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 and complexity of the full-scale converter.
The flexibility issue of the medium voltage perturbation injection converters is addressed through the
hardware and control reconfiguration of the CHB. The ac converter is reconfigured
for dc operation with three different modes of operation possible, depending on the desired voltage
level. The ideas behind the converter flexibility are demonstrated through the simulations covering
the measurement of the terminal characteristics of the MMC.