DC Distribution Systems – An Overview

Abstract

This paper examines the existing and future dc distribution systems which has wide range of applications in data centers, telecommunication systems, residential homes, space crafts, electric vehicles, and aircrafts. The advantages and disadvantages of a dc distribution system are compared against their ac counterpart. Several dc distribution architectures are presented. Dc distribution systems are discussed from the cost, reliability, efficiency, and safety standpoints.

Full text

DC Distribution Systems – An Overview
V. A. K. Prabhala
Electrical Engineering Dept.
Missouri S&T
Rolla, USA
vkpzvf@mst.edu
Bhanu Prashant Baddipadiga
Electrical Engineering Dept.
Missouri S&T
Rolla, USA
bbt68@mst.edu
Mehdi Ferdowsi
Electrical Engineering Dept.
Missouri S&T
Rolla, USA
ferdowsi@mst.edu
Abstract—This paper examines the existing and future dc
distribution systems which has wide range of applications in data
centers, telecommunication systems, residential homes, space
crafts, electric vehicles, and aircrafts. The advantages and
disadvantages of a dc distribution system are compared against
their ac counterpart. Several dc distribution architectures are
presented. Dc distribution systems are discussed from the cost,
reliability, efficiency, and safety standpoints.
I. INTRODUCTION
The growing need for highly reliable power supply for
critical applications like in hospitals, data centers,
telecommunication systems, and semiconductor industry
necessitated extensive use of power electronic converters
(PEC). These converters improve the system reliability,
controllability, efficiency, cost, safety and size of the system.
PECs have been extensively incorporated in the power system
of electric cars, aircrafts, ships, and space stations [1-8]. Also,
interfacing different sources of energy (renewable energy
sources like wind and solar) and energy storage systems
(batteries and ultra-capacitors) is done with PECs. So the
power distribution system should be designed in a way to have
all the aforementioned characteristics. The system
architecture, energy flow control, protection, and power
quality become very important.
Recent developments in the renewable energy technology
and increased penetration of the distributed energy sources are
prompting renewed interest in the dc distribution systems. The
dc power distribution system was first proposed for lighting
purposes and patented by Edison [9] in 1883. Sprague
proposed an ac distribution system in 1886 [10] that
eventually became popular and universally adopted for electric
power distribution. Telecommunication systems and
datacenters are one of the few surviving examples of dc
distribution systems. They are low voltage (48 Vdc) and low
load power systems that have characteristics similar to a
conventional dc distribution system. Increased load levels
would entail increased voltage levels for better system
efficiency, reliability, and cost. In addition, the system should
be able to fully utilize all the energy sources and should have
the flexibility for future expansion.
In case of residential applications, a dc microgrid (or
nanogrid) structure is used for dc buildings which are higher
voltage dc systems. In this paper, a comparative study has
been presented for selecting an optimum dc bus voltage level
that would improve the system efficiency and reliability.
Authors in [11-17], show different power delivery
architectures for both ac and dc systems with different voltage
levels. Ideally, power delivery architecture should provide
power at low cost and should maximize the efficiency. It
should also be reliable to supply critical and sensitive loads as
uninterrupted power is necessary for applications such as data
centers. Moreover, there should be room for further expansion
as and when necessary. In [18-24], authors propose new
topologies for power electronic converters with high
efficiency and high power density that can be used to improve
the overall efficiency of the distribution system.
In this paper, Section II gives a brief overview of different
power distribution architectures which include both ac and dc
distribution systems for different voltage levels. Sections III
and IV discuss the reliability and cost of dc distribution
systems. In sections V, the safety and protection aspects of dc
systems are discussed. Section VI discusses the efficiency of
the dc systems.
