Power Quality Improving with Virtual Flux- Based Voltage Source Line Converter

Antti Tarkiainen Power quality improving with virtual flux-based voltage source line converter Lappeenranta 2005 140 p. Acta Universitatis Lappeenrantaensis 206 Diss. Lappeenranta University of Technology ISBN 952–214–011–2, ISBN 952–214–012–0 (PDF), ISSN 1456–4491 Line converters have become an attractive AC/DC power conversion solution in industrial applications. Line converters are based on controllable semiconductor switches, typically insulated gate bipolar transistors. Compared to the traditional diode bridge-based power converters line converters have many advantageous characteristics, including bidirectional power flow, controllable dc-link voltage and power factor and sinusoidal line current. This thesis considers the control of the line converter and its application to power quality improving. The line converter control system studied is based on the virtual flux linkage orientation and the direct torque control (DTC) principle. A new DTC-based current control scheme is introduced and analyzed. The overmodulation characteristics of the DTC converter are considered and an analytical equation for the maximum modulation index is derived. The integration of the active filtering features to the line converter is considered. Three different active filtering methods are implemented. A frequency-domain method, which is based on selective harmonic sequence elimination, and a time-domain method, which is effective in a wider frequency band, are used in harmonic current compensation. Also, a voltage feedback active filtering method, which mitigates harmonic sequences of the grid voltage, is implemented. The frequency-domain and the voltage feedback active filtering control systems are analyzed and controllers are designed. The designs are verified with practical measurements. The performance and the characteristics of the implemented active filtering methods are compared and the effect of the Land the LCL-type line filter is discussed. The importance of the correct grid impedance estimate in the voltage feedback active filter control system is discussed and a new measurement-based method to obtain it is proposed. Also, a power conditioning system (PCS) application of the line converter is considered. A new method for correcting the voltage unbalance of the PCS-fed island network is proposed and experimentally validated.