Analysis and Implementation of Power Quality Enhancement Techniques in Hybrid...
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1. Smart DC Micro Grids and the FREEDM Approach
Timothy Sonnenberg
Research Experience for Undergraduates
FREEDM Systems Center
North Carolina State University
Raleigh, NC 27607
Email: tasonnen@ncsu.edu
Abstract—There will need to be a change to the conventional
power infrastructure grid in the near future as the use of
renewable energy and electric vehicles are on the rise. Smart
DC micro grid offers the solution to the problem at hand with
the ability to efficiently power the growing amount of digital loads
by drawing DC power straight from local renewable sources or
energy storage devices. This paper will layout the concept of a
smart DC micro grid and further elaborate on the aspects of
control, fault protection and distributed energy storage devices.
The FREEDM approach to smart DC micro grids will also be
laid out for each section.
Index Terms—Smart grid, Micro Grid , DC, FREEDM.
I. INTRODUCTION
The new age of electricity distribution brings about the need
to power many different types of loads and incorporate diverse
sources of energy production. The world is trending towards
a DC distribution system as loads become increasingly digital
and renewable energy use rises [1]. Electric vehicles (EVs)
and Hybrid electric vehicles (HEVs) will add more demand
for energy as they increase in use. As a result, the ideas of a
smart DC grid and DC micro grid are gaining in popularity and
becoming a viable solution to meet future demands. Electronic
loads, fuel cells and batteries are all entirely DC loads.
Additionally, renewable energy sources output DC power or
incorporate a DC link within their power electronic platforms
[1]. Using a DC distribution system to connect these loads and
power sources is the solution that DC micro grids offer. In this
way, DC micro grids effectively eliminate the need for AC to
DC and DC to AC power conversion for renewable energy
sources. The elimination of the power conversion increases
efficiency by up to 35% [2].
Reliability is an important concern with power delivery sys-
tems as blackouts and other power incidents can be very costly.
This is especially true in sectors such as telecommunications
and data centers. DC micro grids with integrated AC delivery
systems can increase the reliability of the network by reducing
the probability and impact of blackouts as well as reduce the
cost of power quality disturbances [3]. As opposed to an AC
system whose power is controlled by adjusting mechanical
generators, DC systems power is controlled by the switching
of a semiconductor device. This semiconductor device can
be controlled much quicker and more accurately [4]. The
speed and ease of control over a DC system not only adds
reliability but also increases compatibility with smart systems.
FREEDMs system of multiple operation modes allows for
quick response to blackouts and faults and the ability to power
critical and non-critical loads during times of disconnect to the
main power grid.
The augmented use of EVs may cause problem for the
current power grid infrastructure; the demand for power will
rise and add strain to the existing grid. Smart DC micro
grids offer fix this problem with energy storage devices.
These devices can be used to charge EVs from excess power
generated from local renewable energy sources. The entire
smart DC system can be autonomously controlled such as
the control scheme for the FREEDM system [5]. Autonomous
control allows for quick and efficient management of faults
and irregular behavior in the system.
This paper highlights the main features of DC smart grids
and the FREEDM system. Section I provides a brief overview
of the fundamentals behind smart grids and DC micro grids
as well as an introduction to the FREEDM system. In section
II, smart DC micro grids are analyzed with respect to control
systems, distributed energy storage device and fault protection.
Section III consists of the conclusion.
II. JUSTIFICATION OF SMART DC MICRO-GRID
The reasons for using a smart DC micro grids are widely
argued in electrical engineering. Micro grids are not a new
idea and have been around since electricity distribution started.
London in 1918 had 50 different power networks which were
operating at 24 different voltages and 10 different frequencies
[6]. Opinions on the use of DC power distribution and use
stem back to the raging debate between Nikola Tesla and
Thomas Edison in the late 19th century. Many loads, all energy
storage devices and renewable energy sources are DC. Smart
DC micro grids offer a way to combine all of these aspects in
an intelligent and managed way. Smart micro grids have the
ability to connect to both DC and AC power sources all while
being able to maintain high quality power through the use
of power electronics, smart controls, and distributed energy
storage devices (DESDs).
The trend towards DC distribution continues while AC
distribution remains the largest source of power by far. The
ability to partner both AC and DC power sources will be
invaluable in the transition period between these distribution
methods. Blackouts in the U.S., India and China have created
great interest in DC micro grids as they are able to provide
2. continuous power, even after the link between the main power
supplier has been lost [6]. Energy independence from the
main grid has also seen attention from the U.S department
of Defense. The ability of DC micro grids to provide secure,
reliable, and economically viable power for defense use is
invaluable for homeland security interests.
The FREEDM smart DC micro grid is autonomously con-
trolled and able to account for faults, voltage and current
stabilization, as well as failures in the bulk AC transmission
grid. The three modes of operation in the FREEDM system
allow quick and efficient response to these potential problems.
Critical loads can still be powered even after the transmission
grid is disconnected. This will prove to be very useful for
industries such as the medical industry, telecommunications
and data centers. Because of these capabilities the FREEDM
system has a viable ability to address future demand for high
quality energy.
