| Due to the high energy storage density and strong controllability,the pulse generator-rectifier power system is suitable to offer flat-top long-pulse high-magnetic field and background magnetic field for pulse high-magnetic field,and its control strategy and operation mode concern the quality of the magnetic field(including the stability,precision and so on).On the basis of the pulse generator-rectifier power system,the research of this thesis focuses on following major contents: modeling of the pulse generator-rectifier power system,phase-locked method for three-phase voltage signals,high-precision control strategy for output current,commutation and control strategy during inverter.In the aspect of power system modeling,a novel twelve-pulse rectifier model based on concept that the average DC voltage is calculated in every duration between two time-sequence adjacent natural commutation points,and the average DC voltages are united in this twelve-pulse rectifier model whether the firing angle is changing or not;A new model in state-space description for generator is established,which adopt the direct-axis and quadrature-axis stator currents as constraint and could describe the transient response exactly,including the effect of damper windings;After analysising the relationship among generator,twelve-pulse rectifier and phase-locked system,a novel model of pulse generator-rectifier power system with phase-locked system is proposed,this model avoids the large computational error in conventional models when the system is with inductive load,could describe the transient response of generator,rectifier and phase-locked system exactly when the firing angle fluctuates quickly in large range,and contributes to the design of phase-locked system and controller;The operation state of the power system is analysised and the influence of the phase-locked system performance to power system are discussed on the basis of this model,and the simulation results are given to prove the accuracy of the proposed model.In the aspect of phase-locked method,the features of three-phase voltage signals and requirements of the power system are analysised,and further a phase-locked method based on the filtering and compensation of the phase deviation,which eradicates the worsening transient response caused by fluctuation of voltage signals amplitude,is proposed.The concept of filter type is introduced as the criterion during the filter design for different occasions,and three types of forth-order filters are designed,improving the transient response and filtering performance;As for the frequency estimation,equivalent transfer function of frequency estimation is decoupled from equivalent transfer function of phase estimation,which contributes to faster response,better filtering and smoother time-domain response;Then an upgraded phase-locked method,which uses two types of filters by changing switch and has better transient response / filtering tradeoff,is proposed and applied to flat-top long-pulse high-magnetic field system;Finally the performance of proposed phase-locked method is verified through simulation.In the aspect of high-precision control strategy for flat-top current,the demand of the control system are analysised according to the characteristics and requirements of the system,and then a feedforward based on the off-line neural network and feedback current control strategy is proposed to produce flat-top long-pulse high-magnetic field.As for feedforward control,the predicable disturbances of the system are evaluated by linear fitting,and further,corresponding linear feedforward is proposed to compensate the predicable disturbances and maintain the magnetic field stability;The feedforward concerns the state of generator and resistance of magnet,so a off-line neural network is used to calculate the optimal feedforward,making it suitable for different initial conditions;As for feedback control,proportional control is adopted because the feedforward control improves the robustness and reduces the difficulty for feedback control,and the principle for setting proportional parameter is that the flat-top magnetic field should approach the step response of critically damped or overdamped system,which could minimize the overshoot and oscillation of flat-top magnetic field;Then this control strategy is applied to dual-coil magnet system;Simulation results show the fine performance of this control strategy.In the aspect of the commutation and control strategy during inverter,the interaction between commutations of two 6-pulse rectifiers in a 12-pulse rectifier is studied,dual-overlapping theory is proposed to explain the commutation failure caused by this phenomenon,obtaining the operating range of firing angle to avoid the commutation failure,and further,operating range for 24-pulse rectifier is deduced;The transient process when the system is transforming from rectification to inverter is studied,the relationship between control parameter and operating range of firing angle are analysised,and the design of control parameter to avoid commutation failure is proposed;The validity of the theory and method is certified through simulation.At last,On the basis of the pulse generator-rectifier power system at the Wuhan Nation High Magnetic Field Center,experimental platforms for flat-top long-pulse high-magnetic field and triple-coil magnet system are established through necessary modification of the pulse generator-rectifier power system according to what is proposed in this thesis;The experiments for flat-top long-pulse high-magnetic field and triple-coil magnet are carried out,and the experimental results show that the proposed methods are capable to improve the controllability of the power system,enhance the stability of the flat-top magnetic field and ensure the inverter and operation. |
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