The Research Of UPS Front-End Rectifier Based On Three Closed-Loop Control

UPS (Uninterrupted Power Supply) makes front-end rectifier, energy storage device(always a group of batteries) and inverter as the core, which can continue to supply power for the users in a short period of time when there are faults in the power supply.Front-end rectifier which is one of the core device in the UPS system, plays a key role in the process of providing high quality power for the inverter.On the basis of further understanding the basic principle, structure, characteristics and related theoretical knowledge of PWM rectifier and battery charger,this paper whose background is the application of UPS power rectifier charging technology, designs an UPS front-end rectifier that based on three closed-loop control.The digital processor TMS320F2812produced by TI company is made as the control chip.The main working content of this paper as follow:Firstly,component, function, classification and basic principle of the UPS were briefly introduced in this paper, as well as the working principle, topological structure and control strategy of PWM rectifier and battery charging circuit. The mathematical model of UPS front-end rectifier was respectively established in the three-phase stationary coordinate system, two-phase stationary coordinate system and two-phase rotating coordinate system. The key control principle of UPS front-end rectifier,such as space vector pulse width modulation, power factor correction and harmonic suppression technology were studied and analyzed.Secondly,three-phase voltage source PWM rectifier topology was designed as the main circuit in this paper, at the same time the battery charging circuit was abandoned. Rectifier and charging are united in the same topology. Traditional predictive current control technique is introduced in the rectifier, which can correct the power factor and restrain the harmonics. The three-stage charging method is used in the charging system, they are current limiting, balanced charging and float charging. Current limiting charging is the constant current charging. Charging current PI loop is added in the double loop of traditional predictive current, then three closed-loop control is formed, which are current outer loop of main circuit, DC voltage inner loop and charging current inner loop. Phase tracking technique is used in the current outer loop of main circuit, which can correct the phase of voltage and current in the grid side. Double stage PI regulation is composed by the DC voltage inner loop and charging current inner loop, which can make DC output voltage and DC charging current track the command value respectively. Balanced charging and float charging are both constant voltage charging process,UPS front-end rectifier which is controlled by the traditional predictive current technique, is working in the state of rectifier. At the same time, batteries are being charged.Finally,for the requirement of design,the experimental prototype of UPS front-end rectifier is built in the laboratory.The rectifier system experiment of UPS front-end rectifier was did on the condition of40kW resistive load, and the three stage charging experiment whose loads are20kW resistance and17pieces of lead-acid battery is also did in the laboratory. The experimental results also directly verify the feasibility and superiority of troditional predictive current control technique and the three closed-loop control.