Research On Cascaded Modular Solid-State Transformer And Its Control Strategy

Solid-state transformer, also called as power electronic transformer is a new type of transformer achieving the function of voltage convert and power transmission in power systems through power electronic devices and technology. It combines power electronic devices and technology with high-frequency transformers to achieve voltage level conversion, isolation functions of the distribution transformers. Comparing with the distribution transformers, it has the following advantages, reduction of the volume and weight of the devices, flexible control of the current on primary side, secondary side voltage, and power, suppression of the reactive power and harmonic current generated by the customers, reduction of the influence to customers from the grid such as the voltage sag and frequency change of the grid, interfaces for DC power supply and network, integrating a variety of distributed power, connecting energy storage device, achieving the harmonization of the power flow distribution in the grid to improve the efficiency and reliability of the power system.Modular cascaded solid-state transformer is consisted of series and parallel low-voltage modules to enhance the level of the voltage and capacity with low voltage and current stress of each module. AC side SPWM wave sinusoidal can be improved with Multi-level approach, leading to a decrease of the volume of the LC filter. But it’s hard to achieve voltage and power balance control among different modules for this topology. The imbalance of voltage and power makes the system prone to fail under the grid voltage fluctuation or load changing.This paper analyzes the operational principle of the cascaded modular solid-state transformer, models and designs the circuit parameters, and makes an experimental platform. Based on this problem, a single-phase d-q vector common duty ratio control for the rectifier with an input-output voltage following control for DAB stage is proposed in this paper to achieve voltage and power balance in each module. A prototype is build up and experimental results confirmed the reliability of the proposed solid state transformer control strategy.