Research On Operating Characteristic And Control Strategy Of Unified Power Flow Controller

Unified power flow controller is a device with comprehensive function and practicalapplication value in flexible AC transmission system, but also is an effective means ofregulating the flow of transmission line. As a series and parallel combined bi-directionalconverter, the unified power flow controller is capable of regulating multiple controlobjectives rapidly and effectively in power system, so as to realize the independent control ofthe active and reactive power flow in lines. Therefore, the study of operating characteristicand control strategy of unified power flow controller has very important significance toensure the economic and stable operation of power system.Firstly, the basic method of power flow control is analyzed, and the principal theory iselaborated that unified power flow controller can realize dynamic regulation of active andreactive power in transmission lines, and improve the transmission capacity of lines. Thepractical operation and control process of unified power flow controller is illustrated from twoaspects of voltage balance and power balance principle. Secondly, the power operation rangesof unified power flow controller aiming at power of input terminal, output terminal and intothe grid are obtained through the derivation of unified power flow controller power balancecharacteristic equations. Based on above, the relationships between power flow of inputterminal, output terminal and into the grid with injected voltage across transmission lines ofthe unified power flow controller are visually indicated by drawing the various parts of powerchange curves. Thirdly, the conditions that satisfy the requirements of series and parallel sideconverter proper operation are studied by analyzing operating characteristics of the unifiedpower flow controller shunt and series converters. Through discussing control methods of theinverter, the control strategies for series and parallel converters are designed according toflow control requirements. Aiming at the interaction problems between the active and reactivepower of unified power flow controller in the regulation process of power line flow, themethod using power decoupling control strategy to meet independent control of lines flow isproposed. Combined with basic approach of power flow regulation, three different outer looppower control methods and flow control modes are designed, making unified power flowcontroller achieve more control function. Finally, the grid-connection simulation model ofunified power flow controller is established, and the proposed power flow control strategy issimulated by MATLAB/Simulink.Through the analysis of power balance, voltage dynamic balance for unified power flowcontroller, the control targets to realize power flow regulation are obtained by control of thevoltage that injects from unified power flow controller into transmission line, the decoupledpower control strategy and the integrated power flow control strategy of unified power flow controller are designed. The correctness and feasibility of flow control strategies designed arevalidated by simulation analysis.