Modelling and Realisation of a Three-Level PWM Inverter Using a DSP Controller to Feed an Asynchronous Machine

Modelling and Realisation of a Three-Level PWM Inverter Using a DSP Controller to Feed an Asynchronous Machine

Denoun Hakim (Mouloud Mammeri University, Algeria), Benyahia Nabil (Mouloud Mammeri University, Algeria), Zaouia Mustapha (Mouloud Mammeri University, Algeria), Benamrouche Nacereddine (Mouloud Mammeri University, Algeria), Salah Haddad (Mouloud Mammeri University, Algeria) and Sadek Ait Mamar (Mouloud Mammeri University, Algeria)
DOI: 10.4018/978-1-4666-7248-2.ch025
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Multilevel inverters have seen an increasing popularity in the last few years for medium- and high-voltage applications. The most popular has been the three-level neutral clamped inverter. Multilevel inverters synthesize output voltage from more than two voltage levels. Thus, the output signal spectrum is significantly improved in comparison with the classical two-level converters. This chapter discusses modelling and control of a Neutral Point Clamped (NPC) inverter which operates with the PWM switching pattern using a DSP. The mathematical model of the NPC inverter is carried out using conversion and connection functions for an easier understanding of the system operation. Simulation results using MATLAB program are reported, and it is shown that the performances obtained for driving an asynchronous motor using this inverter are very promising. Finally, analysis of the theoretical and the experimental results is carried out in order to validate the effectiveness of the proposed control solution.
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1. Introduction

The three level inverters often referred to as Neutral Point Clamped (NPC) inverters offer several advantages over the more common two level inverters. As compared to two level inverters, three level inverters have smaller output voltage steps that mitigate motor issues due to long power cables between the inverter and the motor. These issues include surge voltages and rate of voltage rise at the motor terminals and motor shaft bearing currents. In addition, the cleaner output waveform provides an effective switching frequency twice that of the actual switching frequency. Should an output filter be required, the components will be smaller and less costly than for an equivalent rated two level inverter. Most often the NPC inverter is used for higher voltage inverters. Since the IGBTs are only subjected to half of the bus voltage, lower voltage IGBT modules can be used.

Figure 1 shows the circuit configuration of the NPC inverter. Each leg has four IGBTs connected in series. The applied voltage on the IGBT is one-half that of the conventional two level inverters. The bus voltage is split in two by the connection of equal series connected bus capacitors. Each leg is completed by the addition of two clamp diodes.

Figure 1.

NPC inverter

This topology has been used traditionally for medium voltage industrial applications. In addition to the capability of handling higher voltages, the NPC inverter has several favorable features including lower line-to-line and common mode voltage steps and lower output current ripples for the same switching frequency as that used in a two level inverter.

An attempt is made to present the most common multilevel (three-level) inverters and briefly describe their advantages and disadvantages based on a literature survey. The literature survey is intended to compare the inverters based on their performance aspects and the challenges and restrictions they could impose for application in a given area.

The fundamental purpose of considering multilevel inverters instead of the popular two-level inverters is due to the high value of DC-link voltage that needs to be employed in future developments. With the increase in the DC-link voltage, it is aimed that the performance of the inverter in use does not degrade and hence, multilevel inverter options have been considered.

The increase in power-handling capabilities of the power electronic switches has made the use of multilevel inverters feasible for high-power applications. For high voltage and high power systems, instead of using switches with high voltage ratings, it is beneficial to connect the switches having low-voltage ratings in series (Tolbert et al., 1997). This allows the latter to be switched faster than the switches having higher voltage ratings, thus resulting in switching harmonics of higher frequencies which can be filtered easily.

Due to the dynamic voltage sharing problem when the switches are connected in series, multilevel power inverters have come into existence and they are being used due to several advantages that they offer over the conventional two-level converters. However, in principle, with increased DC-link voltage, there are different ways in which the semiconductor devices can be connected, that is

  • Using switching devices with higher voltage rating. However, this involves high dV/dt stress and renders slower switching due to high voltage level.

  • Series connection of devices, however, dynamic voltage sharing among the devices represents a big challenge for this option.

  • Adopting the multilevel solution which is more preferable due to the limited voltage ratings for the semiconductor devices.

There are a number of multilevel inverters introduced since the year 1975 but the three basic and most well-known topologies are the Cascaded H-Bridge Multilevel Converter (CHB), Neutral Point-Clamped Multilevel Converter (NPC) and the Flying Capacitor Multilevel Converter (FCC). These three basic topologies have been widely accepted for industrial applications (Rodriguez et al., 2002).

Key Terms in this Chapter

DSP (Digital Signal Processor): A specialized microprocessor with an architecture optimized for the operational needs of digital signal processing.

Multi-Level Inverter: A source of high power: used in industrial applications and use either sine or modified sine waves. Instead of using one converter to convert an AC current into a DC current, a multilevel inverter uses a series of semiconductor power converters (usually two to three) thus generating higher voltage.

NPC (Neutral Point Clamping): To convert DC voltage having neutral point into AC voltage

IGBT (Insulate- Gate Bipolar Transistor): Combines the simple gate-drive characteristics of MOSFETs with the high-current and low-saturation-voltage capability of bipolar transistors. The IGBT combines an isolated gate FET for the control input, and a bipolar power transistor as a switch, in a single device.

THD (Total Harmonic Distortion): A measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.

Asynchronous Machine: An AC electric motor in which the electric current in the rotor needed to produce torque is induced by electromagnetic induction from the magnetic field of the stator winding.

PWM (Pulse-Width Modulation): A modulation technique that conforms the width of the pulse,

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