Research On Dynamic Characteristic Of The Cascaded STATCOM And Application To Control The Power System Stability

In recent years, power demand has increased substantially, the link between regional power grids will be continuously strengthened. The huge nonlinear dynamic system characterized by large unit, extra-high voltage, long-range, heavy duty, large area interaction and AC and DC transmission. Regions can effectively carry out power support, achieve efficient allocation of resources and improve the gird reliability and economic efficiency. However, the expending size of grid also brings the problem of power system voltage stability, and the possibility of power failure significantly increases. How to improve the system voltage stability level and prevent the voltage collapse accident has been an issue must be considered in the safe operation of power system. After the emergency of flexible AC transmission system technology, researchers around the world made a lot of research, mainly focusing on the use of FACTS equipment to improve the power system angle stability, prevent sub-synchronous oscillations, control system trend distribution and improve user side power quality. STATCOM is the representative of today's latest technology in reactive power compensation, which is an important component of flexible AC transmission system (FACTS).Starting from the main circuit topology, the dissertation analyzed the advantages of STATCOM device based on chain H-bridge inverter unit cascading multi-level topology; introduced the unipolar multiplier CPS-SPWM modulation strategy applied in cascaded STATCOM; Analyzed and compared the direct detection and indirect detection algorithms of p, q reactive current based on instantaneous reactive power theory on STATCOM current tracking performance and the impact on DC voltage control effect; and analyzed the basic principles of cascaded STATCOM compensation reactive power and common connection point voltage.The dissertation established STATCOM nonlinear mathematical model via equivalent circuit exchange. Later, based on differential geometry theory, it put forward a state feedback exact linearization method to precisely linearize cascaded STATCOM system, and ultimately made the design of cascaded STATCOM active and reactive decoupling controller. The control of cascaded STATCOM was converted into single H-bridge inverter unit control via the change of equivalent circuit. On the basis of the established H-bridge inverter unit mathematical model of considering parameter perturbation and unknown disturbance, the impact of parameter perturbation in error system equation was converted into a part of cycle disturbance. Then it used the new method of combined repetitive control and passive control to realize the current control design of cascaded STATCOM based on robust L2performance criteria. Among them, the repetitive control compensation periodic disturbance and passive control ensured the convergence of repetitive control and progressive track of control objectives, and suppressed the impact of non-periodic disturbance. Through theoretical analysis and simulation verification, cascaded STATCOM current control strategy based on robust L2performance criteria had stronger robustness than the current control strategy based on state feedback exact linearization decoupling.In the next step, the thesis emulates applications of STATCOM at a simple electric system with CONUS software. To have a better view over ability of flexibly controlling and compensating reactive power of STATCOM, the author has compared STATCOM with SVC at the same capacity and installment position. Emulation result is in line with the theory, clearly expressed that STATCOM is preeminent against SVC.Finally, the thesis researches applications of STATCOM in electric system in Vietnam to compensate reactive power, uphold stability of voltage system and improve power supply confidence. Vietnam is geographically narrow and long, stretching from the North to the South with unequally-located power sources and sub-loading along with uneven loading development, which results in a variety of hindrances in balancing effective power and reactive one. Upon calculation, analysis of real electric system operation and establishment of P_V feature at nodes, the author has figured out that voltage nodes have strong fluctuation, which can easily cause voltage collapse at southern region. The author uses PSS/E-33software to calculate OPE in order to opt for suitable location so that STATCOM capacity just compensate reactive power for the southern power grid in2014. The simulation result reveals that after putting STATCOM or SVC at the same location and capacity as calculation result of the thesis, the system can ensure stability of voltage, improve the whole system stability, in which STATCOM performs better than SVC.