Essentially, a mesogrid is a DC grid that connects into an AC grid at several AC nodes (major transformer yards in the above sketch) and so has both transmission and distribution functions. Steve envisioned an HVDC loop around a city as a way to improve reliability and resilience against rolling blackouts. Mesogrids are differentiated from microgrids because of connecting to the AC grid at several points, and are smaller and lower in operating voltage compared to future supergrids. This makes them very well suited as intermediate steps towards supergrids. Below is an abstract for a paper I plan to present at Distributech 2016:
(Update September 22, 2015: the paper was rejected. I still plan to pursue this idea, possibly with different co-authors.)
Mesogrids also make sense as “collector buses” serving several renewable energy farms and energy storage facilities (such as batteries, dispatchable hydro, and pumped storage) as well. By aggregating distant energy farms with several different time-scale storage devices, mesogrids can deliver firm renewable energy at lower aggregate cost than if each energy farm had to firm its own power. In rural areas, segments of a mesogrid loop to serve remote energy farms and storage sites might be overhead lines, though at 6kA as would be required for a 2-4 GW mesogrid loop, multiple parallel overhead lines would be required if part of such a loop is implemented via overhead lines. Because of their high ampacity, elpipe loops are particularly good candidates for initial application of mesogrid technology.
At a smaller scale, mesogrids are an ideal way to distribute power in a DC power environment. The main high-current connections of a mesogrid could be overhead DC lines, elpipes, gas insulated lines (GIL), or HTS lines. If maximum current is 2kA or less, mesogrids based on HVDC cable are also feasible. We present as an example of such a small mesogrid, a datacenter.
- 100 MW datacenter complex, where the advantage comes from sharing backup capacity, and from very low transients on the DC datacenter buses (per our definition of a mesogrid, this needs to tie into at least two different AC substations, otherwise it is a large DC microgrid);
- a small city, where the DC interconnection of substations facilitates two-way power flows in areas with lots of distributed generation;
- a region with numerous energy farms and energy storage facilities, where the DC grid enables firming of power from several energy farms and energy storage facilities, where the primary advantage is due to being able to firm power with less total energy storage online.