The principle and control design of UPFC ( United Power Flow Controller ) are expounded. The system equation of PI ( Proportional Integral ) controller is set up by using the horizontal and vertical components of series output voltage and shunt output current as control inputs. 阐述了统一潮流控制器(UPFC)的原理及其控制器设计,采用串联侧输出电压、并联侧输出电流的横向分量和纵向分量为控制量,建立了比例积分(PI)控制时的系统方程。