Post on 23-Aug-2020
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CHAPTER 5
METHODOLOGY
Modelling is a very important part of any engineering practice. Nowadays with
the use of computers and powerful software extremely complex systems can be
simulated and their performance can be predicted and monitored. This chapter
deals with the virtual real world environment of power system through simulation
studies. A brief detail about MATLAB and SIMULINK is discussed in this chapter.
SimPowerSystems Library blocks and powerlib tools are used for modeling the system
for analysis purpose as the study is on power systems. Modeling of wind and PV has
been explained in detail using the tool. This chapter also presents the details of the
wind farm of the study area.
5.1 Methodology Adopted for the Study
The study area for the research considered is Nittur wind farm, Hassan, Karnataka.
After visiting the wind farm some of the important observations were identified which
are mentioned as follows.
1. The farm is presently using capacitor banks for reactive power support with the
help of auto mechanical switches for maintenance of grid code.
2. The farm has to shut down when ever fault occurs at PCC to avoid voltage
collapse causing heavy loss to the utility.
3. Due to the intermittent nature of wind the continuous switching operation
occurrence induces harmonics in the system and wear and tear of the switches
leads to frequent replacement of switches.
4. As the wind is seasonal, the grid aiding with this renewable DG is not reliable.
With these observations lead the way to consider this as challenging study to give
solutions for the issues and associated problems were formulated. After literature
survey, it was found that STATCOM which belongs to FACTS family is the better
option. The study would suggest the possibility of hybrid generation around the free
land area of Nittur wind farm as the area is marshy land and belongs to forest
department and felt suitable for installation of PV plant.
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Based on the problem formulation, the study was able to identify the objectives to
carry out this research work. To execute the defined objectives the study concentrated
to work on the tools of MATLAB/SIMULINK for simulation studies. Then the
knowledge of this simulation study was implemented to IEEE test system. The results
obtained after modelling and simulation of test system, gave a broad way to work on
the modelling and simulation of the real system. The results of the simulation studies
carried out for both test system and real system are discussed in the next chapter.
Using a practical load profile, real weather data and availability of land area of about
100 acres around the wind farm located near Nittur, Hassan, Karnataka, total capacity
of PV plant that could be installed is proposed. The total capacity was estimated
using the information on installed PV plants in Karnataka.
Simulation study of PV cell, PV module and PV array were carried out to evaluate the
number of arrays that are required to generate the possible estimated PV power plant
capacity. The overall cost of PV system and PV plant capacity are estimated. This
additional renewable energy system, which will compliment the facility well,
showcasing the latest in energy production technology with the existing wind plant,
will provide a reliable power supply. The following section discusses the features of
MATLAB/SIMULINK used as tool for simulation studies.
5.2 MATLAB
MATLAB is a numerical computation and simulation tool that was developed into a
commercial tool with a user friendly interface from the numerical function libraries
LINPACK and EISPACK, which were originally written in the FORTRAN
programming language. MATLAB essentially involves only a single data structure,
upon which all its operations are based. This is the numerical field, or, in other words,
the matrix. This is reflected in the name: MATLAB is an abbreviation for MATrix
LABoratory.
The major advantage of MATLAB is the interaction with the special toolbox
SIMULINK. This is a tool for constructing simulation programs based on a graphical
interface. The simulation runs under MATLAB and an easy interconnection between
MATLAB and SIMULINK is ensured.
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5.3 SIMULINK
SIMULINK is advanced software which is increasingly being used as a basic building
block in many areas of research. As such, it holds a great potential in power system
example to demonstrate the features and scope of SIMULINK based model for
transient stability analysis.
SIMULINK is an interactive environment for modelling, analyzing and simulating a
wide variety of dynamic systems. The complete system can be illustrated in terms of
SIMULUNK blocks in a single integral model. One of the most important features of
SIMULINK is that it is being interactive, which is proved by the display of signals at
each and every terminal. A parameter within any block can be controlled from a
MATLAB command line or through an m-file program. This is used for the transient
stability study since the power system configuration differs before, after and during
the fault. Loading conditions and control measures can also be implemented
accordingly.
