Application of SVC and STATCOM to improve stability of AC/DC hybrid power systems and to suppress subsynchronous oscillation

Abstract

As the power systems continue to grow in size, large interregional AC/DC hybrid grids come into being, ultrahigh-voltage long-distance power transmission lines and ultra-large-capacity generation units are gradually put into operation, and the series capacitor compensators and High Voltage Direct Current (hereinafter referred to as HVDC) systems are adopted to improve the power system stability and transmission capability. Therefore the subsynchronous resonance/oscillation problems and the stability problems in the large-scale AC/DC hybrid power systems resulting from these factors will necessarily become the problems in the secure operation of the grids that can't be ignored. With the implementation of the "Transmission of Power from the West to the East" program, it is of obvious engineering application value to research the stability of AC/DC hybrid grids and the technologies of suppressing subsynchronous oscillations of the power system. This dissertation, in accordance with the applications of the SVC and STATCOM in the large complicated AC/DC hybrid grids, proposes novel control design theories and methods, and tests these theories and methods through calculation analyses and simulation experiments. The SVCs and STATCOMs designed according to these theories and methods have been successfully applied to the 210Mvar SVC at 500kV Wuzhou Substation in China Southern Power Grid, the four sets of SSR-DSs in Guohua Jinjie Power Plant, the 240Mvar SVCs at Chuxiong Converter Station of the Yunnan-Guangdong {171}800kV ultrahigh-voltage DC power transmission project in China Southern Power Grid, and the {171}200Mvar STATCOMs at 500kV Dongguan Substation in China Southern Power Grid. This dissertation contributes to the research and engineering practice of the SVC mainly in the following aspects: (1) Calculates the static voltage stability and transient voltage stability of China Southern Power Grid through simulation experiments, especially researches the transient voltage stability in the cases of abrupt load increases and the faults in tie lines, builds the models for the motor loads at the receiving-end system, calculates and analyses the impact of the proportions of dynamic loads and the abrupt load increases on the stability, simulates the dynamic processes in time of AC/DC faults in the AC/DC hybrid grids, and researches the impact of bipolar block faults in the DC power transmission systems in the cases of different load models on the stability and calculates the reactive power requirements. (2) Introduces the engineering design and calculation methods of the capacity of the SVC used to enhance the system voltage, calculates the voltage fluctuations at the 500kV side of Wuzhou Substation when the SVCs are put into operation and the swing curves of the maximum relative angles among the generator units in the system when the SVCs are in/out of operation, tests the necessity of installing the SVC accordingly, and chooses the capacity of the SVC at Wuzhou Substation according to the comparisons of the stability calculation results in the cases of typical faults. (3) Brings forward the design method of the additional damping controller of the SVC to suppress the low-frequency oscillations, and conducts analysis and verification through time-domain simulation experiments, whose results prove that this design approach is effective, providing ground for the design of the SVC used for suppressing the subsynchronous oscillations of the power systems. The Prony analysis is introduced to get the controlled system model for the SVC damping controller tuning. The most detailed and widely used SVC control system and control strategies for the supplementary controller design and parameters tuning is modeled. (4) Takes the SVCs at Wuzhou Substation as an example, conducts simulation experiments with the RTDS simulator in connection with the real SVC controller in accordance with the IEEE 30-bus systems and tests the concrete results of the SVC used for improving the steady and transient stability and for suppressing low-frequency oscillations of the power system. (5) Proposes the scheme of suppressing subsynchronous resonance/oscillation with the SVC, introduces the concrete system structure design, presents the implementation of the algorithm of introducing the subsynchronous frequency components in the process of fundamental-frequency susceptance regulations, and on the basis of the actual project at the Guohua Jinjie Power Plant and with the methods combining off-line calculation, dynamic simulation and field test, conducts comprehensive and in-depth calculation and analysis of the subsynchronous oscillations resulting from the series compensator at Jinjie Power Plant, and the four sets of SSR-DSs designed successfully accordingly have been put them into operation. (6) Introduces the roles that the SVC plays in ameliorating the stability of the power systems containing HVDC systems, analyses the cause of reactive power oscillations in the context of two sets of SVCs in parallel operation, proposes a novel composite control structure that enables shunt 2 self-governed SVCs to achieve balanced outputs in steady states and transient states and eliminated oscillations, and tests the real SVC controller through the simulation experiments with the RTDS simulator. The SVCs at Chuxiong Converter Station of the Yunnan-Guangdong {171}800kV ultrahigh-voltage DC power transmission project were designed based on this structure, and the field operation results are good. This novel composite control structure also tackle the difficult problems of current-averaging in large power electronics devices of the FACTS equipment, and the 200Mvar STATCOMs at Dongguan, the STATCOM of the largest capacity worldwide, designed based on this structure have been in operation. (7) Researches the STATCOM in providing reactive voltage support for the grid, builds the mathematical model for the cascaded STATCOM, proposes the principle and implementation of the decoupled control of the STATCOM, builds the simulation model of the STATCOM, and tests the effectiveness of the control system designed according to the decoupled control principle under normal and abnormal conditions through time-domain simulation experiments. The 200Mvar STATCOMs designed on the basis of this principle and algorithm are the STATCOMs with the highest voltage level of direct connection worldwide. The works of this thesis can be also adopted to improve performance for distributed generation system such as wind farm which is sensitive to oscillation and demands dynamic reactive power support.