Review on the Development of Solid State Transformer

Introduction

Solid state transformer which has the same working principle as conventional transformer with bi-directional power flow, consisted of using several power electronic switching devices and a high frequency transformer (Ronan et al., 2002),(Qin & Kimball, 2009). SST also known by different name such as Intelligent universal transformer (IUT), power electronics transformer (PET) and Energy control centers (ECC) (She et al., 2013). The solid state transformer (SST), which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology (MIT) (Bowers et al., 1980) in 2010, has gained increasing importance in the future power distribution system. Basically the transformer enables the efficient and long distance transmission (Electronicsforu, n.d.). To overcome the deficiencies of traditional transformer like heavy weight, larger size and current limiting problem, solid state transformer was first introduced by William McMurray in 1970 (McMurray, 1971), but it cannot compete as conventional transformer because it had less efficient as conventional occurred. Figure 1 shows the concept of first electronics transformer.

Figure 1.

Principle of Electronic Transformer

After the development of electronic transformer, the researchers trying to improve the result of solid state devices so that the quality and efficiency of such devices was improved and due to this the efficiency of the electronic transformer also improved. So in this era, in 1980, (Bowers et al., 1980) US Navy sponsored study was conducted to determine the feasibility of replacing electrical power transformer with solid state transformer by J.C. Brows et al.

In 1986, A.L. Goldberg et. al (Goldberg et al., 1987) described the issues related to the design of 1-10 MHz Transformer. They were conduct an analysis of skin and proximity effects in the conductors and measurements of permeability and hysteresis loss in the magnetic material with the computer-assisted study of the relationships between size, efficiency, and frequency.

Pawel M. Gradzki et al proposed a computer-aided tool for design and analysis of high frequency power transformer (Gradzki et al., 1990). This program used a multidimensional minimum search algorithm to design the smallest transformer and size of the magnetic core. The design specification includes current and voltage waveform, desired core shape and material, type of winding etc. the program also calculate the core loss, winding loss, skin effect and proximity effect. Mario Rabinowitz suggested the name of this device as (Rabinowitz, 2003) Compact Transformer.

The solid state transformer was introduced by William McMurray in 1970, but still there is a gap between 1970 to present time in the sense of commercialized application of SST. Because in 1970, this is not technical feasible (McMurray, 1971),(NITC, n.d.) and not comparable with conventional transformer in cost and efficiency. Efficiency is directly related to the ratio of thyristor blocking voltage to the voltage drop during conduction. So initially, it was used in special application (Bowers et al., 1980),(Energy, n.d.) where cost and efficiency are secondary to size and weight. The researchers worked to improve the quality of solid state device and formulate the efficient design of converters that can beneficial to increase the efficiency of SST, and now a day’s some industries manufactured the SST and is also have the global market about some millions of dollar.

Need Of Sst

The conventional transformer have some power quality issues i.e. if the distorted input is given to the primary side of traditional transformer then it results the output also is distorted and (Ronan et al., 2000),(Swathi, 2018),(Wang et al., 2007) the voltage drop under load, inability to mitigate flicker, environmental concern regarding to mineral oil, limited performance under dc offset load unbalances and a need for protection of the primary system, arises the problem inside or beyond the traditional transformer. The development trends of the traditional power transformer are mainly focused on new magnetic materials, manufacturing process and other economic factors (Prabhakaran et al., 2008),(Kunov, 2014). After some development, some complexities or difficulties are also exist in traditional transformer as heavy weight, large size, uncontrolled output, because when input of the primary coil changed then the output of the transformer is also changed. So due to these problem there is a need of some additional features that can able to fulfill such type of requirements.