5.3.1 Simulation and Model Design
SIMULINK is a block diagram environment for multi domain simulation and Model-
Based Design. It supports system-level design, simulation, automatic code generation,
continuous test and verification of embedded systems. SIMULINK provides a
graphical editor, customizable block libraries, and solvers for modelling and
simulating dynamic systems. It is integrated with MATLAB, enabling to incorporate
MATLAB algorithms into models and export simulation results to MATLAB for
further analysis. Key Features are as follows
Graphical editor for building and managing hierarchical block diagrams
Libraries of predefined blocks for modelling continuous-time and discrete-
time systems
Simulation engine with fixed-step and variable-step Ordinary Differential
Equations (ODE) solvers
Scopes and data displays for viewing simulation results
Project and data management tools for managing model files and data.
Model analysis tools for refining model architecture and increasing simulation
speed
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MATLAB Function block for importing MATLAB algorithms into models.
Legacy Code Tool for importing C and C++ code into models
SIMULINK turns the computer into a laboratory for modelling and analyzing systems
that would not be possible for practical otherwise. SIMULINK provides with the tools
to model and simulate almost any real-world problem. It provides a graphical user
interface (GUI) for building models as block diagrams.
SIMULINK also includes a comprehensive block library of sinks, sources, linear and
nonlinear components, and connectors. If these blocks do not meet the needs,
however, there is a possibility to create user‟s own blocks. The interactive graphical
environment simplifies the modelling process, eliminating the need to formulate
differential and difference equations in a language or program.
Models are hierarchical, so user can build models using both top-down and bottom-up
approaches. User can view the system at a high level, and then double-click blocks to
see increasing levels of model detail. This approach provides insight into how a model
is organized and how its various parts interact.
5.3.2 Tool for Simulation and Analysis
After defining a model, user can simulate its dynamic behaviour using a choice of
mathematical integration methods, either from the SIMULINK menus or by entering
commands in the MATLAB command window. The menus are convenient for
interactive work, while the command line is useful for running a batch of simulations.
Using scopes and other display blocks, user can see the simulation results while the
simulation runs. User can then change parameters and see what happens for “what if”
exploration.
SIMULINK software is tightly integrated with the MATLAB environment. It requires
MATLAB to run, depending on it to define and evaluate model and block parameters.
SIMULINK can also use many MATLAB features. Because MATLAB and
SIMULINK are integrated, user can simulate, analyze, and revise their models in
either environment at any point.
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User needs MATLAB running before they can open the SIMULINK Library Browser.
The following procedure is required to open SIMULINK library.
Start MATLAB, and then in the MATLAB Command Window, enter
SIMULINK.
The SIMULINK Library Browser opens.
User can also open the SIMULINK Library Browser from the MATLAB
tool strip, by clicking the SIMULINK Library button.
If user has not already loaded SIMULINK, a short delay occurs while it
loads.
Once the above procedure is carried out General SIMULINK Library Browser
window opens which is shown in Figure 5.1.
Figure 5.1 General SIMULINK Library Browser window
To create a new SIMULINK Model, from the SIMULINK Library browser, the steps
involved are as follows:
From the SIMULINK Library Browser menu, select File> New> Model.
An empty model opens in the SIMULINK Editor. In the SIMULINK Editor,
select File > Save.
In the Save As dialog box, enter a name of the model, and then click Save.
SIMULINK saves the model.
Figure 5.2 shows the new SIMULINK model named untitled, which has to be saved
according to the users choice.
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The SIMULINK Library Browser displays the block libraries installed on the
computer. User can start to build models by copying blocks from a library into a
SIMULINK Editor Model window. For example, in the Library Browser shown in
Figure 5.3 the Sine Wave block is selected. Similarly other blocks according to the
model need to be created are selected.