II. POWER ARCHITECTURE AND VOLTAGE LEVELS
The factors that affect the selection of power delivery
architectures are mainly voltage levels, energy, quality of
power required by the load, overall life, utility ratio, overall
costs, and efficiency. For example, telecommunication loads
require low voltage and high quality power supplied reliably
to them. Moreover, it should be flexible enough to expand and
integrate new sources of energy. Low voltage dc systems for
telecom and datacenter applications operate at 48 Vdc. It has
at least three conversion stages from the ac input to the load
and hence results in lower system efficiency. In [25-33],
different voltage levels for the dc bus have been proposed
based on efficiency, reliability, and cost considerations.
In [11] and [12], three different power architectures
namely conventional ac architecture, rack-level dc
architecture, and facility-level dc architecture used in data and
telecom centers have been described. Conventional dc system
architecture for datacenter applications has a medium voltage
transformer that steps down the utility voltage to 480 Vac
(shown in Fig. 1). The uninterruptible power supply (UPS)
unit supplies power to the power distribution unit (PDU)
which steps down the voltage to 208 Vac. The voltage is
stepped down to keep it in the input range of the power supply
unit (PSU) of the server.
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DC/AC
AC/DC
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480V
3Ø AC
.
DC/DC
AC/DC
380V
PSU
LOADS
VR
VR
FANS
12V
SERVER
RACK
208V
1Ø AC
D
C/
A
C
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D
C
U
P
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U
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.
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S
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R
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F
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1
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S
ER
V
ER
R
A
CK
208
V
1
Ø A
C
Base Line 89.2% x 93.2% x 75.5% x 81.6% = 51.6%
High Efficiency: 94.0% x 94.0% x 87.6% x 87.7% = 68%
D
C/
D
C
A
C/
D
C
380
V
P
SU
Figure 1. Conventional 480Vac architecture.
DC/ACAC/DC
UPS PDU
480V
3Ø AC
.
DC/DC
AC/DC
DC
PSU
LOADS
VR
VR
FANS
12V
SERVER
RACK
208V
1Ø AC
D
C/
A
C
A
C/
D
C
U
P
S
PD
U
4
80
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3Ø A
C
.
A
C/
D
C
LO
AD
S
V
R
V
R
F
A
NS
1
2
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S
ER
V
ER
R
A
CK
208
V
1
Ø A
C
48V DC : 97.1% x 93.8% x 92.4% x 91.5% x 87.7% = 67.4%
380V DC : 97.1% x 93.9% x 96.1% x 89.1% x 87.7% = 68.2%
D
C/
D
C
D
C
P
SU
480V
3Ø AC
Figure 2. Rack-level dc architecture.
AC/DC
UPS PDU
480V
3Ø AC .DC/DC
DC
PSU
LOADS
VR
VR
FANS
12V
SERVER
RACK
A
C/
D
C
U
P
S
PD
U
4
80
V
3Ø A
C
.
LO
AD
S
V
R
V
R
F
A
NS
1
2
V
S
ER
V
ER
R
A
CK
48V DC : 92.9% x 96.1% x 91.5% x 87.7% = 71.9%
380V DC : 95.3% x 96.8% x 89.1% x 87.7% = 72.7%
D
C/
D
C
D
C
P
SU
DC DC
Figure 3. Facility-level dc architecture.
It can be observed that there are several conversion stages
which include the double conversion stage of the UPS where
the input ac voltage is converted to dc where the energy
storage system is connected, again it is converted back to ac
which is stepped down to 208 Vac using a transformer. This
ac voltage is converted to a dc voltage in the range of 380V-
400V in the PSU. Isolated dc-dc converters are used to step
down voltages for further distribution to loads. The overall
system efficiency without any redundancy when operated in
high load condition is typically around 50%. The system
efficiency can be increased to 70% by using high efficiency
and high power density components with increase in
component costs.
In the rack-level dc architecture (shown in Fig. 2) the ac-dc
converter is moved from the PSU of the server to the rack,
thus reducing the cooling requirement for the servers. The
server volume is thus reduced and the whole consolidated rack
has higher power density and improved light load efficiency.
Since the number of conversion stages is the same as the
conventional ac architecture, the overall system efficiency is
not expected to improve any further. If the dc voltage is
increased from 48V to 380-400V, then the system efficiency
can be slightly increased by using non-isolated and more
efficient ac-dc converter.
