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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

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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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

  • 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


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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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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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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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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



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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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Primary

Decoupler

Secondary

Decoupling from Power 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

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 impedance:R(source) = 30/10 = 3 ohms

  • Solution:

    • Connect pipeline to ground through decoupler


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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


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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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Continuous Mitigation

  • Reduces pipeline potentials at alllocations

  • 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: 608-877-9900

Email: [email protected]

Internet: www.dairyland.com


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