Hands-on-OpenIPSL.org using OpenModelica!

CURENT Tutorial
The University of Tennessee, Knoxville, August 22nd 2017
Hands-on-OpenIPSL.org using OpenModelica!
Prof. Luigi Vanfretti
Rensselaer Polytechnic Institute, ECSE, Troy, NY
Web: ALSETLab.com
Email: vanfrl@rpi.edu luigi.vanfretti@gmail.com

Outline
• Startup checklist and set-up
• Intended Learning Outcomes for this Tutorial
• OpenModelica Environment Overview
• OpenIPSL Overview
• Tutorial Package Description
• Example 1: SMIB model implementation
and stability analysis.
o Using OMEdit
o Using OMNotebook
o Using CLI
• Example 2: Stabilizer implementation
• Example 3: IEEE-9 Bus Test System Study
2

Startup Checklist!
Did you get OpenModelica installed? (Y/N)
If “No”, get a copy from one of my USB sticks (It takes too long to
download ~1GB installer, yikes!)
Where you able to simulate the “PID_Controller” example? (Y/N)
If “No” you have an installation problem with OpenModelica.
 Find a partner to work with!
Did you get a copy of OpenIPSL? (Y/N) If “No”:
 Copy the .zip file “OpenIPSL-Tutorial_***.zip” from one of my USB
sticks, or (download about 70 MB…)
 Get it online from:
https://github.com/OpenIPSL/OpenIPSL/releases/tag/Tutorial_NAMUG_2017
3

Intended Learning Outcomes
• To provide a brief introduction to the OpenIPSL giving
you a basic understanding of it’s uses for power system
simulation.
• To familiarize with the OpenModelica environment
• To implement a basic power system model using
OpenIPSL, to simulate it, and to analyze it using
different methods available in the OpenModelica
environment.
• Milestones!
• Complete, at least 1 out of 3 examples, using OMEdit.
• Perform linear analysis on Example 1, using OMNotebook.
4

Setup: Let’s Double Check! (1/2)
• External libraries, such as OpenIPSL,
must be loaded in OMEdit to be used:
• Unzip OpenIPSL-Tutorial_***.zip
• Open OMEdit (Click the gears icon)
• Go to Open Model/Library File(s)
• Browse to the location of the unzipped folder
• Go to the ./OpenIPSL-
Tutorial_***/OpenIPSL folder, and select
package.mo
• Alternative: Drag & Drop
• Drag & drop the package.mo file to the
Library Browser in OMEdit.
5

Setup: Let’s Double Check! (2/2)
• Check the OpenIPSL is loaded (see
previous slide)
• Go to Open Model/Library File(s)
• Browse to the location of the unzipped folder
• Go to the /Application
Examples/_Tutorial/Tutorial folder, and
select package.mo
• Alternative: Drag & Drop
• Drag & drop the package.mo file to the
Library Browser in OMEdit.
6
Let’s do this!

See more here:
https://www.openmodelica.org/openmodelicaworld/tools
The OpenModelica Environment
• OpenModelica is an entire
ecosystem, which at its core has
the OpenModelica Compiler
(OMC).
• There are many ways to interact
with the OMC, and in this tutorial,
we will use some of them, mainly:
• OMEdit
• OMNotebook
• OMShell
7

OpenIPSL Overview
• The library is divided in the following categories:
o Electrical: power system components
o Non-electrical: functions used by electrical
above.
o Interfaces: electrical coupling blocks for
sources/sinks to other power components.
o Types: defines units as kV, MVA…
o Examples: … well, contains “kind of” examples.
 These are actually small network models used for unit
testing of each component.
 Used for library dev. and debugging.
8
Under ./ApplicationExamples/
you can find synthetic and actual
Power System Models’ built using
the library.

./OpenIPSL/Electrical
• Electrical
• The Electrical package contains most of the
components that comprise an actual power
network
• E.g., electrical machines, transmission lines,
loads, excitation systems, turbine+governors,
etc.
• These are used to build the power system
network models
9

./OpenIPSL/NonElectrical
• NonElectrical
• The NonElectrical package is comprised by
functions, blocks or models, which are used to build
the aforementioned power system component
models:
• Transfer functions, logical operators, etc.
• They perform specific operations which were not
available in the Modelica Standard Library (MSL)
• Necessary to replicate the behavior of proprietary
tools for basic functionalities, e.g. integrators with
limiters…, all the way to complex functions
(generator saturation model).
10

./OpenIPSL/Interfaces
• Interfaces
• The Interfaces package contains a set of
specifically developed Modelica connectors to
harmonize the models in this library
• The most important is PwPin a connector, which
contains voltage and current quantities in phasor
representation (real and imaginary components of
the complex number).
• A container to build up a “source” or “sink” sub-
system (e.g. a power plant with multiple machines,
etc.) is also included.
11

