Research On Three-level H-bridge Modulation And Control Strategy Of Fusion Magnet Power Supply

Based on the existing high-power fusion power supply,take the 13th Five Year National Major Scientific Facilities opportunity to carry out the relevant research and development work of the new fusion magnet power supply technology and provide a better power supply scheme for the future construction of fusion reactors.The new fusion magnet power supply adopts PWM-related control technology and high-power IGCT fully-controlled devices.The combination structure of voltage source converter(VSC)and three-level H-bridge can achieve high power factor and low harmonic operation on the grid side and four quadrant operation of the load coil.A load of fusion magnet power supply is a superconducting magnet coil,and its inductance value is very large,usually can reach the level of HENG.To meet the operation requirements of Tokamak,the superconducting magnet coil need to pass a large current.The three-level H design will solve the following problems:1)The influence of the low switching frequency modulation strategy in the H-bridge on the output performance of the converter.2)The operating characteristics of the load inductance coil affect the voltage fluctuation and output performance of the H-bridge DC bus and cause high harmonics of the grid side current.3)The device parameters in the load circuit and the current sharing circuit have deviations,which makes it impossible to establish an accurate system model and control the high-performance output current.Given the above difficulties,the main work of the paper is as follows:(1)Aiming to switch on the over-current and off over-voltage of converter simulation,this paper establishes a more accurate electrical model of IGCT.It constructs a new electrical model by combining the B JT model(Gummel Poon model)and passive components described by mathematical equations.The model combines temperature characteristics with the electrical model,which makes the model unified.The experimental results show that the model’s steady-state voltage and current curves are fitted;During the transient process of the model,the accuracy of voltage MAPE reaches 90%,and the accuracy of current MAPE reaches 80%.The model can provide a reference for some critical parameters in circuit design and control method of Hbridge,such as buffer loop parameters,dead time setting,etc.(2)Aiming at the special application scenario of H-bridge low switching frequency of new magnet power supply,the shortcomings of traditional S VP WM in low switching frequency and long dead time operation are analyzed.An optimized SVPW-8H modulation algorithm is proposed in this paper.By rearranging the order and number of sector vectors,the output average voltage continuity is better,and the utilization of DC bus voltage is higher.Moreover,compared with traditional SVPWM,SVPWM-8H does not reduce the harmonic ability and neutral point balance performance of the system output current.Simulation and experiment verify that the overall performance of SVPWM-8H is better under several indicators.(3)The input power of the superconducting magnet coil is in significant dynamic change,which leads to the fluctuation of the DC bus voltage of the new magnet power supply.In this paper,H-bridge and load coil are equivalent to a nonlinear load.By paralleling a bi-directional DC/DC active energy storage unit with a high dynamic response,bus voltage and grid output power are more stable.Simulation and experimental verification show that the active energy storage unit can reduce the bus voltage fluctuation from 10%to about 2%under the condition of the small prototype;The output power of the power grid is stable,and the three-phase unbalanced current on the grid side changes to the three-phase balanced current,and the current THD is low.(4)Aiming at the unknown inductance,resistance,and other disturbances between the fusion power supply and the superconducting inductance coil,a new control algorithm based on disturbance compensation finite time current sharing is proposed by observing the voltage value generated by the unknown parameters with a finite time observer,which improves the dynamic response of the load current and the dynamic current sharing performance of the branch current during the parallel operation of multiple H-bridges.The experimental and simulation results show that the load current overshoot is reduced from 10%to about 2%under the condition of the small prototype;Each branch current keeps consistent in the dynamic process,which verifies the feasibility of the proposed finite time control algorithm.