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NEPTUNE Power System

NEPTUNE Power System. Ground Return Electrodes Tim McGinnis & Colin Sandwith UW/APL. Background. NEPTUNE uses single conductor cable which necessitates ground return electrodes Shore electrodes (anodes) ~ 10 A Node electrodes (cathodes) ~ 1 A Similar applications include: Gas generation

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NEPTUNE Power System

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  1. NEPTUNE Power System Ground Return Electrodes Tim McGinnis & Colin Sandwith UW/APL

  2. Background • NEPTUNE uses single conductor cable which necessitates ground return electrodes • Shore electrodes (anodes) ~ 10 A • Node electrodes (cathodes) ~ 1 A • Similar applications include: • Gas generation • HVDC Power Distribution • Trans-ocean telecom systems • Impressed current cathodic protection

  3. NEPTUNE Circuit

  4. Important that no current path exists between backbone power circuit and instrument power circuits

  5. Shore Anodes • Current travels in the direction of the net positive charges - opposite to the direction of the net electron flow • Metal from which current leaves to enter an electrolyte is an anode • Several possible materials for ground bed anode (not seawater) • Graphite • Silicon Cast Iron • Lead

  6. Anodic Reactions in a Brackish Marsh (expected conditions on Oregon coast) 2H20 → 4H+ + O2↑ + 4e-Eo(vs. H2/Pt) = 1.23V 2Cl- → Cl2↑ + 2e- Eo(vs. H2/Pt) = 1.36V Fe → Fe+2 + 2e- M → M+n + ne – (general reaction)

  7. High Silicon Cast Iron Anodes (Duriron) • 15% Si and 4% Cr allows formation of a thin protective film of stable, inert silicon dioxide which reduces iron consumption • Surface remains conductive due to iron oxides • Consumption ratings from 0.1 to 0.5 kg A-1yr-1 when buried in backfill • Recommended operating current density is 10-20 A m-2 • Cannot tolerate current reversal

  8. Silicon Cast Iron Consumption • Primary reaction Fe → Fe+2 + 2e- • 0.5 kg A-1 yr-1 x 10 A x 30 years = 150 kg consumption (this is conservative - consumption with backfill as low as 0.1 kg A-1 yr-1) • In order to allow 75% of the electrode to remain, total iron weight should be > 600 kg • Possible Configuration • 2 independent ground beds for redundancy and maintenance • Each with 10 x 30 kg anodes • Selected anodes would be inspected after 1 year

  9. Carbonaceous Backfill • Groundbed anodes are usually surrounded by a carbonaceous backfill – 200 mm (8”) OD. • Reduces the groundbed resistance by improving electrical contact • Porous to allow generated gases to escape • Provides medium in which oxidation reactions can occur which prolongs anode life

  10. Anode Groundbed Design, Connection & Monitoring • Need solid, low resistance connection • Commercial “wedge-lock” devices available with resistance < 1 mΩ • Need to maintain insulation integrity on all wiring and connections • Need to be O2, Cl2 and acid resistant • May need to monitor current to individual anodes to confirm uniform current sharing • Follow all NACE and gov’t anode bed requirements • Lightning protection • Periodic physical inspection of anodes and bed

  11. Node Cathodes • Metal that current enters on leaving an electrolyte is a cathode • Most cathodes are protected from corrosion when properly energized • The primary reaction that takes place at the cathode is: 2 H2O + 2 e- → 2 OH- + H2(g) Eo(vs. H2/Pt) = -0.83 • Calcareous deposits – mostly salts and oxides - can form on the surface of the electrode • Several possible materials: • Copper • Silicon cast iron • Platinum coated substrate • Consider installing 2 cathodes

  12. Platinized Niobium/Titanium Cathodes • Titanium, niobium or tantalum rod substrates covered with a platinum coating • Oxidizing film assures that the substrates will remain stable • The film also makes the substrate surface relatively non-conductive, so electrical discharge occurs through the platinum coating • 50-300 A cm-2 in salt water electrolytes • Titanium has breakdown voltage of 9 V • Niobium and Tantalum have higher breakdown voltages, 115 V and 155 V, respectively • Need large surface area, no horizontal surface, good circulation

  13. Cathode Connection • May be connected to power converter with underwater mateable connector for easy replacement • Need to maintain good insulation on connection and cables • Need to avoid fasteners and other materials susceptible to O2 or H2 damage

  14. Major Technical Concerns • Shore Anode Consumption • can solve by adding mass • Maintain low anode-earth resistance – may need watering capability • Calcareous deposits on Node Cathodes • deposit increases local current density • solve by increasing surface area and configuration • Marine growth and corrosion on Cathodes when not energized • Current and voltage transients or lightning • can cause breakdown voltage to be exceeded which can damage oxide film, platinum plating and/or metal substrate

  15. Environmental Impacts • Formal Environmental Assessment not typically required or done by telecom systems on similar systems • No AC field • Weak DC field very near cable deep in soil and between cable and electrodes • Cable insulation resistivity 10 orders of magnitude greater than sea water so virtually all of the field is confined to the insulation • Many systems over 20 years, no safety or environmental issues

  16. Other Issues • Interaction with science instruments • Instruments will require isolated circuits with no current path to seawater (> 50 M-Ω) • Failure of instruments or cables must not damage or corrode system or other instruments • Need more research on impacts on biological, chemical and electro-magnetic sensors • Corrosion of structures from stray current • Pipes, tanks, other structures on land • Backbone cable conduit to beach • Corrosion of pressure housings and other node components

  17. Any conductive pipes, cables, tanks or other structures can be damaged and consume power

  18. Conclusions • Review all appropriate NACE and Gov’t standards and requirements for remote anode beds • Plan for a safety factor of 10 for current density • Install 2 independent ground beds to allow operation during repair or maintenance • Design for 200 year storms, earthquake and other natural events • Advise all land and marine agencies about anode bed to prevent future interferences • Monitor and inspect all system after 6 months and then annually • Use corrogated, perforated cathode plates for good flow, high surface area and easy handling • Keep all wires and connections insulated > 50 M-Ω from all structures with double seals and insulators for 30++ year endurance

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