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POWERCON 2014 Paper No CP1493 Page 1/8
Abstract— In Voltage Sourced Converter (VSC) based HVDC
systems, the existing harmonics on one side ac system will be
transferred through the dc-link to the other side ac system and
induce a series of harmonics that tend to be more than the origin.
This is also true for the case of one side unsymmetrical ac system;
the fundamental negative-sequence component will be transferred
to the dc-link with the second harmonic is dominant, which in
turn causing a series of harmonics on the other side ac system.
This paper describes the operation of VSC using three switching
functions, which include a fundamental component and carrier
sideband components, using double-edge naturally sampled PWM
to yield their harmonic spectra. The space vector representation
of the VSC’s switching functions is used as a tool for analyzing
and giving the detailed rules of VSC’s harmonic interaction, and
from that, analyzing harmonic propagation through the dc-link.
Based on the methodological analysis, the simulation model of a
suggested project for Northwest power grid - China is built and
implemented using MATLAB/SIMULINK. In the investigation of
harmonic propagation, simulation results show that, the
amplitude of transferred harmonics through the dc-link may be
amplified due to the resonance causing harmonic instability. This
deep understanding of VSC-based HVDC system’s harmonic
propagation characteristic could be beneficial to harmonic
mitigation control.
Index Terms—Voltage sourced converter, VSC-based HVDC,
HVDC harmonic, harmonic propagation, switching function.
I. INTRODUCTION
HE INTRODUCTION of Voltage-Sourced Converter
(VSC)-based HVDC to power system offers the potential
to improve control of power flow and enhance system stability.
However, VSC-based HVDC producing harmonics on both
ac-side and dc-side is inherent by the VSC itself and will
interact with harmonic distortions already existed in the system.
The harmonics of VSC are directly associated with the type of
VSC technologies, modulation techniques and the switching
frequency [1]. A more elaborate understanding of these
harmonics must develop to find adequate approaches to
harmonic elimination, preventing the system from instability
and sizing the components of the system.
Manuscript received July ..., 2014. This work was supported by State Grid
Cooperation of China, Major Projects on Planning and Operation Control of
Large Scale Grid SGCC-MPLG019-2012
Phuchuy Nguyen is with the School of Electrical Engineering, North China
Electric Power University, Beijing 102206, P.R China (e-mail:
huynp@epu.edu.vn).
Minxiao Han is with the School of Electrical Engineering, North China
Electric Power University, Beijing 102206, P.R China (e-mail:
hanminxiao@263.net).
Harmonic interactions between the two sides of the converter
and through the dc-link of HVDC are analyzed in [2] where the
converter is a current-source converter (CSC). In the case of
VSC, a practical case in [3] shows that harmonic instability
may occur if there is an unbalanced second harmonic generated
by a nearby saturated transformer on the ac-side of VSC. Based
on analyzing basic current and voltage equations of CSC and
VSC, the authors in [4] establish simple rules of harmonic
transfer through converters by using switching functions
represented in space vector form. This additional understanding
of harmonic interaction is quite adequate to the analytical and
simulation results in [2] and [3]. The dc equivalent impedance
of VSC is formulated by analyzing the harmonic transfer across
converter in [5]. It is convenient for harmonic resonance
analysis.
This paper presents harmonic transfer rules of VSC. They are
suitable for both two-level and three-level converters, not only
valid for voltage harmonics, but also for current harmonics. The
rules were built after analyzing VSC in both balanced and
unbalanced operating conditions by using switching functions
represented in space vector forms. Unlike previous works just
used the fundamental switching component, this paper also
presents the rules of harmonic transfer under the interaction of
higher order switching components. These rules are adopted
for both positive- and negative-sequence harmonics,
accounting for all harmonics propagated through the dc-link of
VSC-based HVDC. Based on these rules, the dc- and
ac-equivalent impedances of VSC were calculated to give a
clearer view of the harmonic propagation.
The paper has five sections. The first section describes
VSC-based HVDC system suggested for Northwest power grid
– China, used to study harmonic interaction and harmonic
propagation. The second section represents the principle
operation of VSC describing the relationships between both
side quantities under space vector representation. Also, the
switching function of both two-level and three-level converters
are calculated depending on the adopted PWM techniques,
applying double Fourier integral analysis. Thereafter, the rules
of harmonic transfer through VSC are established and applied
to VSC-based HVDC harmonic propagation. The equivalent
impedances are also used to analyze this propagation. Finally, a
Matlab/Simulink model of the VSC-based HVDC system is
built to validate the method.
