Design of Three Phase Solid State Transformer Deployed within Multi-Stage Power Switching Converters  
[PERSON]  
1 Electrical Engineering Department, The University of Lahore, Lahore 54000, Pakistan 1  
[PERSON]  
1 Electrical Engineering Department, The University of Lahore, Lahore 54000, Pakistan 1  
[PERSON]  
2 Faculty of Engineering, \"[PERSON]\" University of Arad, Arad 310130, Romania 2  
[PERSON]  
3 Automatics and Applied Software Department, \"Aurel Vlaicu\" University of Arad, Arad 310130, Romania 3  
[PERSON]  
4 Electrical and Computer Engineering Department, Dalhousie University, Halifax, NS B3H 4R2, Canada 4552  
compensators (STATCOMs) and unified power flow controllers (UPFCs) and so on [4]. Since renewable energy resources (RESs) such as wind, fuel cells and solar have penetrated into the industry, so power electronic converters also find their applications in them [5; 6]. Energy resources like natural coal or gas are limited and the use of RESs is growing constantly with the aim to exchange electric power with the grid by net metering. Owing to the fact that the conventional transformer cannot be interfaced with RESs and the progress made in the field of power electronics, have led to the evolution of another power electronic converter named the solid state transformer, also known as the power electronic transformer, that has gained a lot of attention [7; 8; 9; 10; 11]. With the progress made in the field of the semiconductor industry, high-frequency controlled switches with high-power ratings have been developed. With these advancements and to cope with the future requirement of distributed energy sources integration, research is being carried out in the field of power electronics, which targets the use of solid state transformers (SSTs) in applications of a high-power level. One of the key interests of researchers is to develop a dynamic architecture of SST acquiring minimum controlled switches; the other approach is the integration of SSTs with RESs and other power applications. Solid state transformers respond to control signals but their designs are not simple due to the presence of power electronic converters. The basic principle of operation of SST is that it first transforms low-frequency (50 Hz) AC voltage to high-frequency (frequency more than 20 kHz) voltage. After that isolated dual active DC-DC high frequency converter regulates secondary direct current (DC) voltage, whose size is much smaller because of high frequency operation. The multi-stage power electronic converters use controlled power electronic switches in series. In the first stage active rectifier is used, which converts and controls grid voltage to DC grid voltage and provides extended control of active and reactive power [12]. Thus, it provides the control of power and power factor. One of the most important benefits provided by SST is that it isolates both the primary and secondary side, thus eliminating the coupling effects. Ride through capabilities, and compatibility between RESs and the grid make SST superior to conventional transformers [12]. The ability of SST to generate any variable frequency and voltage enables it to perform better and be efficient in industry applications as it removes the requirement of using variable frequency drives (VFDs). SSTs, in future, will not only replace the conventional bulky transformers dominating at charge stations, locomotive and traction applications, smart grids and at secondary distribution sides, but also provide additional functions, such as the control of the active-reactive power and distribution source integration. Reduced-volume SST with aforementioned features also plays a key role in improving the power quality [13; 14]. Various two-level and multilevel switching converter topologies can be employed in SSTs as discussed in [15]. Some recent configurations of SST are also reported in [16; 17; 18; 19]. SSTs in the form of direct Alternating Current (AC) to AC matrix converters directly convert three-phase alternating current to three phase alternating voltage [20].