| The high-power magnet power supply system is one of the critical systems to ensure the safe and stable operation of the magnetic confinement nuclear fusion device.It is usually based on the thyristor phase-controlled converter technology.With the development of power devices,the power supply schemes based on fully controlled power devices and PWM converter technology are increasingly widely used in highpower fields,with comprehensive performance advantages.The "Comprehensive Research Facility for Fusion Technology(CRAFT),"a central national science Facility during the 13 th Five-Year Plan Period,has carried out pre-research work on a new magnet power supply scheme.It provides a better choice of power supply scheme for the next-generation fusion reactor construction.Based on the unique operating conditions of high power and low switching frequency of the three-phase three-level PWM converter in the new magnet power supply scheme,the key technologies of its control system are studied in this paper.It is mainly related to high-precision threephase phase-locked loop(PLL)design under non-ideal power grid conditions,the modulation technology under low switching frequency,and the control strategy with excellent dynamic and stable performance.The main contents and innovations of the paper are as follows:Aiming at the problem of low accuracy of traditional PLL in the environment of non-ideal power grid with 100 MW installed capacity,an improved PLL based on a frequency-adaptive hybrid filter is designed.This PLL combines the advantages of the second-order generalized integrator(SOGI)and the moving average filter(MAF),enhances the suppression ability of the power grid’s three-phase unbalanced and harmonics,and effectively improves accuracy.The dynamic response speed of the PLL is accelerated by reducing the delay of the integration window of the MAF.Considering the unique operating condition of the magnet power supply with high power and large current,the switching loss of the power device in the converter increases significantly.Since the restriction of heat dissipation and other conditions,the converter’s switching frequency needs to be reduced to hundreds-Hz.Compared with the traditional seven-segment SVPWM,the four-segment SVPWM is proposed to improve the converter’s performance by doubling the control frequency under the same switching frequency.A simplified algorithm of the four-segment SVPWM in the ABC coordinate system is given,significantly reducing computational complexity.On this basis,the midpoint balance control method based on the charge conservation principle is introduced to improve the midpoint potential balance of the DC side.The current inner loop control strategy based on finite control set model predictive control(FCS-MPC)was studied to improve the dynamic response performance of converters.Firstly,it is analyzed that the function value is proportional to the distance between the primary and reference vectors.Then the number of iterations from 27 to 6-7 times is reduced by applying the Nelder-Mead algorithm principle in FCS-MPC optimization.It is significantly reducing the computational load.Secondly,the FCSMPC is combined with four-segment S VPWM to realize fixed switching frequency and effectively improved the AC side harmonic characteristics of the converter operating at the low switching frequency.Considering the characteristics of the rapid change of the load power of the magnet power supply,the traditional voltage outer loop based on the PI controller can not meet the demand of dynamic response.Firstly,the characteristics of the power flow of the new magnet power supply are analyzed.Based on the principle of power balance,a power feedforward control strategy is proposed to speed up the converter output response to load power changes.The PI controller is replaced by the active disturbance rejection controller(ADRC)to accelerate the dynamic response speed,reduce overshoot,and improve the DC side bus voltage stability.In this paper,the effectiveness and feasibility of the proposed PLL scheme,modulation technology and control strategy are verified by theoretical research,system simulation and low-power prototype test.The works provide solutions for future engineering design and system debugging of the new magnet power supply. |
