CHAPTER 3 MODELLING OF PV SOLAR FARM AS...
Transcript of CHAPTER 3 MODELLING OF PV SOLAR FARM AS...
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CHAPTER 3
MODELLING OF PV SOLAR FARM AS STATCOM
3.1 INTRODUCTION
Today, we are mostly dependent on non renewable energy that have
been and will continue to be a major cause of pollution and other
environmental degradation. Finding the sustainable alternative is becoming
increasingly urgent because of these problems and the dwindling supply of
petroleum. Perhaps, the greatest challenge is in devising a sustainable future,
which relies on integration and control of renewable energy sources in grid
distributed generation.
This chapter presents the modelling of current and voltage module of
PV arrays and their characteristics. The formation of PV array using VPV
module is also explained. Also the basics of STATCOM along with the
controller are also presented. The modelling of PV array as STATCOM along
with the MPPT algorithm for constant and variable insolation values are
carried out using MATLAB SIMULINK.
3.2 PV CELL
PV cell is very similar to that of the classical diode with a PN junction.
In figure 3.1, when the junction absorbs light, the energy of absorbed photons
is transferred to the electron–proton system of the material, creating charge
carriers that are separated at the junction. The charge carriers may be
electron–ion pairs in a liquid electrolyte or electron–hole pairs in a solid
semiconducting material. The charge carriers in the junction region create a
potential gradient, get accelerated under the electric field and circulate as
current through an external circuit. The square of the current multiplied by the
resistance of the circuit is the power converted into electricity. The remaining
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power of the photon elevates the temperature of the cell and dissipates into
the surroundings.
Figure 3.1 PV effect converts the photon energy into voltage across the
PN junction
3.2.1 PV CELL TECHNOLOGIES
In comparing alternative power generation technologies, the most
important measure is the energy cost per kilowatt hour delivered. In PV
power, this cost primarily depends on two parameters: the PV energy
conversion efficiency and the capital cost per watt capacity. Together, these
two parameters indicate the economic competitiveness of the PV electricity
[33].
The conversion efficiency of the PV cell is defined as follows
electrical power outputη=solar power impinging cell
The primary goals of PV cell research and development are to improve
the conversion efficiency and other performance parameters to reduce the cost
of commercial solar cells and modules. The secondary goal is to significantly
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improve manufacturing yields while reducing the energy consumption and
manufacturing costs and reducing the impurities and defects. This is achieved
by improving our fundamental understanding of the basic physics of PV cells.
The continuing development efforts to produce more efficient low-cost cells
have resulted in various types of PV technologies available in the market
today in terms of the conversion efficiency and the module cost.
Types of PV Cells
• Single crystalline silicon
• Polycrystalline and semi crystalline silicon
• Thin film cell
• Amorphous silicon
• Spherical cell
• Concentrator cell
• Multi junction cell
3.2.2 MODULE AND ARRAY
The solar cell described in the preceding subsection is the basic
building block of the PV power system. Typically, it is a few square inches in
size and produces about 1 W of power. To obtain high power, numerous such
cells are connected in series and parallel circuits on a panel (module) area of
several square feet. The solar array or panel is defined as a group of several
modules electrically connected in a series–parallel combination to generate
the required current and voltage as shown in figure 3.2 [34].
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Figure 3.2 PV cells, Module and Array
3.3 ELECTRICAL EQUIVALENT CIRCUIT
PV cell can be represented by the equivalent electrical circuit shown in
figure 3.3. [41]. The circuit parameters are as follows.
RS- Internal series resistance
RP - Shunt resistance of the diode
RL – Load Resistance
ISC – Source current
ID – Current through Diode
VD – Voltage across Diode
IPV– output current of PV cell
VPV – Output voltage of PV cell
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Figure 3.3 Equivalent circuit of a solar cell
Applying KCL,
(3.1)
The voltage of the cell VPV is given by the following by applying KVL,
(3.2)
The diode current is given by the expression
(3.3)
3.4 MODELLING OF PV ARRAY
The two models of PV module are current input (IPV module) and
voltage input module (VPV module). The IPV module is well suited for the
case when modules are connected in series and share the same current and
VPV module is well suited for the case when modules are connected in parallel
and share the same voltage.