./ApplicationExamples/_Tutorial/Tutorial/package.mo
• Tutorial
• In this workshop, the Tutorial package will
be used to illustrate basic use examples of
the library
• The package Working_Examples and
corresponding sub-packages will be used
by YOU! to build the models step-by-step.
• In the packages Example_1, Example_2
and Example_3 all steps to build the
models are provided.
• The final ”answer” (i.e. model) is shown with
a “Play” icon.
12

Example 1 - Origins
• This example was originally
presented in the reference
book:
o P. Kundur, “Power System
Stability and Control”, McGraw-
Hill Inc., Palo Alto, California,
1994. See: Example 13.2, pp.
864 – 869.
o IT is NOT exactly the same
as in the book; but can
reproduce the same
phenomena.
13

Example 1 - Origins
• The model is used in senior/graduate courses for analysis of the so-called
“transient stability” and “small-signal stability” (linearized analysis) of the
system including the effects of rotor circuit dynamics and the excitation
control system.
• It aims to represent a power plant comprised represented by one generating
unit (with all it’s control systems) connected via transformer and parallel lines
to an “infinite bus”.
o Hence the name Single Machine Infinite Bus (SMIB) system
o In the French-speaking world they also call it OMIB (One-Machine …)
14
This phenomena observed in early days of
“interconnections” (circa 1960’s), and first
formally explained by de Mello and Concordia
in 1969.
F. P. Dmello and C. Concordia, “Concepts of
synchronous machine stability as affected by
excitation control,” IEEE Trans. Power App.
Syst., vol. PAS-88, pp. 316–329, 1969.

Example 1:
Gathering Parameter Data and the Power Flow Results
To simplify things, we use results from a simulation tool called
PSAT developed by Prof. Federico Milano, a great friend and
exceptional scientist!
• If you are interested in PSAT here is Prof. Milano’s page:
http://faraday1.ucd.ie
• You don’t need to do anything right now!
o The model of Example 1 exists as an example in PSAT and can be
used for power flow calculations and dynamic simulations.
o The parameter data used in the next slides, is that from PSAT’s model
implementation.
o Power flow results were obtained by PSAT
o Reference time-series for comparison with OpenModelica results were
also obtained.
15

Example 1 – Power Flow Calculations and Output
• Example 1 is loaded and the power flow calculations are executed
• The “Static Report” provides all power flow results, and also includes the
initial values of various state variables of the models
16

Example 1 – Power Flow Summary
• The summary of all of the relevant data from the power
flow is given on the figure below
17

Example 1 – Let’s Start Implementing
• Our goal is to build the model in
./Tutorial/Example_1/Example1 step-by-step
18

Example 1 – Generating Plant Model
• First, the package where the generator model will be
located has to be created.
• This is done by right clicking on the Example_1 in the
Working_Examples package
• The package should be named Generator
19
See solution in: ./Tutorial/Example_1/Generator/Step_1

Example 1 – Generator Model “Extends”
• Within the Generator sub-package all components related
to the machine will be included within.
• Extends from OpenIPSL.Interfaces.Generator
20
Fill in: “Generator”
Click
Select
Right
Click
Click

Example 1 – Synchronous Machine and Parameters
• We will use the 6th order model from PSAT
./OpenIPSL/Electrical/Machines/PSAT/Order6
• Do this by dragging the Order6 model from the library
and dropping it to the Generator sub-package that we
just created.
• Double click on the machine
• Parametrize it using
the values in this table 
21
Drag and Drop!
𝑆 𝑛 2220 𝑥′′ 𝑞 0.25
𝑉𝑛 400 𝑇′ 𝑑,0 8
𝑟𝑎 0.003 𝑇′ 𝑞,0 1
𝑥 𝑑 1.81 𝑇′′ 𝑞,0 0.03
𝑥 𝑞 1.76 𝑇′′ 𝑑,0 0.07
𝑥′ 𝑑 0.3 𝑇𝑎𝑎 0.002
𝑥′ 𝑞 0.65 𝑀 7
𝑥′′ 𝑑 0.23 𝐷 0
Fillin

Example 1 – Synchronous Mach. Power Flow Input
• Although we already know the specific values from the power flow
that we need to enter, we would like to be able to enter all of the
data of the Generating Plant (Generator) in a single window.
• Using the variables (V_0, angle_0, etc.) allows to propagate
the parameters to the “upper layer” of the generator component
22
𝑉0 V_0
𝑎𝑛𝑔𝑙𝑒0 angle_0
𝑃0 P_0
𝑄0 Q_0
𝑉𝑏 V_b
𝑆 𝑏 Do not edit
𝑓𝑛 Do not edit
Fill in

