Research On High Voltage Direct Current Power Supply Based On Three-Level Converter

Recently the telecommunication technology has been rapidly developed in many fields. Just as well AC UPS has been widely used, as the power supply of the telecommunication room. In virtue of the inherent characteristics, AC UPS makes it in the process of operating showed up some disadvantages, such as poor reliability, low power density and low conversion efficiency. In view of the above-mentioned shortcomings, there are growing calls here that high voltage direct current(HVDC) power can instead of AC UPS in the telecommunication power field. Compared with AC UPS, HVDC power has higher reliability, higher efficiency, higher power density and more convenient extension.Through the analysis of HVDC power system designing requirements, a power system prototype of HVDC power main circuit with 6k W rated power is set up. A two-stage converter is adopted in the main circuit. As the first stage, VIENNA converter implements power factor correction(PFC). As the second stage, half-bridge three-level LLC resonant converter implements electrical isolation and voltage conversion.In control system, differing from the traditional control structure of two-stage converter, this paper adopts double loop PWM control mode which consists of inner current loop and outer voltage loop. To simplify the control structure, the voltage sampling comes from the output end of the second stage converter in outer voltage loop. Meanwhile the traditional LLC resonant converter exists some drawbacks, such as difficult design of the magnetic components, wide range of frequency change and low efficiency in light load. The half-bridge three-level LLC resonant converter will operate with the fixed-frequency and fixed-duty ratio control mode in this paper. To operate with higher efficiency, the switch frequency of LLC converter will be equal to the resonant frequency in theory. To restrain the disturbance from the resonant component parameter, this paper has presented a new control method of LLC resonant frequency tracking. In view of the dead-time influence on uhf LLC resonant converter, this paper establishes the mathematical relationship between the output voltage of the bridge arm and the dead-time. In terms of the software, the control parameters including the PI controller are designed for the voltage loop and the current loop based on the above control method. The program design of double-loop with voltage and current control is completed.A power system prototype of HVDC power main circuit with 6k W rated power is set up. The TMS320F28335 DSP control chip of TI and the EP3C25Q240C8 N FPGA control chip of Alter have been used for digital control. The simulation results verify the feasibility of the new control method. The prototype achieves power factor correction and a stable output voltage. The experiment results are consistent with theoretical analysis and simulation results.