A study on Development of an algorithm for reducing neutral current in three-phase four-wire PWM rectifier of UPS Kim,Seung Ho (SupervisorSong,JoongHo) Dept.of Electrical Engineering Graduate School of Industry and Engineering Seoul National University of Science and Technology-SOUTH KOREA

ABSTRACT
Recently, the power quality sensitive loads such as computer and communications are so increased that the power supply with good power quality has come to the fore.However,it is possible that the public grid is interrupted by unexpected fault.Therefore,the UPS(uninterruptible power supply)has be come an alternative solution to solve this problem.Generally, the UPS consist of a rectifier,a battery,and a PWM inverter and recently they use IGBT PWM rectifier to reduce input THDi and improve the input power factor.

In this configurations the three-phase four-wire inputUPS is widely used to eliminate the transformers in the UPS system.The aims of control algorithm for the IGBT PWM rectifierin three-phase four-wire UPS is a unity power factor.The SVM(space vector modulation)is one of the most popular and preferable PWM scheme because it reduces the commutation losses and harmonics in the output voltage,and has higher modulation index than SPWM(sinusoidal PWM)but they need a 3D SVM to control three-phase four-wire PWM rectifier and have some difficulties like as selection of inductance of input inductor and controls the neutral current.

To improve this problem the another control algorithm which controls each phase individually has been suggested but this control algorithm use analog based control is sensitive and unstable.So another new algorithm which is added d-q control to control algorithm which controls each phase individually is suggested and it can control stably and can reduce neutral current too.This can be seen by computer simulation and experimental result.