IEEE Electrification Magazine - June 2016 - 27

oven, dishwasher) and laundry rooms (e.g., washing
machine, dryer, iron). For a given power consumption, supplying the loads at 230 V would keep the
same current loading in the wires as in a singlephase 230-VAC system. If high efficiency in the distribution and/or wire-gauge reduction is required,
the voltage can be increased up to 400 V. Beyond
this level, the efficiency improvement should be
negligible, and the protection system would be
highly demanding.
3) High-power elements (≥538 V, ≥10 kW) are expected
to be a part of the common-facilities elements for a
building, including DGs (e.g., PV panels, WTs, FCs,
µCHP), ESSs, and the building's air-conditioning system, elevators, and EV charging posts.

Integration of Future LVdc Distribution Systems
The following discussion analyzes the selection of the
voltage and topology of the distribution system in building
applications. The standards, guidelines, and codes necessary to widely
implement LVdc systems to supply
the electricity to homes and buildings
have not yet been developed, even
though several organizations, such as
the IEC or EMerge Alliance, are actively working in this area. In the near
future, a reasonable first step to apply
dc power to buildings, as depicted in
Figure 1, is to use an LVdc system to
interconnect the local RESs, ESSs, and
loads common in buildings, such as
lighting systems and elevators,
whereas the ac distribution system is
used to connect the building to the grid and supply the
apartments. The purpose of this configuration is that not a
single change is required in the apartments, and therefore
there is no need for the consumers to change electrical
equipment in their homes.
If we take a look forward, once the regulations and
standards for dc system are developed and mature,
hybrid ac/dc distribution systems for buildings, as the one
proposed in Figure 3, will likely be widely accepted and
implemented, and most important, there will be dc-compatible devices broadly available in the market, such as
appliances and electronics that can be used with either
an ac or a dc system. In the literature, most of the proposed architectures for dc distribution in buildings use a
unipolar bus running at 380-400 V. As mentioned previously, this is the solution adopted by the data-center
industry, even though it might not be the most optimized
solution for buildings and homes. It is an easier step to
take, rather than developing a new solution for the distribution system. This solution is simple, convenient, and
more efficient than the traditional ac system. It is convenient because it has been tested in different applications,

therefore most of the solutions already developed in that
field can be used for this particular systems as well. It is
more efficient because first, the voltage level is higher,
hence the losses in the conductors are lower, and second,
using a dc system to distribute the energy between mostly dc-based devices (e.g., PV panels, batteries, lights,
motor drives) allows a reduction in the number of conversion stages.
It seems that unipolar 400-V distribution systems are
widely accepted as a suitable solution for dc distribution for building applications. However, aside from the
convenience of adapting an already tested solution, the
following question must be asked: Is the 400-V system
the best option for distribution in buildings? For
instance, regarding the topology of the system, we discussed earlier the benefits and disadvantages of
unipolar, bipolar, and multibus architectures. Bipolar
configurations offer significant advantages, especially
for this kind of application, which has different
elements (e.g., generators, loads,
storage systems) with a wide range
of power ratings. Therefore, the
availability of different voltage levels
to supply the elements in the systems enables a better compromise
between distribution efficiency and
safety. As a consequence, a bipolartype LVdc distribution system seems
a better solution.
Then the discussion moves to
which voltage levels should be used
for the distribution. We have discussed that dc voltage levels within
230-400 V are a good compromise
between, efficiency, safety, and compatibility with existing ac systems. Furthermore, once established that a
bipolar configuration is a more convenient solution for
distribution in building/residential applications, it
seems advantageous to apply the same topology for the
distribution grid between the buildings themselves. In
Figures 4 and 5, a two-level bipolar distribution grid is
shown. The IEC 60038 standard sets the upper limit for
LVdc systems at 1500 V, therefore, to maximize the efficiency in the distribution, a bipolar !750 V is proposed,
from which it is possible to extract a second-level
!375 V bipolar line, which fits the suitable voltage levels
for both high and medium power-rating elements in
buildings. It should be also noted that the use of a twolevel bipolar distribution system restricts the voltage
level to be used in the system, therefore doubling up
twice the voltage level 380-400 V would exceed the LV
limit set by the standards. The grounding and protection scheme for the proposed two-level bipolar LVdc
distribution system is not trivial, and some modifications are needed to comply with the safety requirements. The neutral conductor of the !375-V distribution

DC provides a
promising solution
for modern power
systems to improve
efficiency, power
quality, resiliency,
and reliability.

	

IEEE Electrific ation Magazine / j une 2 0 1 6

27



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