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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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Bi-directional converter – A Nanogrid
1
Manjula. J, 2T.B.Dayananda
Department of Electrical & Electronics, Visvesvaraya Technological University, 1IV Sem, M.Tech, ,
2
Associate Professor, Department of Electrical and Electronics,
Dr. Ambedkar Institute of Technology, Bangalore-560056
Email: 1 [email protected]
Abstract— Due to increase in population, there is increase
in power demand in our day to day life, so there is a need
for development of an alternative solution which reduces
the dependency on the grid. Since fossil fuels are
exhaustible, we can consider renewable energy as an
alternative source of energy. Whenever there is an outage
or black-out we need to have alternate source as dcnanogrid which then supplies energy to the residential
applications and also it can be used to inject back the
energy to the grid itself.
In this paper, a nanogrid which consists of a bi-directional
converter used for power conversion is analysed.
I. INTRODUCTION
In today’s world “global warming” is posing a serious
threat to mankind. As we all know fossil fuels are
subjected to exhaustion after a certain period of time.
Down the line in another 50 years we will not be left
with these fossil fuels. Hence nature is alarming us to
take a call to preserve them and also at the same time to
make use of other alternative sources of energy such as
wind energy, biomass, solar etc. By this way greenhouse
emissions can be reduced to a greater extent.
Renewable energy sources such as solar, wind, biomass,
hydropower, and geothermal can provide sustainable
energy services, based on the use of routinely available,
indigenous resources.[1] Due to cost fluctuations of oil
& gas, the demand for renewables-based energy systems
is increasing. Fossil fuels are non-renewable resources
of energy. The oil, natural gas and coal we use today are
gone forever. However, biomass fuels are renewable
because the growth of new plants and trees replenishes
the supply.
taken care of by a bi-directional converter. The
centralized power generation model is more economical
and also more reliable source of energy production.
An electric power system (EPS) is the greatest and most
complex machine ever built. It is a network of electrical
components which consists of wires, cables,
transformers, towers, circuit breakers all bolted together
in some fashion. It is used to supply, transmit and use
electric power, which can be divided into the categories
of power generation, power transmission, power
distribution, and power consumption. The EPS is a
mechanical system with only modest use of sensors,
minimal electronic communication and almost no
electric control. The traditional model of large base-load
AC centralized electrical power generation and its
distribution via long transmission lines causes huge
losses of energy and costs required to operate such
systems.
Many steps have been taken to mitigate the potential
blackouts, particularly to give way for new technologies
which makes EPS more reliable and also can sustain
high economy which is based on power sensitive
equipment.
Battery-based hybrid and electrical vehicles and solidstate based lighting are transforming the transportation
and lighting industries, both of which are powered by
electric current. A DC nanogrid is the key enabler of the
“zero energy module” with a very minimum wastage in
transmission and conversion. So, it can be used to treat
100% energy needs of a building.
II. DG SYSTEMS
Distributed Generation (DG) is a type of electrical
The term “nanogrid” (fig 1) may be defined as a smallgenerator or static inverter producing alternating current
scale generators and loads upto 20KW in size, and loads
that (a) has the capability of parallel operation with the
are located within short radius of the sources. The
utility distribution system, or (b) is designed to operate
available renewable energy sources can also be directly
separately from the utility system and can feed a load
interfaced to the nanogrid at higher reliability. In
that can also be fed by the utility electrical system. This
rectification mode, a buck converter steps down the
is sometimes referred to simply as “generator”.
level of voltage and is stored in the battery whereas in
regenerative mode, a boost converter comes into
Distributed Generation is a back-up electric power
scenario to supply back the power to the grid. The
generating unit that is used in many industrial facilities,
charging and discharging action of the battery action is
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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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hospitals, campuses, commercial buildings &
departmental stores. These back-up units are primarily
used by customers to provide emergency power when
grid-connected power is unavailable and they are
installed within the consumer premises where the
electric demand is needed. The transportation cost is
reduced since it is installed very close to the demand
centre; hence there are no associated transmission losses.
DG systems.
Distributed
generators
include
induction and
synchronous electrical generators as well as any type of
electrical inverter capable of producing A/C power. An
Emergency or Standby Generation System is designed
so as to never electrically interconnect or operate in
parallel with the utility system. An Interconnected
Generation System is any generator or generation
system that can parallel (or has the potential to be
paralleled via design or normal operator control), either
momentarily or on a continuous basis, with the utility
system.
The uses of DG are:

Future demand can be handled with ease without
any investment in expanding the existing system.

