MATLAWSIMULINKAND PSPICE AS MODELLING
TOOLS
FORPOWER SYSTEMS AND POWER ELECTRONICS
Gilbert Sybille
Hoang Le-Huy
Power Networks Simulation Laboratory
Institut de recherche d’Hydro-Qu&ec (IREQ)
Varennes, Quhbec, Canada
Dept. of Electrical and Computer Engineering
Universith Lava1
Ste-Foy, Qukbec, Canada
ages and inductor currents.
Simulation results are displayed on Simulink
scopes while the simulation is running. The user can access
a variety of MATLAB fknctions and toolboxes for processing and plotting of waveforms stored in the MATLAB workspace.
I. MTLAB/SIMULINK AS MODELLING AND
SIMULATION TOOL FOR POWER SYSTEM
TRANSIENTS
A. Presentation of the Power System Blockset in MATLAB/
Simulink environment
B. Stmng points of the PSB in power systems simulation
The Power System Blockset (PSB) simulation tool
uses the MATLAB/Simulinkenvironment to represent common components and devices found in electrical power networks. It consists of a block library that includes electrical
models such as RLC branches and loads, transformers, lines,
surge arrester, electric machines, power electronics devices,
etc. Diagrams can be assembled simply by using click and
drag procedures in Simulink windows. The Power System
Blockset uses the same drawing and interactive dialogue
boxes to enter parameters as in the standard Simulinkblocks.
In the PSB,the power system is represented in two
parts: a state-space model for the linear circuit and a feedback model (using current injection) for the nonlinear elements. The differential equations of a linear circuit
consisting of resistors, inductors, capacitors and mutually
coupled inductors can be written in the form of two state
equations.In the linear circuit., the state variables are capacitor voltages and inductor currents. Inputs are the voltage and
current sources. Outputs are the measured voltages and currents.
State variable formulation allows the use of a wide
variety of futed step and variable step integration algorithms.
For small and medium size systems, variable time steps algorithms are usually faster because the number of steps will be
less than with a fixed-time step method. However, for large
systems which contain many states of many non-linear
blocks such as power electronic switches, it is advantageous
to discretize the electrical system.
Nonlinear elements such as transformer saturation
branches, varistors, nonlinear inductances, switches and
electric machines are modeled using nonlinear v-i relations.
Initialization
An initialization process is executed each time the
simulation is started. It computes the state-space representation of the circuit and verifies if the circuit is consistent with
electrical rules and builds the Simulink model of the electrical network. The PSB graphical interface also includes an
interactive tool to set initial conditions of the capacitor volt-
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Simulink’s variable-step event-sensitive integration
algorithms allow increased accuracy in zerocrossing
detection of currents as compared with (non-interpolated) fixed-step algorithms.
Simulation either with continuous variable time-step
integration algorithms or with a discretized system is
possible. For large systems which contain many states
or many non-linear blocks such as power electronic
switches, discretizationof the electrical system allows
much faster simulation than variable time step methods.
Simulink’s graphical interface provides a user-friendly
environment where the power circuit and control system
are represented in the same diagram. The results are displayed while the simulation is running.
The processing power of MATLAB allows the designer
to perform complex post-processing on simulation
results.
The SimulinWSB libraries offer a variety of models
(machines,power electronic devices and control blocks,
etc.) allowing fast development of models for drives,
FACTS and custom power devices.
C. Drawbacks of the PSB in power systems simulation
Use of variable time step integration algorithms is
limited to small and medium size systems (typically less
than 30 electrical states and 12 switches)
11. PSPICE AS MODELLING AND SIMULATION TOOL
FOR POWER SYSTEM TRANSIENTS
A. The PSPice Program
SPICE (Simulation Program with Integrated Circuit Emphasis) is a general-purpose circuit simulation program that was developed at the University of CalifomiaBerkeley in 1975. SPICE can perform nonlinear dc, nonlin-
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ear transient, and linear ac analyses. In addition to the standard R,L and C elements, models for transmission lines and
several semiconductor devices types are also available
In the time domain analysis, the circuit equations
are formulated using modified nodal approach. Both dc and
transient solutions are obtained by an iterative process which
is terminated when the branch currents and node voltages
converge to within a tolerance.
