Study On The Control Strategy Of The Series-Parallel Double Conversion System For UPFC

Since 1980's FACTS system, which is based on the technology of power electronics, have gotten more and more attention. Among the FACTS devices Unified Power Flow Controller composed of a parallel and a series conversion system can not only work separately as a STATCOM or a SSSC but also control the real and reactive power flow alone the transmission line independently. Such characteristics attract the eye ball of the researchers. To solve the problems existed in the research field of UPFC, this paper works in the six areas as listed below. First, the real/reactive power absorbed/injected by the components of the UPFC system during power flow changes is plotted in 3D space. That provides an effective method to determine the power rating of the power conversion parts of the UPFC and judge the availability of the control scheme for UPFC. By observing the power map, it can be found that every point of the transmission real & reactive power is corresponding to a certain voltage injected by the series part of the UPFC. However, for the power map of the injected power by the series part of the UPFC one power operating point may corresponding to more than one voltage injected by the series part of the UPFC. So only the series voltage injected by the series part of the UPFC can be controlled to manipulate the transmission power flow. According to the discussion of the voltage control characteristic of UPFC, to maintain the input port voltage of the UPFC constant, the parallel part of the UPFC must compensate proper reactive power and there is a linear relationship between the output reactive power of the parallel part and the input port voltage amplitude of the UPFC. On the other hand the analysis also indicates that when the voltage amplitude of the input port of the UPFC is controlled to be constant during the power flow variation, it is the shunt part of the UPFC that provides all the reactive power needed by the transmission reactive power variation and the variance of the reactive power absorbed by the series part of the UPFC. And it is shown that the input port voltage of the UPFC can be controlled by the shunt part of the UPFC as well as by the series part of the UPFC, while the reactive power flow of the transmission line can be controlled in two different way too. Second, according to the operation requirement of UPFC and STATCOM this paper presents effective methods and formulations to calculate and select the best circuit parameters for the parallel part of the UPFC. With the proper circuit parameters it is easier for the parallel part to realize it control target, and that ensures the stable operation of the UPFC system. Based on the analysis, the decoupling control strategy and the controller design procedure are presented. With the proposed control scheme and control parameter the parallel part works separately as a STATCOM is tested with experimental works, and the effects of several voltage control schemes are discussed. Third, in this paper the control system of the SSSC/DVR and the procedure of inserting the series device smoothly into the transmission line are discussed. The little signal model of the SSSC is established and used to design the dc bus controller for the SSSC. Experimental results indicate that the control system has good performance. Would SSSC cause Sub-Synchronous Resonance? Till now there are different opinions. In this paper analysis and experimental works are done to prove that SSSC will cause SSR with non-proper control in the capacitor simulation mode. At last, the main power flow control strategies for the UPFC are analyzed and compared. Simulation and experimental works are done to test the conclusions. The results prove that the cross coupling power flow control scheme has slow dynamic response performance, and the influence between the real and reactive power flow is small. The cross decoupling control scheme gets a faster response but during the power flow changes there will be a large power flow variation caused by coupling phenomenon. Coordination control add forward-feed signal to the real & reactive power reference of the parallel part of the UPFC, and it can improve the dynamic performance of the dc bus voltage control and the input port voltage control, however this control method can not improve the reactive power flow dynamic control performance. Based on the real & reactive power balance analysis, this paper proposes a novel control scheme for UPFC firstly, which is different to the conventional strategies. In this control scheme the parallel part of the UPFC is used to control the dc bus voltage and the transmission reactive power, the series part of the UPFC is applied to control the UPFC input port voltage and the transmission real power flow. Because the series part controlsthe UPFC input port voltage directly and acts as a DVR with real power flow control function, a less input port voltage variation is expected during the power floe changes. On the other hand because the reactive power output of the parallel part is controlled directly to manipulate the transmission reactive power flow, a faster reactive power flow performance is expected. Simulation and experimental works proved the effectiveness of the proposed control scheme, although this scheme does nothing to improve the dynamic performance of transmission real power flow control, its reactive power flow control performance is better than cross coupling control and there is little real power variation caused by coupling during the reactive power changes. Analysis indicates that this control scheme is insensitive to the transmission line parameter variation, so this proposed control scheme is an effective method to improve power flow control performance, when the transmission line parameter is not properly known. In this paper a hardware experimental system is established to test various control schemes of UPFC. And those are the first effective experimental results gotten in the nation. These works study the control characteristics of UPFC and make a useful exploration of the powerful FACTS device for future application in China.