Transcript Dec99Overheads - Western Regional Gas Conference

AC Mitigation Overview
Mike Tachick
Dairyland Electrical Industries, Inc.
Mitigation requirements
Connection to low impedance grounding
DC isolation of CP system
Addressing lightning, AC faults
Complying with codes
AC Power Effects
Steady-state induction
– Can be measured
Fault current/voltage
– Usually estimated/modeled
Conductors, grounds sized for estimated
fault current
Amplitude
Background: AC and Lightning
Compared
Time (milliseconds)
Alternating Current
Time (microseconds)
Lightning
AC Voltage Mitigation
Create a low impedance AC path to
ground
Have no detrimental effect on the CP
system
Provide safety during abnormal conditions
(AC fault, lightning)
Factors Affecting Mitigation
Key Factors…
Soil resistivities, layers
Proximity to power lines
Power line loading
Quality of pipeline coating
…and many others
AC Mitigation Design
Best performed by consulting engineers
using specialized software
Evaluates coating stress, step and touch
potentials, fault current, steady-state
current, grounding arrangements, etc.
Typical for larger projects, new construction
AC Mitigation Design
CP personnel can roughly estimate or
experiment
Practical for small projects, single
locations
Trade-off: optimizing design vs. efficiently
installing estimated requirements
AC Mitigation Design
Induced voltage is ongoing nuisance
AC faults are significant risk
Design should consider lightning hazards
Control step and touch potentials
Step Potential
Touch Potential
Typical Field Values
Open circuit voltage: up to 50V
Short circuit current: up to 30A
Can exceed these values
Field Measurements
Open circuit voltage:
voltmeter from pipe to
grounding system
Short circuit current:
AC ammeter – reads
current in a bond
between pipe and
ground
Mitigation Example
Problem:
– Open-circuit induced AC on a pipeline = 40 V
– Short-circuit current = 15 A
– Then, source impedance:
R(source) = 40/15 = 2.7 ohms
Solution:
– Connect pipeline to ground through decoupler
Mitigation Example
Typical decoupler impedance:
X = 0.01 ohms
0.01 ohms << 2.7 ohm source
15A shorted = 15A with decoupler
V(pipeline-to-ground) = I . X = 0.15 volts
Result: Induced AC on pipeline reduced from
40 V to 0.15 V
Local Mitigation
Reduces pipeline potentials at a specific
point (typ. accessible locations)
Commonly uses existing grounding
systems
Needs decoupling
Local Mitigation
Voltage
on
Pipeline
High
Low
Continuous Mitigation
Reduces pipeline potentials at all locations
Provides fairly uniform over-voltage
protection
Typically requires design by specialists
Continuous Mitigation
Gradient control wire choices:
Zinc ribbon
Copper wire
Not tower foundations!
Mitigation Wire Installation
Reasons to DC Decouple
From Grounding Systems
If not decoupled, then:
CP system attempts to protect grounding
system
CP coverage area reduced
CP current requirements increased
CP voltage may not be adequate
But simultaneous AC grounding is also needed…
Decoupler Characteristics
High impedance to DC current
Low impedance to AC current
Passes induced AC current
Rated for lightning and AC fault current
Fail-safe construction
Third-party listed to meet electrical codes
Typical Decoupler Ratings
Threshold: 2-3V peak
AC impedance: 0.01Ω
DC leakage: <<1mA @ 1V
AC fault: 2 to 10kA
Lightning: 75-100kA
Typical Decouplers
Typical Decouplers
Zinc Ribbon for Mitigation
Provides CP if not isolated
Affects CP if impressed current system is used
Doesn’t allow instant-off readings when not isolated
Grounding effectiveness changes with zinc consumption
Can pick up stray currents
Copper for Mitigation
Common, low cost material
Must be decoupled with suitable device
Grounding Materials
If decoupled…
CP system not affected
Allows instant-off readings
Lengthens grounding system life
Avoids stray current pick up
Typical Mitigation Site
Typical Mitigation Site
Typical Mitigation Site
Typical Mitigation Site
Typical Mitigation Site
Other Grounds
Casings at road crossings
Station grounding system
Existing metallic vault
Abandoned pipeline
Culvert
Other metallic structure with low
resistance to earth
Mitigation - Other Issues
Risks at insulated joints
Other affected facilities
Hazardous locations
Mitigating at an Insulated Joint
Mitigating at an Insulated Joint
Provides AC mitigation for pipeline
Provides over-voltage protection for
insulated joint
Easy location to test, install
Other Affected Facilities
Transfer of AC fault conditions to other
structures
Goal is to keep voltage from rising, control
current flow
Doesn’t mean that current will not get on
your pipeline or others
Accomplished by bonding, grounding
Example site
Road
Metering
Station
Casing
IJ
Power line
Substation
Pipeline
IJ
Example site
Road
Metering
Station
Casing
IJ
Power line
Substation
Pipeline
IJ
Hazardous Locations
Accomplish mitigation while complying
with codes
Determine your site classification
Use certified (listed) products and
methods
Keep conductors short to limit overvoltage, possible arcing, due to lightning
Hazardous Locations
CFR 192.467 – combustible atmospheres
and insulated joints
CFR 192 incorporates the National
Electrical Code “by reference”
NEC defines hazardous locations and
product requirements
Questions?
For follow up questions:
[email protected]