Figure 5.2 New SIMULINK Model
Figure 5.3 Sine wave block from library
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Once all the blocks are selected and the model is developed then last step is to
simulate the model. Before simulating a model, simulation options are required to
setup. For this specific options, such as the stop time and solver, using the model
configuration parameters dialog box as shown in Figure 5.4. For example, in the Stop
time field, user can enter as 20 and in the Max step size field as 0.2. Then once
clicked OK the SIMULINK software updates the parameter values with changes and
close the configuration parameters dialog box which is shown as in Figure 5.4.
Figure 5.4 Configuration Parameters Window
After entering relevant configuration parameter, model is ready to simulate the simple
model and visualize the simulation results. The simple procedure is as follows:
In the SIMULINK Editor, select Simulation > Start. The simulation runs. The
simulation stops when it reaches the stop time specified in the configuration
parameters dialog box. Alternatively, controlling the simulation is possible by
clicking the Start simulation button and Pause simulation button on the
SIMULINK Editor toolbar.
Double-click the Scope block. The Scope window opens and displays the
simulation results.
From the Scope block toolbar, click the Parameters button. Select the Style
tab. The Scope Parameters dialog box displays Figure editing options.
Change the appearance of the Figure. For example, select white for the Figure
color and Axes background color. To see the changes, click Apply.
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Figure 5.5 shows the window of option for style changes of scope parameters. The
scope block displays its input with respect to simulation time and it also display
signals generated during simulation.
Figure 5.5 Style changes of scope parameters
The Scope block can have multiple axes (one per port) and all axes have a common
time range with independent y-axes. The scope block allows the user to adjust the
amount of time and the range of input values displayed. User can move and resize the
Scope window and can modify the scope's parameter values during the simulation.
If the signal is continuous, the scope produces a point-to-point plot. If the signal is
discrete, the scope produces a stair-step plot. Figure 5.6 shows the scope and its
various parameters.
Figure 5.6 Scope Parameters
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5.4 SimPowerSystems
SimPowerSystems software and other products of the physical modeling product
family work together with SIMULINK software to model electrical, mechanical, and
control systems. SimPowerSystems software operates in the SIMULINK
environment.
SimPowerSystems provides component libraries for modelling and simulating
electrical power systems. It includes models of three-phase machines, electric
drives, FACTS, and wind power generators. Abstracted models of power
electronics components are also included, enabling to assess the impact of
switching events on system-level behaviour. These components are used to model
the generation, transmission, distribution, and consumption of electrical power.
SimPowerSystems models can be discretized to speed up simulations and
configured for phasor simulation, which helps t o determine the transient stability
of electrical power systems. Key features of SimPowerSystems are:
Application-specific models, including common AC and DC electric drives,
flexible AC transmission systems and wind-power generators
Discretization and phasor simulation models for fast model execution
Ideal switching algorithm for fast simulation of power electronic devices
Functions for obtaining equivalent state-space representations of circuits
Tools for computing load flow and for initializing models of three-phase
networks with machines
Frequency domain analysis methods, including FFT and harmonics
Demonstration models of key electrical technologies
5.4.1 Role of Simulation in Design
SimPowerSystems software is a modern design tool that allows scientists and
engineers to rapidly and easily build models that simulate power systems. It uses the
SIMULINK environment, allowing the user to build a model using simple click and
drag procedures. Not only user can draw the circuit topology rapidly, but analysis of
the circuit can include its interactions with mechanical, thermal, control, and other
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disciplines. This is possible because all the electrical parts of the simulation interact
with the extensive SIMULINK modelling library. Since SIMULINK uses the
MATLAB® computational engine, designers can also use MATLAB toolboxes and
SIMULINK blocksets. SimPowerSystems software belongs to the physical modelling
product family and uses similar block and connection line interface.
5.4.2 SimPowerSystems Library
SimPowerSystems libraries contain models of typical power equipment such as
transformers, lines, machines and power electronics. These models are proven ones
coming from text books, and their validity is based on the experience of the Power
Systems Testing and Simulation Laboratory of Hydro-Québec, a large North
American utility located in Canada.
