The concept of the future smart grid mostly depends on Voltage Source Inverter (VSI). The VSI mainly consists of power electronic devices i.e. IGBT, MOSFET, etc. with high switching frequency operation. High power converters are now being considered for the transmission and distribution of power systems such as in high voltage DC (HVDC) transmission, flexible AC transmission systems (FACTs), and STATCOM, etc. Along with these applications, solid state transformer (SST) is getting much attention for the high-power transmission and distribution system. It facilitates with HVDC power transmission, reduced transformer size, low cost, and easy mobility. The size of the transformer coil can generally be measured from the rms voltage equation of the transformer, E_rms=4.44 f N ф, where, N is the number of coil winding, ф is the flux, and f represents the frequency. Hence, the number of coil winding is inversely proportional to the frequency which indicates the possibility of using high frequency in order to reduce the number of coils as well as the cost and weight. However, the large number of power electronic devices increases the complexity of the control of the SST system. A three-stage three-port SST has been designed based on cascaded H-bridge converter topology for smart grid application. Two H-bridge converters were cascaded connected at the front-end stage for HVAC input. The front-end stage AC/DC and the tale end-stage DC/AC converters were switched at 10 kHz switching frequency and used fundamental operating frequency of 60 Hz whereas the DC/DC stage used 5kHz switching frequency with 2kHz operating frequency.
The main research goal of this paper is to develop a reliable control structure for high/medium frequency SST. In this paper we introduced a d-q vector-controlled SST system for single phase three-stage three-port structure. The advanced power electronic switch such as SiC or GaN can be used to increase the power density of the converters which can further minimize the system size and cost. The main challenge is to control the high dv/dt in a high voltage and high frequency operating conditions. Moreover, there are some leakage current flow occurs for grid-connected PV system which can easily be reduced by SST topology that provides the required galvanic isolation to the PV system from the grid. The most efficient application of SST is the hybrid distribution system that comprises a power converter along with the traditional transformer. The first stage consists of two h-bridge converters that are cascade-connected in the primary side supplying high voltage DC at the HVDC bus. The second stage mainly consists of high/medium frequency step down transformer in order to reduce the voltage at the low voltage dc bus. This stage operates DC-AC conversion on the primary side of the transformer and AC-DC conversion on the secondary side with h-bridge converter topology. The third stage remains separated with low voltage dc link and DC/AC inverter supplies the inverted output to the load through LC filter. The effectiveness and validation of the proposed model are simulated in the MATLAB Simulink platform.
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