Glance Into Solid-State Transformer Technology: A Mirror For Possible Research Areas

ABST CT: Solid-State Transformer (SST), a power electronics based transformer is an emerging technology in electric power system. e transformer is being investigated to completely replace existing Line/Low Frequency Transformer (LFT). SST is composed of either of the two topologies: AC-DC-AC, two steps approach; or AC-AC, single-step approach. e two steps approach consists of three stages: AC-DC; DC-DC; and DC-AC stages. e DC-DC stage is made up of a boost DC-DC converter, a DC-AC inverter and a High Frequency Transformer, HFT. erefore, SST performs the tasks of LFT by means of power electronic converters and HFT. e main essence of SST is to provide solution to the problem of bulkiness and heaviness of the LFT in the power distribution network. is is with the view to providing reduction in construction cost, cost of maintenance and transportation. e power electronics transformer provides numerous advantages which are grouped into: e transformer has high power density; it functions in blackouts and brownouts; and it provides easy means of distributed renewable energy integration into associated grid. erefore, this paper provides a glance into the technology of the SST for its be er understating and promotion of research activities in the area.


INTRODUCTION
Power Electronics (PE) is one of the most emerging areas of Electrical Engineering [1]. e technology is a branch of engineering that combines elds of electrical power, electronics and control [1]. PE is the technology associated with efficient conversion, control and conditioning of electric power by static means from its available input into a desired electrical output form [2]. e In other words, the technology is concerns with application of solid-state electronics to the control and conversion of electric power. PE enables efficient and exible control over power from the grid. erefore, this technology has been a racting research a ention globally [3]. e research in Power Electronics is mainly aim at miniaturization of practical electronic applications, and effectiveness of cost and dynamic performance of electrical equipment [4]. One of the areas of application of PE is the transformer.
Low/Line Frequency Transformer (LFT) otherwise known as conventional transformer had been in use since 1880s. Since when it was invented, the transformer has been signi cantly improved. However, basic concepts of the transformer remain unchanged [5] and [6]. e transformer is relatively efficient, reliable etc. However, LFT gives losses at no load condition. is implies that it consumes power even if no load is connected at its secondary side. It only allows for unidirectional power ow. LFT is sensitive to harmonics causing it to heat up and to DC offset load imbalances, which causes them to saturate. In some of the LFTs, oil is used as a coolant, which can cause pose fear due to possibility of re hazards and a racts environmental concerns [7]. In addition, the LFT is associated with bulky size and heavy weight. ese disadvantages, coupled with the fact that the emerging grid in electric power system requires new functionalities, controllability and exibility from the distribution make the LFT undesirable [8]. ese and many more constraints of LFT permit researchers to conclude that the conventional transformer cannot effectively meet up with challenges of modern grid [4]. erefore, a smart power electronics based transformer (Solid-State Transformer, SST) has been investigated. e SST has relatively reduced size and weight due to use of High Frequency Transformer (HFT) in its design. Additional advantages of SST over the LFT are: mitigation of icker, on-demand reactive power support to grid, power quality improvement, fast control of bidirectional active power ow (Forward and reverse ow), protection from system disruptions and overloads, provision for energystorage capacity among others [9] and [10].  [9] is paper conducted a research on evolution trend of SST, Its concept architecture, its conversion stages, HFT, and provides a general overview of the SST technology. Also, Obstacle of the SST to universal acceptance and future research possibilities are highlighted by the paper. Conclusion is nally drawn at the end of the paper.

EVOLUTION TREND OF SST
Concept of SST was rst discovered in 1968 by McMurray, a Los Angeles born Engineer who worked with General Electric Company [9]. e 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.
e ET circuit was later modi ed to include inverse-paralleled pairs of Turn-OFF switches at 50% modulation index of both input and output stages by the inventor as shown in Figure 2.
In 1980, Brooks proposes a SST design which was known as Intelligent Universal Transformer (IUT). IUT was produced in 1982 by Electric Power Research Institute (EPRI) [12]. e IUT circuit diagram is as shown in Figure  3.
Several names were given to SST by many Institutes and researchers: Power Electronics Transformer (PET); Energy Control Center (ECC); Energy Router etc. [9] 3. SST CONSEPT e SST can conveniently replace the LFT a er addressing most of the basic challenges it has and provide the following functionalities in the electric grid [14]: AC or DC bidirectional power ow; 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]. ese in addition to the functions of the LFT it also offers: provision of galvanic isolation; and step up/down of input voltage. e additional functionalities SST can provide will enable it solve challenges of smart grid [15]. Opportunity to condition the power ow 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. e 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 signi cantly reduced. e SST circuit is composed of two or more static converters isolated by a transformer that operates at frequency considerably higher than the conventional 50/60Hz. e SST concept is as represented in Figure 5 [8].
Comparison between SST and LFT based on some selected transformer parameters is as shown in Table 1.

SST ARCHITECTURE
Many SST architectures exist. e architectures have different design features, and circuit performances [16]. e SST architectures are classi ed into four groups. ese are [4] and [11]: 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) ree stages with low and medium voltage DC link. Circuit diagrams of these groups are as shown in Figures 6-9.

