Lightning, AC Faults, and Over-Voltage Protection

Report
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: <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
iac vs. Soil Resistivity and Vac
100
Shock
Hazard
Limit
Shock
Hazard
Limit
AC Voltage (V)
15
10
2
iac
100
A/sqm
Iac=at
100A/m
5
2
iacat
= 20A/m
20 A/sqm
Iac
2
iac at
= 50
A/sqm
Iac
50A/m
1
0.1
100
1,000
Resistivity (Ohm-cm)
Courtesy of Bob Gummow – Corrosion Service
10,000
20,000

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