IEEE Electrification Magazine - June 2016 - 34

devices or in uninterruptible power supply (UPS) systems;
usually the battery is only ever used when the primary
supply is lost, and then it supplies all demand. LPD
enables the best use of batteries all the time, taking into
consideration lifetime impacts of battery cycling. Without
communications, a storage device will generally not know
if it should be charging or discharging (or neither) and at
what rate. Some systems use changes in voltage levels to
communicate about system status, but this is not always
reliable and greatly limits the scope of possible information passed. In complex systems with multiple local generation and multiple local storage devices, only with
effective communications can proper, efficient, and economic use of electricity storage be accomplished.

Privacy and Cybersecurity
Communication in LPD is only ever point to point-
there is no multihop communication as in data networks. This dramatically reduces the scope for concern
for security and privacy, because only devices with a
direct wired connection can attempt to disrupt or spy
on one another other. In addition, because the communications are so simple, there is little opportunity for
mischief. This is greatly different from the risks and
security vulnerabilities that occur in ac grid-tied systems, particularly as large numbers of devices owned by
different people are connected.

New Powering Models
In the past, end-use devices were only ever connected to a
single external power source. There are rare exceptions,
such as electronic devices in data centers or telecom

Utility
Grid

Utility
Grid

Meter

Utility
Grid
Meter

Meter

Unreliable ac

Unreliable ac

Unreliable ac

Inverter/Rectifier

Inverter/
Rectifier

Islanding
Disconnect
Reliable ac

dc Grids

Power Flows
Data Flows

dc Grids

Inverter

Inverter/Rectifier
dc
Grids

Reliable ac
(a)

(b)

(c)

Figure 6. Alternative methods to integrate dc grids and reliable ac.
(a) Attaching dc systems to the existing ac infrastructure with a rectifier or inverter/rectifier, (b) creating reliable ac infrastructure that is
strictly subsidiary to the dc grids, and (c) carving out a portion of the
ac infrastructure that can be powered from the dc side during times
of grid failure.

34

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2016

facilities. However, managed dc creates the possibility of
devices easily able to use power from more than one
source, at different times, or at the same time. This is particularly useful when resiliency is of concern, because a
device can be powered through one means most of the
time, but by another means when the first is not working or
is more expensive. For example, it would be convenient if
refrigerators could take in dc power from a vehicle or local
generation or storage during times when the utility grid is
down, or expensive, and use grid ac power otherwise.
AC circuits are usually multidrop-many devices can
be attached to a single wire. DC topologies more commonly provide only a single device per power port. This
ensures that capacity limits on the cable, and cable length,
are not exceeded. However, with communications, it could
be possible to create multidrop capability for technologies
such as Ethernet, with cost advantages for many devices,
including lighting, to enable easy daisy-chaining.
The recent 380-V dc standard lacks any mechanism for
communicating about power. A good option to remedy this
is to use standard Ethernet links, in parallel to each power
link. The Ethernet link could provide small amounts
of power (relative to what the 380-V path could provide)
to energize the end-use device for communications to
negotiate aspects of the 380-V line before it is energized.

DC Deployment Architectures
Direct dc, particularly when combined with LPD, raises the
question of how LPD should be deployed in buildings with
most economy and benefit. The dc portion of building
infrastructure can be treated as a single cloud of technology for this purpose, including generation, storage, and
connections to vehicles. DC infrastructure can be integrated with ac buildings in several ways that differ primarily
in how reliable device operation is accomplished. Power
reliability and quality are often key drivers for dc adoption.
The first way to integrate dc is to simply attach dc
systems to the existing ac infrastructure with a rectifier
or inverter/rectifier, as shown in Figure 6(a). The ac system is unchanged in topology or capability. The dc system can be reliable in the face of grid failure, as long as
power is prevented from being exported to the ac side
during these times. The data line between the meter and
dc grids passes price information; it must come from the
meter, not the grid, in cases when the price is different
depending on whether the building is buying power from
or selling power to the grid, as some regions are soon
planning to do.
A second method is to create some reliable ac infrastructure as strictly subsidiary to the dc grids, as shown in
Figure 6(b). The reliable ac power is always produced from
dc and thus can also be insulated from power quality
issues that may be present on the utility grid; needs for
quality and reliability are highly correlated. The reliable ac
is decoupled from the ac grid at all times. Typically, the
amount of power in the reliable ac domain is small in



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

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