Dc isolation over voltage protection on cp systems
1 / 57

DC Isolation & Over-Voltage Protection on CP Systems - PowerPoint PPT Presentation

  • Uploaded on

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.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about 'DC Isolation & Over-Voltage Protection on CP Systems' - oshin

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Dc isolation over voltage protection on cp systems l.jpg

DC Isolation & Over-Voltage Protection on CP Systems

Mike Tachick

Dairyland Electrical Industries

Typical problems l.jpg
Typical Problems

  • AC grounding without affecting CP

  • Decoupling in code-required bonds

  • AC voltage mitigation

  • Over-voltage protection

  • Hazardous locations

Conflicting requirements l.jpg
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 l.jpg
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 l.jpg
Isolation problems

  • Insulation strength/breakdown

  • FBE coating: 5kV

  • Asphalt coating: 2-3kV

  • Flange insulators: 5-10kV?

  • Monolithic insulators: 20-25kV

Over voltage protection l.jpg
Over-Voltage Protection

  • From:

    • Lightning (primary concern)

    • Induced AC voltage

    • AC power system faults

Over voltage protection goal l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
Key Parameters of Lightning Waveform

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

Crest Amperes


Slope = di/dt

(Rate of rise,


1/2 Crest Value

0 8 20 Time in microseconds

Ac and lightning compared l.jpg
AC and Lightning Compared


Time (milliseconds)

Time (microseconds)

Alternating Current


Over voltage protection best practices l.jpg
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 l.jpg
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 l.jpg
Grounding System Review

  • Secondary (user) grounding system

  • Primary (power co) grounding system

    These systems are normally bonded

Grounding system schematic l.jpg
Grounding System Schematic



Situation pipeline with electrical equipment l.jpg
Situation: Pipeline with Electrical Equipment

  • Grounded electrical equipment affects CP system

  • Code requires grounding conductor

  • Pipeline in service (service disruption undesirable)

Decoupler characteristics l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 power utility30 l.jpg




Decoupling from Power Utility

Decoupling from utility34 l.jpg
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 l.jpg
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 l.jpg
Case study – station decoupling

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

Induced ac voltage l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
Mitigation of Induced AC

  • Two general approaches:

    • Spot mitigation

    • Continuous mitigation

Spot mitigation l.jpg
Spot Mitigation

  • Reduces pipeline potentials at a specific point (typ. accessible locations

  • Commonly uses existing grounding systems

  • Needs decoupling

Continuous mitigation l.jpg
Continuous Mitigation

  • Reduces pipeline potentials at alllocations

  • Provides fairly uniform over-voltage protection

  • Typically requires design by specialists

Continuous mitigation49 l.jpg
Continuous Mitigation

  • Gradient control wire choices:

    • Zinc ribbon

    • Copper wire

    • Not tower foundations!

Hazardous locations l.jpg
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 l.jpg
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 l.jpg
Hazardous Locations

Division 1

Division 2

Cfr 192 467 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
For further application questions…

Mike Tachick

Dairyland Electrical Industries

Phone: 608-877-9900

Email: [email protected]

Internet: www.dairyland.com