DC Isolation

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DC Isolation Over-Voltage Protection on CP
Systems
Mike Tachick Dairyland Electrical Industries
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Typical Problems

• AC grounding without affecting CP
• Decoupling in code-required bonds
• AC voltage mitigation
• Over-voltage protection
• Hazardous locations

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Conflicting Requirements

• Structures must be cathodically protected (CP)
• CP systems require DC decoupling from ground
• All electrical equipment must be AC grounded
• The conflict DC Decoupling AC Grounding

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Reasons to DC Decouple From Electrical System
Ground

• 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

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Isolation problems

• Insulation strength/breakdown
• FBE coating 5kV
• Asphalt coating 2-3kV
• Flange insulators 5-10kV?
• Monolithic insulators 20-25kV

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Over-Voltage Protection

• From
• Lightning (primary concern)
• Induced AC voltage
• AC power system faults

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Over-Voltage Protection Goal

• Minimize voltage difference between points of
  concern
• At worker contact points
• Across insulated joints
• From exposed pipelines to ground
• Across electrical equipment

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Step Potential
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Touch Potential
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Over-voltage Protection Products and Leads

• Both the protection product and the leads have
  voltage across them
• Lead length can be far more significant than the
  product conduction level

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Effect of Lead Length

• Leads develop extremely high inductive voltage
  during lighting surges
• Inductive voltage is proportional to lead length
• Leads must be kept as short as possible
• Not a significant effect seen with AC

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Key Parameters of Lightning Waveform
Crest Amperes
1.0
Slope di/dt (Rate of rise, Amps/µsec)
1/2 Crest Value
0 8 20 Time in microseconds

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

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AC and Lightning Compared
Amplitude
Time (milliseconds)
Time (microseconds)
Alternating Current
Lightning
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Over-Voltage Protection Best Practices

• Desired characteristics
• Lowest clamping voltage feasible
• Designed for installation with minimal lead
  length
• Fail-safe (fail shorted not open)
• Provide over-voltage protection for both
  lightning and AC fault current

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Example Insulated Joint
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Example Insulated Joint
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Example Insulated Joint
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Insulated Joint Protection Summary

• Rate for
• AC fault current expected
• Lightning surge current
• Block CP current to DC voltage across joint
• AC induction (low AC impedance to collapse AC
  voltage) rate for available current
• Hazardous location classification

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Grounding System Review

• Secondary (user) grounding system
• Primary (power co) grounding system
• These systems are normally bonded

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Grounding System Schematic
Primary
Secondary
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Situation Pipeline with Electrical Equipment

• Grounded electrical equipment affects CP system
• Code requires grounding conductor
• Pipeline in service (service disruption
  undesirable)

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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

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Grounding System After Decoupling
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Issues Regarding Decoupling

• NEC grounding codes apply 250.2,
• 250.4(A)(5), 250.6(E)
• Decoupler must be certified (UL, CSA, etc.)
• No bypass around decoupler

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Rating for Equipment Decoupling

• Rate for
• AC fault current/time in that circuit
• Can rate by coordinating with ground wire size
• Decoupler must be certified (UL, etc)
• Steady-state AC current if induction present
• DC voltage difference across device
• Hazardous area classification

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Example MOV
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Decoupling Single Structures When is it
Impractical?

• Too many bonds in a station from CP system to
  ground
• Bonds cant be reasonably located

• Solution Decouple the entire facility

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Decoupling from Power Utility
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Decoupling From the Power Utility

• Separates user site/station from extensive
  utility grounding system
• Installed by the power utility
• Decoupler then ties the two systems together

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Decoupling from Power Utility
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Decoupling from utility
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Decoupling from utility
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Decoupling from utility
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Decoupling from utility

• Primary and secondary have AC continuity but DC
  isolation
• CP system must protect the entire secondary
  grounding system

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Rating for Utility Decoupling

• Rate for
• Primary (utility) phase-to-ground fault
  current/time
• Ask utility for this value
• Select decoupler that exceeds this value

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Case study station decoupling
Station Before After
A 870mV 1130
B 800 1175
C 950 1570
D 1140 1925
P/S readings at the station before and after
decoupling from the power company grounding system
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Induced AC Voltage

• Pipelines near power lines develop induced
  voltage
• Can vary from a few volts to several hundred
  volts
• Voltages over 15V should be mitigated (NACE
  RP-0177)
• Mitigation reduction to an acceptable level

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Induced AC Mitigation Concept

• Create a low impedance AC path to ground
• Have no detrimental effect on the CP system
• Provide safety during abnormal conditions

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Example Mitigating Induced AC

• Problem
• Open-circuit induced AC on pipeline 30 V
• Short-circuit current 10 A
• Then, source impedanceR(source) 30/10 3
  ohms
• Solution
• Connect pipeline to ground through decoupler

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Example Mitigating Induced AC, Continued

• Typical device impedanceX 0.01 ohms 0.01
  ohms ltlt 3 ohm source
• 10A shorted 10A with device
• V(pipeline-to-ground) I . X 0.1 volts
• Result Induced AC on pipeline reduced from 30 V
  to 0.1 V

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Mitigation of Induced AC

• Rate for
• Induced max AC current
• DC voltage to be blocked
• AC fault current estimated to affect pipeline

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Mitigation of Induced AC

• Two general approaches
• Spot mitigation
• Continuous mitigation

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Spot Mitigation

• Reduces pipeline potentials at a specific point
  (typ. accessible locations
• Commonly uses existing grounding systems
• Needs decoupling

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Mitigation example sites
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Mitigation example sites
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Mitigation example sites
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Mitigation example sites
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Continuous Mitigation

• Reduces pipeline potentials at all locations
• Provides fairly uniform over-voltage protection
• Typically requires design by specialists

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Continuous Mitigation

• Gradient control wire choices
• Zinc ribbon
• Copper wire
• Not tower foundations!

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Hazardous Locations

• Many applications described are in Hazardous
  Locations as defined by NEC Articles 500-505
• Most products presently used in these
  applications are
• Not certified
• Not rated for hazardous locations use

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Hazardous Location Definitions
Class I explosive gases and vapors
- Division 1 present under normal conditions
(always present)
- Division 2 present only under abnormal
conditions
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Hazardous Locations
Division 1
Division 2
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CFR 192.467

• (e) An insulating device may not be installed
  where combustible atmosphere is anticipated
  unless precautions are taken to prevent arcing.

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CFR 192.467, continued

• (f) Where a pipeline is located in close
  proximity to electric transmission tower footings
  
• . . . it must be provided with protection
  against damage due to fault current or lightning,
  and protective measures must be taken at
  insulating devices.

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CFR 192 link to NEC

• CFR 192 incorporates the National Electrical Code
  (NEC) by reference
• This classifies hazardous locations
• Defines product requirements and installation
  methods

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Guidance Documents (Haz Loc)

• AGA XF0277 gas facilities
• API RP-500 petroleum facilities
• CFR 192.467 gas pipeline regs
• NEC section 500-505 - haz loc definitions,
  requirements
• CSA C22.2 No. 213 product requirements
• UL 1604 product requirements

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For further application questions

• Mike Tachick
• Dairyland Electrical Industries
• Phone [phone]
• Email mike_at_dairyland.com
• Internet www.dairyland.com