In the facility-level dc architecture as shown in Fig. 3, the
dc-ac conversion stage in the UPS and the ac-dc conversion
stage in the PSU of the server are removed. Again the
transformer in the PDU is also not required . Since the number
of conversion stages are reduced, the power delivery
efficiency is increased significantly. The efficiency can be
further increased by using higher dc voltages. The copper
losses are higher for a 48Vdc system compared to 380V/400V
dc system. Low voltage systems have to distribute power at
high currents which limit the efficiency of the system.
For residential buildings and homes, the so called ac and
dc nanogrid architectures can be used where the renewable
energy sources and storage systems are integrated along with
the existing power grid [14]. It has been envisioned that future
homes would be self-sustainable with a mix of renewable and
alternative energy sources like wind, solar, generators, micro-
turbines, and batteries along with the utility grid. Fig. 4 shows
one such ac nanogrid architecture for residential applications
where plug in hybrid electric vehicles (PHEVs) are also going
to be part of the scheme in the near future. It will have an
energy control center (ECC) that would monitor the power
flow and would communicate with the utility grid for energy
trading that would eventually reduce the cost of energy by
reducing the energy consumption from the utility grid during
peak hours. It will also have smart metering and can remotely
control the breakers.
Another architecture that has been reported extensively is
the dc nanogrid (shown in Fig. 5) architecture for homes and
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SOLAR
ARRAY
WIND
TURBINE
ENERGY
STOR AGE
PLUG-IN
HYBRID
GRID
Energy
Control
Center
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AC-DC DC-DC DC-DC
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Appliances:
Stove/Range….
y
Figure 4. Ac nano grid for residential homes.
DC BUS
SOLAR
ARRAY
WIND
TURBINE
ENERGY
STOR AGE
PLUG-IN
HYBRID
GRID
ECC
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Consumer El:
TV, Computer,….
Appliances: Washer,
Dryer,..
Appliances:
Stove/Range….
LED Light
Figure 5. Dc nano grid for residential homes.
future buildings [14]. Dc systems have inherent advantages
over ac system like high overall efficiency, easier integration
of renewable energy sources and energy storage systems.
Moreover there is no reactive power and frequency
stabilization issues which results in reduced copper losses.
Many loads like LED lighting systems, consumer electronics,
and appliances using variable speed motor drives can be easily
powered by dc distribution systems.
In dc distribution systems, the dc voltage level should be
selected to maximize the efficiency and reliability and to
reduce the costs and increase the flexibility of the system for
future expansion. Conventional dc distribution systems for
telecom and data centers operate at 48 Vdc. Higher system
voltages is preferred for the aforementioned factors but there
are challenges like safety and protection that can lead to fire
hazards. Again reliability of system is also affected as the
voltage and current stresses are increased on the components
as it increases the risk of component failure.
In [27], it has been reported that Telecom New Zealand
deployed a 220 Vdc system to replace the existing 50 Vdc
system for increased power capacity. The new system has
reportedly reduced the installation costs and copper costs. The
proposed system uses 30-40% smaller batteries and maximizes
available floor space. In [32], for a 220 V single-phase supply
or 380 V three-phase system, the dc bus is chosen ±110 V
with a grounded center tap as shown in Fig. 6. The dc bus
voltages are selected such that the existing electric appliances
can be used in the proposed dc system with little
modifications.
In [33], the (DC)2concept has been introduced for dc
integrated data centers which has higher efficiency and
reliability compared to ac systems. The distribution bus
voltage of the proposed system is in the range of 500-550 Vdc
with distributed energy sources that increase the reliability of
the system. The improved efficiency decreases the power
required for the cooling systems and hence there is 21%
savings in total energy consumption compared to ac systems.
