IEEE Electrification Magazine - June 2016 - 31

When generation and loads
are compatibly sized, the vast
majority of electricity generated
in the dc domain can remain
there, and the vast majority of
energy used in the dc domain is
also generated there. A result is
(a)
(b)
(c)
that only small amounts of energy cross the ac-dc link, and the
Figure 2. The standard dc connectors for (a) a USB, (b) an Ethernet, and (c) a 380-V dc.
electrical capacity of the link can
thus be significantly downsized.
Many end uses such as lighting and electronics require
(nanogrids) of arbitrary topology and scale. Products
relatively consistent daily use of electricity, so generaare unavailable today that implement networked dc,
tion and storage for such local dc grids can be sized to
but the local power distribution (LPD) technology
the needed consumption level. In Figure 1(c), if either
described below would make this possible. Similar to
the generation or storage is eliminated, some of the
managed dc, on which it is built, networked dc has
benefits of direct dc are retained, but with decreasing
more in common with network communications
costs, it is likely that systems with both generation and
technologies than with traditional power distribution
storage will be the norm.
technologies. Figure 3 shows an example network of
dc nanogrids, including local generation and vehicles,
DC Technology Architectures
with a connection to ac infrastructure.
Although direct dc is possible to implement today-and
Networked dc makes use of a relatively new feature
is effectively used in some installations-it is uncomof some managed dc technologies-bidirectional
mon and lacks standard designs on which to draw. Some
power-for which the direction of power flow may be
voltages (e.g., 12 V) lack standard connectors, although
different at different times. This exists in the latest verin recent years progress in the area of connectors has
sion of the USB Power Delivery specification as well as
been made by the Emerge Alliance for 380- and 24-V dc
with HDBaseT, a variant of Ethernet. Networked dc
(Figure 2 shows standard dc connectors for three techdoes not require any innovation in how electricity is
nologies). Even if common deployment schemes were
transferred, but it does require new capabilities in comavailable, it is likely that the use of direct dc would
munications and control. Figure 4 shows a USB hub
remain uncommon if the only benefit delivered was
and an Ethernet switch; with the addition of modest
energy savings. Key to making direct dc more successful
additional communications technology, each of these
is to focus on other benefits and develop new technolocould become a fully functional nanogrid controller,
gy to create more benefits and/or decrease costs.
although adding some electricity storage as well will
DC power distribution exists today in two primary
likely be the norm.
forms-and a third form (networked dc) could be develA key principle of LPD is to decouple how power distrioped-in terms of how it is organized and structured and
bution is managed from functional control protocols in
the way in which it operates within buildings.
which end-use devices participate, similar to how physical
1) Traditional dc moves power though circuits as
layer and application layer protocols are separated from
determined by Kirchhoff's laws, similarly to the
each other with the Internet Protocol (IP) in IP networks.
way in which ac systems operate. For example, in
vehicles, power distribution is organized into circuits, similar to power distribution of circuits in ac
Microgrid
building infrastructure.
ac
Controller
Infrastructure
2) Managed dc moves power across a single cable from
one device to another, with characteristics (including voltage and current) determined principally by
Nanogrid Controller
Loads
digital management as communicated between the
two devices and implemented with modern power
electronics. USB and Ethernet both perform this
process and regularly increase capability. Managed
PV
dc derives from modern communications technologies more so than from traditional power distribution (ac or dc).
3) Networked dc extends managed dc from single power
Figure 3. An example network of dc nanogrids.
links to a network or mesh of power entities
IEEE Electrific ation Magazine / j une 2 0 1 6

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2016