Analysis, design and implementation of high performance control schemes for three phase PWM AC-DC voltage source converter

Abstract

This thesis reports the investigation of high performance control of three phase PWM ACDC voltage source converters under the generalized supply voltage operating conditions. Based on this proposed model in the positive and negative synchronous rotating frames, the explanations of the appearance of the even-order harmonics at the dc output voltage and the low frequency odd-order harmonics in the input ac currents are provided under the unbalanced and distorted supply voltage conditions. In order to eliminate the even-order harmonics at the dc link voltage and the odd-order harmonics in the ac line side currents, the proposed control scheme can be highlighted as two parts, (1) DC link voltage harmonics control scheme and (2) AC line side current harmonics control scheme. In the DC link voltage harmonics control scheme, the singular perturbation method has been proposed to design the cascaded four current PI controllers in the positive and negative sequence d-q frame with a voltage PI controller. The closed-loop system can achieve the desired output transient performance by ensuring the stability conditions and properly selecting the time constants for two-time-scale motions, namely, fast motion sub-system and slow motion sub-system. A plug-in time domain based repetitive controller (TDRC) is developed and employed in the current harmonics control scheme. The proposed plug-in digital repetitive control scheme can minimize the harmonics in the line side currents while maintaining the dc link voltage constant. Since the repetitive controller in the frequency domain needs only to learn and update these two parameters, magnitude and phase, the controller design would be easier in the frequency domain than in the time domain. Hence, frequency domain based repetitive controller (FDRC) is introduced to enhance the robustness of the control system and improve the tracking performance for the supply line side currents. The experimental results conducting on a 1.6 kVA prototype PWM AC-DC voltage source converter validate that by using the proposed dual frame control schemes, the even-order harmonic component at the dc link voltage and the odd-order harmonic in the supply ac line side currents can be minimized and the supply side power factor can be kept close to unity under the unbalanced and distorted supply voltage operating conditions.