IEEE Electrification Magazine - June 2016 - 33

link. Some technologies allow for
bidirectional power today; others
could accomplish reversing power
flow with two parallel links, one for
each direction. Power links to vehicles will become a common example
of bidirectional power flows. We can
also expect vehicles to be able to connect directly to one another, so that
one can charge another anywhere.
Management of power distribution
with and within vehicles should use
the same technology as is used
everywhere else.

Plug-and-Play End-Use Devices

A key feature of LPD
is that it will enable
both storage and
generation of
electricity to become
plug and play, i.e.,
able to be connected
and disconnected at
will by anyone
without the safety
risk otherwise
present when using
higher voltage levels.

In existing managed dc technologies,
initially only a small amount of
power is provided to enable communications. Each device communicates
to the other about its capabilities and
preferences to allow for adjusting
and optimizing the link. The voltage
can be negotiated to the highest that
can be provided and then used and is also appropriate to
the cable (based on its capacity and length). This minimizes resistance losses and maximizes power capacity.
Communications can be used to determine cable length. In
some USB links, the cable itself reports its characteristics to
the devices, and a variety of combinations of voltage and
current are available that the electricity-providing and
electricity-consuming devices might support. This allows
for the most efficient combination that both devices can
use to be selected. When a device is plugged in that is not
compatible electrically for any reason, this can be detected and reported.

will cut power to the entire circuit
(and hence all devices on it) when the
limit is reached. This is inconvenient
if it occurs frequently. To avoid it, ordinary practice is to substantially oversize wires and circuits, and building
codes generally require this, even on
circuits that will never carry such
high current. This is a waste of the
copper and capital and increases
installation costs.
With managed dc, actual power
levels being used can be tracked. In
addition, what devices can potentially use at maximum and what
they are actually consuming at any
particular moment can be considered to then install equipment of
appropriate capacity. That is, capacity limits can be respected through
pricing and communication rather
than oversizing and blowing breakers. In emergency situations, devices
can be summarily disconnected on
an individual basis as needed to reduce total demand
rather than interrupting power to the entire circuit.

Plug-and-Play Generation

By communicating before delivering power, conditions
that would otherwise be unsafe can be avoided. If a cable
is cut or improperly connected, this will be detected by an
interruption in the communication. This signal can be
used to terminate ongoing power delivery or to stop delivery from the beginning. In such a case, one proprietary
mechanism cuts power within 3 ms to enable 400-V delivery over Ethernet cabling. Capacity and voltage limitations
of the cable or either device can also be respected automatically. Cable overheating could be detected through
the two devices comparing the supplied and received
power levels and recognizing when the difference
between these is inappropriately large.

Traditional electricity systems (ac and dc) use generators
that can vary their output to follow electricity demand
based on voltage changes, thus maintaining balance in
the system. Communications enable systems in which
demand can respond to supply conditions, and in systems with more than one generator, communications
can determine generator on-times and output levels.
When connected to a utility grid, the best balance of
utility power and local generation can be determined.
Generation technologies are often nondispatchable
(as with many renewables), lose power on conversion
between ac and dc and between different voltages, and
have variable part-load efficiencies, minimum times to
be on or off, and losses on each cycle up and down. With
communications, these factors can be coordinated to
maximize efficiency and equipment use and improve
reliability and safety. Communication can ensure that a
generator can safely deliver power of the quantity and
type desired, before it begins doing so. Without this ability, safe and efficient operation requires careful system
design and management, as well as extra hardware.
Optimal system operation is impossible without
effective communications.

Managed Limited Supply

Plug-and-Play Storage

Each electrical circuit has a maximum current or power
level that it can manage, due to wire size, length, temperature, and other factors. In ac systems, a circuit breaker

Electricity storage as a common feature in electricity systems is only now emerging. Existing use of storage has
been for reliable or disconnected operation of individual

Improved Safety

IEEE Electrific ation Magazine / j une 2 0 1 6

33



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

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