IntroductionUntil termed a voltage swell or a momentary

 IntroductionUntil the 19th century main motive of electricity consumers was the continuity of power supply also termed as reliability of the electrical supply. Now in 20th century consumers not only require reliability but also concentrates on power quality. Power quality has been a hot research topic for power system due to the substantial financial losses caused by inadequate power quality 1. Very sensitive  and  expensive loads like hospitals, manufacturing industries, air traffic control towers and financial institutions requires reliability as well as high quality of power supply if not any unexpected voltage distortions (voltage sags) causes a fault in some equipment of the systems which lead to failure or disconnection of the load with the loss of million euros. IEEE 1159 also defines an increase in voltage between 1.1p.u. to 1.8 p.u. in the RMS voltage and a frequency deviation from 0.5 cycles to one cycle as fundamental caused by switching off a large inductive load or energizing a large capacitor banks termed a voltage swell or a momentary over voltage. As a result power system stability becomes a very challenging problem that has to be taken into account.Active power filters (APF) 2,3, flexible alternating currents systems (FACTS) 4-12 such as static VAR compensator (SVC),  static synchronous compensator (STATCOM) 4-6, unified power flow controller (UPFC)7-9 , interline power flow controller (IPFC), and some custom power devices 11  are the common solutions and widely applied to achieve customized power quality .The conventional method of these 2-11  power quality converter are with voltage source inverters. Here this paper aims to design the same with solid state transformers (SSTs) . In conventional method of interfacing the grid is supported with high voltage like 33kV, which has several drawbacks 12. Among all the weight of the transformer is the major one which weighs 7 tons for a 2-MW wind based interfaced system 13. Despite of its weight, to reduce I2R loses in the cable technology, many manufacturers placing solid state converters for interconnection. So this weight can be reduced by increasing the frequency as a factor of 300 times where the operating frequencies are in terms of several kHz, for example 20kHz. In addition, the solid state transformers are attractive to run the electric motor drives such as traction motors in electric locomotives, pumps in compressor for application in oil and gas industries etc. 14. Over all the SSTs make the system stable and reliable with compact size.In this paper, we can see the principle and operation of a solid state transformer or High-Frequency Inverter with simulated results and like to be serving as a power quality compensating device with some proposed circuit topologies in the area of FACTS and Custom power devices. At last, power quality analysis is performed from a different IEEE-System bus by high frequency static compensator (HFSC) and a high frequency interline power flow controller (HFIPFC).Chapter 1Introduction to Solid State Transformers When compared to the conventional, steel-core transformer a solid state transformer (SST) is light weight and more compact device. This makes it more suitable for mobile power deployments, dc transmission lines and smart and micro grid applications 15. Among all offshore uses this feature which helps to keep overall weight of the platform as low as possible 16. Furthermore, SST has some functionality like unity power factor, active voltage regulation, and programmable ac frequencies 17. Additionally, reactive power compensation, quality of the power output from renewable energy systems and dual power output like ac and dc are some key factors offered by SSTs 15.  The basic idea of an SST is to achieve better voltage regulation and to reduce the weight and volume when compared to steel-core transformers 18. In addition, it can function as a flexible alternating current system (FACTS) device as it provides control voltage and current, active power flow, voltage sag problems, power quality enhancements etc. 19. SST is further observed in different topologies that can regulate the voltage in dc bus systems which can be connected to dc micro grids for evaluating the relatively new smart grid systems 20. Several projects applicable to electrical traction from single phase to three-phase topologies based on SST, have been proposed to replace steel-core transformers 21.  The 15kV and 10kV SiC-based SSTs have a wide range of applications in medium and high voltage STATCOMs, active filters, power electronic converters for integration of renewable energy systems, distribution generation systems and FACTS devices 22. The authors in 19-22, concentrate the improvement of power quality by SSTs with steady state analysis and tried to reduce the voltage ripples with some feedforward controllers. Conventional FACTS devices possess many undesirable properties like bulky size, and separate transformer and voltage source inverters. The authors in 23 presented transformerless STATCOMs which, however, come with several draw-backs like complexity in structure, low efficiency, utilization of large dc capacitors and the need for protection of each individual cells which render realization of high power STATCOMs difficult. In contrast, SST based FACTS devices give high performance and offer smaller sizes and volumes 15-19.  Fig.1 First solid state transformer patented in 196 as power electronic transformer1.1 TOPOLOGIES IN SOLID STATE TRANSFORMERSDepending on the particular application the SST has different designs and features. The generic design, as proposed and patented by McMurray 24 in 1968, is presented in Fig.1. This design was used to provide galvanic isolation with high-frequency or medium-frequency isolated converters.   Fig.1. First patented SST as power electronics transformerLater, autonomous operation and soft-switching based converters were developed,  but they were sprightly applicable for increasing the isolation transformer rather than increasing the voltage level which rendered them unsuitable for connecting several kilo amps to kilovolts. Again in 1968, a cascaded H-bridge based transformer was patented with multi-cell topology for medium and high voltage applications. This could be used either as connected input-series or output parallel cells. 24.Until now, five topologies prevail in both industry and academia 25. These are matrix type, isolated back-end, isolated front-end, isolated modular multilevel converter and single-cell approach.   Fig.2. DAB based solid state transformerFig. 2 depicts a matrix type topology suitable for ac-ac dual active bridges (DAB) applications. This topology offers flexibility and low complexity and is particularly used where volume and weight are key constraints An ac-ac DAB has a symmetric configuration (with a high-frequency transformer) and energy storage capability (with leakage inductances – although non-zero leakage inductance facilitates better voltage regulation and soft switching). Similar ac-ac configurations 68, DAB based converter holds many advantages like airborne applications 25 and power quality improvement techniques26.The isolated back-end is mainly used for ac-dc and ac-ac design applications with less complexity, like traction systems 20. The isolated front-end topology is used in high power applications like interconnection of renewable energy systems with the medium voltage grid and smart grid applications 15, 16. The isolated modular multilevel converter and single-cell approach are simple and do not need individual dc supplies, which makes their circuit control very easy and simple, as shown in Fig.6.