This reduces right of way costs.

Reduces fossil fuel consumption.

Reduces peak supply burdens of the utility grid.

Improves power quality.
Distributed Generation can be easily interfaced with
non-generating technologies such as power storage
devices like batteries and flywheels in addition to
generators, while DG is limited to small scale (less than
20 MW) electrical generation located close to point of
use. Unlike central power plant generation, DG often
utilizes the waste heat from the generation process as an
additional form of energy for space or process heating,
dehumidification, or for cooling through absorption
refrigeration.
Distributed generation, which can be referred as smallscale electricity generation, is a fairly new concept in the
economics literature about electricity markets, but the
idea behind it is not new at all. In the early days of
electricity generation, distributed generation was the
rule, not the exception. The first power plants only
supplied electricity to customers in the close
neighbourhood of the generation plant. The very first
grids were DC based, and therefore, the supply voltage
was limited, as was the distance that could be used
between generator and consumer. Balancing demand
and supply was partially done using local storage, i.e.
batteries, which could be directly coupled to the DC
grid. Along with small-scale generation, local storage is
also returning to the scene.
When compared with AC transmission, DC transmission
is far more efficient with improved controllability and
reduced cost over long distances. These days, HVDC
technology is used because of fast power flow control
and also for reactive power compensation. Also, DC is
more stable when compared to AC and also can be
easily integrated with energy storage devices.[7]
A typical DG conversion system is made of two main
energy converting stages. The first stage is the prime
fuel converting block wherein prime fuel is converted
into mechanical energy and in the second stage
mechanical energy is converted into electrical power
using an induction generator or synchronous alternator.
Fuel cells and PV converter are very good examples of
Fig 1. Nanogrid structure of home
(source:www.google.com)
III LITERATURE SURVEY
In recent years, due to the growing concern over energy
shortage and environmental pollution, the concepts of
distributed generation (DG) systems, smart grid systems,
and dc-based hybrid power systems have become
progressively more popular, especially with the
decreasing costs of various clean renewable energy
sources like wind, bio-mass, solar, and fuel-cell systems.
These DG systems would be connected to the utility grid
under normal operating conditions; but additionally,
these systems have the capability to sustain a local
system by sourcing power directly from the renewable
energy sources and energy storage devices if
necessary.[5] The frequency-response based design
procedure for the proposed control system is presented
in detail for all the converter operating modes.
Autonomous load shedding can be implemented in a
nanogrid that uses DC bus signalling for source
scheduling by shedding loads when the dc bus voltage
decreases to a level that signals an overload condition.
This paper explains the control requirements for the
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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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system interface converters that is required to permit
load shedding and explains a procedure for
implementing a prioritized load shedding scheme in a
practical system.[7]
In this paper DC bus signaling acts as a means of
generator scheduling and power sharing in a nanogrid
under steady-state conditions. [20] DC bus signaling is a
novel control strategy that is a hybrid of the voltage
level signaling and voltage droop schemes. Discrete
voltage levels on the nanogrid bus indicate the state of
the system and determine the behaviour of each source.
In dc-microgrid applications, a power distribution
system needs a bi-directional inverter to control the
power flow between dc bus and ac grid, and to regulate
the dc bus to a certain range of voltages, in which dc
load may change abruptly. This will result in high dcbus voltage variation. In this paper, we take into account
this variation and propose an on-line regulation
mechanism according to the inductor current levels to
balance power flow and enhance the dynamic
performance. [21] Additionally, for power compensation
and islanding protection, the bi-directional inverter can
shift its current commands according to the specified
power factor at ac grid side.
A dc nanogrid - hypothetical future sustainable home
electronic power distribution system with dc-based
distribution and the presence of renewable energy
sources is dealt in this paper.. Description of the
nanogrid and the testbed for its system-level operation
analysis is presented in this paper together with initial
experimental results and conclusions.[32]
All the sub-systems like micro, mini & nano grids are
interconnected with the help of energy control centres
