Flexible AC Transmission System Control Strategy Research

Along with the rapid development of national economy, the electrical power demand is increasing. The reverse distribution between energy and load demands that the energy should be optimized in a wide range and the UHV strong smart grid should be constructed in national range. However,the wide range interconnection of power grids makes the network structure and operation states more complex, which need the higher requirement of operators. The FACTS is the typical high-power electronic technology,which overcomes the shortcomings of traditional control strategy, such as slow response speed and discrete control function. The FACTS also improve the control ability of AC grid and transmitting capacity of transmission lines.The application of multi-type FACTS devices achieves the global/local objective optimization. The high-performance of FACTS devices depends on the control system and special electronic devices. The researches and analysis of FACTS devices are of great significance, which will improve the application value and usable range of FACTS in practical AC transmission project.From the prospective of grid-connection modes, the aim of this paper is to analyse and improve related issues on control strategies of FACTS devices, including the series FACTS, the parallel FACTS and the hybrid FACTS. The specific research works are as follows:(1) The control strategy of series FACTS: Based on the principle of controlled series compensator, the impedance characteristic, damping characteristic and harmonic characteristics of TCSC are analyzed. The control system, stable control strategy and impedance control strategy are proposed. And the PID impedance control strategy of TCSC is emphasized,which provides the important basis for the practical application of TCSC.The basic principle of static synchronous series compensator is analyzed.The constant voltage compensation mode and constant impedance compensation mode are both illustrated. Based on the dq synchronous rotating reference frame, the decoupled control scheme is given, which includes negative feedback and cross decoupling.(2) The control strategy of paralled FACTS: The basic principle of STATCOM is analyzed. The reactive power transmission and operating modes of STATCOM are illustrated based on the relation of the inverter output voltage and network voltage. The double-loop vector controller is established for STATCOM and the typical simulation model is given on the platform of PSCAD/EMTDC.(3) The control strategy of hybrid FACTS: The basic principle of UPFC is analyzed. Based on the existed control system of UPFC, the inverter with indirect current control is adopted in UPFC control strategy. The typical time-domain simulation model of UPFC is established, and the corresponding device-level control and bottom-level control models are designed.(4) Simulation models of STATCOM and UPFC are established in PSCAD/EMTDC according to the engineering cases. STATCOM model is used to verify the validity of double-loop vector control strategy; meantime,it’s used to verify the validity of dc voltage control and reactive power control. UPFC model is established to validate the indirect current control strategy of paralleled and series converters.