A. Smart Grid
In future energy distribution, consumers will actively play a
part in energy production. In order for this to work out, future
grids need to have the ability to control the flow of power and
be able to switch between local renewable sources and bulk
production sources as needed [3]. The output of renewable
energy generators depend on weather conditions such as solar
radiation and wind speed. In order to maintain proper energy
supply to the grid, a balancing of power supply and demand
load is a must [4]. This necessitates the use of intelligent
features to be incorporated in generation, distribution and
consumption of energy.
Smart grids have been defined as an electricity network that
can intelligently integrate the actions of all users connected to
it, in order to efficiently deliver sustainable, economic, and se-
cure electricity supplies by the European Union [7]. Fig. 1 and
2 both illustrate the smart grid concept as an overhead view.
Fig. 2 contrasts the smart grid to the outdated model of power
distribution. By reworking the energy infrastructure model,
smart grids can incorporate smart meters, home automation,
power systems and energy storage. This results in increased
efficiency, security and sustainability to the grid. Smart grids
are also able to deliver real time information to enable smart
energy decisions.
B. DC Micro Grids
A DC micro grid is a small electricity network that is
connected to the main grid as well as other micro grids only
using a single point of connection. DC micro grids can be
categorized in two operation modes: islanded mode and grid-
connected mode. Islanded mode is generally used in more
remote areas where a connection to a main power supply
would be too difficult or too costly. In the grid-connected
mode, the micro grid is able to draw power from main power
supplies if the local renewable sources fail to generate enough
to supply the loads. A simple example of a DC micro grid can
be seen in fig. 3.
Fig. 1. Smart Grid Concept [4]
Fig. 2. Smart Grid Comparison [4]
Fig. 3. Simplified DC Micro Grid Structure [9]
Perhaps the biggest advantage that DC MGs offer is the
economic value it can provide. Smart DC MGs with bidirec-
tional flow of power to and from bulk sources can typically
bring about financial benefits that are at least three times larger
than the cost of implementation. Improvements in outage
duration, frequency, power quality and efficiency, peak demand
reduction and reduced emissions all play a major role in the
financial benefit [2]. The ability to sell excess power generated
from local sources using bidirectional distribution only adds
to the value smart DC MGs offer.
When compared to traditional low voltage AC, 380V DC
architectures can reach up to 30 percent higher efficiency [1].
3. Fig. 4. FREEDM System Overview [12]
The usual approach to DC micro grids is to have a 380V bus
running through the grid to be used to power loads with higher
energy demands such as a washing machine or fast charger. A
renewable source of energy, such as solar panels, could also
be tied directly into the 380V bus. Electric Vehicles can be
charged more efficiently using a DC power source and can
also be used as backup storage in case of cut off from service
energy [2]. Directly connecting DC generation to DC loads is
the main feature which makes DC micro grids such a feasible
and efficient system for the future.
C. FREEDM System Background
The FREEDM systems center, an US National Sceince
Foundation (NSF) generation-III research labratory, developed
the FREEDM approach. It is a feasible system for powering
future needs based on solid state transformers (SSTs) and solid
state fault isolation devices (FIDs). The system contains both
an AC and a DC microgrid powered by a medium voltage
AC bus which is in turn powered from the 69kV sub-station.
Bi-directional communication permit the DC micro grids, AC
micro grids and solid state transformers to send and receive
information to the control center [8]. Fig. 4 shows the basic
configuration of the FREEDM system.
The FREEDM system is also highly scalable and able to
accommodate a growing community. Residential users are able
to use the plug and play interface to satisfy their energy
demand. If excess power is generated and the DESD is at
full capacity, then the excess energy can be sent back to the
grid with the intelligent energy management (IEM). The IEM
Fig. 5. Solid State Transformer Function [12]
in turn uses the SSTs to perform the actual power control and
voltage regulation.
The SST is the back bone to the FREEDM systems opera-
tions. It performs AC to DC power conversion, high frequency
DC to DC conversion, regulation of DC bus, and a DC to
AC stage. A functional configuration of the SST can be seen
in fig. 5. The SST isolates voltage and frequency parameters
from the DRED and the DESD side of the system [9].
This capability strengthens the system stability as the low-
4. voltage side is decoupled from the grid side. The SST is
what enables the FREEDM system to have control over all
necessary parameters.
III. TECHNOLOGIES OF SMART DC MICRO GRIDS
A. Fault Protection
As DC distribution increases, protection of DC smart grid
will become more and more important. Multi-terminal DC
power systems have not had the same standards and practical
experience over the last century that AC power systems have
had [4]. This is one of the reasons why protecting against a
DC fault is more difficult than an AC fault.
At FREEDM, a solid state FID is used to isolate the network
in the event of a fault or irregular circumstances. In order to
keep the bi-directional flow of power, four diodes are used
in the device. Also a solid-state switch is needed in order to
interrupt the current. The knee point current is controlled by
using the gate voltage of the device. Should a fault occur,
the current will exceed the knee point which will increase the
terminal voltage of the FID. A fault can be detected by sensing
the terminal voltage [5].
