Research On Voltage And Power Balancing Control Method For Cascaded Solid-State Transformer

Since the second industrial revolution started the process of electrification,electric energy,which is an efficient and high-quality secondary energy,has gradually become an indispensable part in the normal operation and development of human society.The development and use of electric energy are closely related to the power system,in which the transformer plays a crucial role.At present,power frequency transformer with simple structure,high reliability and low price is widely used in power system.However,with the large-scale grid connection of renewable energy power generation and energy storage equipment and the deepening of electrification in the field of transportation,the power frequency transformer can not meet the development needs of the above fields.In contrast,the cascaded solid-state transformer(SST)has the advantages of ability of power quality management,smaller size and lower weight,which can provide better hardware support for power system.Therefore,it is of great significance to study its topology,related characteristics and control technology.In this paper,single-phase three-stage SST(cascaded H-bridge high-voltage stage,double active bridge isolation stage and three-phase inverter low-voltage stage)is taken as the research object,focusing on the high DC voltage balancing control technology,power balancing control technology and positive and negative sequence component detection technology of power grid signal.The main research contents are as follows:(1)The switching model and average model of single-phase three-stage SST are established,the main transfer functions of dq decoupling control for high-voltage stage,parallel voltage control for isolation stage and P/Q control for low-voltage stage are derived.Then,the parameters of PI controller are designed according to the Bode diagram of transfer function.A single-phase SST simulation platform is built in Matlab/Simulink software to verify the design and function of each controller.(2)The novel additional balancing control strategy for high-voltage stage is proposed,which effectively improves the reliability of the system.Firstly,based on the instantaneous power calculation method of dq vector control,this paper analyzes the influence of active-duty cycle modification of DC voltage balancing control on the reactive power when the highvoltage stage operates in both the unit and non-unit power factor condition.It reveals that the active-duty cycle modification will lead to the unbalanced distribution of reactive power when the high-voltage stage operates in the non-unit power factor condition.By adjusting the reactive-duty cycle relatively based on the modifiable amount of the active-duty cycle,the average distribution of reactive power of high-voltage stage is realized under all operating conditions.In addition,based on the analysis of the dynamic equations of active and reactive current in the high voltage stage,the reason that the DC voltage and reactive power balancing control will interfere with the dq decoupling control is studied.Through the open-loop adjustment of the active-duty cycle of any module in the high voltage stage,the sum of the product of the active-duty cycle modification and the DC voltage of each module is zero,and the coupling effect between additional balancing control and dq decoupling control is basically eliminated.(3)The input voltage balancing control strategy for isolation stage is proposed,which effectively improves the power transmission quality of the system.Firstly,the output DC current characteristics of H-bridge modules in high voltage stage are studied when they are controlled by the same modulation ratio,and the equivalent model of isolation stage with inputseries-output-parallel structure is established.Then,based on the dynamic equation of output voltage,the coupling relationship between the parallel voltage control and the voltage balancing control which directly adjust the phase-shift ratio of dual active brisge(DAB)module is analyzed in the dynamic operation condition.It is found that there is a nonlinear coupling between them,which makes it difficult to design the decoupling control.By introducing intermediate control variables which makes them indirectly adjust the phase-shift ratio of each module,the coupling relationship between two controls is converter from nonlinear to linear,thus reducing the difficulty of decoupling control design.Finally,the decoupling control method of linear coupling is studied.By making the sum of the product of the intermediate control variable modification and DC voltage of each module zero,the steady and dynamic decoupling between the parallel voltage control and the voltage balancing control is realized.(4)The current sensorless power balancing control strategy of isolation stage is proposed,which effectively improves the economy and reliability of the system.Firstly,based on the average model of SST,the power detection method of isolation stage is analyzed,and the power flow relationship among high voltage stage,isolation stage and DC capacitor is studied.By estimating the transmission power of DAB module according to the output power of H-bridge module and the disturbance power of DC capacitor,the high-resolution current sensor is omitted.At the same time,based on the dynamic equation of output voltage and the expression of transmission power of DAB module,the influence of power balance control on parallel voltage control and total transmission power is studied respectively.It can be found that the decoupling control method used in voltage balancing control strategy of isolation stage is also suitable for power balance control strategy.(5)The positive and negative sequence component detection strategy is proposed when the power grid is in complex operation conditions,which effectively improves the stability of the system.Firstly,based on the P/Q control strategy of low-voltage stage when the power grid is in unbalanced condition,the influence of the accuracy of positive and negative sequence component detection results of power grid signal on power control and synchronous reference frame phase-locked-loop performance is analyzed.Then,the positive and negative sequence detection method in αβ coordinate system is studied,the output characteristics of adaptive notch filter are analyzed,and the reason of its output signal will be interfered by the DC component in the input signal is revealed.By modifying its structure,the DC component in the output signal is eliminated.Finally,the suppression method of high-order harmonics in positive and negative sequence components is studied.By combining the improved PR controller and multivariable filter,the interference of high-order harmonics to the control system is effectively reduced.