Power System Blockset

Simulation of Switches and Power Electronic Devices

Two methods are used for simulation of switches and power electronic devices:

• If the switch is purely resistive the switch model is considered as part of the linear circuit. The state space model of the circuit, including open and closed switches, is therefore recalculated at each switch opening or closing, producing a change in the circuit topology. This method is always used with the Circuit Breaker and the Ideal Switch because these elements don`t have internal inductance. It is also applied for Diode and Thyristor with Ron>0 and Lon=0, and for the Universal Bridge with forced commutated devices.
• If the switch contains a series inductance (Diode and Thyristor with Lon>0, IGBT, MOSFET, or GTO), the switch is simulated as a current source driven by voltage across its terminals. The nonlinear element (with a voltage input and a current output) is then connected in feedback on the linear circuit, as shown on Figure 3-2.

You have therefore the choice to simulate diodes and thyristors with or without Lon internal inductance. In most applications, it is not necessary to specify an inductance Lon. However, for circuit topologies resulting in zero commutation or overlap angle you will have to specify a switch inductance Lon in order to help commutation.

Let us consider for example the circuit shown on Figure 3-3. This circuit is available in the psbrectifier_ideal.mdl file. The thyristor bridge is fed from an infinite source (zero impedance) so that the commutation between thyristors is quasi instantaneous.

Figure 3-3: Three-Phase Thyristor Rectifier on Infinite Source

If you simulate this circuit without internal thyristor inductances (Lon=0), you will observe high current spikes flowing in the three lines. This happens because during commutation two thyristors connected to the same positive or negative terminal of the bridge are in conduction for a short period of time, applying a line-to-line short circuit on the source (see Figure 3-4). During commutation, the current is limited only by the internal resistance of thyristors (with Ron= 0.01 ohm, the current reaches 208*sqrt(2)*sin(30)/(2*0.01)= 7.35 kA or 245 times the normal DC current of 30 A). These short circuits can be avoided by using a small Lon= 1 µH in the thyristor model. If you repeat the simulation, you will get "ideal" square current waveforms with a peak value of 30 A.

If you zoom on the line current during a commutation, you will discover that the commutation is not instantaneous. The commutation time depends on the Lon value and the DC current.

Figure 3-4: Source Currents and DC Load Voltage with Lon =0 and Lon =1 µH

Simulating with Continuous Integration Algorithms

Simulating Discretized Electrical Systems