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.