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3.4.1 IPV Module
Figure 3.4 IPV Module
The MATLAB SIMULINK circuit of the IPV module is given by
Figure 3.5 Circuits inside the Current IPV Model
3.4.2 VPV Module
Figure 3.6 VPV Module
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Figure 3.7 Circuit inside the Voltage VPV Module
3.4.3 I-V AND P-V CURVES
The electrical characteristic of the PV cell is generally represented by
the current vs. voltage curve. The MATLAB SIMULINK Simulation diagram
for obtaining the electrical characteristics of PV cell is shown in figure 3.8.
Figure 3.9 shows the I-V characteristic of a PV module under different values
of Insolation. (200, 400, 600, 800, 1000 W/m2)
Figure 3.8 SIMULINK Circuit for PV Module characteristics
2Ppv
1Ipv
Ipv
Insolation
Vpv
Ppv
PV module (I)
f (z) zSolve
f(z) = 0
Algebraic Constraint2Insolation
1Vpv
Vpv
Vpv
Insolation
Ipv
Ppv
PV module (V)
PV1
PV power
Insolation
I-V characteristic
VpvVpv
Ipv
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Figure 3.9 Current vs. Voltage characteristic of the PV module for
different insolation levels
The power output of the panel is the product of the voltage and current
outputs. Figure 3.10, shows the P-V characteristic of a PV module under
different values of Insolation. (200, 400, 600, 800, 1000 W/m2)
Figure 3.10 Power vs. Voltage characteristic of the PV module
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3.5 MAXIMUM POWER POINT TRACKING
A controller that tracks the maximum power point locus of the PV
array is known as the Maximum Power Point Tracking (MPPT). The points of
maximum array power form a curve is termed as the maximum power locus.
Due to high cost of solar cells, it is necessary to operate the PV array at its
maximum power point.
Several MPPT algorithms have been proposed from time-to-time.
Some of the popular schemes are the hill climbing method, incremental
conductance method, constant voltage method, modified hill climbing
method, β method, system oscillation method and the ripple correlation
method, perturb and observe method, open and short circuit method, fuzzy
logic and artificial neural network [40].
3.5.1 Perturb and Observe Method
The perturb and observe method, also known as perturbation method,
which is the most commonly used MPPT algorithm in commercial PV
products. This is essentially a “trial and error” method. The PV controller
increases the reference for the inverter output power by a small amount and
then detects the actual output power. If the output power is indeed increased,
it will increase again until the output power starts to decrease, at which the
controller decreases the reference to avoid collapse of the PV output due to
the highly non-linear PV characteristic [39].
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Figure 3.11. Perturb and Observe Method flow chart
Figure 3.11 shows the flow chart of the P&O method. The present
power P(k) is calculated with the present values of PV voltage V(k) and
current I(k), and is compared with the previous power P(k-1). If the
incremented power increases, keep the next voltage change in the same
direction as the previous change. Otherwise, change the voltage in the
opposite direction as the previous one. [40]
3.5.2 Incremental Conductance Algorithm
In the incremental conductance method, the MPP is tracked by
matching the PV array impedance with the effective impedance of the
converter reflected across the array terminals. The latter is tuned by suitably
increasing or decreasing the value of ‘M’. [39].
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Figure 3.12. Incremental Conductance Algorithm flow chart
The main task of the incremental conductance algorithm is to find the
derivative of PV output power with respect to its output voltage, which is
dP/dV. The maximum PV output power can be achieved when its dP/dV
approaches zero. The controller calculates dP/dV based on measured PV
incremental output power and voltage. If dP/dV is not close zero, the
controller will adjust the PV voltage step by step until dP/dV approaches zero,
at which the PV array reaches its maximum output.
