Research On Key Techniques For LCC Resonant Two Stage High Voltage DC Power Supply

The high voltage dc power supply has been extensively applied in the fields of industry, medical instruments, environment governance, etc, which continually promote the development of national economy. LCC-SPRC is one of the ideal topologies for high voltage power supply,since it is compatible with the parasitic parameters of the transformer and possesses good ability of zero-voltage switching (ZVS),output voltage regulation and short circuit protection. The nonlinearity of LCC-SPRC mainly appears as complicated operation modes. As a result, the research on steady-state analysis,parameter design and control optimization is necessary to achieve good performance of the converter. Besides, high voltage power supply conventionally employs diode rectifier as the front-end converter. The main drawbacks are lower power factor and high current harmonics injection into the grid. The study of active rectifiers is of great importance to improve the line power quality of high voltage power supply. This dissertation mainly discusses a two-stage high voltage dc power supply which employs a LCC-SPRC and a VIENNA rectifier as front-end converter. Modeling, steady-state analysis, parameter design and control strategy are further studied. The main contents are as follows.A dynamic phasor-based model for front-end VIENNA rectifier is developed. The issue of neutral point(NP) voltage is studied. The imbalance of dc-link voltages and low frequency ripples (LFRs) are caused by nonzero NP current in a cycle. A zero sequence voltage injection method is considered to balance the NP voltage. The required zero sequence voltage is decomposed into its dominant components, i.e. dc term and 3rd harmonic component. The relationship between NP current and zero sequence voltage is derived. A modified one cycle control (OCC) scheme with zero sequence voltage injection is then proposed. The dc term and 3rd harmonic component of zero sequence voltage are calculated independently and regulated by a distribution parameter. The proposed OCC scheme can balance the NP voltage and eliminate the LFRs.The validity of the proposed method is verified by both simulation and experimental results.In order to study the nonlinearity of LCC-SPRC with capacitive filter, an equivalent circuit is developed by representing the parallel resonant capacitor,the transformer,the rectifier and the output filter with a complex impedance. A large signal model of LCC-SPRC is built based on phasor transformation method.The differential equations of state variables in time domain are transformed into complex form. As a result,the simulation speed is improved and the steady-state behavior of the LCC-SPRC is well captured. The steady-state characteristics of LCC-SPRC are studied including voltage conversion ratio, resonant current and ZVS operation. The effect of load, switching frequency, input impedance angle, rectifier nonconduction angle and resonant capacitor ration is investigated. The guidelines for parameter design are provided. A parameter design method for impedance angle restriction is proposed.The proposed method can optimize the resonant current while satisfying the requirements of output power and ZVS operation. The theoretical analysis and parameter design method are verified by simulation and experimental results.Based on the large signal model proposed in chapter 3, the power stage small signal models of LCC-SPRC for conventional phase shift (PS) modulation and asymmetrical phase shift (APS) modulation are developed respectively. The small signal models of digital PS modulator and APS modulator are further studied. The transfer functions of digital PS and APS modulators are derived based on dynamic phasor method, which indicate that a small signal delay exists in the digital modulator. The small signal delay for digital PS modulator is a function of PS angle and switching period, depending on the modulation strategy.As for the digital APS modulator, the small signal delay is also a function of PS angle and switching period,while independent of duty ratio. The couple effect of two input of digital APS modulator is evident only when high harmonic components are considered. Simulation and experimental results verified the small signal models and the theoretical analysis.An APS control is proposed to improve the efficiency of LCC-SPRC. Two control variables, i.e. duty ratio and phase shift angle are introduced, which can be independently regulated with a pulse width modulation (PWM) and phase shift combined modulator. As a consequence, additional degrees of freedom in control can be achieved. The steady-state characteristics with APS are analyzed, such as output power,ZVS operation and resonant current. The results indicate that a specific output power can be achieved by different combinations of control variables. In other words, infinite operation points exist for a given output power. However, the operation points are not equivalent in terms of ZVS operation and resonant current. A efficiency optimization strategy is then proposed to search for optimal operation point with minimal resonant current in all output power range, while ZVS is maintained. The proposed strategy reduces the conduction losses and the overall efficiency of LCC-SPRC is improved. The influence of small signal delay on control loop is investigated. The validities of the APS control method and efficiency optimization strategy are verified by simulation and experimental results.A high voltage dc power supply comprising of VIENNA front-end rectifier and LCC resonant converter is studied in this paper. Dynamic phasor based models are developed for VIENNA rectifier and LCC resonant converter. A neutral point voltage balance strategy is proposed for VIENNA rectifier. Steady-state analysis is provided and an asymmetrical phase shift control method is proposed for LCC resonant converter. The validity of theoretical analysis and proposed methods are verified by experimental results.