How SimPowerSystems Software Works

Every 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® software. This process performs the following steps:

  1. Sorts all SimPowerSystems™ blocks, gets the block parameters and evaluates the network topology. The blocks are separated into linear and nonlinear blocks. Each electrical node is automatically given a node number.

  2. Once the network topology has been obtained, the state-space model (A, B, C, D matrices) of the linear part of the circuit is computed. 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.

    If you are using the phasor solution method, the state-space model is replaced with the complex transfer matrix H(jω) relating inputs and outputs (voltage and current phasors) at the specified frequency. This matrix defines the network algebraic equations.

  3. Builds the Simulink model of your circuit and stores it inside the Powergui block located at the top level of your model.

The Simulink model uses an S-Function block to model the linear part of the circuit as well as the switches and power electronic devices. Predefined Simulink models are used to simulate nonlinear elements. Simulink Source blocks connected at the input of the State-Space block are used to simulate the electrical source blocks.

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.

Interconnection of Linear Circuit and Nonlinear Models

Once SimPowerSystems software has completed the initialization process, the simulation starts. You can observe waveforms on scopes connected at the outputs of your measurement blocks. Through the Powergui block, you can access the LTI viewer and obtain transfer functions of your system between any pair of input and output. The Powergui block also allows you to perform a FFT analysis of recorded signals to obtain their frequency spectrum.

If you stop the simulation and double-click the Powergui block, you have access to the steady-state values of inputs, outputs, and state variables displayed as phasors. You can also use the Powergui block 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 frequency range that you want, visualize impedance curves, and store results in your workspace for Impedance Measurement blocks connected in your circuit.

Limitations of the Nonlinear Models

Because nonlinear models are simulated as current sources, they cannot be connected in series with inductors and their terminals cannot be left open.

If you feed a machine through an inductive source, SimPowerSystems prompts you with an error message. You can avoid this error by connecting large resistances in parallel with the source inductances or across the machine terminals.

A series RC snubber circuit is included in the model of the Breaker block and power electronics blocks. If you keep these snubber circuits in service, you do not encounter any issues. The snubber can be changed to a single resistance by setting Cs to Inf, or to a single capacitor by setting Rs = 0. To eliminate the snubber, specify Rs = Inf or Cs = 0.

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