The main advantage of this algorithm over the P&O method is its fast
power tracking process. However, it has the disadvantage of possible output
instability due to the use of derivative algorithm. Also the differentiation
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process under low levels of insolation becomes difficult and results are
unsatisfactory. [42].
3.5.3 β method
The other method is based on β tracking which has the advantage of
both fast and accurate tracking. It is observed that the value of β remains
within a narrow band as the array operating point approaches the MPP.
Therefore by tracking β, the operating point can be quickly driven to close
proximity of the MPP using large iterative steps. Subsequently, small steps
(i.e. conventional MPPT techniques) can be employed to achieve the exact
MPP. Thus, β method approximates the MPP while conventional MPPT
technique is used to track the exact MPP. Flow chart for the β method
algorithm is given in figure 3.13. [42]
Figure 3.13. β method flow chart
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3.5.4 Open and Short circuit Method
The open and short circuit current method for MPPT control is based
on measured terminal voltage and current of PV arrays. By measuring the
open-circuit voltage or short-circuit current in real-time, the maximum power
point of the PV array can be estimated with the predefined PV current-voltage
curves. This method features a relatively fast response and do not cause
oscillations in steady state. However, this method cannot always produce the
maximum power available from PV arrays due to the use of the predefined
PV curves that often cannot effectively reflect the real-time situation due to
PV nonlinear characteristics and weather conditions. Also, the online
measurement of open-circuit voltage or short-circuit current causes a
reduction in output. [40]
3.5.5 Fuzzy Logic and Other Algorithms
Since the PV array exhibits a non-linear current-voltage or power-
voltage characteristic, its maximum power point varies with the insolation and
temperature. Some algorithms such as fuzzy logic or artificial neural network
control with nonlinear and adaptive in nature fit the PV control. By
knowledge based fuzzy rules, fuzzy control can track maximum power point.
A neural network control operates like a black box model, requiring no
detailed information about the PV system. After learning relation between
maximum power point voltage and open circuit voltage or insolation and
temperature, the neural network control can track the maximum powerpoint
online. [40].
In the proposed MPPT algorithm, the following conditions are considered
• It is assumed that the Boost output voltage Vout= VDC is constant
• Iref is used as the control variable for the Boost DC-DC converter
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• PV array current ideally tracks the Boost input current reference: IPV=Iref
The Perturb and observe algorithm is applied in this method where the
value of Ipvi s adjusted to Iref to operate at MPP. The flowchart for the perturb
and observe method applied is given in figure 3.14.
Figure 3.14 Flowchart for Perturb and Observe Algorithm
3.5.6 MPP Tracking Operation
For a six module PV of 85 Watts each connected in series at full sun
the maximum power of 510.8 W is achieved as shown in the figure 3.15 and
the MPP tracking results are shown in the figure.3.16.
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Figure 3.15 MPP for Six 85W module connected in series
Figure 3.16 MPP tracking operation
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3.6 FORMATION OF PV ARRAY
The PV array is formed by connecting six IPV modules, in series as
shown in the figure 3.17.
Figure 3.17 Formation of PV Array
3.7 PRINCIPLE AND OPERATION OF STATCOM
STATCOM is a shunt-connected reactive-power compensation device
that is capable of generating and absorbing reactive power and in which the
output can be varied to control the specific parameters of an electric power
system. It is in general a solid-state switching converter capable of generating
or absorbing independently controllable real and reactive power at its output
terminals when it is fed from an energy source or energy-storage device at its
input terminals. Specifically, the STATCOM considered in this chapter is a
voltage-source converter that, from a given input of dc voltage, produces a set
of 3-phase ac-output voltages, each in phase with and coupled to the
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corresponding ac system voltage through a relatively small reactance (which
is provided by either an interface reactor or the leakage inductance of a
coupling transformer) [7].
