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INTEGRATION OF DISTRIBUTED SOLAR POWER GENERATION USING BATTERY ENERGY STORAGE SYSTEM K.MOUNIKA
Sri.G.VEERANNA
M.E(Power Systems & Automation)
Asst.Professor
Department Electrical & Electronics Engineering, S.R.K.R Engineering college, Bhimavaram, Andhra Pradesh
Abstract : This paper presents an overview of
sunlight into electricity using semiconductors that
the challenges of integrating solar power to the
exhibit the photovoltaic effect. Photovoltaic effect
electricity distribution system, a technical
involves the creation of voltage in a material upon
overview of battery energy storage systems, and
exposure to electromagnetic radiation. The solar cell
illustrates a variety of modes of operation for
is the elementary building block of the photovoltaic
battery energy storage systems in grid-tied solar
technology. Solar cells are made of semiconductor
applications. . Battery energy storage systems are
materials, such as silicon. One of the properties of
increasingly being used to help integrate solar
semiconductors that makes them most useful is that
power into the grid. These systems are capable of
their conductivity may easily be modified by
absorbing and delivering both real and reactive
introducing impurities into their crystal lattice. The integration of significant amounts of
power with sub -second response times. With these capabilities, battery energy storage systems
photovoltaic (PV) solar power generation to the
can mitigate such issues with solar power
electric grid poses a unique set of challenges to
generation as ramp rate, frequency, and voltage
utilities and system operators. Power from grid-
issues. Specifically, grid-tied solar power
connected solar PV units is generated in quantities
generation is a distributed resource whose output
from a few kilowatts to several MW, and is then
can change extremely rapidly, resulting in many
pushed out to power grids at the distribution level,
issues for the distribution system operator with a
where the systems were often designed for 1-way
large quantity of installed photovoltaic devices.
power flow from the substation to the customer. In climates with plentiful sunshine, the widespread
Index Terms— Battery energy storage systems,
adoption of solar PV means distributed generation on
photovoltaic, renewable, solar.
a scale never before seen on the grid. Grid-connected
I.
INTRODUCTION
solar
PV
dramatically
changes the load pro-file of an electric utility
Photovoltaic is the field of technology and
customer. The expected widespread adoption of solar
research related to the devices which directly convert
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generation by customers on the distribution system
counteract the change in generation. In small power
poses significant challenges to system operators both
systems, frequency can also be adversely affected by
in transient and steady state operation, from issues
sudden changes in PV generation. Battery energy
including voltage swings, sudden weather -induced
storage systems (BESS), whether centrally located at
changes in generation, and legacy protective devices
the substation or distributed along a feeder, can
designed with one-way power flow in mind. When
provide power quickly in such scenarios to minimize
there is plenty of sunshine during the day, local solar
customer interruptions. Grid-scale BESS can mitigate
generation can reduce the net demand on a
the above challenges while improving system
distribution feeder, possibly to the point that there is
reliability and improving the economics of the
a net power outflow to the grid. In addition, solar
renewable resource.
power is converted from dc to ac by power electronic
This paper describes the operation and
converters capable of delivering power to the grid.
control methodologies for
Due to market inefficiencies, the typical solar
designed to mitigate the negative impacts of PV
generator is often not financially rewarded for
integration,
providing reactive power support, so small inverters
distribution system efficiency and operation. The
are often operated such that they produce only real
fundamentals of solar PV integration and BESS
power while operating a lagging power factor,
technology are presented below, followed by specific
effectively taking in or absorbing reactive power, and
considerations in the control system design of solar
increasing the required current on the feeder for a
PV coupled BESS installations. The PV-coupled
given amount of real power. A radial distribution
BESS systems described in this paper utilize the XP-
feeder with significant solar PV generation has the
Dynamic Power Resource (XP-DPR).
potential to generate most of its own real power
II.
while
a grid-scale BESS
improving
overall
power
PHOTOVOLTAIC INTEGRATION
during daylight hours, while drawing significant reactive power. Modest
Solar power’s inherent intermittency poses
levels of solar PV generation
on
distribution circuits can be easily managed by the
challenges in terms of power quality and reliability.
distribution system operator (DSO). However, both
A weather event such as a thunderstorm has the
the DSO and the customers of electric retail service
potential to reduce solar generation from maximum
may soon feel the undesirable impacts on the grid as
output to negligible levels in a very short time. Wide-
PV penetration levels increase.