which acts as energy routers. Smart metering, on-line
communication, data collection & evaluation, sensing
faults, regulation and bi-directional power flow
operation are the pre-requisites of an energy control
centre.
A Smart grid is the use of sensors, communications,
computational ability and control in some form to
enhance the overall functionality of the EPS. This
system becomes smart by sensing, communicating,
applying intelligence, exercising control and through
feedback, continually adjusting. Optimization is
achieved this implementation which also ensures high
reliability by mitigating environmental impact, manage
assets and also the cost of installation & maintenance.
Fig 2. Smart Grid
(souce:www.google.com)
As in fig 1, a 380v dc bus is connected to many
renewable energy sources, storage devices & loads via
power converters. Nowadays, 380v is used as a standard
voltage in dc data centres and 48v is used for consumer
devices and is accessible by 48v batteries. This system is
referred as electronic-based dc nanogrid. This is a zero
net energy consumption module. In this voltage is
regulated by connecting battery directly to the dc bus.
The operating voltage of DC bus is chosen between
360V to 400V easily so that power sharing can be
carried out with voltage regulation. [7]The static V-I
graph of ECC is shown in fig 3. ECC takes energy from
the grid in the first quadrant hence output current Ig is
positive whereas in the second quadrant it is sourced
back to the grid. The converter can reprogram its V-I
characteristic in the second quadrant to limit the current
value which corresponds to demanded power on the
request of operator. A droop control scheme is used to
detect changes in the system and to adjust the operating
points accordingly.
IV PROPOSED SYSTEM
Fig 3. Single phase-full bridge converter
This topology is very widely used to transfer the energy
between ac grid to dc storage system. This system is
connected to a buck converter so that the voltage level is
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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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stepped down before storing it to the battery. Here the
circuit operates in two-modes: one in rectification mode
and the other in re-generative mode. In rectification
mode, the ac- is converted into dc and is used to charge
a dc-link capacitor, which is stepped down and stored in
battery. Like-wise during re-generation mode, the d
from the battery is stepped up to much higher level and
is capacitor is charged and is inverted with the help of
bi-directional converter so that dc is converted to ac.
With the help of filter present before the grid, unwanted
ripple is removed. A controller is developed to control
the firing angle of the IGBT’s used in the circuit.
An additional requirement of the inverter is to have the
capability of preventing the DG from islanding.
Islanding is a condition which occurs when a generator
or an inverter and a portion of the grid system separate
from the remainder of large distribution system and
continues to operate in an energized state This poses a
potential threat to other equipments which is in the
vicinity and hence it should be taken care of well in
advance that is during planning itself.[15-17]
The high-frequency switching of the current and voltage
in power electronics can generate fast di/dt and dv/dt
noises referred as EMI noises. This electromagnetic
interference can harm the neighbor component and also
deteriorates the system operation. By proper grounding
techniques and by the use of EMI filter the EMI can be
reduced to a greater extent.
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V CONCLUSION
In the field of power electronics, this bidirectional
converter gives fast dynamic response and also faults are
regulated easily which can be practically implemented in
our day to day life. This can be used to replace existing
electromechanical devices with digital control system. A
nanogrid can be used as an alternate source of energy at
home. It reduces the burden on utility grid and when the
battery is full can also be used to share the load along
with the grid. This is also very economical and is an
efficient way to manage power. Renewable energy
sources can be directly connected to the grid which
further reduces the cost.
VI FUTURE WORK
Since large number of power converters are injected to
the grids, it causes instability in the grid. Therefore
proper care should be taken to synchronize it. The
ability of the converter can be increased with softswitching and interleaving converter technologies. All
the EMI aspects need to be taken care of at the design
level itself which reduces the total cost of nanogrid.
Adaptive voltage control can be used to optimize the
system’s performance. DC stability study, ground-fault
& short-circuit protections need to be considered for the
converter implementations.
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International Journal of Electrical, Electronics and Computer Systems (IJEECS)
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