Three-Phase High Performance Double-Conversion UPS

Nowdays,UPS is widely used to critical device.In order to reduce the pollution made by front stage of UPS,PWM rectifier is adopted.The voltage source rectifier(VSR) can provide nearly sinusoidal input current ,unity power factor ,regenerating capacitity and contant controllable DC bus. The main drawback of VSR is the input line current contains high frequency component around the switch frequency.This frequency ripple may pollute the grid. This problem can be solved at the price of choosing a high value ac inductors making it expensive and bulky.Moreover,the dynamic performance of the system will becomes poor with that so high value of inductance.Adopting an LCL-filter configuration allows to use reduced value of the inductance and to reduce the switching frequency pollution emtted to grid. Meanwhile,adopting an LCL-filter configuration will reduce the stability of system and lead to excessive damping and made unnecessary increase of losses.Both passive and active damping can solve it.In this paper,we mention two methods:virtual resistor and lead-lag element and adopt lead-lag element in application.Morevoer,we also use 4-leg inverter as the DC/AC part of the UPS to realize three phase four wire system.Investigate the relationship between Space-Vector Modulation and Three-Phase Carrier-Based PWM.Based on this relationship,we can use a simple Carrier-Based PWM method for three-phase four-leg voltage source converter which can achieve the same performance of using 3D-SVPWM for it.Last we will illustrate an double-conversion UPS containing LCL-filter VSR as the AC/DC part connected to grid and 4-leg inverter as the DC/AC part.Thanks to unified modulation method for both AC/DC and DC/AC part,the two parts using carier-based PWM with same zero sequence will equivalent to using SVPWM method.