Transient Modeling And Control Analysis Of A Pulse-Generator/Rectifier Power System

The controlled long-pulse magnetic field, also called semi-continuous, quasi-continuous, etc., which integrates the advantages of both continuous and pulsed magnetic fields, is put forward by scientific specialists to meet the requirements of some scientific experiments on the field stability and intensity for a certain period of time. Although the kind of flat-top long-pulse high-magnetic field has successfully been achieved by a few global laboratories by using pulse generator-rectifier power system, its quality now available is quite from the rigorous demand of some scientific experiment with high resolution. To increase the field-ripple quality of the long-pulse magnetic field would be of great theoretical significance and practical use. This thesis is mainly a contribution on the theoretical research of attaining low-ripple flat-top magnetic field by utilizing the pulse generator-rectifier power system at the Wuhan Nation High Magnetic Field Center (WHMFC). It covers the following major contents:transient average modeling of a varying-speed pulse generator with controlled line-commutated converter, synchronization method for the three-phase power converters, control strategy for the long-pulse magnet system with low-ripple flat-top high-magnetic field, and on-line measurement method for the magnet’s non-linear time-varying resistance.In Chapter2, a new transient average-value model of the varying speed pulse-generator line-commutated converter pair, including the six-pulse rectifier, the twelve-pulse series-connected rectifier and the two distinct twelve-pulse series-connected rectifiers with different dc loads, is obtained which accurately accounts for the rotor and stator transients, the subtransient saliency of the machine, the nonlinearly varying firing angle of the converters and the time-varying magnetic coil loads. As is predicted and observed in the periodical characteristics of the space current vector of the internal armature windings during the commutation and non-commutation states of each semiconductor device, it is convenient for the transient variables on both sides of the converter to be averaged with respect to a prototypical switching interval in the symmetric operation of the machine-converter pair. To ensure the accuracy of the mathematical average-model and to solve the problem of the continuity with the mathematically dynamic model, the instantaneous value of the d and q-axis armature winding currents at the switching time instants is simultaneously divided into low-frequency and high-frequency components, and the magnitude of the high frequency current component is directly calculated as the difference between the instantaneous value and average value of the d and q-axis armature winding currents. The harmonic current balance theory and modulation theory of converter on the basis of the Fourier-series expansion of the switching functions are proposed each to calculate the average value of the d and q-axis armature winding currents during the transient. Characteristics of the DC output voltage of the generator-converter pair is presented here in details, and a kind of an equivalent DC-generator model is proposed to contribute to part of the DC output voltage. Validity and high accuracy of the proposed transient average-value model is proved through a thorough comparison with the detailed valve-by-valve switch-value model.In Chapter3, new synchronization methods for the pulse-generator rectifier power system under the conditons of large frequency changes, sudden phase jumps and serious distortion, are proposed and modified on the basis of the control structures of traditional phase-tracking systems from the point of the specific composition of the phase information. The transient characteristics of the pulse-generator converter pair, esepecially the harmonic contents and varying phase in the fundamental component of the distorted synchronous AC voltage, is fully analyzed. In order to solve the problem of harmonic-component attenuation and anti-phase mutation interference, the type of the filter in the zero-crossing detection method is attentively chosen and its related main parameters are carefully designed. To efficiently improve the performance of the zero-crossing detection method in phase-tracking and noise and impulsive disturbance rejection, a creative solution is proposed by partly or fully adding the calculated phase error between the two adjacent zero-crossings directly into the output phase and simultaneously decreasing the period forgetting ratio. By analyzing the characteristics of the transformed voltage Uqe in the synchronous reference frame, a digital moving average filter is proposed and added into the forward control path in the Synchronous Reference Frame PLL (SRP-PLL) to effectively filter the switching harmonic components in the distorted synchronous AC voltages. Its control parameters are optimized briefly based on the specified condition of AC voltage’s magnitude and frequency of the pulse-generator converter pair. The Routh-Hurwitz stability criterion on the linearized model is utilized to find the stable range of the control parameters in the SRP-PLL with the moving average filter.In chapter4, an optimal and adaptive double closed-loop controlled strategy with a feedforward compensator is proposed to solve the problem of unavoidable large field overshoot and low-frequency field oscillation during flat-top period in the traditional single-loop field (or current) control mode, which is operated simply by the PI controller to regulate error signal between magnetic field setpoint and actual field to zero. On the basis of analyzing the specific electrical characteristics of the sing-coil long-pulse magnet, an instantaneous voltage and current double closed-loop controlled strategy with a current feedforward compensator for the controlled twelve-pulse rectifier energizing one magnet coil is proposed and designed. State differential equations of the long-pulse magnet system with two coaxially-nested and mutually-coupled coils are mathematically obtained for detailed analysis. A special control method by operating one12-pulse rectifier in open-loop control mode and the other in feed-back control mode is implemented to solve the existing coupling problem in the constant current control mode in the two rectifier modules due to the different time constants of DC-loads. Furthermore, an analogous double closed-loop control strategy, with a feedforward compensation loop, is adopted for the power rectifier of the inner coil while operating the other converter for the out coil in open-loop mode to decrease the field overshoot and to improve the stability during flat-top period. To validate the good dynamic performance of the proposed control method, simulation results in generating flat-top low ripple field in the magnets also are presented.In chapter5, for the identification of the long-pulse magnet’resistance needed for the high-stability control of the flat-top magnetic field, a novel computation method employing the gradient descent technique for on-line caculation of the dynamically varying resistance of the long-pulse magnet coils is proposed to solve the disadvantages of poor anti-interference ability and low accuracy of the traditional directly-calculating method. The detailed computating model of the single magnet coil is first achieved with its main control parameters appropriately designed on the basis of the magnet’s load time constant. With the corresponding gradient control coefficients, its performance about time response and capacity of disturbance rejection is analyzed brifely. Next displayed is the on-line resistance computation model of the long-pulse magnet with two coaxial nested coils. Its main control parameters are carefully designed considering the different current pulse width and time-response requirements between the inner and outer magnet coils. The high accuracy of the proposed on-line computation method is finally validated with its performance simulated on the condition with variable dc voltages and serious harmonic interferences.