The Research On Theory And Applications Of Filter Commutated Converter And Its Impacts On The HVDC System

The power grids are developing quickly in China so far. With the application of"nation-wide power grid interconnection, China's power transmission from the West to the East, mutual supply between the North and the South, and power system market"strategy plans. The constructions of High Voltage Direct Current (HVDC) systems for long distance and large capacity power transmission are absolutely going to be formed in the future. However, that will bring about some technical problems such as harmonic pollutions in the grids, the harmonic transfer between AC and DC side in HVDC system and the harmonic instability, various kinds of distributions in power systems, which can finally lead to the breakdown of the operations in HVDC systems. The dissertation is funded by the key science & technology project 05GK1002-1 of the Tenth Five Year Plan and 06GK1003-1 of the Eleventh Five Year Plan of Hunan Province which is called"The research and exploitations of new-type HVDC system and its electric apparatus", focuses on the patent of invention and the patent of utility model:"Self-coupled and Harmonic Suppression Converter Transformer"and coordinating with thyristor converters, establishes the theoretical analytical system of novel Filter Commutated Converter (FCC) in HVDC transmission system. To implementing the industrialization of FCC, a great deal of theoretical analysis and experiments are programmed. The main works are organized as follows:Introductions of the present applications and development status of HVDC in China. The history and technical characteristics of converting apparatus, converters and filtering equipments in HVDC systems are descried. Series of problems come from the applications of Line Commutated Converter (LCC), Capacitor Commutated Converter (CCC) and Thyristor Controlled Commutated Converter (CSCC) are summarized and a novel circuit topological structure of FCC is presented, which has the abilities of harmonic suppression at valve side and reactive power compensated at the same time. Also the stabilities of HVDC systems are strengthen by FCC. The winding connections scheme of FCC is presented, the vector relations and ratios between the ac windings and valve windings are analyzed in 12 pulse FCC-HVDC systems. Four kinds of structures of the new systems are introduced according to different power capacitors and transportations. The inductive harmonic suppression mechanism and reactive power compensated in FCC are studied, the parameters of valve side filtering systems which working on the resonant points are designed and a dc transmission platform is used to modeled and compared with LCC & FCC, the simulation results show that the theoretical analysis of FCC is correct, parameters of the platform are proper, and the filtering equipments work effectively.Taking the particularity of the connection scheme of FCC into account, the port voltage equation of valve side and commutated reactance calculation formula are obtained based on the theory of multi-winding transformer,Kirchhoff's law and port voltage & current equations, as a result the FCC structure could be converted into the equivalent circuit model of Graetz bridge. The steady-state and transient-state characteristics of LCC & FCC are compared with each other based on the mathematical models. A dc transmission exploitation platform is used to verify the correctness of the theoretical analysis, and the simulation results and experiment data agree well with each other.The impedance-frequency characteristics of converter are important for analysis and solve harmonic instability in HVDC system. The computing formulas of equivalent dc impedance and equivalent ac impedance of FCC are derived theoretically based on switching function method considering the communication course. The dc transmission system exploitation platform is taken as an example to comparing the calculations of impendence- frequency characteristics between traditional line commutated converter(LCC) and FCC, the simulation results show that the impedance-frequency characteristics of converter has an un-ignorable impact on ac system resonant frequency, the stability of ac system would be strengthen and improved by FCC, the equivalent ac impedance reduces remarkably at ac resonant frequency, the harmonic instability in HVDC can be avoided more effectively as a result.The Qusai-Steady-State (QSS) model of the FCC-HVDC is established based on the CIGRE standard HVDC model and the small signal state equations are derived and the dynamic response processes are calculated when the rectify side is under constant current control mode and the inverter side is under constant voltage control mode, the calculated results are coherent with the simulation results. The characteristic roots curve of the changeable control system parameters is analyzed to obtained the suitable PI parameters, it is verify that FCC-HVDC equipped with the control parameters has a better stability and dynamic response feature.The commutation failure of converters is a kind of common fault of DC system when faults of AC system occur. The concrete diversity of commutation processed between symmetrical three-phase conditions and unsymmetrical three-phase conditions, the calculation formulas of the actual extinction angle are given and the relationship among the inverter sudden commutating voltage reductionΔV, extinction angle, dc current and the phase shift has been studied. Then the steady mathematical model of FCC is established and the influences of the compensation factor of series capacitors in FCC's valve side filtering system have been investigated on the HVDC system's extinction angle , commutation angle, voltage of the bus at converter station and commutation reactance; a detailed analysis of dynamic characteristics comparisons between FCC and line commutated converter(LCC) working as an inverter under AC faults are simulated by PSCAD/EMTDC based on the DC transmission platform, the results shows that single-phase faults are somewhat more onerous for the probability of commutation failures than three-phase faults, the harmonic contents is decreasing significantly and FCC can complete the commutation process under more serious sudden voltage reduction, and also the compensation factor of the valve side in FCC should be selected in such a way that the commutation failure could be avoided and commutation reactance could be limited as much as possible. Also the faults on the dc bus lines and convert bridges are analyzed as the LCC & FCC working as a rectify converter respectively, the results show that FCC has a better suppression effects on the short currents of dc bus line and the converter valve sets.The structure and testing functions of the dc transmission platform are introduced at length. According to the present testing functions and measurements conditions, parts of experiments are programmed and carried out to verified the theoretical analysis of FCC, a series of more credible technical information are obtained from the testing data and wave curves to verify the practically applications in projects of FCC.