Discontinuous PWM techniques in three-phase power converters

Abstract

Discontinuous pulse-width modulation (DPWM) techniques are used three-phase power converters to improve their efficiency. When power losses are reduced, less cooling equipment is needed. Therefore, smaller devices can be manufactured with fewer expenses. In this thesis, various DPWM techniques are examined, and they are compared to other conventional PWM methods. The goal of this thesis is to research the advantages and disadvantages of using these DPWM techniques.

The first part of this thesis explains the operating principles of three-phase inverters and rectifiers. The concept of an active rectifier is also introduced. Inverters are used to convert direct current into alternating current, whereas a rectifier performs the opposite operation. As the production of renewable energy becomes more common, the demand for these power converters increases. For instance, the DC power produced with photovoltaic cells must be converted to AC power suitable for the power grid.

In a conventional three-phase inverter, a phase leg with two switches is connected to each phase. The switches in these phase legs are generally controlled with pulse-width modulation (PWM). With PWM, the input voltage of the inverter can be chopped into discrete pieces, and thus form a desired output signal. In general, the desired waveform resembles a sine wave. The modulation is often implemented by comparing the values of a carrier signal and a reference signal. While the reference signal value is greater than the carrier signal, the phase leg it is connected to is clamped to the positive DC rail of the inverter. In the opposite case, the phase leg clamps to the negative DC-rail.

The switching frequency of the PWM is often very high, leading to the switching losses making up a significant part of the power losses of the entire inverter. In DPWM techniques, each phase leg of the inverter is clamped to either the positive or the negative DC rail for one-third of each period of the reference signal. This results in a major reduction of the switching losses. The unmodulated period can be implemented in either one or multiple segments. The losses can be reduced the most by positioning the unmodulated periods close to the peak current values.

The drawback of using the DPWM methods is the increased total harmonic distortion as well as the increased motor leakage current in motor drive applications. In this thesis, adjustable PWM (APWM) and near state PWM (NSPWM) are presented as possible solutions to this problem.