The basic SPICE input and output interfaces are
done through text files. The input file provides the description of the circuit and the output file contains the numerical
results of the simulation and details on the simulation process.
The SPICE code has been ported to different commercial packages with additional features such as model and
subcircuit libraries, behavioral models, graphical pre-proms’
sor and post-processor, etc. The discussion here is specifically on PSpice A/D (from Orcad), one of the most popular
versions of SPICE.
Data input to PSpice AID is done through a graphical pre-processor (Orcad Capture) that allows the user to
build circuit diagrams using element models and subcircuits
from different libraries. The basic libraries include sources,
analog, digital, and behavioral elements.
There are also several libraries from semiconductor
manufacturers that contain specific models of discrete components and ICs. A “Library Editor” allows the user to build
his or her own libraries for the need of the application. For
the simulation o f power converters, power semiconductor
models (thyristors, MOSFETs, IGBTs,...) are available from
manufacturers. Electric machine models are not available so
that we have to develop the models by using standard
machine models ( d q models) as subcircuits hidden behind
icons that we can draw with the “Model Editor”. The circuit
diagram can be on several interconnected pages and they can
be divided into several f u n ~ t i ~ nlevels
a l (subsystems).
Before the simulation can start, Orcad Capture performs an electrical rule check and generates a circuit file containing the netlist and circuit description that will be
processed by PSpice.
The computation engine of PSpice is based on the
SPICE algorithm. The dynamic elements (capacitors and
inductors) are model by companion models. Thus the circuit
equation system is algebraic and is solved at each simulation
point using a particular algorithm based on trapezoidal and
Gear integration methods. The time steps are automatically
adjusted by the program based on the error estimation fonnulas. The user has no choice on the integration method. User
specified options include maximum step size, relative and
best accuracy of voltage and currents, best accuracy of electric charge, minimum conductance for any branch, DC and
bias iteration limit, transient time point iteration limit, and
nominal temperature.
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PSpice can handle nonliearities in circuit elements
(saturated transformer, MOV,...) and in control systems (limiter, hysteresis, nonlinear transfer characteristic,...). PSpice
A/D can perform mixed-signal simulation of circuits containing both analog and digital components.
The simulation results are stored in a data output
file that is processed by a graphical post-processor (Probe)
which permits data plotting in addition to supporting some
data analysis functions.
B. Strengths of PSpice in power system simulation
These include:
An User-friendly interface for data entry (schematic cap-
ture) and output data processing.
Detailed models of electronic components
Abundant libraries of electronic components (including
several power electronicdevices) and control ICs.
Capability of simulating mixed-signal (analog and digital)systems.
C. Drawbacks of PSpice in power systems simulation
These include:
The simulation is not interactive.
The user has little control on the integration process
Electric machine and power component models (in particular three-phase components) are not available. The
development of such models may be take a large amount
of time.
111. CONCLUSIONS
The PSB is well suited to the simulation of medium
size power systems and power electronics using variable or
fixed step algorithms from Simulink. The PSB libraries contain basic elements as well as many ready-built sub-systems.
Control systems using Simulink blocks can be naturally integrated into the power system model. The computation capabilities of MATLABKimulink can be advantageously
exploited in post-processing of the simulation results.
PSpice is a very popular circuit simulation program
with many features such as ease of usage, large library of
available device models and affordability.
In the study of power systems, PSpice is well suited
for device-level modeling of small size systems: study of
voltages and currents in power converters, snubber circuit
design, study of transformer transients, etc. Also, PSpice performs well for system-level modeling of small size power
systems (less than 30 states and 6 switches) using ideal
switch models instead of detailed semiconductor models.
However, in the simulation of larger size power systems the
execution time may be excessive.
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