Power Flow Studies of HVDC Grids with DC Power Flow Controllers
Date
2023-04-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0002-2224-5450
Type
Thesis
Degree Level
Masters
Abstract
High-Voltage Direct Current (HVDC) transmission, especially based on voltage source converters
(VSCs), have attracted significant research interests due to renewable energy sources integration
in power grids, notably offshore wind farms. Despite recent research contributions in the literature
on HVDC systems, a number of challenges remain unsolved, such as lack of a comprehensive
study regarding power-electronics-based devices in HVDC systems, suitable modelling
approaches for sophisticated DC power flow controllers, power loss modeling of DC power flow
controllers, powerful and practical DC power flow solvers, and the highly-meshed test structure
of HVDC grids for power flow.
To address these research gaps, in this thesis, a comprehensive literature review has been
conducted on power electronics devices in HVDC systems in Chapter 2. These devices are divided
into three categories in the review: 1) power converters; 2) DC/DC converters; and 3) DC power
flow controllers (DCPFCs). As an emerging power electronics device being introduced less than
a decade ago, DCPFCs are the main focus of this thesis.
A novel unified Newton-Raphson (NR)-based DC power flow solver (DCPFS) is presented in
Chapter 3 to solve the DC power flow problem in multi-terminal HVDC (MT-HVDC) grids by
employing a novel DCPFC, the multi-port interline DC power flow controller (MIDCPFC). The
proposed DCPFS modifies physical and control state variables of the whole system (MIDCPFC
and the MT-HVDC grid) simultaneously to control power flow in HVDC lines, especially
overloaded lines. The static model and the power injection model of the MIDCPFC are obtained
and their equations are embedded within the designed DCPFS. The absence of the fictitious bus
preserves the original conductance matrix of the system and its symmetry, and thus, the original
system's Jacobin matrix only needs minor modifications in the developed unified NR-based
DCPFS. Additionally, the proposed DCPFS is straightforward for implementation since the
voltage of the intermediate capacitor of MIDCPFC is treated as an independent variable, as a result,
there is no need to use external processes to control its value. The shunt conductance of HVDC
lines is also considered. The comprehensive models have been proposed to model power losses of
MIDCPFC and VSCs for the first time. Finally, a new modified 15-bus MT-HVDC grid is
proposed and implemented for verification purposes. The obtained results verify the accuracy and
efficacy of the proposed concepts, models, and formulations of this study.
A novel sequential NR-based DCPFS is proposed in Chapter 4 to solve the DC power flow problem
in MT-HVDC grids by employing MIDCPFC and decoupling the power flow equations of the
MIDCPFC and the MT-HVDC grid. In the proposed sequential NR-based DCPFS, there is no
trace of fictitious buses, the original conductance matrix of the system and its symmetry are
preserved, and the shunt conductance of HVDC lines is considered for precise modeling. The
structure of the proposed DCPFS is sequential, which decouples the MIDCPFC and grid related
power flow equations. A prominent feature of the DCPFS is that it fully preserves the system's
original Jacobin matrix and does not require any modification to that matrix, which reduces the
computational burden. In addition, power losses of the MIDCPFC and VSCs are embedded in DC
power flow equations. The proposed sequential NR-based DCPFS is straightforward to implement
as the voltage of the MIDCPFC is treated as an independent variable, and consequently, no external
process is needed to control it. Various scenarios are tested on a modified 15-bus MT-HVDC grid
to verify the proposed sequential NR-based DCPFS. The accuracy and efficacy of the proposed
approach is validated through these case studies.
Description
Keywords
HVDC systems, Multi-port Interline DC Power Flow Controller, Power Flow Study, Newton-Raphson, Multi-terminal HVDC Grids
Citation
Degree
Master of Science (M.Sc.)
Department
Electrical and Computer Engineering
Program
Electrical Engineering