Page 31 - Modern Control of DC-Based Power Systems

xxx Introduction

Further advantages are reduction of power system weight-to-space
ratio and the reconfigurability in case of fault and enhanced power
quality.
An ISPS consists typically of multiple generators, loads, power lines,
power electronic devices, and filters. The fact that a small isolated distri-
bution system with limited generation capability and low rotating mass
has to accommodate fast changing load levels allows certain analogies to
the terrestrial DC microgrid or other application fields as aircrafts, subma-
rine vehicles, or space stations. In this context, the developed and pre-
sented concepts can be a vital contribution to the terrestrial DC
microgrids or DC distribution systems in general, since every ship offers a
greenfield deployment of new technology.
Here is a short introduction about the background and motivations of
this book. The main research objectives and contributions are presented
in a compact way. Finally, a structured outline is presented.
In DC ISPSs the individual loads, or groups of loads, are normally fed
through power electronic converters directly connected to the distribu-
tion Medium Voltage DC (MVDC) bus. Power converters are Variable
Structure System (VSSs): they are by definition nonlinear. When observ-
ing the system from the DC bus the load converters exhibit a Constant
Power Load (CPL) behavior, which is also nonlinear.
In order to maintain the load power constant while being under the
influence of fast current and/or voltage disturbances, the load converters
are tightly controlled; as a consequence, the CPLs present towards the
DC bus a negative incremental resistance behavior. Under certain condi-
tions, this behavior may lead to instability of the MVDC bus [2].
Conditions under which the instability takes places and methods for the
stability analysis of MVDC systems have been reviewed and discussed in
Ref. [3].
Traditionally, the voltage stability assessment of DC systems is focused
on ensuring stability with small variations around one given operating
point, thus small-signal stability. The small-signal assessment has been exe-
cuted in cascaded systems with the stability analysis in the frequency
domain, and the Middlebrook criterion and its further developments [1].
Up to now, adequate methods for assessing stability at large-signal level
are still not widespread or only available at the level of AC systems.
The MVDC ISPSs efforts have been concentrated in the design of
power converters interfacing the loads towards the MVDC distribution.
In order to prevent the MVDC ISPS from voltage instability, the load