SimPowerSystems

Connecting Your Model with Other Nonlinear Blocks

You now learn how to avoid error messages that can appear with nonlinear blocks when they are simulated by a current source. Obviously, a current source cannot be connected in series with an inductor, another current source, or a an open circuit. Such circuit topologies are forbidden in the Power System Blockset.

Similarly, if your nonlinear model uses a Controlled Voltage Source block, this model could not be short-circuited or connected across a capacitor.

Suppose, for example, that you want to study the inrush current in a nonlinear inductance when it is energized on a voltage source. Using blocks from powerlib library and my_PSBlibrary, you can build the circuit shown here. Change the Breaker block parameters as follows.

• External control:not checked
  Switching times:[ 1/60]
  
  
  

Figure 1-32: Circuit Topology Causing an Error

If you try to simulate this circuit, you get the following error message.

This topology is forbidden because two nonlinear elements simulated by current sources are connected in series: the Breaker block and the Nonlinear Inductance block. To be able to simulate this circuit you must provide a current path around one of the two nonlinear blocks. You could, for example, connect a large resistance, say 1 M, across the Breaker block or the Inductance block.

In our case, it is more convenient to choose the Breaker block because a series RC snubber circuit is provided with the model. Open the Breaker block dialog box and specify the following snubber parameters:

• Snubber resistance Rs (Ohms) : 1e6
  Snubber capacitance Cs (F) : inf
  

Notice that in order to get a purely resistive snubber you have to use an infinite capacitance.

Note    Using an inductive source impedance (R-L series) instead of a purely resistive impedance would have produced another error message, because the current source modeling the nonlinear inductance would have been in series with an inductance, even with a resistive snubber connected across the breaker. In such a case, you could add either a parallel resistance across the source impedance or a large shunt resistance connected between one breaker terminal and the source neutral terminal.

Make sure that the phase angle of the voltage source is zero. Use the ode23tb integration algorithm and simulate the circuit for 1 second. Voltage and current waveforms are shown here.

Figure 1-33: Current and Flux Waveforms When Energizing the Nonlinear Inductance with Maximum Flux Offset

Figure 1-33 shows that energizing the inductor at a zero crossing of voltage results in a maximum flux offset and saturation.

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