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

DC Isolation & Over-Voltage Protection on CP Systems

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DC Isolation & Over-Voltage Protection on CP Systems

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  1. DC Isolation & Over-Voltage Protection on CP Systems Mike Tachick Dairyland Electrical Industries

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

  3. 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

  4. 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

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

  6. Over-Voltage Protection • From: • Lightning (primary concern) • Induced AC voltage • AC power system faults

  7. 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

  8. Step Potential

  9. Touch Potential

  10. 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

  11. 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

  12. 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

  13. AC and Lightning Compared Amplitude Time (milliseconds) Time (microseconds) Alternating Current Lightning

  14. 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

  15. Example: Insulated Joint

  16. Example: Insulated Joint

  17. Example: Insulated Joint

  18. 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

  19. Grounding System Review • Secondary (user) grounding system • Primary (power co) grounding system These systems are normally bonded

  20. Grounding System Schematic Primary Secondary

  21. Situation: Pipeline with Electrical Equipment • Grounded electrical equipment affects CP system • Code requires grounding conductor • Pipeline in service (service disruption undesirable)

  22. 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

  23. Grounding System After Decoupling

  24. 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

  25. 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

  26. Example: MOV

  27. 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

  28. Decoupling from Power Utility

  29. 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

  30. Primary Decoupler Secondary Decoupling from Power Utility

  31. Decoupling from utility

  32. Decoupling from utility

  33. Decoupling from utility

  34. Decoupling from utility • Primary and secondary have AC continuity but DC isolation • CP system must protect the entire secondary grounding system

  35. 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

  36. Case study – station decoupling P/S readings at the station before and after decoupling from the power company grounding system

  37. 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

  38. 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

  39. 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

  40. 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

  41. Mitigation of Induced AC Rate for: • Induced max AC current • DC voltage to be blocked • AC fault current estimated to affect pipeline

  42. Mitigation of Induced AC • Two general approaches: • Spot mitigation • Continuous mitigation

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

  44. Mitigation example sites

  45. Mitigation example sites

  46. Mitigation example sites

  47. Mitigation example sites

  48. Continuous Mitigation • Reduces pipeline potentials at alllocations • Provides fairly uniform over-voltage protection • Typically requires design by specialists

  49. Continuous Mitigation • Gradient control wire choices: • Zinc ribbon • Copper wire • Not tower foundations!

  50. 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