| With the application of power electronic converter technology in power system, a new type of intelligent transformer, Electronic Transformer Power (PET), has received more and more attention. Compared with the traditional transformer, power electronic transformer has many advantages and has high power density, good controllability, intelligent and strong compatibility characteristics. The PET has important significance to the improvement of the grid quality, the construction of digital network and green network.At present, three level topology of the input stage, intermediate isolation stage and output stage is widely applied in PET. The input stage is three-phase PWM rectifier which converts the three-phase power grid into DC voltage and the control target for input current is a sine curve with unity power factor. The output stage is three-phase inverter which converts the DC voltage into three-phase AC voltage of the desired amplitude and frequency. The intermediate isolation stage of isolated DC/DC converter is the key part of PET and the main research section with the function of energy transmission, voltage level transformation and electrical isolation.The isolated bidirectional DC/DC converter consists of a full bridge inverter, a high (medium) frequency transformer, a full bridge rectifier and an output filter circuit. In the control aspect, two square wave signals with 50% duty cycle, same frequency and different phases are adopted. Taking LC resonant bidirectional DC/DC converter as an example, the mathematical model is established in view of the circuit structure, and then the DC/DC converter power transmission expression is derived including active power and reactive power. The different effects of phase shift angle, resonant frequency, switching frequency and other parameters on the power transmission characteristics are analyzed. The non LC resonant DC/DC converter can be regarded as a special case of the LC resonant DC/DC converter, so the power transmission formula of non LC resonant DC/DC converter can be obtained based on the derivation of the formula of LC resonant DC/DC converter and the power transmission characteristics are analyzed. Then, several control strategies of the isolated DC/DC converter are introduced, including the power feed forward compensation control strategy, the power direct feedback control strategy and the simplified control strategy. Due to the LC resonant DC/DC converter, soft switching is realized. The switching loss is small, so it is suitable for high frequency and high power direction.The overall circuit of DC/DC converter and PET main circuit design and parameter calculation is given, including the selection of input and output filter design, high and low voltage DC side capacitor design, and high-frequency transformer Then a DC/DC converter and the PET system simulation model simulation is built in the simulation software MATLAB/Simulink. Through the simulation, the correctness of the formula derivation and theoretical analysis are verified. |
PDF Full Text Request