II. VSC-BASED HVDC SYSTEM
POWERCON 2014 Paper No CP1493 Page 1/8
Abstract— In Voltage Sourced Converter (VSC) based HVDC
systems, the existing harmonics on one side ac system will be
transferred through the dc-link to the other side ac system and
induce a series of harmonics that tend to be more than the origin.
This is also true for the case of one side unsymmetrical ac system;
the fundamental negative-sequence component will be transferred
to the dc-link with the second harmonic is dominant, which in
turn causing a series of harmonics on the other side ac system.
This paper describes the operation of VSC using three switching
functions, which include a fundamental component and carrier
sideband components, using double-edge naturally sampled PWM
to yield their harmonic spectra. The space vector representation
of the VSC’s switching functions is used as a tool for analyzing
and giving the detailed rules of VSC’s harmonic interaction, and
from that, analyzing harmonic propagation through the dc-link.
Based on the methodological analysis, the simulation model of a
suggested project for Northwest power grid - China is built and
implemented using MATLAB/SIMULINK. In the investigation of
harmonic propagation, simulation results show that, the
amplitude of transferred harmonics through the dc-link may be
amplified due to the resonance causing harmonic instability. This
deep understanding of VSC-based HVDC system’s harmonic
propagation characteristic could be beneficial to harmonic
mitigation control.
Index Terms—Voltage sourced converter, VSC-based HVDC,
HVDC harmonic, harmonic propagation, switching function.
I. INTRODUCTION
HE INTRODUCTION of Voltage-Sourced Converter
(VSC)-based HVDC to power system offers the potential
to improve control of power flow and enhance system stability.
However, VSC-based HVDC producing harmonics on both
ac-side and dc-side is inherent by the VSC itself and will
interact with harmonic distortions already existed in the system.
The harmonics of VSC are directly associated with the type of
VSC technologies, modulation techniques and the switching
frequency [1]. A more elaborate understanding of these
harmonics must develop to find adequate approaches to
harmonic elimination, preventing the system from instability
and sizing the components of the system.
Manuscript received July ..., 2014. This work was supported by State Grid
Cooperation of China, Major Projects on Planning and Operation Control of
Large Scale Grid SGCC-MPLG019-2012
Phuchuy Nguyen is with the School of Electrical Engineering, North China
Electric Power University, Beijing 102206, P.R China (e-mail:
huynp@epu.edu.vn).
Minxiao Han is with the School of Electrical Engineering, North China
Electric Power University, Beijing 102206, P.R China (e-mail:
hanminxiao@263.net).
Harmonic interactions between the two sides of the converter
and through the dc-link of HVDC are analyzed in [2] where the
converter is a current-source converter (CSC). In the case of
VSC, a practical case in [3] shows that harmonic instability
may occur if there is an unbalanced second harmonic generated
by a nearby saturated transformer on the ac-side of VSC. Based
on analyzing basic current and voltage equations of CSC and
VSC, the authors in [4] establish simple rules of harmonic
transfer through converters by using switching functions
represented in space vector form. This additional understanding
of harmonic interaction is quite adequate to the analytical and
simulation results in [2] and [3]. The dc equivalent impedance
of VSC is formulated by analyzing the harmonic transfer across
converter in [5]. It is convenient for harmonic resonance
analysis.
This paper presents harmonic transfer rules of VSC. They are
suitable for both two-level and three-level converters, not only
valid for voltage harmonics, but also for current harmonics. The
rules were built after analyzing VSC in both balanced and
unbalanced operating conditions by using switching functions
represented in space vector forms. Unlike previous works just
used the fundamental switching component, this paper also
presents the rules of harmonic transfer under the interaction of
higher order switching components. These rules are adopted
for both positive- and negative-sequence harmonics,
accounting for all harmonics propagated through the dc-link of
VSC-based HVDC. Based on these rules, the dc- and
ac-equivalent impedances of VSC were calculated to give a
clearer view of the harmonic propagation.
The paper has five sections. The first section describes
VSC-based HVDC system suggested for Northwest power grid
– China, used to study harmonic interaction and harmonic
propagation. The second section represents the principle
operation of VSC describing the relationships between both
side quantities under space vector representation. Also, the
switching function of both two-level and three-level converters
are calculated depending on the adopted PWM techniques,
applying double Fourier integral analysis. Thereafter, the rules
of harmonic transfer through VSC are established and applied
to VSC-based HVDC harmonic propagation. The equivalent
impedances are also used to analyze this propagation. Finally, a
Matlab/Simulink model of the VSC-based HVDC system is
built to validate the method.
II. VSC-BASED HVDC SYSTEM