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Simulating Power Electronic Models

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Introduction Circuit Using Forced-Commutated Power Electronics Circuit Using Naturally Commutated Power Electronics

Introduction

There are two types of circuits using power electronics:

• Circuit using forced-commutated power electronics

• Circuit using naturally commutated power electronics

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Circuit Using Forced-Commutated Power Electronics

In this type of circuit, the semiconductor such as IGBT GTO FET turns off instantaneously when the gate signal is removed. This switching off may in turn cause instantaneous switching on of another device (usually a diode).

This problem of instantaneous switching of two or more devices in the same time step has been solved in the SimPowerSystems software, both for continuous and discrete fixed time step solvers. It is now possible to discretize circuits with any type of topology, like multilevel converters. This was possible before only with the use of the Universal Bridge.

The power electronic models are implemented directly in the S-functions that compute the state-space equations of the model.

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Circuit Using Naturally Commutated Power Electronics

In this type of circuit, the diode or thyristor is turned off at zero crossing of current. It means that if the model is discretized at fixed time step, the natural zero crossing will most probably occur between two time steps. As current inversion is detected at a time step following the real zero crossing, a small negative current will be chopped. This current chopping in inductive circuits will cause numerical oscillations that can be observed in simple rectifier circuits.

This problem is not yet solved in the SimPowerSystems software because we still use a fixed-step solver. (The system is discretized using a trapezoidal solver.) The way to avoid this problem is to increase the RC snubbers connected across diodes or thyristors. But if large time steps are used, this may lead to unacceptable leakage current in the snubbers.

Fortunately, the trend of power electronics is to use forced-commutated power electronics and PWM rather than naturally commutated power electronics.

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Simulating Discretized Electrical Systems

Increasing Simulation Speed

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