SimPowerSystems

How the Power System Blockset Works

Once you have built your circuit with the blocks of powerlib, you can start the simulation just like any other Simulink model. Each time you start the simulation, a special initialization mechanism is called. This initialization process computes the state-space model of your electric circuit and builds the equivalent system that can be simulated by Simulink.

The power2sys function is part of the process. It gets the state-space model and builds the Simulink model of your circuit. power2sys can also be called from the command line to obtain the state-space model of the linear part of the circuit. When called by the initialization process, power2sys performs the following four steps as shown in Figure 3-1:

• Sorts all Power System Blockset blocks, gets the block parameters and evaluates the network topology. The blocks are separated into linear and nonlinear blocks, and each electrical node is automatically given a node number.
• Once the network topology has been obtained, the state-space model of the linear part of the circuit is computed by the circ2ss function. All steady-state calculations and initializations are performed at this stage.
• If you have chosen to discretize your circuit, the discrete state-space model is computed from the continuous state-space model, using the Tustin method.
• Builds the Simulink model of your circuit and stores it inside one of the measurement blocks. This means that you need at least one measurement block (Current Measurement block, Voltage Measurement block, Three-Phase VI Measurement block, or Multimeter block) in your model. The connections between the equivalent circuit and measurements blocks are performed by invisible links using the Goto and From blocks.

The Simulink model uses a State-Space block or an S-Function block to model the linear part of the circuit. Predefined Simulink models are used to simulate nonlinear elements. These models can be found in the powerlib_models library available with Power System Blockset. Simulink source blocks connected at the input of the State-Space block are used to simulate the electrical sources blocks.

Figure 3-1: Power System Blockset Flowchart

The next figure represents the interconnections between the different parts of the complete Simulink model. The nonlinear models are connected in feedback between voltage outputs and current inputs of the linear model.

Figure 3-2: Interconnection of Linear Circuit and Nonlinear Models

Once power2sys has completed the initialization process, Simulink starts the simulation and you can observe waveforms on scopes connected at the outputs of your measurement blocks.

If you stop the simulation and drag a copy of the Powergui block into your circuit window, you will have access to the steady-state values of inputs, outputs, and state variables displayed as phasors. You can also use the interface to modify the initial conditions. The Powergui block interface allows you to perform a load flow with circuits involving three-phase machinery and initialize the machine models so that the simulation starts in steady state. This feature avoids long transients due to mechanical time constants of machines. The Powergui block allows you to specify the desired frequency range, visualize impedance curves, and store results in your workspace for Impedance Measurement blocks connected in your circuit.

Advanced Topics

Choosing an Integration Method: Continuous or Discrete