Research On The Operation Characteristics And Control Strategies Of UPFC

With the help of FACTS technology, our AC transmission system will become more controllable, effective, security, economy. As one the most complex element in the FACTS'family, UPFC is a multi-functional device including the functions of both STATCOM and SSSC. With proper control strategy, UPFC can be used to adjust real and reactive power of the transmission line, maintain voltage of the bus where UPFC is connected, improve transient stability and damp out low frequency oscillate of power systems. This thesis devotes itself to the in-depth studies of UPFC, including the issues of the dynamic characteristic analysis, the control strategy design, the experiment equipment development and the application of UPFC in power systems.Firstly, A decoupling systemic control strategy based on the proportion-integral (PI) control for the UPFC shunt- series VSCs is proposed, The main objective of the proposed control is to realize the following functions of the UPFC, in which the shunt side of the UPFC is mainly used to maintain both the voltage of the AC system where the UPFC is connected as well as the voltage of the DC capacitor, and the series side of the UPFC is used to control the power flow of the transmission line. Time-domain simulation model of UPFC base on its component devices is set up in detail using PSCAD/EMTDC. Simulation results show excellent dynamic performance of UPFC, including voltage control, power flow adjustment, and oscillate damping against the fault.Secondly, the damp characteristic of UPFC is studied in detail using eigenvalue method. Based on the main controller for power flow adjustment, a supplement controller is designed specially to improve the ability of low frequency oscillation damping, it is realized based on the principle of PSS. The oscillation frequency of the power system is used as the feedback signal. The performance of the UPFC as well as the supplement controller is investigated by simulation using PSCAD/EMTDC. Simulation results show that UPFC has the ability of damping out the low frequency oscillation and the supplement controller can further enhance power system low frequency oscillation ability.Thirdly, A PI self-tuning control strategy based on the fuzzy logic control theory is proposed for the UPFC series VSC. Details of designing the fuzzy rules and the determination table for the fuzzy logic control are given in this paper. In order to show the effectiveness of the proposed control, simulations are carried out for the proposed UPFC models as well as the control using PSCAD/EMTDC. Simulation results show that the dynamic characteristic of UPFC with the proposed control is much better than that with the conventional fixed parameter PI control. The power flow adjustment speed is greatly increased, and the stability of the system against the fault is also considerably improved.At last, a 15kVA UPFC experiment equipment is realized successfully in a dynamic simulation laboratory environment. Details of the supervisory control system of UPFC equipment is discussed in detail together with the detail experiment results.