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, as well as the integration of power electronics equipment, is shaping the requirements for stable grid infrastructure. These requirements mainly come from the stability-related phenomena arising from different subsystem interactions. Namely, medium voltage systems require special attention due to the lack of equipment and solutions intended for their impedance measurement. Hence, this thesis provides the perception of the problem of medium voltage impedance measurement and system identification from the point of view of perturbation injection converters. A cascaded H-bridge the converter supplied from a medium-voltage multi-winding phase-shifting transformer is proposed in this thesis. Moreover, in combination with wideband injection signal, it imposes itself as a viable solution to this problem. The thesis commences with an overview of readily available converter topologies and injection signals. The converter topologies are discussed, their downsides are pointed out and it is outlined how these issues can be addressed by using the cascaded H-bridge topology. Additionally, the lack of flexibility of the state-of-the-art solutions is highlighted. Furthermore, the modelÄ ing approach of the single power electronics building block of the cascaded H-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 the single-phase H-bridge inverter. An estimation of the source-load affected dynamics is provided on the basis of the closed-loop control modelling. This question needed to be answered out of the concern for the interaction between the active front end and the H-bridge inverter. For efficient perturbation injection, the active front end should not limit the output dynamics of the H-bridge inverter. Real-time simulations in combination with additional single-phase dq-frame measurement and identification methods revealed that there is in fact very little to no influence between two sub-converters. Moreover, the terminal characteristics of the active front end, i.e. its input admittance and output impedance are measured in an experimental setup, providing the first result in the full experimental verification of the notions proposed. The hardware and control designs of the active front end are subsequently verified through the power circulation tests in a setup including two active front ends and the medium voltage multi-winding transformer. On one side, the verification is made possible due to the transformer primary side synchronization 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, the control verification is performed on the basis of an industrial control system required owing to the size