Concept of SST was first discovered in 1968 by McMurray, a Los Angeles born Engineer who worked with General Electric Company [9]. The discovery of the Electronic Transformer (ET) as it was then called was announced in 1970 [11]. Circuit diagram of the ET is as shown in Figure 1.

The SST can conveniently replace the LFT after addressing most of the basic challenges it has and provide the following functionalities in the electric grid [14]: AC or DC bidirectional power flow; reduced size and weight as a result of use HFT in its construction; correction of power factor, fast fault detection and protection e.t.c [12]. These in addition to the functions of the LFT it also offers: provision of galvanic isolation; and step up/down of input voltage. The additional functionalities SST can provide will enable it solve challenges of smart grid [15]. Opportunity to condition the power flow as provided by the SST will enable easy integration and subsequent management of Renewable Energy Sources, RES in the grid and handling of different loads. The traction systems will be blessed of achieving high performances as a result of reduced size and weight of the SST if it is employed to work with the systems. Additionally, cost of transformer, its transportation and maintenance will be significantly reduced.

Many SST architectures exist. The architectures have different design features, and circuit performances [16]. The SST architectures are classified into four groups. These are : i) Single stage without Direct Current, DC link; ii) two stages with low voltage DC link; iii) two stages with medium voltage DC link; and iv) Three stages with low and medium voltage DC link [11,4]. Circuit diagrams of these groups are as shown in Figure 6 to Figure 9.

There are three main conversion stages in the SST. These are : i) AC-DC conversion stage (Rectifier); ii) DC-DC conversion stage; and DC-AC conversion stage (Inverter) [16,17].

The AC-DC stage is the first stage of SST and is responsible for the connection of MV AC side of the distribution network and the MV DC link [16]. Multilevel topology is mainly used. The commonly used multilevel are: i) cascade H-bridge; Neutral Point Clamped (NPC) otherwise known as diode-clamped; and iii) Flying capacitor converter.

At this stage, the input voltage is the line voltage of the grid for the desired Solid-State Transformer. The DC voltage is calculated using the expression [15]:

\begin{equation} V_{DC}=\frac{2}{\frac{2\pi }{6}} \tag{1} \end{equation}

DC-DC conversion stage comprises three main sections. These are: i) DC-AC converter (Inverter); ii) HFT; and iii) AC-DC converter (Rectifier). This stage is the most crucial stage of the SST. Much architecture for this stage is available with different circuit topologies, performances and characteristics. Some of these architectures are: i) Dual Active Bridge (DAB), which consists of two H-bridges isolated by an HFT; ii) Bidirectional Isolated Current Doublers (BICD) converter; and iii) The LLC resonant converter. At the High frequency transformer stage, the peak Line Voltage (V_Lpeak) and Root Mean Square (RMS) value of the Line Current (I_{L RMS}) are given as follows [15]:

\begin{equation} V_{Lpeak}=\frac{\pi }{3}V_{DC} \tag{2} \end{equation}

\begin{equation} I_{LRMS}=\frac{VA }{V_{RMS}\sqrt{3}} \tag{3} \end{equation}

where V_DC is the DC voltage at the stage, and VA is the apparent power rating of the SST.

DC-AC conversion stage, here the DC link medium voltage is converted to AC low voltage to drive AC loads. In a situation whereby DC port is required, it is created at the input of this stage.

High frequency Transformer (HFT) is one of the major components of SST [18]. It plays vital role for the performance and overall efficiency of the transformer. It provides the following advantages: electric isolation between the input and output stages; large voltage and current ratios between input and output; size and weight reduction among others [19,20]. Due to the fact that the HFT operates at high frequency it is connected to high voltage in case of medium voltage grid, its design is more complex compared to that of the conventional transformer. Use of HFT is responsible for size and volume reduction as well as high power density obtained with the SST. This is in line with the relation between the frequency ‘f’ and overall transformer size as approximated in the following expression [11].

\begin{equation} A_{w}XA_{c}\propto \frac{S}{B_{m}XfXJ} \tag{4} \end{equation}

where, A_c is the core area, A_w is the window area S is the output apparent power B_m is the peak flux density, and J is the current density of the conductor. A review of the technology of HFT will be presented later in the paper.

In general, while designing SST harmonics are mitigated to the lowest minimum. This action is carried out by an input and output inductors. Value of each of these inductors when multilevel converters are employed in the design, is calculated using the expression [21]:

\begin{equation} L=\frac{1}{n}X\frac{V_{DC}}{\Delta I_{peak}Xf_{sw}} \tag{5} \end{equation}

Where n is a variable that depends on number of levels of converters to be used for the SST and equals 2, if a 4 levels converter is to be used, then n= 8, f_sw is the switching frequency.

The LC filter capacitor value is calculated as follows [21]:

\begin{equation} C_{out}=\frac{1}{\left ( 2\pi f_{c} \right )^{2}L_{inv}} \tag{6} \end{equation}

where L _inv is the value of the output inductor. If input filter capacitor is to be calculated then L_inv in equation (6) will mean input inductor.

In many designs of the SST use of DC link is also employed. Value of capacitor at the DC link can be calculated as follows [21]:

\begin{equation} C_{DC}=\frac{V_{a-peak}XI_{n-peak}}{4\pi Xf_{g}XV_{DC}X\Delta V_{c}} \tag{7} \end{equation}

where V_a-peak is the output voltage peak value and I_n-peak is the neutral current peak value.

Technology of the SST uses majorly converters to convert signal voltage from AC to DC and vise-versa, modulation technique to mitigate harmonics, filters at both input and output, control strategy for successful operation of the transformer. Technology of some of these units is briefly reviewed under this section of the paper.

It is good to re-emphasize that critical need to increase energy efficiency and safety in the electrical industry is one of the major problems being faced by Electrical power industry that necessate search for solution. Other problems are: Inherent nature of renewable energy, ii) traditional monitoring schemes (e.g. SCADA) may not provide enough fidelity to adequately monitor generation.

In view of the foregoing problems, SST is proposed to replace LFT due to some of its advantages as enumerated earlier. However, SST does not penetrate the electric network globally. This is due to the following [38]:

i) Reliability: LFT has been seen to be highly reliable. It operates in harsh environment. On ther other hand, SST due to use of semiconductor may not be as reliable as the LFT. This change can be significantly reduced.

ii) Cost : Cost is a factor that is putting more confidence in the minds of the users of electricity and the utility as regard use of LFT as against the SST. Use of semiconductors and other devices, requires huge maintenace cost. However, researchers may inprove on this area.

iii) Efficiency : The National Electrical Manufacturers Association’s class 1 efficiency rating indicates that, a 30kVA LFT must have an efficiency of greater than 97.5%. A comparable SST to this LFT may have an efficiency of far less than this value. SST involves many convresion stages which sometimes reduces its overal efficiency. However, this could be a thing of the fast in a near future.

The following are some of the areas require rigorous research to enable the SST completely replce the LFT in the electric network.

i) SST reliability improvement

ii) Improvemment of SST protection

iii) Use of Low voltage devices to improve efficiency

iv) In caase of use Multi-level converters in the design of the SST, optimum number of level should be identified.

v) Possibility of enabling low loss high current MF interconnections opportunity

vi) Selection of propor insulation materials under MF voltage Stress.

The paper glanced in the SST technology with the view to identifying where its technology started, where is it? And how can it be improved in the future. It is evident that with the areas of cries against SST being raised by researchers resolved, the transformer can replace the LFT in the electric network.