area weather related output fluctuations can be
A PV system consists of a number of
strongly correlated in a given geographical area,
interconnected components designed to accomplish a
which means that the set of solar PV generators on
desired task, which may be to feed electricity into the
feeders down-line of the same substation has the
main distribution grid. There are two main system
potential to drastically reduce its generation in the
configurations – stand-alone and grid-connected. As
face of a mid-day weather event. The resulting output
its name implies, the stand-alone PV system operates
fluctuations can adversely affect the grid in the form
independently of any other power supply and it
of voltage sags if steps are not taken to quickly
usually supplies electricity to a dedicated load or
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INTERNATIONAL CONFERENCE ON CURRENT INNOVATIONS IN ENGINEERING AND TECHNOLOGY
loads. It may include a storage facility (e.g. battery
= 0.01
+
ISBN: 378 - 26 - 138420 - 5
−
bank) to allow electricity to be provided during the −
night or at times of poor sunlight levels. Stand-alone
−1
(6)
systems are also often referred to as autonomous systems since their operation is independent of other
Based on (6), it is evident that the power
power sources. By contrast, the grid-connected PV
delivered by the PV array is a function of insolation
system operates in parallel with the conventional
level at any given temperature.
electricity distribution system. It can be used to feed electricity into the grid distribution system or to power loads which can also be fed from the grid. The PV array – characteristic is described by the following; =
−
−1
(2)
In (2), q is the unit charge, k the Boltzman’s constant, A is the p-n junction ideality factor, and Tc the cell temperature. Current irs is the cell reverse Fig. 1. Simplifi ed one-line diagram of a BESS in parallel with a Solar PV fa-cility connected to the grid.
saturation current, which varies with temperature according to =
−
(3)
III.
BATTERY ENERGY STORAGE
In (3), Tref is the cell reference temperature, the reverse saturation current at Tref. and EG
A. Battery Energy Storage Basics
the
band-gap energy of the cell. The PV current iph
A grid-scale BESS consists of a battery bank,
depends on the insolation level and the cell
control system, power electronics interface for ac - dc
temperature according to
power
conversion, protective circuitry, and a
transformer to convert the BESS output to the = 0.01
+
−
(4)
transmission or distribution system voltage level. The
In (4), iscr is the cell short-circuit current at
one- line diagram of a simple BESS is shown in Fig.
the reference temperature and radiation, Kv a
1. A BESS is typically connected to the grid in
temperature coefficient, and the insolation level in
parallel with the source or loads it is providing
kW/m . The power delivered by the PV array is
benefits to, whereas tradi-tional uninterruptible
calculated by multiplying both sides of (2) by vpv.
power supplies (UPS) are installed in series with their
=
−
−1
loads. The power conversion unit is typically a bi-
(5)
directional unit capable of four quadrant operation,
Substituting iph from (4) in (5), Ppv becomes
means that both real and reactive power can be delivered or absorbed independently according to the
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needs of the power system, up to the rated apparent power of the converter.
Most BESS control systems can be operated via automatic generation control (AGC) signals much
The battery bank consists of many batteries connected
in
a
combination
like a conventional utility generation asset, or it can
series-parallel
be operated in a solar-coupled mode where real and
configuration to provide the desired power and
reactive power commands for the converter will be
energy capabilities for the application. Units are
generated many times per second based on real -time
typically described with two numbers, the nameplate
PV output and power system data. In the case of the
power given in MW, and the maximum storage time
XP -DPR, three -phase measurements from potential
given in MWh. The BESS described in this paper is a
and current transducers (PTs and CTs) are taken in
1.5/1 unit, means it stores 1 MWh of energy, and can
real-time on an FPGA device, and once digitized
charge or discharge at a maximum power level of 1.5
these signals become the input for proprietary real
MW. In renewable energy applications, it is common
time control algorithms operating at kHz speeds.
to operate a BESS under what is known as partial
Various control algorithms have been used for PV
state of charge duty (PSOC), a practice that keeps the
applications, providing control of ramp rates,
batteries partially discharged at all times so that they
frequency support.
are capable of either absorbing from or discharging power onto the grid as needed.
Fig.2.Configuration of the grid-connected hybrid PV /Battery generation system
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generators. Frequency deviation is caused by a
B.Ramp Rate Control Solar PV generation facilities have no
mismatch in generation and load, as given by the
inertial components, and the generated power can
swing equation for a Thevenin equivalent power
change very quickly when the sun becomes obscured
source driving the grid. The system inertia is
by passing cloud cover. On small power systems with
typically described using a normalized inertia
high penetrations of PV generation, this can cause
constant called the H constant, defined as
serious problems with power delivery, as traditional
=
thermal units struggle to maintain the balance of
ℎ
power in the face of rapid changes. During solar -
H can be estimated by the frequency response of the
coupled operation, the BESS must counteract quick
system after a step-change such as a unit or load trip.