To summarize, a STATCOM controller provides voltage support by
generating or absorbing reactive power at the point of common coupling
without the need of large external reactors or capacitor banks.
3.7.1 The Principle of Operation
A STATCOM is a controlled reactive-power source. It provides the
desired reactive-power generation and absorption entirely by means of
electronic processing of the voltage and current waveforms in a voltage-
source converter (VSC). A single-line STATCOM power circuit is shown in
figure. 3.18 (a), where a VSC is connected to a utility bus through magnetic
coupling. In figure3.18 (b), a STATCOM is seen as an adjustable voltage
source behind a reactance—meaning that capacitor banks and shunt reactors
are not needed for reactive-power generation and absorption, thereby giving a
STATCOM a compact design, or small footprint, as well as low noise and
low magnetic impact. The exchange of reactive power between the converter
and the ac system can be controlled by varying the amplitude of the 3-phase
output voltage, Es, of the converter, as illustrated in figure3.18 (c). That is, if
the amplitude of the output voltage is increased above that of the utility bus
voltage, Et, then a current flows through the reactance from the converter to
the ac system and the converter generates capacitive-reactive power for the ac
system. If the amplitude of the output voltage is decreased below the utility
bus voltage, then the current flows from the ac system to the converter and the
converter absorbs inductive-reactive power from the ac system. If the output
voltage equals the ac system voltage, the reactive-power exchange becomes
zero, in which case the STATCOM is said to be in a floating state. Adjusting
the phase shift between the converter-output voltage and the ac system
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voltage can similarly control real-power exchange between the converter and
the ac system.
Figure 3.18 The STATCOM principle diagram: (a) a power circuit;
(b) an equivalent circuit; and (c) a power exchange
The reactive- and real-power exchange between the STATCOM and
the ac system can be controlled independently of each other. Any combination
of real power generation or absorption with var generation or absorption is
achievable if the STATCOM is equipped with an energy-storage device of
suitable capacity, as depicted in figure 3.19. With this capability, extremely
effective control strategies for the modulation of reactive and real output
power can be devised to improve the transient and dynamic system stability
limits.
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A STATCOM can improve power-system performance in the following areas:
1. dynamic voltage control in transmission and distribution systems
2. power-oscillation damping in power-transmission systems
3. transient stability
4. voltage flicker control
5. control of not only reactive power but also (if needed) active power in
the connected line, requiring a dc energy source.
Figure 3.19 The power exchange between the STATCOM and the ac
system
Figure 3.20 An elementary 6-pulse VSC STATCOM
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An elementary 6-pulse VSC STATCOM is shown in figure 3.20,
consisting of six self-commutated semiconductor switches (IGBT, IGCT, or
GTO) with antiparallel diodes. In this converter configuration, IGBTs
constitute the switching devices. With a dc-voltage source (which may be a
charged capacitor), the converter can produce a balanced set of three quasi-
square voltage waveforms of a given frequency by connecting the dc source
sequentially to the three output terminals via the appropriate converter
switches.
3.8 CONVENTIONAL STATCOM AND SOLAR FARM PV ARRAY
BASED STATCOM
There has been many compensating devices performing reactive power
compensation, voltage regulation, etc. but device that has the structural
advantage is necessary for the system assumed. STATCOM proves to have
the structural advantage to act as the compensating device for the assumed
system. PV array and the inverter setup are analogous to the design of
conventional STATCOM. From figure 3.22 it is clear to understand how PV
array setup can be utilized as STATCOM.
Figure 3.21 conventional STATCOM VS PV based STATCOM
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A structural advantage that helps in utilization of PV array as
STATCOM is PV array output is dc voltage which is used as capacitor as in
conventional STATCOM. Also, inverter design is made to operate as
converter in the PV array arrangement. So PV array along with inverter is
been conveniently used as STATCOM for the assumed system.
