IEEE Electrification Magazine - June 2016 - 32

(a)

(b)

Figure 4. (a) A USB hub and (b) an Ethernet switch, devices that
could be readily modified to be nanogrid controllers.

These separations make each part simpler and more
effective. In addition, the topologies of power distribution,
data communications, and building structure can all be
disjoint. Communication is needed to determine where
power should flow in these networks. In networked dc,
power flows are determined by computation.

What is LPD?
LPD is a network model of power. Electricity is managed in
arbitrary and dynamically changing topologies of sources,
storage, and end-use devices. Power connections are peer
to peer between two devices, not via buses with many
devices attached. All power exchanges are digitally mediated. End-use devices are organized into nanogrids-single domains of power for voltage, capacity, reliability, and
management. A nanogrid controller manages the power
distribution to end-use devices connected to the controller
and exchanges power with other nanogrids; local generation (or a utility grid connection) is a special form of a
nanogrid controller. Controllers almost always have local
storage to aid in balancing supply and demand over time.
In LPD, there are only two device types and two types of
links, paralleling the architecture of the Internet. Figure 5
shows a diagram of a nanogrid controller with attached
end-use devices, internal storage, and connections to
other controllers.
In data networks, all packets are different, addresses
provide their destination, and routing mechanisms

determine the path required to get to the destination. In
contrast, all electrons are the same, so how do they
know where to flow in a power network? The answer is
that they should flow from places at which they are
more available (plentiful supply) to those where they
are less available (more in demand). Balancing supply
and demand is a basic function of any grid-and of our
economy in general. Elsewhere in our economy, price is
the basic mechanism used to accomplish resource allocation. LPD, based on a local price set by each nanogrid
controller, directs electricity to flow toward nanogrids
with higher prices, much as gravity forces water to
flow downhill.
With generation and storage in many places in the network, power might flow in different directions at different
times, as supply and demand change. The prices are local
to each nanogrid. Local pricing is the core mechanism
used for coordinating among devices on when to generate
power, when to charge or discharge storage, and how
much and when to use electricity in end-use devices. The
price includes a nonbinding forecast of future prices to
help inform decisions about modulating device behaviors
over time.
LPD capabilities can be added to managed dc technologies, such as USB and Ethernet, and to new physical layers
of power that implement managed dc. Some will have
higher power capacity. 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. This makes changing
power infrastructure something that can be done simply,
inexpensively, and frequently, as local conditions require.

How LPD Can Enable DC Success
Direct dc with LPD together provide a compelling value
proposition and a plausible deployment path that can tip
dc power from being a small player on the edges of electricity distribution to a sizeable component-to become a
success in the market by delivering value to users and
saving energy. The following are benefits that LPD can
bring to power systems in any building.

Optimal Operation with a Local Price
Gateways to Other
Controllers (nG or µG)
or Local Generation

Nanogrid Controller

Storage

Loads
Figure 5. A nanogrid controller.

32

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

A core element of the definition of a nanogrid is a local
price that correctly indicates the relative scarcity of power
and so drives efficient operation of end-use devices, local
generation, and local storage. It can also be used to optimize exchange of power with a utility grid. Analog voltage
levels can be used to indicate scarcity, but this is not accurate and precludes including a forecast of future prices
and negotiating between devices. Forecasts are essential
to moving generation, storage, and end use across time for
better system operation.
Communications are also needed for devices to know
when they should switch the direction of power flow in a



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