The SimPowerSystems main library, powerlib, organizes its blocks into libraries
according to their behaviour. The powerlib library window displays the block library
icons and names. Double-click a library icon to open the library and to access the
blocks. The main powerlib library window also contains the Powergui block that
opens a graphical user interface for the steady-state analysis of electrical circuits
which is shown in Figure 5.7.
Figure 5.7 Powerlib Library Window
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5.4.3 Block Parameter Configurations
Modelling of each component of power system which would be required for the
power system analysis and study using Simpowersystem library blocks explained
along with the corresponding block parameters configurations. The main components
of power system such as Bus, Transmission Line, Source, Wind Farm, STATCOM etc
have been discussed.
Bus: This block is used to measure three phase voltages and currents in a circuit. So
it can be used as BUS for power system studies. Figure 5.8 and 5.9 represents the Bus
and its parameter window respectively. In Figure 5.9 it is observed that all the details
that are required for measurement needed to fill up.
Figure 5.8 Bus Representation
Figure 5.9 Bus Parameters
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Transmission Lines: This block models a three phase transmission line with single
PI section. The model consists of one set of RL series elements connected between
input and output terminals and two sets of shunt capacitances lumped at both ends of
the line which is shown in Figure 5.10. In transmission line block parameter, all the
necessary details of the line are entered as shown in Figure 5.11.
Figure 5.10 PI Section Transmission Line
Figure 5.11 Transmission Line Block parameter
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Transformer: Figure 5.12 and 5.13 shows the two winding three phase transformer
and this block implements three single-phase transformers to form three phase
transformers. The block model shown is step down transformer from 120 KV to 25
KV. But according to the requirement of the model the ratings can be adjusted.
Figure 5.12 Step Down Three Phase Transformer
Figure 5.13 Three Phase Transformer Block Parameter
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Grounding Transformer: This block implements a three phase transformer by using
three single phase transformers. Here the winding connection is set to ʏn to access the
neutral point of the Y. Also click measurements to none. Grounding transformer
representation and parameter window is shown in Figure 5.14 and 5.15.
Figure 5.14 Grounding Transformer
Figure 5.15 Grounding Transformer block parameter
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Three Phase Mutual Inductance: Three phase mutual inductance representation is
shown in Figure 5.16. This block implements three phase impedance with mutual
coupling between phases. Self impedance and mutual impedances are set by entering
positive and zero sequence parameters and shown in Figure 5.17.
Figure 5.16 Three phase Mutual Inductance
Figure 5.17 Three phase Mutual Inductance Block parameter
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Three Phase Programmable Voltage Source: This block implements a three –phase
zero- impedance voltage source and it is shown in Figure 5.18. The neutral is
accessible via input 1 of the block. Time variation for amplitude, phase and frequency
of the fundamental can be pre programmed which is shown in Figure 5.19.
Figure 5.18 Three phase Programmable Voltage Source
Figure 5.19 Three phase Programmable Voltage Source block parameters
Wind Farm: The wind farm block shown in Figure 5.20 constitutes subsystem
wherein six 1.5 MW rating Wind Turbine Generators are modelled which is shown in
Figure 5.21. As shown in Figure 5.22 the turbine details such as output power, wind
speed and pitch angle can be assigned according to the designed model.
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Figure 5.20 Wind Farm block
Figure 5.21 Wind Turbine Generator
Figure 5.22 Block parameter of Wind Turbine Generator
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Three Phase Fault: Using this block a fault can be created wherein the user can
create either LG or LLG or three phase fault according to requirement of the study of
the system. Figure 5.23 shows its representation and Figure 5.24 where any type of
short circuit can be created to see the performance and effects on the system
Figure 5.23 Fault Representation block
Figure 5.24 Three phase fault block parameter window
STATCOM: This power Electronic device is of phasor type representation used for
controlling reactive power and is represented in Figure 5.25. The converter rating can
be changed in the block parameter depending upon the user‟s application which is
shown in Figure 5.26. It can be made connected to the circuit with the help of manual
switch to No trip position, in contrast can be disconnected at Trip position.
