Iaetsd integration of distributed solar power generation

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
the challenges of integrating solar power to the
electricity distribution system, a technical
overview of battery energy storage systems, and
illustrates a variety of modes of operation for
battery energy storage systems in grid-tied solar
applications. . Battery energy storage systems are
increasingly being used to help integrate solar
power into the grid. These systems are capable of
absorbing and delivering both real and reactive
power with sub -second response times. With
these capabilities, battery energy storage systems
can mitigate such issues with solar power
generation as ramp rate, frequency, and voltage
issues. Specifically, grid-tied solar power
generation is a distributed resource whose output
can change extremely rapidly, resulting in many
issues for the distribution system operator with a
large quantity of installed photovoltaic devices.
Index Terms— Battery energy storage systems,
photovoltaic, renewable, solar.
I. INTRODUCTION
Photovoltaic is the field of technology and
research related to the devices which directly convert
sunlight into electricity using semiconductors that
exhibit the photovoltaic effect. Photovoltaic effect
involves the creation of voltage in a material upon
exposure to electromagnetic radiation. The solar cell
is the elementary building block of the photovoltaic
technology. Solar cells are made of semiconductor
materials, such as silicon. One of the properties of
semiconductors that makes them most useful is that
their conductivity may easily be modified by
introducing impurities into their crystal lattice.
The integration of significant amounts of
photovoltaic (PV) solar power generation to the
electric grid poses a unique set of challenges to
utilities and system operators. Power from grid-
connected solar PV units is generated in quantities
from a few kilowatts to several MW, and is then
pushed out to power grids at the distribution level,
where the systems were often designed for 1-way
power flow from the substation to the customer. In
climates with plentiful sunshine, the widespread
adoption of solar PV means distributed generation on
a scale never before seen on the grid.
Grid-connected solar PV dramatically
changes the load pro-file of an electric utility
customer. The expected widespread adoption of solar
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generation by customers on the distribution system
poses significant challenges to system operators both
in transient and steady state operation, from issues
including voltage swings, sudden weather -induced
changes in generation, and legacy protective devices
designed with one-way power flow in mind. When
there is plenty of sunshine during the day, local solar
generation can reduce the net demand on a
distribution feeder, possibly to the point that there is
a net power outflow to the grid. In addition, solar
power is converted from dc to ac by power electronic
converters capable of delivering power to the grid.
Due to market inefficiencies, the typical solar
generator is often not financially rewarded for
providing reactive power support, so small inverters
are often operated such that they produce only real
power while operating a lagging power factor,
effectively taking in or absorbing reactive power, and
increasing the required current on the feeder for a
given amount of real power. A radial distribution
feeder with significant solar PV generation has the
potential to generate most of its own real power
during daylight hours, while drawing significant
reactive power.
Solar power’s inherent intermittency poses
challenges in terms of power quality and reliability.
A weather event such as a thunderstorm has the
potential to reduce solar generation from maximum
output to negligible levels in a very short time. Wide-
area weather related output fluctuations can be
strongly correlated in a given geographical area,
which means that the set of solar PV generators on
feeders down-line of the same substation has the
potential to drastically reduce its generation in the
face of a mid-day weather event. The resulting output
fluctuations can adversely affect the grid in the form
of voltage sags if steps are not taken to quickly
counteract the change in generation. In small power
systems, frequency can also be adversely affected by
sudden changes in PV generation. Battery energy
storage systems (BESS), whether centrally located at
the substation or distributed along a feeder, can
provide power quickly in such scenarios to minimize
customer interruptions. Grid-scale BESS can mitigate
the above challenges while improving system
reliability and improving the economics of the
renewable resource.
This paper describes the operation and
control methodologies for a grid-scale BESS
designed to mitigate the negative impacts of PV
integration, while improving overall power
distribution system efficiency and operation. The
fundamentals of solar PV integration and BESS
technology are presented below, followed by specific
considerations in the control system design of solar
PV coupled BESS installations. The PV-coupled
BESS systems described in this paper utilize the XP-
Dynamic Power Resource (XP-DPR).
II. PHOTOVOLTAIC
INTEGRATION
Modest levels of solar PV generation on
distribution circuits can be easily managed by the
distribution system operator (DSO). However, both
the DSO and the customers of electric retail service
may soon feel the undesirable impacts on the grid as
PV penetration levels increase.
