Study On The Control Strategy Of UPFC Based On Node Current Injection Method

Flexible AC Transmission Systems (FACTS) devices have enriched andenhanced the regulation and control methods of the power system to a great extend,due to their fast response, flexible control, flexible adjustment, etc.. Meanwhile,unified power flow controller (UPFC) is considered to be the most creative and themost powerful FACTS components, which is because of its functional combinationof more than one of the FACTS devices, making it capable of controlling the busvoltage, the active and reactive power flow of the line separately or simultaneouslyin any combination.As the software industry is growing more and more mature, a variety ofcommercial software has been integrated with the power flow calculation program,making it possible to reduce workload greatly if a UPFC model adaptive to thecurrent commercial software is proposed. However, the behavior of the currentmodels of UPFC is very likely to depend on the bus type of the access node, whichmakes the application of the model limited.The UPFC was studied in depth in this paper, including its mathematical modeland control strategy. It is found through the analysis of the mathematical model ofUPFC that, the control of UPFC to the power flow of the line can be realizedthrough the control of the injection current of the shunt side and the series side;therefore, a control strategy of UPFC based on the node current injection method,which is adaptive to PSASP so as to make full use of its internal resources to reducethe programming work, was proposed in this paper. When the node current injectionmethod is adopted to establish the control model of UPFC, the convergence of thepower flow might be worsen and the tuning of the control parameter might be moredifficult if the traditional control method of UPFC is adopted; therefore, animproved control model was proposed in this paper, in which the bias control wasadopted and, the active and reactive power was controlled, respectively, by thed-axis and q-axis component of the forecasted current. The transient control modelof UPFC was proposed with the consideration of the dynamic characteristics of thecapacitor, in order to analyze the control effort of the UPFC to the power flow andthe bus voltage during the transient process. The user program of both thesteady-state model and the transient-state model were established with C++ language. The calculation was completed with the alternative call of the userprogram and the internal power flow program through the user interface of PSASP.The simulation results show that the proposed model of UPFC based on nodecurrent injection method had a good performance in the control of the power flow inthe steady state as well as the transient state. It can not only improve thetransmission power of the controlled lines, but also help to control the power flowand the bus voltage to a steady state during the transient process so as to improve thetransient stability of the system.