In this section, you
The circuit in the figure represents an equivalent power system feeding a 300-km transmission line. The line is compensated by a shunt inductor at its receiving end. A circuit breaker allows energizing and de-energizing of the line. To simplify matters, only one of the three phases is represented. The parameters shown in the figure are typical of a 735-kV power system.
Circuit to Be Modeled
The graphical user interface uses Simulink functionality to interconnect various electrical components. The electrical components are grouped in the Simscape Power Systems Specialized Technology library.
Open the Simscape Power Systems main library by entering the following command at the MATLAB® prompt.
From the File menu
of the simscapepowersystems_ST window, open a
new blank model to contain your first circuit and save it as
Open the Fundamental Blocks > Electrical Sources library and copy the AC Voltage Source block into the circuit1 window.
Open the AC Voltage Source dialog box by double-clicking the icon and enter the Amplitude, Phase, and Frequency parameters according to the values shown in Circuit to Be Modeled.
Change the name of this block from AC Voltage Source to Vs.
The resistance Rs_eq of the circuit can be obtained from the Parallel RLC Branch block. Duplicate the Parallel RLC Branch block, which is already in your circuit1 window. Select R for the Branch Type parameter and set the R parameter according to Circuit to Be Modeled.
Once the dialog box is closed, notice that the L and C components have disappeared so that the icon now shows a single resistor.
Name this block Rs_eq.
Resize the various components and interconnect blocks by dragging lines from outputs to inputs of appropriate blocks.
The model of a line with uniformly distributed R, L, and C parameters normally consists of a delay equal to the wave propagation time along the line. This model cannot be simulated as a linear system because a delay corresponds to an infinite number of states. However, a good approximation of the line with a finite number of states can be obtained by cascading several PI circuits, each representing a small section of the line.
A PI section consists of a series R-L branch and two shunt C branches. The model accuracy depends on the number of PI sections used for the model. Copy the PI Section Line block from the Elements library into the circuit1 window, set its parameters as shown in Circuit to Be Modeled, and specify one line section.
The shunt reactor is modeled by a resistor in series with an inductor. You could use a Series RLC Branch block to model the shunt reactor, but then you would have to calculate and specify the R and L values manually based on the quality factor and reactive power specified in Circuit to Be Modeled.
Therefore, you might find it more convenient to use a Series RLC Load block that allows you to specify directly the active and reactive powers absorbed by the shunt reactor.
Copy the Series RLC Load block, which can be found in the Fundamental Blocks/Elements library. Name this block 110 Mvar. Set its parameters as follows:
As no reactive capacitive power is specified, the capacitor disappears on the block icon when the dialog box is closed. Interconnect the new blocks as shown.
You need a Voltage Measurement block to measure the voltage at node B1. This block is found in the Fundamental Blocks/Measurements library. Copy it and name it U1. Connect its positive input to the node B1 and its negative input to a new Ground block.
To observe the voltage measured by the Voltage Measurement block named U1, a display system is needed. This can be any device found in the Simulink Sinks library.
Open the Sinks library and copy the Scope block into your circuit1 window. If the scope were connected directly at the output of the voltage measurement, it would display the voltage in volts. However, electrical engineers in power systems are used to working with normalized quantities (per unit system). The voltage is normalized by dividing the value in volts by a base voltage corresponding to the peak value of the system nominal voltage. In this case, the scaling factor K is
Copy a Gain block from the Simulink library and set its gain as above. Connect its output to the Scope block and connect the output of the Voltage Measurement block to the Gain block. Duplicate this voltage measurement system at the node B2, as shown below.
Add a Powergui block to your model. The purpose of this block is discussed in Using the Powergui Block to Simulate Simscape Power Systems Models.
Select Simulation > Run.
Open the Scope blocks and observe the voltages at nodes B1 and B2.
While the simulation is running, open the Vs block dialog box and modify the amplitude. Observe the effect on the two scopes. You can also modify the frequency and the phase. You can zoom in on the waveforms in the scope windows by drawing a box around the region of interest with the left mouse button.
The Voltage Measurement block acts as an interface between the Simscape Power Systems blocks and the Simulink blocks. For the system shown above, you implemented such an interface from the electrical system to the Simulink system. The Voltage Measurement block converts the measured voltages into Simulink signals.
Similarly, the Current Measurement block from the Fundamental Blocks/Measurements library can be used to convert any measured current into a Simulink signal.
You can also interface from Simulink blocks to the electrical system. For example, you can use the Controlled Voltage Source block to inject a voltage in an electrical circuit, as shown in the following figure.
Power Systems modeling environment is similar to that
of other products in the Physical Modeling family. Its blocks often
feature both normal Simulink input and output ports
special electrical terminal ports :
Lines that connect terminal ports are special electrical connection lines. These lines are nondirectional and can be branched, but you cannot connect them to Simulink ports > or to normal Simulink signal lines.
You can connect Simulink ports
to other Simulink ports and electrical terminal ports only to other electrical terminal ports.
Converting Simulink signals to electrical connections or vice versa requires using a Simscape Power Systems block that features both Simulink ports and electrical terminal ports.
Some Simscape Power Systems blocks feature only one type of port.
When you measure a current using a Current Measurement block, the positive direction of current is indicated on the block icon (positive current flowing from + terminal to – terminal). Similarly, when you measure a voltage using a Voltage Measurement block, the measured voltage is the voltage of the + terminal with respect to the – terminal.
Pay particular attention when you connect capacitor elements together with voltage sources, or inductor elements in series with current sources. When you start the simulation, the software displays an error message if one of the following two connection errors are present in your diagram:
You have connected a voltage source in parallel with a capacitor, or a series of capacitor elements in series, like in the two examples below.
To fix this problem, you can add a small resistance in series between the voltage source and the capacitors.
You have connected a current source in series with an inductor, or a series of inductors connected in parallel, like in the example below.
To fix this problem, you can add a large resistance in parallel with the inductor.
The Powergui block is the environment block for Simscape Power Systems Specialized Technology models. It is used to store the equivalent Simulink circuit that represents the state-space equations of the Simscape Power Systems blocks. It also opens tools for steady-state and simulation results analysis and for advanced parameter design. When you start the simulation, you will get an error if no Powergui block is found in your model.
See powergui for more information on how to use the powergui block in your models.