This dissertation is dedicated to the development of reliable, practical and cost-effective solutions to the issues caused by CMVs in power converter systems. A novel integrated ac choke which incorporates the differential-mode (DM) filtering and common-mode (CM) suppression functions, and new modulation techniques for current-source converters (CSCs) are presented.
The CMVs are divided into three frequency ranges: 1) the low frequency range with triplen harmonics, 2) the medium frequency range referring to the switching frequency and sidebands of its integral multiples, and 3) the high EMI frequency range. In voltage-source converters (VSCs), CMVs are mainly distributed in the medium frequency range, and no low-frequency CMV component is produced by sinusoidal pulse-width modulation (SPWM). This is favorable for the passive filter and CM choke design. The CMVs in CSC-based drives are dominant in the low frequency range. Under normal operating conditions, space vector modulation (SVM) generates a larger CMV magnitude than other modulation schemes in CSCs.
A three-phase integrated choke is proposed to suppress the CMV in the medium frequency range in VSC systems. Different magnetic paths for the CM and DM fluxes generate the required high CM-to-DM inductance ratio. The magnetic integration brings great benefits in savings of iron and copper materials, reduction of weight and cost, and improvement in efficiency. The finite element analysis and experiment of a prototype are carried out for verification.
Transformerless photovoltaic inverters and neutral-connected motor drives are identified as applications of the integrated choke. Appropriate system grounding and filter components, e.g. CM capacitors and damping resistors, are employed for coordination. The CM behaviors are explained by circuit modeling. With evaluated parameters, simulations of the VSC systems using the proposed choke are conducted, verifying the analysis of the CM voltage/current mitigation effect.
Modified SVM techniques that avoid zero states are adapted for use in CSCs to decrease the CMV magnitude. The nearest-three-state methods present superior harmonic performances with a high modulation index and no increase in the switching frequency. For a lower modulation index, the combined active-zero-state technique can be implemented as compensation. The proposed modulation schemes are supported by both simulation and experimental results.