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3PHASE AC
DISTRIBUTION
SMART
ENERGY METER
N3PHASE
AC BUS
3-PHASE AC
LOADS
1-PHASE AC
LOADS
POWER FACTOR
CORRECTED AC-DC
PRE REGULATOR
MICRO COMPUTE R
CONTROLLER
TO ALL BLOCKS
-110V 0V +110V
DC BUS
EC BATTERY
CHARGER
BIDI RECTIO NAL
DC-DC CONVERTER
DC-DC
CONVERTER
DC-DC
CONVERTER
DC LOADS
AC LOADS
DC-AC
CONVERTER
EC
BATTERIES
BACKUP
BATTERIES
SOLAR
CELLS
BACKUP
GENERATOR
D
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ERTER
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Figure 6. ±110V dc power system for residential homes.
Studies show that the dc voltage level of 380 V is an ideal
voltage considering the number of battery cells that are
required to be connected for energy storage system,
availability of components with suitable ratings, and in terms
of safety (see Fig. 7). Electric Power Research Institute (EPRI)
demonstrated a prototype 380 Vdc system that has 7%
improved efficiency compared to the state-of-the-art ac power
distribution [34].
III. RELIABILITY
The reliability of a dc system depends on its system
architecture and redundancies resulting in component failure.
Increasing the system voltage adversely affects the voltage and
current stresses experienced by the switching components and
thereby reducing the reliability of the system. Switching
components like MOSFETs and diodes should always be
operated within their safe operating area. Hence they are
selected considering a factor of safety so that there are enough
margins to take care of the voltage and current transients,
which may result in the failure of these components. If the
operating voltages/currents are more than its rating during
transients, the electrical stress on the component will lead to
breakdown and result in its failure.
It has been observed that an increase in the main bus
voltage reduces the life of the capacitors [35]. The lifetime of
electrolytic capacitors is calculated using
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900 BS (UK)
750 Ordinance (JPN)
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300
428 (192 cells)
374 (168 cells)
321 (144 cells)
Law, Regulation,
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Dist.Gen. No. of Battery
Cells
Voltage Rating
of Parts
374
(
168
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s
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405
350
320
Bulk Operating
Voltages
AC Peak Input
Voltages
373 (UK)
354 (EU)
324 (US)
311 (JPN)
Advantages of
Higher V oltage
Sys tems
Dist ri butio n
Eff ici en cy
Cable Length
ETSI
Stan da r d
In cr eas e s
In cr eas e s
420
260
380
380
600
200
800
1300
DC Voltage (V)
Figure 7. DC bus voltage level selection.
where, Lis the lifetime (hours), L0is the base lifetime at
maximum core temperature (hours), Tcore and Tcore,max are the
normal and maximum operating temperatures of the core
respectively (qC), and Mvis the voltage multiplier, which is the
ratio of applied dc voltage to the rated dc voltage. When
capacitors are subjected to higher stress levels, the core
temperature is close to its maximum temperature which causes
degradation and results in reduced life of the capacitor. DC
systems with higher voltages require smaller capacitances
because for the same amount of power the capacitance is
inversely proportional to the square of voltage. Thus capacitors
are selected with sufficient voltage ratings to account for
increase in the dc bus voltage without reducing the reliability
of the system. Also, series-parallel combination of capacitors
can be employed to improve system reliability.
If batteries are used for energy storage system, a more
number of battery cells is required to be connected in series to
attain the required voltage. This leads to reduced system
reliability as failure in any one cell in the string of cells would
result in failure of the whole battery pack.
IV. COST
Higher dc bus voltage improves the system efficiency thus
reducing the operational costs of the system. The amount of
copper required to limit the copper losses decreases with
increasing voltage, and is inversely proportional to the square
of the voltage. Thus the copper costs are reduced as
conductors with smaller cross-sectional areas can be used. The
cabling and installation costs are reduced as the conductors are
flexible and easy to lay. Conductor costs may be between 10
to 15% of the total hardware cost and the savings associated
by increasing the dc bus voltage can represent less than 5% of
the total hardware costs [28]. Higher voltages require
elaborate protection schemes which require costly fusing,
wiring and non-standard connectors that can offset the
savings. Again the capacitors required should also have higher
voltage ratings which are expensive. Thus these design factors
cause some offset in the savings.