Example 1 – Excitation Control System
• The AC voltage at the terminals of a
generator (stator) is controlled by varying
the DC voltage of the field winding (rotor).
• Here, we model the so-called “Automatic
Voltage Regulator” using PSAT’s AVR
Type III model, and parametrize it with the
values in the table.
• All control systems need a set-point,
however, the value of the reference
depends on the measured voltage and
other variables.
• In this case, the set-point, is computed
during initialization using the initial
equation construct.
• A constant block from the MSL pss_off
will be used as a zero.
23
𝑣 𝑓,𝑚𝑎𝑥 7
𝑣 𝑓,𝑚𝑖𝑛 -6.4
𝐾0 200
𝑇2 1
𝑇1 1
𝑇𝑒 0.0001
𝑇𝑟 0.015
Fill in

Example 1 – Interfacing all components
• To finish the generator model,
different signals need to be
connected
• Optionally, the “icon”
generator model can be
altered from the “Icon View”
for visual appearance.
24
1. Machine's terminal voltage to AVR's input signal
2. AVR's output field voltage to machine's input field
voltage
3. Initially calculated mechanical power to input signal
of the machine's mechanical power
4. Constant pss_off to the PSS input at the AVR
5. Initial generator field voltage to initial AVR field
voltage
6. Generator pin to External pin
1
2
3
6
4
5

Example 1 – Power Network Package
• We now create the Network sub-package within:
• Example_1 by right clicking on the Example_1 in the
Working_Examples package
25
See solution in: ./Tutorial/Example_1/Network/Step_1
Fill in: “Network”

Example 1 – Power Network Model
• The power network model will be implemented in the
Network package we just created
• Right clicking on the Network package
• The name of the network model will be “Example_1”
26
Fill in: “Example_1”

Example 1 – Power Network Components
• Drag and drop the generator model that we created (name it G1) and 3
bus models (name them B1, B2, and B3)
• We can now propagate common parameters used by all models.
• Drag and drop OpenIPSL.Electrical.SystemBase block
• It defines base capacity (MVA) and frequency (fn)
parameters for all of the components in the network model
• In text view add the inner keyword in front of the component declaration
27
𝑆 𝑏 100
𝑓𝑛 60

• Add the following transmission lines and transformer models
• And parametrize them:
Example 1 – Power Network Branches
28
2017-08-23
Transformer
𝑅 0.0 𝐺 0.0
𝑋 0.5*100/2220 𝐵 0.0
𝑆 𝑏 Don’t edit
Line 1
𝑅 0.0 𝐺 0.0
𝑋 0.93*100/222
0
𝐵 0.0
𝑆 𝑏 Don’t edit
Line 2𝑆 𝑏 Do not edit 𝑓𝑛 Do not edit
𝑆 𝑛 2220 𝑘𝑇 1
𝑉𝑏 400 𝑥 0.15
𝑉𝑛 400 𝑟 0

• It is quite common to use an “Infinite Bus” to model the connection of a plant to the rest of the
power transmission network.
• This is a very rudimentary form of an “External Equivalent Model”
• The assumption is that at the point of connection, the network is capable of absorbing any
amount of power and to define the voltage at it’s bus… this is changing due to many factors
• Anyway… drag and drop the ”Infinite Bus” model and parametrize both the generator and
infinite bus using the power flow data!
Example 1 – Power Network External Equivalent
29
𝑉 0.90081 𝑎𝑛𝑔𝑙𝑒 0
P -1998 Q 87.066
Infinite bus
𝑉 1 𝑎𝑛𝑔𝑙𝑒 0.4946
P 1997.999 Q 967.92
G1
Note: such type of power flow would be unfeasible in an actual network.
Observe the voltage magnitude and powers – this is truly a textbook example.