A PV system consists of a number of
interconnected components designed to accomplish a
desired task, which may be to feed electricity into the
main distribution grid. There are two main system
configurations – stand-alone and grid-connected. As
its name implies, the stand-alone PV system operates
independently of any other power supply and it
usually supplies electricity to a dedicated load or
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loads. It may include a storage facility (e.g. battery
bank) to allow electricity to be provided during the
night or at times of poor sunlight levels. Stand-alone
systems are also often referred to as autonomous
systems since their operation is independent of other
power sources. By contrast, the grid-connected PV
system operates in parallel with the conventional
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
saturation current, which varies with temperature
according to
= − (3)
In (3), Tref is the cell reference temperature,
the reverse saturation current at Tref. and EG the
band-gap energy of the cell. The PV current iph
depends on the insolation level and the cell
temperature according to
= 0.01 + − (4)
In (4), iscr is the cell short-circuit current at
the reference temperature and radiation, Kv a
temperature coefficient, and the insolation level in
kW/m . The power delivered by the PV array is
calculated by multiplying both sides of (2) by vpv.
= − − 1 (5)
Substituting iph from (4) in (5), Ppv becomes
= 0.01 + −
− − 1 (6)
Based on (6), it is evident that the power
delivered by the PV array is a function of insolation
level at any given temperature.
Fig. 1. Simplifi ed one-line diagram of a BESS in parallel with
a Solar PV fa-cility connected to the grid.
III. BATTERY ENERGY STORAGE
A. Battery Energy Storage Basics
A grid-scale BESS consists of a battery bank,
control system, power electronics interface for ac - dc
power conversion, protective circuitry, and a
transformer to convert the BESS output to the
transmission or distribution system voltage level. The
one- line diagram of a simple BESS is shown in Fig.
1. A BESS is typically connected to the grid in
parallel with the source or loads it is providing
benefits to, whereas tradi-tional uninterruptible
power supplies (UPS) are installed in series with their
loads. The power conversion unit is typically a bi-
directional unit capable of four quadrant operation,
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.
The battery bank consists of many batteries
connected in a combination series-parallel
configuration to provide the desired power and
energy capabilities for the application. Units are
typically described with two numbers, the nameplate
power given in MW, and the maximum storage time
given in MWh. The BESS described in this paper is a
1.5/1 unit, means it stores 1 MWh of energy, and can
charge or discharge at a maximum power level of 1.5
MW. In renewable energy applications, it is common
to operate a BESS under what is known as partial
state of charge duty (PSOC), a practice that keeps the
batteries partially discharged at all times so that they
are capable of either absorbing from or discharging
power onto the grid as needed.
Most BESS control systems can be operated via
automatic generation control (AGC) signals much
like a conventional utility generation asset, or it can
be operated in a solar-coupled mode where real and
reactive power commands for the converter will be
generated many times per second based on real -time
PV output and power system data. In the case of the
XP -DPR, three -phase measurements from potential
and current transducers (PTs and CTs) are taken in
real-time on an FPGA device, and once digitized
these signals become the input for proprietary real
time control algorithms operating at kHz speeds.
Various control algorithms have been used for PV
applications, providing control of ramp rates,
frequency support.
Fig.2.Configuration of the grid-connected hybrid PV /Battery generation system
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B.Ramp Rate Control
Solar PV generation facilities have no
inertial components, and the generated power can
change very quickly when the sun becomes obscured
by passing cloud cover. On small power systems with
high penetrations of PV generation, this can cause
serious problems with power delivery, as traditional
thermal units struggle to maintain the balance of
power in the face of rapid changes. During solar -
coupled operation, the BESS must counteract quick
changes in output power to ensure that the facility
delivers ramp rates deemed acceptable to the system
operator. Allowable ramp rates are typically speci-
fied by the utility in kilowatts per minute (kW/min),
and are a common feature of new solar and wind
power purchase agree-ments between utilities and
independent power producers. Here the ramp rate
refers only to real power, and that the reactive power
capabilities of the BESS can be dispatched simultane-
ously and independently to achieve other power
system goals.
Ramp Rate Control algorithm used in the XP-DPR
continuously monitors the real power output of the
solar generator, and commands the unit to charge or
discharge such that the total power output to the
system is within the boundaries defined by the
requirements of the utility. The system ramp rate is
maintained to less than 50 kw/min, whereas the solar
resource alone had a maximum second-to- second
ramp rate of over 4 MW/min.
C. Frequency Response
Even with ramp- rate control, there are still going
to be occasional frequency deviations on the system.
On small, low-voltage systems, it is common to see
frequency deviations of 1–3 Hz from the nominal 50
or 60 Hz frequency. Frequency deviation has adverse
effects on many types of loads as well as other
generators. Frequency deviation is caused by a
mismatch in generation and load, as given by the
swing equation for a Thevenin equivalent power
source driving the grid. The system inertia is
typically described using a normalized inertia
constant called the H constant, defined as
=
ℎ

H can be estimated by the frequency response of the
system after a step-change such as a unit or load trip.
