Control Of Unified Power Flow Controller (upfc)

As the most flexible controller of the Flexible AC Transmission System (FACTS), a Unified Power Flow Controller(UPFC) can control transmission line impedance, voltage magnitude and phase angle either selectively or simultaneously in appropriate combinations and can also control independently the real and reactive power flow in a transmission circuit. This paper introduces the principle, performance and basic operating modes of the UPFC.The PWM inverter, one of the devices of the UPFC, is studied and the direct current control scheme of the PWM rectifier is discussed in this paper .A single neuron self-adaptive PID controller is designed in the scheme, which decreases the response time of DC voltage sharply when the load is changed. A hysteresis band current controller is analyzed and the calculating formulas about its switching frequency are deduced in the paper. According to the determinative factors of switching frequency, this paper presents a current control scheme with alterable hysteresis band, which reduces the range of switching frequency and improves the performance of the controlled current.By means of vector analysis, a mathematics model of the series part of the UPFC is deduced in dqo orthogonal co-ordinate within synchronously rotating reference frame. A state feedback decoupling method is applied in the model. Consequently, the real and reactive power can be controlled independently. Finally, a BP neuron network PI control scheme is presented for the mathematics model, and the parameter of the PI controller can be found automatically and adjusted online. Simulation results prove that the scheme improves the self-adaptive capability of PI controller, and lowers interaction of the real and reactive power.