Ph.D. Thesis
POWER CONVERTER SYSTEMS FOR HVDC
TRANSMISSION INTEGRATED WITH WIND FARM
Advisor: Professor Dong-Choon Lee
Presented as Ph.D. Thesis
2013 June
Graduate School of Yeungnam University
Department of Electrical Engineering
Control and Electric Machinery ∙ Power Conversion Major
Author :Thanh Hai Nguyen
Abstract
In this thesis, a novel power converter system for high-voltage direct current
(HVDC) transmission integrating the offshore wind farm (WF) into the grid is proposed,
in which a hybrid of a twelve-pulse diode rectifier (12P-DR) and a voltage-source
converter (VSC) is employed. For this topology, the 12P-DR is capable of delivering a
part of wind farm power, the rest of which is absorbed by the wind farm VSC (WFVSC)
since the WFVSC is controlled and operated as a voltage source of a constant frequency.
Also, the current at the PCC becomes almost sinusoidal since the WFVSC functions as
an active power filter for the 11th- and 13th-order harmonic current components. By
virtue of the diode rectifier, the voltage rating of the diode rectifier and the VSC is
reduced by configuring them in series connection. Then, the cost and power loss of
HVDC converters is reduced compared with the case of the conventional fully-rated
VSC, whereas the performance is kept almost the same.
By a simple VSC with two levels, the efficiency of the proposed HVDC converter
is about 99.07% high, compared with 98.4% in the fully-rated VSC system, and the cost
of power semiconductor devices including the gate drivers in the HVDC converter
system is about 53.47% compared with that of in the VSC-based HVDC links. In
addition, to improve the performance and the efficiency of the power converter system
for HVDC links, the modular multilevel converter (MMC) is utilized instead of the
simple 2-level VSC, in which the efficiency of the proposed HVDC system is about
99.27% compared with 99.0% and 98.4% of the HVDC transmission systems based on
the fully-rated capacity of the MMC and the neutral-point clamped (NPC) multilevel
converter, respectively.
For integrating the HVDC transmission system with the distorted and unbalanced
voltage of the network, a novel control scheme for the grid-side converter of the HVDC
transmission systems based on a composite observer is proposed to mitigate the grid
current distortions due to unbalanced and distorted grid voltage conditions for the gridconnected
PWM converter. The composite observers can extract the fundamental and
harmonic components of the grid voltages and currents precisely without any magnitude
reduction or phase delay. Then, the positive- and negative-sequence components are
separated from the fundamental components by all-pass filters. The grid current
components are regulated by a multi-loop harmonic current controller, in which the
positive-sequence component is regulated by PI (proportional-integral) controllers and
the negative-sequence and harmonic components are regulated by PR controllers. This
approach allows the unbalanced and distorted components in the grid current to be
eliminated from the system.
For the wind farm operation, a control strategy to smooth the output power of the
wind farm before delivering to the grid is also proposed, in which the inertial effect of
the wind turbines and coordinated operation of individual wind turbines (WT) are
utilized. A two-level control scheme is applied to control the wind farm, which consists
of a high-level control for the wind farm and low-level controls for individual WTs. The
power references of the wind farm and each wind turbine generator are produced by the
high-level controller, whereas the individual WTs are controlled to produce the power as
their commands by the low-level controllers. With the control scheme, the output power
of the WF is smoothened, even though the output powers of individual WTs are
fluctuated. For this control strategy, when the reference power is lower than the
available power, some of individual wind turbines are operated in the kinetic energy
charge operation by increasing the turbine speeds. Then, some of individual wind
turbines release the power by reducing the turbine speed, when the power command is
higher than the available power.
The PSCAD/EMTDC simulation results for a 500-MW HVDC transmission system
integrating the wind farm into the grid have been shown to verify the validity of the
proposed scheme. Also, the experimental tests have been carried out for the laboratoryscaled
system of the HVDC link and wind turbine system.
