Research On Key Techniques Of Cascaded Power Electronic Transformer

With the development of smart gird, an ever increasing number of renewable energy has been connected to power system and higher requirements on power system are brought forward for power quality, stability and flexibility. As one of the most important devices of power system, traditional power transformer is undesirable for these new functions of modern power system owing to its excessively single function. Due to the development of power electronics technology, new kinds of power transformer have opportunity to form and improve. Power electronic transformer achieves isolated AC/AC conversion by power electronic devices and middle (high) frequency transformer. Power electronic transformer can not only achieve functions of common transformer, like voltage-varying, isolation and power-conversion, it also has high controllability and flexibility. Therefore, power electronic transformer meets the needs of the future power system and it has great significance economic and social value. This article centers on the key control technology of cascaded power electronic transformer (PET) and studies in several aspects, namely interphase DC voltage balancing control of the input stage, parallel control of the DC output stage, unbalanced reactive power compensation, fault-tolerant control. The main works and innovative achievements are as follows:1) The mathematical model and control method of cascaded PET input stage are established and studied. Mathematical model of single phase cascaded H-bridge is established and existing methods of single phase balance control are compared. The active vector correction method has more advantages in DC voltage balancing. Three-phase cascaded H-bridge model is established further. The overall control method of three-phase cascaded grid-connected converter is proposed, referring to the decoupling control of the three-phase full bridge rectifier.2) A novel interphase DC voltage balancing algorithm is propsed for the input stage of PET. Based on the analysis of average power of each phase model, switching-style methods of interphase DC voltage balancing are proposed for two three-phase configurations——zero and negative-sequence voltage injection method for the wye configuration, and zero-sequence current injection combined with individual phase control method for the delta configuration. Both of the methods have good performance in steady and transient situation, and both the output performance and power adjustment ability of inverter can be taken into consideration. Simulation and experiment results verify the correctness.3) The dual active bridge (DAB) parallel control of the DC output stage is optimal designed. Base on the analysis of unipolar phase-shifted control DBA model and soft switching conditions, research of the control method is carried out. According to the structural features of cascaded power electronic transformer, a two-stage integrated control method is put forward. The proposed new method is compared with common paralleled current sharing control method in theory. The proposed method simplifies the control system with the same effect. Contrast simulation results verified the argument above.4) The characteristics of cascaded PET are analyzed for the unbalanced reactive power compensation. Different compensation domains of two three-phase configurations are analyzed under hierarchical control. For the delta configuration, any unbalanced degree of the grid current can be compensated but with big current margin. For the wye configuration, it is much suitable for occasions where unbalanced degree of the grid current is small, due to the structural advantages. The analysis above can also be applied to cascaded static compensator (STATCOM). Furthermore, the two-stage integrated control is applied to the compensation of any unbalanced degree, making full use of the current margin of DC output stage and eliminating the device margin provided by the input stage for DC voltage balancing control.5) A new fault-tolerant control strategy of cascaded PET is proposed and studied. On the base of bypassing the fault power units, this article proposes a fault-tolerant control strategy based on double zero-sequence voltage injection and DC voltage optimization. By applying zero-sequence voltage injection, this method solves the problem effectively that the DC voltages of healthy units are balanced slowly after bypassing the fault ones. This method can also improve the reliability of the converter, by preventing current mutation. In addition, optimizing the DC voltage increases the amount of tolerated faulty units and strengthens the reliability of the converter. Simulation and experiment results verified the argument above.In the last part, some designing principles of the crucial parameters are given. Moreover, a designing method of the control system is put forward. Finally, an experimental prototype verified the overall function of the whole system.