3.9 MODES OF OPERATION OF PV BASED STATCOM
The operation of the proposed STATCOM has been divided into three
modes. The modes are (i) day time excess power mode, (ii) day time mode,
(iii) night time mode.
i. Day time excess power mode: In this mode, the output voltage of the
PV array drives the boost converter based STATCOM for
compensating the source as well as charges the battery.
ii. Day time mode: When continuous compensation is required, if the PV
output voltage is equal to the requirement of the boost converter input,
the PV array can directly connect to the boost converter so as to step-
up the voltage and match the dc link voltage of the three-leg VSC. In
this mode, the battery is not charged.
iii. Night time mode: In this mode, PV output is absent and only the
battery supplies the boost converter for providing compensation at the
night time.
3.10 CONTROL OF DC LINK VOLTAGE WITH BOOST
CONVERTER
The boost converter is used to step up the input voltage to obtain a
desired output voltage. The circuit operation is divided into two modes. In
mode 1, when the switch is in on condition, the input current supplies energy
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to the inductor for a period Ton. Similarly in mode 2, when the switch is off,
the inductor voltage adds to the source voltage and current is forced to flow
through diode and the load for a period Toff. The PV or battery voltage is fed
to the boost converter and the output voltage of the boost converter is
obtained to maintain the dc link voltage of the three-leg voltage source
converter. The output voltage, Vout is greater than the input Voltage Vin and
the output equation is shown in the following equation.
(3.4)
Where Vout=Vdc, Vin=V
(3.5)
where V is the PV or battery voltage, D is the duty cycle, Ton is the ON time
and Toff is the OFF time.
3.11 CONTROL OF PV BASED STATCOM
There are many control algorithms available for the generation of
reference source currents for the control of proposed STATCOM in the
literature such as, synchronous reference frame theory, instantaneous reactive
power theory (p–q theory), power balance theory etc.[44],[47],[71]. The
synchronous reference frame theory is found suitable for the control of VSC.
A block diagram of the controlling algorithm is shown in figure 3.22. The
feedback signals are sensed from the load currents, PCC voltages and dc bus
voltages of STATCOM. The load currents from the a–b–c frame are first
converted to a–b–0 frame and then to d–q–0 frame using the following
equation,
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(3.6)
A three phase PLL (phase locked loop) is used to synchronize these
signals with the PCC voltage. The dc component of id and iq are obtained by
passing a d–q–0 current component through the low pass filter. The input of
first PI (Proportional Integral) controller is the error between the reference dc
bus voltage (Vdc*) and the sensed dc bus voltage (Vdc) of STATCOM. The
output of PI controller is the loss component of the current (iloss).
(3.7)
where Vde(n) is the error between reference and sensed dc voltage at the nth
sampling instant. Kpd and Kid are the proportional and integral gains of the
DC bus voltage PI controller. Therefore the reference source current is,
(3.8)
Similarly, the amplitude of actual PCC voltage and its reference value
are fed to another PI controller for regulating the PCC voltage. The output of
the PI controller is added to the dc component of iq because this is a
quadrature component of current required for regulating the ac voltage.
(3.9)
where Vde(n) is the error between reference (Vs*) and sensed supply voltage
(Vs(n)) amplitude at the nth sampling instant. The proportional and integral
gains of the PCC voltage PI controller are Kpq and Kiq.
The reference supply quadrature axis current is,
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(3.10)
By using reverse Park’s transformation, the resultant d–q–0 currents
are again converted back to reference source currents. The reference currents
in all the three phases (isa*, isb*,isc*) are used for generating the gate pulses
for three-leg VSC based STACOM. A PWM current controller is used for
generating the gating signals for the IGBT’s in VSC by using the reference
and sensed source currents.
