Z-SOURCE INVERTER FOR ADJUSTABLE SPEED DRIVERS (A NOVEL SPEED SYSTEM)

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This paper presents a Z-source inverter system and control for adjustable speed drives (ASD). The Z-source inverter employs a unique LC network to couple the inverter main circuit to the diode front end. By controlling the shoot-through duty cycle, the Z-source can produce any desired output ac voltage, even greater than the line voltage. As results, the new Z-source inverter system provides ride-through capability under voltage sags, reduces line harmonics, and extends output voltage range. Simulation results will be presented to demonstrate the new features

1. INTRODUCTION

The Traditional ASD system is based on a voltage-source inverter (V-source inverter), consisting of a diode rectifier front end, dc link capacitor, and inverter bridge Because of the V-source inverter, the ASD system suffers the following common limitations and problems.

• Obtainable output voltage is quite limited below the input line voltage. The V-source inverter is a buck (step-down) inverter. For a 230–V motor, the low obtainable output voltage significantly limits output power that is proportional to the square of the voltage. This is a very undesirable situation for many applications where the motor and drive system has to be oversized.

• Voltage sags can interrupt an ASD, thus shutting down critical loads and processes. Over 90% of power quality related problems are from momentary (typically 0.1–2 s) voltage sags of 10–50% below nominal. The dc capacitor in an ASD is a relatively small energy storage element, which cannot hold dc voltage above the operable level under such voltage sags. Lack of ride-through capacity is a serious problem for sensitive loads driven by ASDs.

Inrush and harmonic current from the diode rectifier can pollute the line. A recently developed new inverter called Z-source inverter has a niche for ASD systems to overcome the above problems. A Z-source inverter-based ASD system can produce any desired output ac voltage, even greater than the line voltage;

• provide ride-through during voltage sags without any additional circuits;

• reduce in-rush and harmonic current.

This paper presents the basic idea of an ASD system using the Z-source Inverter, its main circuit configuration, an equivalent circuit, and control. Simulation results are included to demonstrate the idea and features of the new ASD system