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Figure 5.25 STATCOM Block (Phasor type)
Figure 5.26 Block parameter of STATCOM
Various other components of power system that are required for modelling the power
system can be obtained from the library and blocks can be chosen as discussed above.
After the study of all the blocks that are required for the desired power system, next
step is to modelling the power system.
Powergui: The Powergui block is necessary for simulation of any SIMULINK model
containing SimPowerSystems blocks. It is used to store the equivalent SIMULINK
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circuit that represents the state-space equations of the model. Rules to be followed
when using this block in a model are as follows:
Always place the Powergui block at the top level of diagram for optimal
performance. However, user can place it anywhere inside subsystems for their
convenience; its functionality will not be affected.
There can be a maximum of one Powergui block per model
Must name the block powergui
Figure 5.27 shows the representation of Powergui block which is essential to include
in the model created for study purpose. Figure 5.28 shows various power system
analysis that can be performed from Powergui block parameter.
Figure 5.27 Powergui Block
Figure 5.28 Analysis tools from Powergui block parameter
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5.5 Wind System Modeling using MATLAB/SIMULINK
As discussed in previous sections usage of SimPowerSystems software for Modelling
requires MATLAB and SIMULINK.
In addition to SimPowerSystems software, the physical modelling product family
requires other products for modelling and simulating mechanical and electrical
systems. There are also a number of closely related tool boxes and other products
from the MathWorks that can be used with SimPowerSystems software.
The graphical user interface makes use of the SIMULINK functionality to
interconnect various electrical components.
The electrical components are grouped in a library called blocks and it is shown
Figure 5.29. Each component is represented by a special icon having one or several
inputs and outputs corresponding to the different terminals of the component.
Figure 5.29 Power system components library
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To build a required model, first step is to copy the blocks from the SIMULINK
Library Browser to the new model window as explained earlier. The power library
and the new model window are shown in Figure 5.30
Figure 5.30 Power system components library and new model window
The blocks that are needed to build the model on the new untitled file are to be
carried out. The step by step procedure is as follows:
Select the Sources library in the SIMULINK Library Browser.
Right click on the block to be used.
Select “open to untitled”.
Then the selected block will gets added to the „untitled‟ file as shown in Figure 5.31
and 5.32. Like this any block that need to be loaded, to create a model can be added
to the untitled file for simulation.
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Figure 5.31 Three phase π section line selection to add to new model in
untitled file
Figure 5.32 Loading of selected file to untitled file from the
simpowersystem library
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Similarly by following above steps required power system can be created
which includes the following components which were clearly explained in
previous subchapter.
Bus
Transmission line
Ground
Three-Phase Source
Load
Wind Farm
STATCOM
To show how the components are loaded to the untitled file, STATCOM block and
wind farm block are taken as example and are shown in Figure 5.33 and 5.34
respectively.
Figure 5.33 STATCOM loaded to untitled model
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Figure 5.34 Wind Farm loaded to Untitled new Model
After selecting the required power system components from the simpowersystem
library, desired model can be obtained. The modelling of a sample system is
modeled with the help of procedure explained which is shown in Figure 5.35.
Figure 5.35 Sample Power System Model
Once the power system is modelled next step is to simulate the system and if there are
no errors the results are observed from relevant scopes which are explained in the next
coming section.
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5.6 PV System Modelling with MATLAB/SIMULINK
PV module represents the fundamental power conversion unit of a PV generator
system. The output characteristics of PV module depends on the solar insolation, the
cell temperature and output voltage of module. Since PV module has nonlinear
characteristics, it is necessary to model it for the design and simulation of maximum
power point tracking (MPPT) for PV system applications.
