Research On Hihg Voltage DC UPS System Based On Three-phase VSR And LLC Converter

Uninterruptible Power Systems(UPS)is usually regarded as a standby for electrical system.It works in case of urgency when a failure occurs in main power due to external factors.In view of traditional AC and low voltage DC UPS,there are many shortcomings such as poor reliability of power supply and difficulty in capacity expansion.Meanwhile,the demands on UPS reliability and its power supply capacity are more and more strict.Under this background,this paper carried out the research of high voltage DC(HVDC)UPS.In this paper,the theoretical rationality of high voltage DC UPS cascaded by voltage-type three-phase PWM rectifier(three-phase VSR)and full bridge LLC resonant converter is studied first.In the system structure,the first stage three-phase PWM rectifier is used to improve the power factor of the input side of the UPS system,and the full bridge LLC resonant converter is used to achieve high voltage conversion and electrical isolation.In the aspect of system control,this paper proves that the traditional dual closed-loop cascade control strategy has the limitation of the unbalanced grid.On the basis of analyzing the traditional instantaneous reactive power concept,an improved anti-grid voltage unbalance control algorithm to ensure that the three-phase PWM rectifier can still maintain the power factor unit,the sinusoidal of net current and DC bus voltage constant.Then,based on analyzing the gain characteristics of traditional full bridge LLC converters and its relationship with the parasitic capacitance and inductance at high frequency,gain characteristic expression and curve of LLC converter including parasitic parameter are derived with the fundamental frequency analysis method.And improved measure on the performance of full bridge LLC resonant converter is given.Finally,taking into account the full bridge LLC resonant converter with variable frequency control has the disadvantages of low efficiency for light load,switching frequency unstable,magnetic components design difficulties and no-load output voltage is not adjustable,this paper presents a unified control scheme of three-phase VSR and fixed-frequency full-bridge LLC resonant converter.The control scheme realizes the dual purpose of simplifying thewhole machine control structure and improving the working efficiency of high-voltage DC UPS system.On the basis of the TMS320F2812 DSP control chip,a 1KW power experimental platform is built.The experiment results verify the correction of the improved control algorithm proposed and the feasibility of the unified control scheme.Meanwhile,the test results are consistent with the theoretical analysis.