By having FIDS at different steps along the stream of power,
the FREEDM system is able to have optimum protection of its
radial system. Each zone along the stream will be programmed
in a way that allows a fault to be detected before it can impact
other areas. This is done by making the gate voltage of each
FID lower upstream than downstream. Making the knee point
current on each FID equal to two times the maximum load
current ensures that at least one FID will interrupt the flow of
current in each path [5].
Should a failure occur at the transmission grid, the sub-
station solid state transformer will detect the failure. The
FREEDM system will then be disconnected from the trans-
mission grid and immediately start islanding mode. This will
be covered in greater detail in the control systems section of
this paper.
B. Distributed Energy Storage Device
Storage systems are a vital part of smart grids that will
maximize their benefits. Storage systems can help regulate
voltage, distribution losses, transmission congestion, and price
arbitrage. In the event of cut-off from main AC transmission
lines, energy storage devices are vital to keeping power
flowing to critical loads.
FREEDM has developed its own DESD with the goals of
delivering safe, efficient, fiscally appealing energy storage to
support the modern grid. With the use of advanced power
electronics, the FREEDM DESD is able to communicate with
the SST and other parts of the grid. Algorithms have also been
developed to accurately estimate the state-of-charge and state-
of-health of the battery. This estimation allows for efficient
utilization of DESD in grid applications such as bringing
stability to uneven sources of generation such as solar.
More specifically, The DESD at FREEDM is controlled
through different communication protocols as well as a bea-
glebone black platform. The beaglebone black can act as
both a communication gateway and application development
platform. The protocol used for the DSP is MODBUS. For
communication with the Beagle bone black to the FREEDM
distribution grid, the MQTT protocol is used. The Toshiba
batteries used communicate to the ARM board system using
the CAN protocol. The batteries themselves have a built in
battery management system (BMS) which can measure the
voltage, temperature, current and estimate the state-of-charge
of the battery [11].
C. Control Systems
Control systems are the heart of any smart grid system.
They manage the behavior of all the devices connected. In
order to maintain system stability in smart DC systems, smart
devices must be able to control and change the power flow
to the necessary parameters. Intelligence in the DC system
means that all activity is recorded, stored and analyzed in real
time. This allows for the system to make the most informed
decision. This is achievable through the use of smart meters.
Smart meters are able to communicate and execute control
commands, send real time information to the control system
and other smart meters as well as account for the energy stored
in DESD when informing the control system [3].
Fig. 6. FREEDM System Modes of Operation [9]
Communication between the different distributed energy
sources is generally too slow to reliably pass on signals in
time for power management. In some cases, communication
systems may not even be obtainable. Droop control is a useful
solution to the communication problem. Droop control is
used to quickly regulate voltage, frequency and power sharing
between resources. This power management tool is used in
the FREEDM system to regulate parameters in all operation
modes.
5. The control system for DC smart grids in the FREEDM
system is based on hierarchical power management consisting
of three modes of operation [10]. These modes can be seen
in fig. 6. These three modes are: transmission control mode,
islanding mode, and SST in islanding mode. In order for this
type of control system to work, all modes must be capable
of controlling voltage and frequency [5]. During transmission
control mode, the system is under normal operation taking in
power from the renewable energy sources as well as taking
power from the main AC transmission grid as needed to
accommodate for the loads in the system.
Islanding mode is entered when the substation SSTs detect
a transmission grid failure. The FREEDM system is then
disconnected from the transmission grid. The local energy
sources as well as the DESD sources will be drawn on for
supplying the loads. During this mode, load control is essential
to achieve power balance. Non-essential loads can be shed
based on predefined levels in order to maintain stability. Load
shedding parameters can be seen in fig. 7. Energy storage
sources will be drawn on for suppling the loads. During this
mode, load control is essential to achieve power balance. Non-
essential loads can be shed based on predefined levels in order
to maintain stability. Load shedding parameters can be seen
in fig. 7.
Fig. 7. FREEDM Load Shedding Parameters [9]
To enter SST islanding mode, the medium voltage DC micro
grid frequency will have to drop below 55 hz. This occurs
when the DC micro grid cannot supply enough power for
the medium voltage (MV) part of the grid to be regulated.
This means that the system is unbalanced. When this mode
is activated, the DC/DC stages of the SST will be stopped
and the load will be supplied purely by the DC micro grid. If
the SST detects that the MV grid is recovering, the stopped
processes will start again and the system will transition back
to the transmission control mode. With these three modes the
FREEDM system is
IV. CONCLUSION
Smart DC micro grids offer many solutions to rising
problems in the currently electricity transmission, distribution
and consumption methods. These problems include black-
outs, greater incorporation of electric vehicles, and efficiently
powering remote areas. An overview of what a smart DC
micro grid is as well as the approach FREEDM took in its
model for the future is presented. Controls, Distributed energy
storage device, and fault protection are covered in more detail.
Solutions are highlighted to show the feasibility and validity
of smart DC micro grids as a major part of the future of energy
consumption.
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