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Roger W. Faulkner

HVDC Converter Station Grounding

President & Founder at Electric Pipeline Corporation
I learned at a recent HVDC course organized by EUCI that HVDC ground returns are normally shallow; the presenter said that most are buried in a trench or sited in a lake or the sea. When they are connected to drilled wells, the deepest the presenter (Ramy Azar of SNC-Lavalin) knew of involved multiple 500 foot deep wells. I was truly shocked by this, as it seems positively primitive when viewed in the context of modern oil & gas drilling technology. A gas well can be drilled as deep as 18,000 feet. 3 km is positively routine, and costs ~$.5 million.

In most areas, there are saline aquifers encountered somewhere along a deep well bore. Can anyone explain why it is not routine to lay a drill pipe down in a saline aquifer for the ground return for an HVDC converter? Directional drilling can easily be used to provide plenty of contact area with the saline aquifer. Such deep-sited ground returns could eliminate concerns with ground currents, I believe.

Comments

  • Stig
    Stig Nilsson
    Principal Engineer at Exponent and Consumer Electronics Consultant
    You may have a point. I guess the answer is that there has so far not been a need to do so. Also, shallow electrodes arfe maintainable. However, many newer dc systems have a metallic return that adds costs to the dc link. If deep electrodes were to be an answer, then it might be possible to save the metallic neutral line.

    Stig
  • Roger W. Faulkner
    Roger W. Faulkner
    President & Founder at Electric Pipeline Corporation
    It becomes particularly relevant in the scenario where both large and small HVDC converters are linked to a supergrid. The supergrid itself needs to be bipolar HVDC, but smaller power taps, say < 400 MW, are much less expensive is they can be monopoles; here is an excerpt from my blog post on this:

    The really high power lines of the HVDC supergrid (>10kA) must always be bipoles, but for small lines, monopole configurations would be OK, and would save money. For example, a 100 MW substation could power a remote town. To move 100 MW based on a /-800kV supergrid can be accomplished by a monopole with ground return, which would imply an imbalanced current of only 125 amps. This seems to me to be OK. Such a supergrid would be based on a backbone of underground bipoles designed for high current. Up to 400MW or so, converter stations off of the supergrid main line would be fed by a monopole with ground return, which greatly reduces the cost of the elpipe connector. By taking different small converter stations off the and - poles of the supergrid, one tries to balance pole current with - pole currents at all points in the bipolar supergrid but of course with monopolar connectors, the bipole circuit cannot be in balance everywhere.This problem is analogous to the placement of single phase transformers on a three phase power distribution line.

    The larger converter stations > 400MW will still be bipoles, but with grounded neutrals. This allows an imbalance of the two poles, which will be used to re-balance the current in the two major bipoles at the point where the /- 800kV tap into the main loop occurs.
  • Tarek Abdelgalil, P. Eng., SM.IEEE
    Tarek
    Tarek Abdelgalil, P. Eng., SM.IEEE
    Manager, System Studies
    Nice discussion. From reliability prespective, it would be tough to lose the supply for the city when you have problem with your line or the converter station. Planning Criteria and reliability requirements would make this scenario less likely.

    If the use of shallow grounding electrode is meeting regulation in place, the need for deeper electrode ( extra cost and maintainability issue) is not proven.

    However, it could be a good idea for further research.
  • Roger W. Faulkner
    Roger W. Faulkner
    President & Founder at Electric Pipeline Corporation
    The whole point if my proposal is to change the regulation so that at least 100 amps, and maybe thousands of amps could be entrusted to ground return in an HVDC supergrid. This would save $billions, perhaps without causing the kind of problems that occur with shallow ground return. I do not know if a surface steel pipeline could still develop a substantial current if it lies between two deeply buried ground electrodes which are embedded in underground saline aquifers. It probably depends on whether there exists a salt water path through porous rock all the way from one electrode to the other.