Protection

January 13, 2018 | Author: Anonymous | Category: Engineering & Technology, Electrical Engineering
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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

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

Crest Amperes 1/2 Crest Value

0 8 20 Time in microseconds

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

Amplitude

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

Cable sizing chart Source: NACE SP0177-2007

AC Faults

Manufacturer’s fault data

Arc Distance – AC Faults 40

138 kV

Distance (m)

30

230 kV 500 kV

20

10

0 0

6500

15000

30000

Soil Resistivity (ohm-cm)

Courtesy of Bob Gummow – Corrosion Service

45000

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 overvoltage, 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

Change

Induction result

Soil resistivity

Increase

Increase

Coating resistance

Increase

Increase

Load current

Increase

Increase

Dist from tower

Increase

Decrease

Change in distance

Any change

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:
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