Medium Voltage Impedance and Admittance Measurement and System Identification Techniques

The recent trend of an increasing share of renewable energy sources in modern power systems, aswell as the integration of power electronics equipment, is shaping the requirements for stable gridinfrastructure. These requirements mainly come from the stability-related phenomena arising fromdifferent subsystem interactions. Namely, medium voltage systems require special attention dueto the lack of equipment and solutions intended for their impedance measurement. Hence, thisthesis provides the perception of the problem of medium voltage impedance measurement and systemidentification from the point of view of perturbation injection converters. A cascaded H-bridgethe converter supplied from a medium-voltage multi-winding phase-shifting transformer is proposedin this thesis. Moreover, in combination with wideband injection signal, it imposes itself as a viablesolution to this problem.The thesis commences with an overview of readily available converter topologies and injectionsignals. The converter topologies are discussed, their downsides are pointed out and it is outlinedhow these issues can be addressed by using the cascaded H-bridge topology. Additionally, the lack offlexibility of the state-of-the-art solutions is highlighted.Furthermore, the modelčing approach of the single power electronics building block of the cascadedH-bridge converter is presented. Initially, the open-loop control model in the dq-frame is presented.The dq-frame modelling approach is adopted for both the three-phase active front end and for thesingle-phase H-bridge inverter. An estimation of the source-load affected dynamics is providedon the basis of the closed-loop control modelling. This question needed to be answered out of theconcern for the interaction between the active front end and the H-bridge inverter. For efficientperturbation injection, the active front end should not limit the output dynamics of the H-bridgeinverter. Real-time simulations in combination with additional single-phase dq-frame measurementand identification methods revealed that there is in fact very little to no influence between twosub-converters. Moreover, the terminal characteristics of the active front end, i.e. its input admittanceand output impedance are measured in an experimental setup, providing the first result in the fullexperimental verification of the notions proposed.The hardware and control designs of the active front end are subsequently verified through the powercirculation tests in a setup including two active front ends and the medium voltage multi-windingtransformer. On one side, the verification is made possible due to the transformer primary sidesynchronization method, effectively alleviating the need for the filter part of the active front end.Namely, the transformer leakage inductances are used as the filter inductors. On the other side, thecontrol verification is performed on the basis of an industrial control system required owing to thesize and complexity of the full-scale converter.The flexibility issue of the medium voltage perturbation injection converters is addressed through thehardware and control reconfiguration of the CHB. The ac converter is reconfiguredfor dc operation with three different modes of operation possible, depending on the desired voltagelevel. The ideas behind the converter flexibility are demonstrated through the simulations coveringthe measurement of the terminal characteristics of the MMC.