Example 1 – Fault Event
• We would like to see how the system behaves when exposed to a
disturbance.
• The disturbances that are most critical for power networks are 3-phase-to-
ground faults.
• To model this, we add the block from ./OpenIPSL/Electrical/Events/PwFault
30
𝑅 0 𝑡1 0.5
𝑋 0.01*100/2220 𝑡2 0.57
Fault

Example 1 – The Final Model!
• The network model is completed by connecting all of the
components together.
• Now, finally!, the model can be simulated and linearized
31

Example 1 – Loading Reference Results
• Loading Results from PSAT
• The system will be simulated with 3-phase-to-ground fault at t=0.5s with a duration
of 70ms.
• The simulation results will be compared with the reference results from PSAT that
will be loaded first
• PSAT results are provided in a file “PSAT_dyn.csv”
• To load the file, the view should be switched to “Plotting” tab
• Result file can be opened by navigating the menu to:
File  Open Result File(s)
• In the pop-up menu, one has to
select “Comma Separated Values”,
navigate to the directory where the
file is located and open.
32

Example 1 – Plot Reference Results
• In the variable browser, three
three time-series from PSAT
results are loaded which can
be displayed on the plot as it is
shown in the figure.
• Loaded waveforms are
generator terminal voltage,
excitation field voltage and the
generator speed
33

Example 1 – Finally, let’s simulate this!
• Before the simulation, solver and its parameters are set to be the similar as in
PSAT.
• Click on or go to
• A set-up window
will appear.
• Use these settings:
34

Example 1 – Run!
• By pressing the “Simulate” button on the toolbar,
simulation of the model is executed
• Once the simulation is completed,
the Variable Browser is populated with
the simulation results
35

Example 1 – Choose Variables and Plot
• To display the simulation results or compare it with the results from PSAT, one
can mark the check-box next to the variable which will be shown on the plot
• To show the terminal voltage of the generator in PSAT and OpenModelica,
variables “PSAT_dyn.v” and “Example_1.G1.machine.v” have to be selected
36

Using OMNotebook for Interactive Analysis
• The OpenModelica installation includes a very handy
tool called OMNotebook.
• OMNotebook allows you to interact with the
OpenModelicaCompiler (OMC), and at the same time
analyze the model.
• This gives you a basic “lab notebook” where you do
your tests before you create a script for automated
analysis (called .mos scripts).
• We use OMNotebook next to load the model
developed, and simulate it interactively.
37

OMNotebook: Open the Notebook
• Under C:your pathOpenIPSL-
masterApplicationExamples_TutorialNotebook_Tutorial; you will find
the OMNotebook ”Example_1.onb”
• Double click it to open it using OMNotebook
38

OMNotebook: Set-up your path & evaluate cells!
39
Set your patch to where the decompressed version
of your tutorial .zip is available.
Evaluate each cell using “Shift+Enter”
Instantiate and Simulate the Model!

OMNotebook: Plotting Results
40
2017-08-23

OMNotebook: Parametric Sweep
• Using OMNotebook, we can interact with the
OMC in order to analyze the behavior of the
system.
• The AVR’s output looks saturated.
• The type of instability that we are observing it
is typically due to negative feedback from the
AVR.
• This was shown in the early 1900’s by
Concordia and De Mello!
• A parametric sweep is used in OMC to
simulate the system’s response for different
values of the gain of the AVR, namely, K0.
41
Results from K=10 to K=50
Evaluate from this part of the
notebook

OMNotebook: Linear Analysis
• Let’s now do a bit more analysis on the system’s stability
by using linear analysis.
• This will allow us to determine the system’s damping with
the current value of the AVR’s gain (K=200).
• Two methods are shown in the following slides, using
OMNotebook and the OMShell (optional – see the
Appendix).
42

OMNotebook: Linearization
• The following script is used to extract the eigenvalues of the system:
• This is the A-matrix:
• And these the Eigenvalues:
43This is the complex unstable mode we were looking for!

Example 2 – Power System Damping Control
• In the Example 1, it was shown that the
system was unstable with a pair of poles
on the right side of the stability plane.
• In the Example 2, Power System
Stabilizer (PSS) will be added to the
generator in order to stabilize the system.
• The use of power system stabilizers
became commonplace in the 1980’s after
the seminal work by Larssen and Swann.
• They are now re-designed and calibrated
routinely in the Western US.
44
E. Larsen and D. Swann, “Applying power
system stabilizers, parts I, II and III,” IEEE
Trans.Power App. Syst., vol. PAS-100, pp.
3017–3046, Jun. 1981

Example 2 – PSS Implementation
• The work on Example 2 should continue with the files
prepared in a package
Tutorial.Working_Examples.Example_2
• The first step is to add the model of the PSS Type II to
the model of the generator
• The internal control structure of the PSS can be
accessed by right-clicking on the PSS block and
selecting “Open Class”
45
1
2
1
2
1
0
0

Example 2 – Control Design/Parametrization
• Ideally, the parameters of the PSS should be design to move the
RHP pole, and many methods exist to achieve this… topic for a
different tutorial.
• The PSS here is only parametrized according to a previous
control design.
46
OpenIPSL Tutorial 2017-08-23
PSS
𝑣𝑠,𝑚𝑎𝑥 0.2 𝑇1 0.154
𝑣 𝑠,𝑚𝑖𝑛 -0.2 𝑇2 0.033
𝐾 𝑤 9.5 𝑇3 1
𝑇 𝑤 1.41 𝑇4 1

