Page 50 - Modern Control of DC-Based Power Systems

CHAPTER TWO

Small-Signal Analysis of Cascaded
Systems

2.1 MVDC SYSTEM CONSIDERATIONS INFLUENCING
VOLTAGE STABILITY
The scope of this chapter is to present to the reader how certain
simplifications or assumptions impact the system behavior with respect to
stability. Starting from a control oriented description of a CPL, it is then
indicated to the reader how the system behavior is impacted by choosing
different converter topologies and their respective control schemes. This
is then later extended to different types of loads which are typically not
considered in stability studies and an analysis is undertaken on the impact
on a controlled system under the classic control theory (single input single
output (SISO)) is altered. For the classical control theory, practical design
examples for a cascaded system are given, including small-signal stability
assessments at the design phase and during operation with the help of the
online system identification approach.
In the medium voltage direct current (MVDC) modeling and simula-
tion in shipboard power systems, normally the zonal or load interface
converter is modeled by a step-down converter controlled as a voltage
source. The step-down converter is designed and controlled for the pur-
pose to represent the CPL characteristics as a load interface converter.
However, in reality the ideal constant power load (CPL) behavior could
not be valid in the MVDC system, because of the limitations mentioned
as follows [1,2]:
1. Saturation of converter: When the converter saturates (i.e., duty cycle
cannot exceed 1) the converter goes into open loop and therefore the
forced behavior by the control is no longer valid. Therefore the nor-
mal load behavior, e.g., resistive, will be presented rather than the
forced CPL behavior. This saturation exists commonly in the real sys-
tem, as typically the duty cycle of a step-down converter is set for
economic reasons close to the saturation limit. On one hand, this

Modern Control of DC-Based Power Systems. © 2018 Elsevier Ltd.
DOI: https://doi.org/10.1016/B978-0-12-813220-3.00002-8 All rights reserved. 15