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DC Isolation & Over-Voltage Protection on CP Systems. Mike Tachick Dairyland Electrical Industries. Typical Problems. AC grounding without affecting CP Decoupling in code-required bonds AC voltage mitigation Over-voltage protection Hazardous locations. Conflicting Requirements.

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dc isolation over voltage protection on cp systems

DC Isolation & Over-Voltage Protection on CP Systems

Mike Tachick

Dairyland Electrical Industries

typical problems
Typical Problems
  • AC grounding without affecting CP
  • Decoupling in code-required bonds
  • AC voltage mitigation
  • Over-voltage protection
  • Hazardous locations
conflicting requirements
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
reasons to dc decouple from electrical system ground
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
isolation problems
Isolation problems
  • Insulation strength/breakdown
  • FBE coating: 5kV
  • Asphalt coating: 2-3kV
  • Flange insulators: 5-10kV?
  • Monolithic insulators: 20-25kV
over voltage protection
Over-Voltage Protection
  • From:
    • Lightning (primary concern)
    • Induced AC voltage
    • AC power system faults
over voltage protection goal
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
over voltage protection products and leads
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
effect of lead length
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
key parameters of lightning waveform
Key Parameters of Lightning Waveform
  • Lightning has very high di/dt (rate of change of current)

Crest Amperes

1.0

Slope = di/dt

(Rate of rise,

Amps/µsec)

1/2 Crest Value

0 8 20 Time in microseconds

ac and lightning compared
AC and Lightning Compared

Amplitude

Time (milliseconds)

Time (microseconds)

Alternating Current

Lightning

over voltage protection best practices
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
insulated joint protection summary
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
grounding system review
Grounding System Review
  • Secondary (user) grounding system
  • Primary (power co) grounding system

These systems are normally bonded

situation pipeline with electrical equipment
Situation: Pipeline with Electrical Equipment
  • Grounded electrical equipment affects CP system
  • Code requires grounding conductor
  • Pipeline in service (service disruption undesirable)
decoupler characteristics
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
issues regarding decoupling
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
rating for equipment decoupling
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
decoupling single structures when is it impractical
Decoupling Single Structures: When is it Impractical?
  • Too many bonds in a station from CP system to ground
  • Bonds can’t be reasonably located
  • Solution: Decouple the entire facility
decoupling from the power utility
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
decoupling from utility34
Decoupling from utility
  • Primary and secondary have AC continuity but DC isolation
  • CP system must protect the entire secondary grounding system
rating for utility decoupling
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
case study station decoupling
Case study – station decoupling

P/S readings at the station before and after decoupling from the power company grounding system

induced ac voltage
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
induced ac mitigation concept
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
example mitigating induced ac
Example: Mitigating Induced AC
  • Problem:
    • Open-circuit induced AC on pipeline = 30 V
    • Short-circuit current = 10 A
    • Then, source impedance:R(source) = 30/10 = 3 ohms
  • Solution:
    • Connect pipeline to ground through decoupler
example mitigating induced ac continued
Example: Mitigating Induced AC, Continued
  • Typical device impedance:X = 0.01 ohms0.01 ohms << 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
mitigation of induced ac
Mitigation of Induced AC

Rate for:

  • Induced max AC current
  • DC voltage to be blocked
  • AC fault current estimated to affect pipeline
mitigation of induced ac42
Mitigation of Induced AC
  • Two general approaches:
    • Spot mitigation
    • Continuous mitigation
spot mitigation
Spot Mitigation
  • Reduces pipeline potentials at a specific point (typ. accessible locations
  • Commonly uses existing grounding systems
  • Needs decoupling
continuous mitigation
Continuous Mitigation
  • Reduces pipeline potentials at alllocations
  • Provides fairly uniform over-voltage protection
  • Typically requires design by specialists
continuous mitigation49
Continuous Mitigation
  • Gradient control wire choices:
    • Zinc ribbon
    • Copper wire
    • Not tower foundations!
hazardous locations
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
hazardous location definitions
Hazardous Location Definitions

Class I = explosive gases and vapors

- Division 1: present under normal conditions (always present)

- Division 2: present only under abnormal conditions

hazardous locations52
Hazardous Locations

Division 1

Division 2

cfr 192 467
CFR 192.467

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

cfr 192 467 continued
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.”

cfr 192 link to nec
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
guidance documents haz loc
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
for further application questions
For further application questions…

Mike Tachick

Dairyland Electrical Industries

Phone: 608-877-9900

Email: mike@dairyland.com

Internet: www.dairyland.com