(Update September 22, 2015: the paper was rejected. I still plan to pursue this idea, possibly with different co-authors.)
Mesogrids
for Flexible Energy Distribution
Roger Faulkner and Clay Taylor (Alevo R&D); Randell Johnson (Alevo Analytics); and Steve Eckroad (EPRI): DistribuTech
2016 abstract
Mesogrids are proposed DC grids positioned between large long
distance transmission lines or central generators and smaller distribution substations,
up to the scale of backing up a city power supply. The concept was introduced
by Steve Eckroad of EPRI in a patent[i],
mainly directed at protecting an urban area from a rolling blackout, but the
same basic design also facilitates two-way energy flow due to distributed
generation, and increased use of shared, non-local energy storage. This patent
describes a mesogrid sized for an urban area, taking the form of an HVDC loop
around a city, connecting all the major transmission substations around the
city by DC. Eckroad envisioned high temperature superconducting (HTS) lines to
accomplish the DC connection, while we envision conventional conductor-based
HVDC mesogrids based on elpipes[ii],
overhead lines, and cables. Elpipes are polymer-insulated metallic extrusions,
linked through flexible couplings, which run on wheels within a pipe, which are
being developed by Alevo. Current for an elpipe can be much higher than a
cable, simply because more aluminum/meter can be used economically with the
elpipe design. Elpipes make it feasible to transmit more than 6kA, allowing
GW-level transmission at 80kV. If elpipes form the backbone of a mesogrid,
resistive loss through the mesogrid conductors can be kept below one percent.
Mesogrids are between current AC
grids and typical microgrids in size and voltage. As with DC microgrids,
mesogrids facilitate two-way energy flows in a system containing a lot of
distributed generation. Mesogrids are distinct from DC microgrids both by moving
more power and because they may link into the AC grid at multiple different
substations. Like the future supergrid, mesogrids are envisioned as DC grids
that parallel and reinforce the AC grid. Mesogrid voltage will typically be
around 80 kV, and represent an evolutionary step towards a future supergrid, at
voltage levels which are safe and compact, yet realistic for GW-scale
transmission.
Systems that could be powered by
mesogrids range from a large industrial or data center complex, to a city, or a
region with numerous energy farms and energy storage sites. Optimum voltage for
mesogrids is well below typical HVDC transmission voltage (300-800kV). Like the
future supergrid, mesogrids are envisioned as strictly DC grids; voltage will
be most economical for the envisioned applications around ±(30-180) kV, which
places them in the voltage range bridging between MVDC and HVDC. Power levels
at ±40kV can conveniently be 2 GW (6250 amps) if the connections are based on
elpipes (lower power mesogrids could be based on cables). This work
demonstrates the utility of mesogrids at several power levels, including urban
area power reliability enhancement as originally proposed by Steve Eckroad.
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.
Elpipes are high capacity segmented DC conductors that are
based on polymer-insulated pipe-shaped conductors (think bus pipes) rather than
wires; the segmented nature of elpipes allows far more conductor per meter of
line to be used than is possible for cables, which must be able to wrap on a
reel. Elpipes are capable to carry much higher current than cables or overhead
lines. By moving high capacity DC transmission underground, elpipes make
installation of a mesogrid linking substations in an urban area more
politically feasible. Unlike cables, elpipes will be readily repairable, as
they sit on wheeled carriages inside underground pipes. Elpipe repairs can be
made rapidly by swapping out faulted segments, and the elpipe will not have to
be dug up for most repairs.
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.
HVDC mesogrids make sense for
underground energy exchange over a region with many terminals. We present for
consideration three particular examples of power systems linked by mesogrids:
- 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.
These three cases represent realistic size scales as
particular examples of mesogrids. The smallest mesogrid, Example 1 corresponds
to a demonstration project at Alevo's 2000-acre North Carolina factory campus
that would be capable to provide fully backed-up, high quality (low voltage fluctuation
& low transients) DC power to 100 GW of new advanced industrial and
datacenter loads, with high reliability and power quality guaranteed by on site
back-up resources. Example 2 extends the
mesogrid to the region around our factory (the town of Concord, NC or the city
of Charlotte; not yet decided), via elpipe links and VSC AC/DC converters at numerous
substations in the town of Concord, NC. Finally, Example 3 considers a mesogrid
extending around a city with high solar penetration and nearby wind farms (site
not picked). These cases are not modelled in full detail, but thoroughly enough
to visualize the cost/benefits at each scale.
Any DC grid with power levels over
a megawatt has a significant problem with circuit protection, and circuit
protection generally costs much more than for an AC grid at the same power. In
order to simplify modeling, all mesogrid cases analyzed use a single DC loop
(MVDC or HVDC), as shown in the cited EPRI patent; there are both main loop
circuit breakers (rated for maximum power of the entire loop), and fast
disconnect switches on the main loop between every pair of power taps. One can
obtain the n-1 redundancy level for every tap on such a loop, by placing a
circuit breaker between every next neighbor set of power taps on the mesogrid.
Such fast isolation is very expensive if the main loop circuit breakers are
based on power electronic hybrid breakers capable of operating within a few
milliseconds; for full redundancy one would require as many full-load circuit
breakers as there are power taps. We consider an economically optimized
solution for each power level, in which there are only a few full-load circuit
breakers on the main loop, supplemented by fast main loop disconnect switches between
each next-neighbor pair of power taps (AC/DC converters). In addition, each
AC/DC converter is protected by relatively small circuit breakers.
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