changes in output power to ensure that the facility
The equation can be re-written so that the system H is
delivers ramp rates deemed acceptable to the system
easily calculated from the change in frequency of the
operator. Allowable ramp rates are typically speci-
system after a generator of known size has tripped
fied by the utility in kilowatts per minute (kW/min),
off, according to
and are a common feature of new solar and wind
1 =2
power purchase agree-ments between utilities and
=
independent power producers. Here the ramp rate
1 −∆ 2
=
−∆ 2
refers only to real power, and that the reactive power capabilities of the BESS can be dispatched simultane-
where the unit of H is seconds,
is system angular
ously and independently to achieve other power
speed,
system goals.
remaining generation online after the unit trip, and
Ramp Rate Control algorithm used in the XP-DPR
∆
is the system frequency,
is the
is the size of the generator that has tripped.
continuously monitors the real power output of the When frequency crosses a certain threshold, it is
solar generator, and commands the unit to charge or
desirable to command the BESS to charge in the case
discharge such that the total power output to the
of over-frequency events, typically caused by loss of
system is within the boundaries defined by the
load, or to discharge for under-frequency events,
requirements of the utility. The system ramp rate is
which often result when a generator has tripped
maintained to less than 50 kw/min, whereas the solar
offline. Using proportional control to deliver or
resource alone had a maximum second-to- second
absorb power in support of the grid frequency
ramp rate of over 4 MW/min.
stabilization is referred to as droop response, and this
C. Frequency Response
is common behavior in generator governors equipped
Even with ramp- rate control, there are still going
with a speed-droop or regulation characteristic.
to be occasional frequency deviations on the system.
Droop response in a governor is characterized as a
On small, low-voltage systems, it is common to see
proportional controller with a gain of 1/R, with R
frequency deviations of 1–3 Hz from the nominal 50
defined as
or 60 Hz frequency. Frequency deviation has adverse effects on many types of loads as well as other
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Where
ISBN: 378 - 26 - 138420 - 5
is the grid frequency,
frequency dead band, and Where
is steady-state speed at no load,
steady-state speed at full load, and
is the
is the power
rating of the BESS in KVA.
is
A set of droop characteristic curves for a 1 MW
is the nominal
BESS is depicted in Fig. 3.
or rated speed of the generator. This means that a 5% droop response should result in a 100% change in power output when frequency has changed by 5%, or 3 Hz on a 60 Hz power system. Since the BESS uses a power electronics interface, there is no inertia or speed in the system, and we must approximate this desirable behavior found in thermal generators. The straight forward implementation is to digitally calculate an offset for the BESS output power command as response proportional to the frequency. The response has units of kW and is determined as
Fig. 3. Frequency droop response curves for 5% response on a 1 MW BESS.
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IV.
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SIMULATION RESULTS
The photovoltaic and battery energy storage system are combined and connected to the grid and is simulated in Simulink /MATLAB R2009a.
Fig.4., Results For Solor Power Measured Over 24 Hours
Fig.5., Ramp Rate control to 50 kW/min for a 1 MW photovoltaic installation and a 1.5 MW/1 MWh BESS for a full day
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Fig. 5., depicts the operation of an XP-DPR BESS
[3] C. Hill and D. Chen, “Development of a real-
smoothing the volatile power output of a 1 MW solar
time testing environment for battery energy
farm. Here the system ramp rate is maintained to less
storage
than 50 kW/min, whereas the solar resource alone had a
applications,” in Proc. IEEE Power Energy Soc.
maximum second-to- second ramp rate of over
Gen. Meeting, Detroit, MI, Jul. 2011.
4MW/min.
systems
in
renewable
energy
[4] A. Nourai and C. Schafer, “Changing the electricity game,” IEEE Power Energy Mag., vol. 7, no. 4, pp. 42–47, Jul./Aug. 2009.
V.
CONCLUSION
[5] R. H. Newnham, W. G. A Baldsing, and A.
Integration of energy storage systems into
Baldsing, “Advanced man-agement strategies for
the grid to manage the real power variability of solar
remote-area power-supply systems,” J. Power
by providing rate variation control can optimize the
Sources, vol. 133, pp. 141–146, 2004.
benefits of solar PV. Using the BESS to provide
[6] C. D. Parker and J. Garche, “Battery energy-
voltage stability through dynamic var support, and
storage systems for power supply networks,” in
frequency regulation via droop control response
Valve-Regulated Lead Acid Batteries, D. A. J.
reduces integration challenges associated solar PV.
Rand, P. T. Mosely, J. Garche, and C. D. Parker,
Coupling solar PV and storage will drastically
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increase reliability of the grid, enables more effective
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grid management, and creates a dispatchable power
[7] N. W. Miller, R. S. Zrebiec, R. W. Delmerico,
product from available resources. Battery energy
and G. Hunt, “Design and commissioning of a 5
storage systems can also improve the economics of
MVA, 2.5 MWh battery energy storage,” in
distributed solar power generation by reduced need
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for cycle traditional generation assets and increasing
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asset utilization of existing utility generation by
[8] “Analysis of a valve-regulated lead-acid battery
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VI.
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