Figure 3.22 Control Algorithm of STATCOM
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3.12 SIMULATION OF PV ARRAY SYSTEM
In this case, the model parameters are the standard PV module data-
sheet parameters:
• short-circuit current Isc
• open-circuit voltage Voc
• rated current IR at maximum power point (MPP)
• rated voltage VR at MPP
• Under standard test conditions (1kW/m2, 1.5 AM, 25oC). A bypass
diode (a single diode across the entire module) can be included.
Temperature effects are not modelled. PV array consisting of 6 PV
modules connected in series.
• PV array is operated at the maximum power point (MPP) under all
conditions. Vpv, Ipv is set as operating point for MPP. 6-module (85 W
each) PV array with full sun (1,000 W/m2 insolation).
• PV array operates at MPP: Ppv = 6 * 85 W = 510 W.
• The DC output is then fed to boost converter and then to DC-AC
converter for obtaining the required AC output voltage Vinv
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3.12.1 PV Array Simulation for Constant Insolation
Figure 3.23 PV array system combined with DC-DC boost convertor and
DC-AC Inverter for constant Insolation
Figure 3.24 Simulink Block of Constant insolation
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The PV array combined with DC-DC boost convertor and DC-AC
inverter for a constant insolation value of 1000 kW/m2 is simulated and the
output voltage, output current, input and output power of inverter is obtained.
The output voltage from the inverter gives a constant value of 158 W
for a constant insolation as shown in the figure 3.25.
Figure 3.25 Output Voltage of Inverter for constant insolation
Figure 3.26 Output current from inverter for constant insolation
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The output current is also a constant value of 5.8 A for a constant
insolation value as shown in the figure 3.26. Similarly the various outputs
such as Inverter duty and input and output power of a PV array is also a
constant value for the constant insolation as shown in figure 3.27 and 3.28
respectively.
Figure 3.27 Inverter Duty cycle for constant insolation
Figure 3.28 Input and Output power for constant insolation
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3.13.2 PV Array Simulation for Variable Insolation
Figure 3.29 PV array system combined with DC-DC boost convertor and
DC-AC Inverter for variable Insolation
The PV array combined with DC-DC boost convertor and DC-AC
inverter for a variable insolation value as shown in figure 3.29 is simulated
and the output voltage, output current, input and output power of inverter is
obtained.
The insolation values chosen here are 0, 400, 850, 950, 1000, 950, 850,
400, 0 kW/m2 for a time duration 0, 60, 120, 180, 240, 300, 360, 420, 480
seconds respectively as shown in figure 3.30.
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The simulation outputs of voltage, current, input and output power is
shown in the figure 3.31, 3.32, 3.33, 3.34 and 3.35 respectively. The values of
all the parameters are varying with respect to the insolation values.
Figure 3.30 Simulink block for variable insolation
Figure 3.31Output Voltage of Inverter for Variable insolation
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Figure 3.32Output current of Inverter for Variable insolation
Figure 3.33 Efficiency of Inverter for Variable insolation
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Figure 3.34 Duty cycle of Inverter for Variable insolation
Figure 3.35 Ideal, input and output power of Inverter for
Variable insolation
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Thus the simulation results shows that the PV array responds to the
different insoltaion values by giving the corresponding variations in the
outputs. Hence this model can be used for different insolation values.
3.13 SUMMARY
A new concept of using a PV solar power plant as STATCOM is
introduced here. A MATLAB/SIMULINK based model of PV array is
discussed in this chapter. The Utilization of the PV array and the converter as
a STATCOM device is explained with simulations. Various MPPT algorithms
are introduced and perturb and observe method is utilized. The simulation
results show the various outputs of PV array for different insolation levels.
This newly developed system thus can act as a FACTS device providing a
flexible control over both active and reactive power on a transmission line.
The PV based STATCOM can be implemented during night hours when PV
solar plant produces no real power. The configuration can possibly be realized
during daytime hours too. The PV based STATCOM can be used to regulate
the transmission/distribution line voltages, to support inductive load VAR
requirements, to improve the system performance during dynamic
disturbances and to suppress harmonics.