The equation can be re-written so that the system H is
easily calculated from the change in frequency of the
system after a generator of known size has tripped
off, according to
=
1
2 =
1
2
−∆
=
−∆
2
where the unit of H is seconds, is system angular
speed, is the system frequency, is the
remaining generation online after the unit trip, and
∆ is the size of the generator that has tripped.
When frequency crosses a certain threshold, it is
desirable to command the BESS to charge in the case
of over-frequency events, typically caused by loss of
load, or to discharge for under-frequency events,
which often result when a generator has tripped
offline. Using proportional control to deliver or
absorb power in support of the grid frequency
stabilization is referred to as droop response, and this
is common behavior in generator governors equipped
with a speed-droop or regulation characteristic.
Droop response in a governor is characterized as a
proportional controller with a gain of 1/R, with R
defined as
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Where is steady-state speed at no load, is
steady-state speed at full load, and is the nominal
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
Where is the grid frequency, is the
frequency dead band, and is the power
rating of the BESS in KVA.
A set of droop characteristic curves for a 1 MW
BESS is depicted in Fig. 3.
Fig. 3. Frequency droop response curves for 5% response on a
1 MW BESS.
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IV. 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
smoothing the volatile power output of a 1 MW solar
farm. Here the system ramp rate is maintained to less
than 50 kW/min, whereas the solar resource alone had a
maximum second-to- second ramp rate of over
4MW/min.
V. CONCLUSION
Integration of energy storage systems into
the grid to manage the real power variability of solar
by providing rate variation control can optimize the
benefits of solar PV. Using the BESS to provide
voltage stability through dynamic var support, and
frequency regulation via droop control response
reduces integration challenges associated solar PV.
Coupling solar PV and storage will drastically
increase reliability of the grid, enables more effective
grid management, and creates a dispatchable power
product from available resources. Battery energy
storage systems can also improve the economics of
distributed solar power generation by reduced need
for cycle traditional generation assets and increasing
asset utilization of existing utility generation by
allowing the coupled PV solar and BESS to provide
frequency and voltage regulation services.
VI. REFERENCES
[1] F. Katiraei and J. R. Aguero, “Solar PV
Integration Challenges,” IEEE Power Energy
Mag., vol. 9, no. 3, pp. 62–71, May/Jun. 2011.
[2] N. Miller, D. Manz, J. Roedel, P. Marken, and E.
Kronbeck, “Utility scale battery energy storage
systems,” in Proc. IEEE Power Energy Soc.
Gen. Meeting, Minneapolis, MN, Jul. 2010.
[3]C. Hill and D. Chen, “Development of a real-
time testing environment for battery energy
storage systems in renewable energy
applications,” in Proc. IEEE Power Energy Soc.
Gen. Meeting, Detroit, MI, Jul. 2011.
[4]A. Nourai and C. Schafer, “Changing the
electricity game,” IEEE Power Energy Mag.,
vol. 7, no. 4, pp. 42–47, Jul./Aug. 2009.
[5]R. H. Newnham, W. G. A Baldsing, and A.
Baldsing, “Advanced man-agement strategies for
remote-area power-supply systems,” J. Power
Sources, vol. 133, pp. 141–146, 2004.
[6]C. D. Parker and J. Garche, “Battery energy-
storage systems for power supply networks,” in
Valve-Regulated Lead Acid Batteries, D. A. J.
Rand, P. T. Mosely, J. Garche, and C. D. Parker,
Eds. , Amsterdam, The Netherlands: Elsevier,
2004, pp. 295–326.
[7] N. W. Miller, R. S. Zrebiec, R. W. Delmerico,
and G. Hunt, “Design and commissioning of a 5
MVA, 2.5 MWh battery energy storage,” in
Proc. 1996 IEEE Power Eng. Soc. Transm.
Distrib. Conf., pp. 339–345.
[8] “Analysis of a valve-regulated lead-acid battery
operating in utility en-ergy storage system for
more than a decade,” 2009.
[9] A. Nourai, R. Sastry, and T. Walker, “A vision &
strategy for deploy-ment of energy storage in
electric utilities,” in Proc. IEEE Power En-ergy
Soc. Gen. Meet., Minneapolis, MN, Jul. 2010.
[10]P. Kundur, Power System Stability and Control.
New York: Mc-Graw-Hill, 1994, pp. 589–594.
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Iaetsd integration of distributed solar power generation

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