Research On Main Circuit Topology And Power Characteristics Of Unified Power Flow Controller

Unified Power Flow Controller(UPFC) is an effective way to control power system states. Main difficulties in application of UPFC exist in the design and modulation of converter structure for high voltage and large capacity application, together with the design and realization of a control system which can guarantee the validity, safety and reliability of UPFC. Base on thorough investigation of recent research and application situation of UPFC, this article analysis the structure, basic principles and output voltage waveforms of several optional converter topologies at the beginning, compare equipment requirements and output voltage harmonics of multi-pulse converters with modular multi-level converter(MMC) as applied to UPFC, then propose the idea of UPFC using modular multilevel converter. After that, this paper studies power flow control and voltage control characteristics of UPFC based on the steady state phasor model, together with study of external characteristic parameters design of UPFC. Through equivalent circuit model analysis of MMC, the commonness of MMC with other voltage source converters is proved. Then this paper established the dynamic differential equation model of UPFC, deduced small signal\models for control targets, and designed outer loop controller together with damping controllers suitable for UPFC. Simulations for testifying the effectiveness of control strategies are done under a seven nodes test system by PSCAD, power flow control, system oscillation damping and system single fault response characteristics of UPFC are tested. At the end of this paper, a design of230kV,50MVA UPFC based on MMC is discussed. After parameters design including DC voltage, arm reactance, modular number and sub-module capacitors, control system for UPFC according to the conclusions of earlier chapters and the characteristics of MMC topology is established. Then two UPFC start-up schemes are proposed and testified. Power flow control and shunt reactive power compensation of the MMC based UPFC are simulated under PSCAD. Simulation results show that the new topology of UPFC can work well, control objectives can fast tracking the set values and realize a stable operation.