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 –HVDC Power Distribution –Trans-ocean telecom systems –Impressed current cathodic protection

NEPTUNE Circuit

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

Shore Anodes 1.Current travels in the direction of the net positive charges - opposite to the direction of the net electron flow 2.Metal from which current leaves to enter an electrolyte is an anode 3.Several possible materials for ground bed anode (not seawater) –Graphite –Silicon Cast Iron –Lead

Anodic Reactions in a Brackish Marsh (expected conditions on Oregon coast) 2H 2 0 → 4H + + O 2 ↑ + 4e - E o (vs. H 2 /Pt) = 1.23V 2Cl - → Cl 2 ↑ + 2e - E o (vs. H 2 /Pt) = 1.36V Fe → Fe e - M → M +n + ne – (general reaction)

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 -1 yr -1 when buried in backfill Recommended operating current density is A m -2 Cannot tolerate current reversal High Silicon Cast Iron Anodes (Duriron)

Silicon Cast Iron Consumption Primary reaction Fe → Fe e 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

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 Carbonaceous Backfill

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 O 2, Cl 2 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

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 H 2 O + 2 e- → 2 OH- + H 2 (g)E o (vs. H 2 /Pt) = 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

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 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

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 O 2 or H 2 damage

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

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

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

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

Conclusions 1.Review all appropriate NACE and Gov’t standards and requirements for remote anode beds 2.Plan for a safety factor of 10 for current density 3.Install 2 independent ground beds to allow operation during repair or maintenance 4.Design for 200 year storms, earthquake and other natural events 5.Advise all land and marine agencies about anode bed to prevent future interferences 6.Monitor and inspect all system after 6 months and then annually 7.Use corrogated, perforated cathode plates for good flow, high surface area and easy handling 8.Keep all wires and connections insulated > 50 M-Ω from all structures with double seals and insulators for 30++ year endurance