Page 141 - Modern Control of DC-Based Power Systems

Generation Side Control 105

For the implementation of the DC DC conversion stage, various
technologies could be used, e.g., buck converter, DAB, or MMC. The
buck converter is selected, considering that unidirectional power flow is
necessary.
Van der Broeck et al. stated about buck converters [6]:
• They provide a low-cost and robust topology, exhibit well known
steady state and dynamic properties and the design procedures for
these converters have been studied intensively.
• However, the control of a one quadrant buck converter over a wide
load range and varying voltage transfer ratios is challenging.

4.2 STATE-SPACE MODEL
The modeling of the ISPS in this book is based on Ref. [7], which
is a radial concept topology. It features a DC bus and the loads are con-
nected either through a DC DC converter, which throughout this book
has been referred to as a point of load converter directly bus or via a load
zone. The generation devices are connected via a rectifier and a following
DC DC converter to the MVDC bus, this DC DC converter will be
abbreviated as line regulating converter (LRC). A more detailed represen-
tation is depicted in Fig. 4.2 [8]. It will be presumed that each load zone
can be represented by a lumped load.
In the model, four generating systems (G1 G4) are considered, which
are connected through rectifiers (C1 C4) in cascade with buck conver-
ters, i.e., LRC (C5 C8) to the MVDC bus. The MVDC bus feeds all the
shipboard loads, through service power converters (C9 C15). In Fig. 4.2
the loads M1 and M2 are ship propulsion motors, each fed by inverters.
The loads and propulsion motors are connected to the MVDC bus
through cables (CA1 CA6). Loads L2 and L3 are LVDC (low voltage
DC utility) and LVAC (low voltage AC utility) loads and are respectively
fed by buck converters and inverters. It has to be highlighted that the stor-
age components which are mentioned in Ref. [7], such as supercapacitor,
batteries, or flywheel, are omitted from the analysis in this book.
From Fig. 4.2 the general circuit model representation of the system
can be derived. It is depicted in Fig. 4.3. The representation shows n DC
generating systems in parallel with m loads. In this circuit model the DC