Research On Improved Multimodule Parallel UPQC Based On Thyristor And Ultracapacitor

With the rapid development of industry,society to the increasingly strict requirements of power quality,in order to improve more power quality problems,shrinking UPQC device volume and expand function,on the basis of this paper designed a set of bidirectional thyristor and a new type of multiple modules in parallel UPQC ultracapacitors,harmonic current and reactive power compensation capacity and dynamic voltage recovery.In this paper,the topological and mathematical models of the three-phase four-wire UPQC are first studied.The series module of the UPQC is removed and only the parallel module compensation current is retained.Three operating modes are derived: APF,forced shutdown and UPS.The system designed in this paper adopts the ip-iq detection algorithm in the APF mode,and the control strategy combining PI control and repetitive control is adopted in the control algorithm.In the detection of voltage sag,instantaneous voltage decomposition method is adopted and its effectiveness is verified.Then,the turn-off characteristics of bidirectional thyristor are analyzed.On the basis of RC buffer circuit parallel on both sides of bidirectional thyristor,a strategy of using inverter to generate back voltage to force turn-off is designed and its effectiveness is verified in simulation.In this paper,the APF mode and the UPS mode are respectively controlled,and the APF mode and the UPS mode are respectively controlled by the droop control method without interconnection line.An isolated symmetrical half-bridge DC/DC converter circuit is selected on the DC side to connect the inverter and the supercapacitor,and is used as the two-way control of the energy conversion module.The steady state commutation process of DC/DC bi-directional converter is analyzed,a closed loop control system based on phase-shift control is designed and its bi-directional commutation function is verified in the simulation.In the end,the new parallel energy storage UPQC proposed in this paper is modeled and simulated,and the multi-module parallel simulation verification of APF mode,forced shutdown process and UPS mode is carried out.The simulation of voltage fluctuation fault verifies that the energy storage link in the whole parallel system guarantees the stability of dc voltage,and the system can compensate the current and voltage well to ensure the current quality and voltage quality under the voltage sag fault.