Example 2 – Signal Routing
• The PSS uses the generator speed as an input signal,
and provides a “damping” signal output to modulate the
action of the AVR.
• The signals of the generator model are connected as
shown, and the model of the generator is completed.
47

Example 2 – Simulation and Comparison
• Similarly to Example 1, we will compare
the simulations from OpenModelica
and reference results from PSAT.
• This time, reference simulation results
from the PSAT can be found in the file
“PSAT_dyn_PSS.csv”
• After the simulation is executed,
variable browser should look as in the
screenshot.
• The supplementary signal provided by
the PSS is compared in the figure.
48

Example 2 – Linear Analysis
• The Appendices include the detailed steps on how to perform
linear analysis using the CLI and OMShell.
• Here we compare the results:
• The unstable pole was successfully moved to the LHP!
49
Without PSS With PSS

Example 3 – Exploring the IEEE 9 Bus System
• Example 3 contains the model of the
IEEE 9 Bus system
• This is a very typical “test case” used in
the literature and text books for basic
stability and control analysis.
• It is pre-configured with all of the power
flow and dynamic data
• In the previous two examples, you
learned how to build the models of the
power system, introduce the faults, run
the dynamic simulations and perform the
linearization of the model
• In Example 3 you are free to explore the
model and introduce various faults
50

Example 3 – Different Simulation Scenarios
51
You can, for instance, introduce
the bus fault …
... or open the line at the given
time instant*
… or introduce a step disturbance to the voltage reference of the generators by setting the desired
refdisturb_x parameter to true

53
Luigi Vanfretti Tin RabuzinAchour
Amazouz
Mohammed
Ahsan Adib
Murad
Francisco
José Gómez
Jan Lavenius Le Qi Maxime
Baudette
Mengjia
Zhang
Tetiana
Bogodorova
Giusseppe
Laera
Joan Russiñol
Mussons
The OpenIPSL can be found online
• http://openipsl.org
RaPId, a system identification software that uses OpenIPSL can be
found at:
• https://github.com/ALSETLab/RaPId
• http://dx.doi.org/10.1016/j.softx.2016.07.004
Thanks to all my current and former
students, friends and developers that
have supported the effort!
Our work on OpenIPSL has been published in the
SoftwareX Journal:
• http://dx.doi.org/10.1016/j.softx.2016.05.001
Marcelo Castro Miguel Aguilera

Appendix for Example 1
Web: ALSETLab.com

Example 1 – Linearization using OMShell
• To linearize the system, OpenModelica scripting will be needed
• Along with the library, a set of commands was provided (Command_List.txt)
to linearize the model and extract the A matrix
55

• Linearization
• Copy and paste each line from the Command_List.txt for
Example 1 to the command prompt in OpenModelica
56
OpenIPSL Tutorial 2017-08-23

• The third command will
save the A matrix of the
linearized state-space
model in the variable a as
a string
57
• Copy the output from the
previous command without
the quotation marks by
pressing Ctrl+C

• To save the matrix A as a matrix of
Real values type A := and then
press Ctrl+V to paste the copied
matrix
58
• It is known that the eigenvalues of the
linearized system can be found by solving the
following equation:
• This can be done by executing the last
command
(eval,evec) :=
Modelica.Math.Matrices.eigenValues
(A);
𝑑𝑒𝑡 𝑨 − 𝜆𝑰 = 𝟎

• The eigenvalues are now stored in
the eval variable and they can be
listed by executing eval
• Groups of numbers are listed where
the first number is real part of the
system’s pole and the second one is
the imaginary part
59
• It can be seen that the pair of
conjugate poles exists on the right
side of the stability plane and thus,
the behavior of the system is
unstable

Appendix for Example 2
Web: ALSETLab.com

Example 2 – Linear Analysis of Model with PSS
• To linearize the system, OpenModelica
scripting will be needed
• Along with the library, a set of
commands was provided
(Command_List.txt) to linearize the
model and extract the A matrix
61
• Copy and paste each line from the
Command_List.txt for Example 1 to the
command prompt in OpenModelica

Example 2
• The rest of the steps shall be
repeated as it was shown in
Example_1
• The same procedure with a linearized
system from Example 2 results in the
new set of eigenvalues
62
• The conjugate pair of poles that was
on the right side of the plane in
Example 1 was, by introducing the
PSS, moved to the left side of the
stability plane and, thus, the system
is now stable

Hands-on-OpenIPSL.org using OpenModelica!

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