  This paper presents a novel direct-injection modular universal power flow and
quality control topology exclusively using lower power components. In addition
to conventional high-voltage applications, it is particularly attractive for
the distribution and secondary grids, e.g., in soft open points, down to low
voltage as it can exploit the latest developments in low-voltage high-current
semiconductors. In contrast to other concepts that do not interface the grid
through transformers, it does not need to convert the entire line power but
only the injected or extracted power difference. The proposed power flow and
quality (f/q) controller comprises a shunt active front end, together with
high-frequency links serving as a power supply for a series floating module per
phase. Each of the floating modules is in series with one phase of the line,
floating with the electric potential of that particular phase, avoiding any
ground connection. Omitting bulky and dynamically limited line transformers of
conventional universal power flow controllers, the presented direct-injection
f/q controller enables exceptionally small size and volume, high power density,
high frequency content, and fast response. In contrast to direct-injection
concepts with full back-to-back converters, it only needs to handle a fraction
of the power. The circuit combines grid-voltage low-current electronics in the
shunt unit and low-voltage high-current modules in the floating series
injection units. Simulations and experiments demonstrate and validate the
concept.