In [28], it has been shown that the cost of a 270 Vdc
system is 15% lower than that of a 48 Vdc system. Due to
lower number of conversion stages, the equipment cost is
reduced. Again, the cabling costs are also lower compared to
the 48 Vdc system since smaller diameter wires are used. The
battery cost is higher for the 270 Vdc system since more
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30-KW system
1
0.85
Cabling
Battery
Power
conversion
equipment
48-V DC power
supply system
270-V DC power
supply system
Cost (normalized)
1.0 --
0.5 --
0
Figure 8. Cost breakdown of 48 Vdc and 270Vdc systems.
Figure 9. Configuration of plug and socket outlet for a dc system [36].
number of cells are connected in series. The cost breakdown
for a 30 kW 270 Vdc and a 48 V dc system is shown in Fig. 8.
V. SAFETY AND PROTECTION
Dc distribution system protection is different from that of
an ac system as it is difficult to break a dc current compared to
an ac current. Protection devices like circuit breakers and
fuses for both the ac and dc systems are similar. In case of dc
systems, they have to withstand more stresses because of the
persistent nature of the arc while breaking a dc current. For dc
buildings and residential applications, circuit breakers can be
used instead of fuses as it can be reset when the fault is
cleared. Again the voltage and current rating of the circuit
breaker is lower for a dc system compared to an ac system.
Same components can be used for protection both on the ac
and the dc side. In addition, in the event of short circuits,
individual converters will have the short circuit protection and
can be easily detected by observing the dc bus voltage. If the
dc bus voltage is below a certain threshold, the controller can
PFC
AC/DC
DC/DC
~380V
1ࢥAC Loads
DC/DC Loads
.
.
.
.
Power Supply Unit
Figure 10. 380 Vdc power delivery architecture for server
applications.
signal a short circuit and shut down the system.
Dc distribution systems for data centers and commercial
buildings require special plug and socket which should
provide arc extinction, prevent electric shocks, and mechanical
locks for human safety. The Nippon Telegraph and Telephone
Corporation and Fujitsu Component Limited have developed a
400 Vdc, 10 A plug and socket (shown in Fig. 9) that can be
used for integration with the dc distribution system [36].
When a dc current is cut off, then there is an arc generated that
can damage the equipment or can be lead to fatal injuries. To
prevent that, the plug should not be taken from the socket
when the power supply is on, but there can be instances when
the plug is removed from the socket accidentally or during
emergencies when the power in still on. During those cases,
arc extension and shock prevention should be done by the plug
and socket.
VI. EFFICIENCY IMPROVEMENT
In [18], a 380 Vdc modified power delivery architecture
(shown in Fig. 10) for server applications has been proposed
which is more efficient than the conventional 48 Vdc system.
In the modified power delivery architecture, the efficiency is
increased by reducing the number of power conversion stages
and placing isolated dc-dc converters close to the load. High
efficiency resonant topologies such as the LLC resonant
topology is used for the dc-dc conversion stage with high
power densities. Also in [23-24], several multi-input dc-dc
converter topologies have been introduced where various
storage systems and energy sources such as renewable sources
can be integrated to achieve high power density and
efficiency.
VII. CONCLUSION
Dc distribution has advantages like improved efficiency
and reduced costs compared to ac distribution. Major
applications of dc distribution systems are in the field of
telecommunications, data centers, dc buildings, and
microgrids. In current scenario, several challenges exist that
need to be addressed before the dc distribution system can be
used for commercial purposes. One major challenge is
selecting a suitable voltage level that makes the system more
efficient and reliable. Studies till date propose that a 380 Vdc
system has both higher reliability and efficiency. Another
critical challenge is the advancement in safety and protection
technology for dc systems. Therefore, a suitable industry
standard is needed to be developed to make such systems
3rd International Conference on Renewable Energy Research and Applications Milwakuee, USA 19-22 Oct 2014
ICRERA 2014 311

more compatible and ready to integrate with existing power
systems.
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