However, the SimPowerSystem tool in Matlab/SIMULINK package offers wind
turbine models but no PV model to integrate with current electronics simulation
technology. Thus, it is difficult to simulate and analyze in the generic modelling of
PV power system. This motivates to develop a generalized model for PV cell, module
and array using Matlab/ SIMULINK.
This model of PV cell, PV characteristics module and PV array was based on a
MATLAB model from ECEN2060. The MATLAB window is shown in Figure 5.36.
Figure 5.36 MATLAB window
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The SIMULINK model of a PV cell is shown in Figure 5.37. The description of each
block of the PV cell model is shown in Figure 5.38. The P-V power and I-V
characteristics represents X-Y plot of its inputs in a MATLAB window and
workspace block to save the input as an N-dimensional array and are discussed in
results and discussion chapter. These PV cells are grouped together to obtain PV
modules. The SIMULINK model of PV module is shown in Figure 5.39.
Figure 5.37 PV Cell SIMULINK Model
Figure 5.38 Description of each block of the PV Cell Model
ECEN2060
PV cell characteristics
Vpv
PV
To WorkspaceProduct
PV power
1e-9*(exp(u/26e-3)-1)
PN-junction characteristic
1/1000
Insolation to
ISC current gain
1000
Insolation
I-V characteristic
ISC
Id
Ipv
Ipv
Ppv
Ppv
Vpv
Vpv
PV cell characteristic
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Figure 5.39 PV Module SIMULINK Model
Depending upon the requirement of PV system the modules are classified as current
input PV module as shown in Figure 5.40 and voltage input PV module as shown in
Figure 5.41. The current input module is well suited for the case when modules are
required to be connected in series and share the same current. Whereas the voltage
input module is suited for the case when modules are needed to be connected in
parallel and share the same voltage.
Figure 5.40 Current input PV Module
Figure 5.41 Voltage input PV Module
Based on the requirement data sheet in MATLAB/SIMULINK window is filled
accordingly. The parameters for the study are conFigured on the basis of ECEN2060
details and the window parameter is shown in Figure 5.42. The modules are finally
connected together to obtain PV array. Here six module output are added with
summer to show how the output can be increased as shown in Figure 5.43.
ECEN2060
PV Module Characteristics
Vpv
Vpv
Insolation
Ipv
Ppv
PV module (V)
PV1
PV power
Insolation
I-V characteristic
Vpv
Vpv
Ipv
PV Module characteristic
Characteristic
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Figure 5.42 Parameters to be configured in a PV Model block
Figure 5.43 PV Array Model
ECEN2060
6-module PV Array
XY power
XY V-I
PV
To Workspace
Product
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV6
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV5
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV4
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV3
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV2
Ipv
Insolation
Vpv
Ppv
PV module (I)
PV1
Ipv Ramp
1000
Insolation
Add
Ipv
Ipv
Vpv
Vpv
Ppv
Ppv
6 Module PV Array
characteristic
Characteristic
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5.7 Scope Details
After modelling, the simulation is carried out for analysing the system. If there exists
any error in the model, simulation terminates showing the related compilation errors
that needed to be corrected for the modelling. Simulation time for analysis can be
selected suitably. Finally to observe the results scopes are used as explained in
previous chapter.
For this study details like Bus voltage V, current I, real power P and reactive power Q
at PCC are required and hence they are measured and selected for modelling in the
scope. Figure 5.44 shows the bus scope and Figure 5.45 shows the measurement of all
the parameters of the bus. Similarly wind speed, real power, reactive power and pitch
angle measurements are required and hence scopes were modelled according to the
requirements which are shown in Figures 5.46 and 5.47. Finally from STATCOM t
he measurements such as voltage and reactive power are necessary for the study and
hence scopes are modelled as shown in Figures 5.48 and 5.49.
Figure 5.44 Bus Scope details
Figure 5.45 Measurement of various parameters of the bus
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Figure 5.46 Wind Turbine Scope details
Figure 5.47 Measurement of various parameters of wind turbine
Figure 5.48 STATCOM Scope details
Figure 5.49 Measurement of various parameters of STATCOM