OVERVIEW OF SST CONVERSION STAGES
ere are three main conversion stages in the SST. ese are [16] and [17]: i) AC-DC conversion stage (Recti er); ii) DC-DC conversion stage; and DC-AC conversion stage (Inverter). e AC-DC stage is the rst 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. e commonly used multilevel are: i) cascade H-bridge; Neutral Point Clamped (NPC) otherwise known as diode-clamped; and iii) Flying capacitor converter.  (1) 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) e 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]: 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
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] and [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. is is in line with the relation between the frequency 'f' and overall transformer size as approximated in the following expression [11].
where, A c is the core area, A w is the window area S is the output apparent power B m is the peak ux 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. is 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]: 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. e LC lter capacitor value is calculated as follows [21]: where L inv is the value of the output inductor. If input lter 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]: where V a-peak is the output voltage peak value and I n-peak is the neutral current peak value.

GENERAL OVERVIEW OF THE SST TECHNOLOGY
Technology of the SST uses majorly converters to convert signal voltage from AC to DC and vise-versa, modulation technique to mitigate harmonics, lters at both input and output, control strategy for successful operation of the transformer. Technology of some of these units is brie y reviewed under this section of the paper.   Some DC-DC converters that cannot provide Electric isolation can be modi ed to provide it. is can be achieved by means of unidirectional core excitation. Example: of such improvement are: Fly-back converters (Obtained from buck -boost converters). It is derived by a second winding to the inductor in the Buck-boost converter to achieve electrical isolation as in Figure 16; Forward converters (derived from Buck converters). is is derived from Buck converter. e voltage ratio in this form of converter depends on the duty ratio/modulation index in a similar manner to a Buck converter. e voltage transformation ratio is the same in either way of this type of converter. Circuit diagram of this converter is as shown in Figure 17.   Pulse Width Modulation is used to regulate output of these converters to achieve desired output. In order to obtain bidirectional core isolation as in SST, the following architectures are used to produce a square-wave form AC at the input of the high-frequency isolation transformer: Push-pull; Half Bridge; and Full Bridge. Circuit diagrams of these converters are as shown in  c. Series Resonant Converters (SRC).
ese are combination of converter topologies and switching strategies that result in zero-voltage and/or zerocurrent switching. Addition of Inductance 'L' and Capacitance 'C' enables resonant behavior. An LC tank is used to cause oscillation of the capacitor voltage and inductor current. is results in Zero-Voltage Source (ZVS) and/or Zero-Current-Source (ZCS). Circuit diagram of these Converters is shown in Figure 21.
ese converters act as current source.
ese converters can operate in the following modes: (i) Discontinuous-Conduction Mode (DCM) with f s < 1/2 f 0. . In this case it operates at a frequency lower than the resonant frequency. Merits of this type of converter are: e switches naturally turn OFF at zero current and zero voltage, as the inductor current goes to zero; It has be er controllability over no load conditions. However, it is associated with higher conduction losses due to large peaks in the inductor current every cycle. is is a result of the fact that the switching frequency is very low, and therefore it allows both the inductor current and capacitor voltage to fully resonate through a complete cycle; (ii) Continuous-Conduction Mode (CCM) with 1/2 f 0 < f s < f 0 . In this mode, operation only takes place between half of the resonant frequency and the resonant frequency. Here, the switches turn ON at nite current and voltage, thus resulting in a turn-ON switching loss. However, it enables continuous inductor current. Turn OFF of the switches occurs naturally at ZCS and ZVS in this mode of operation; (iii) Continuous-Conduction Mode with f s > f 0 In this mode, operation is limited above the resonant frequency. Here, the switches turn ON at nite current, but turn ON at a zero current and zero voltage also. is operation creates signi cant turn-OFF switching loss. However, Lossless snubbers can be used in parallel with the switches to correct this problem.

d. Parallel Resonant Converters.
In this type of converters, the resonant tank is connected in parallel with the resonant capacitor 'C r. Circuit

Applied Materials and Technology
diagram of this converter is as shown in Figure 22. e Converters act as voltage source. e SAB converters consist of two H-bridges: An active bridge at the input which converts DC to AC; and passive output bridge which performs the AC to DC conversion. e HFT isolates the two bridges [20]. SAB converter circuit schematic is as shown in Figure 23. is and some technical reasons slow down penetration of SST in the distribution network. [23].
As earlier mentioned, a converter can operate at DCM and CCM under steady-state operation condition. When SAB is subjected to operate at DCM, plot of currents and voltages are as shown in Figure 24. It is observed in the Figure 24, that currents and voltages have half-wave symmetry.
e load current 'I 0 ', Output Voltage 'V 0 ' and Active power P 0 at this operation of the SAB are given in the following expressions considering Figure 24 respectively [20]. On the other hand, under CCM, the SAB voltages and currents are shown in Figure 25. Under this condition, the current shi angle is calculated as shown in the following expression. e Load current 'I 0 ' output voltage 'V 0 ' and active power 'P 0 'delivered under the CCM condition are given by the following expressions [20].

Review of high frequency transformer technology
It was earlier mentioned that the SST consisted of converters at both input and output being separated by the HFT. e use of HFT to provide galvanic isolation in the development of the SST is responsible for the achievement of the low size and light weight of the SST.
is was expressed in equation (4).
To achieve HFT some important points are to be put into considerations. ese include [24]: (i) proper magnetic material selection to achieve the required high power density and low losses; (ii) ermal behavior to avoid breakdown especially for high-voltage and high-power designs; (iii) the winding con guration of the HFT can signi cantly affect its efficiency at high frequency; (iv) Insulation requirement at high-voltage operation should be considered. Some magnetic materials suitable for high powers and their merits and demerits are tabulated in Table 1 [20]. Table 2 Magnetic materials comparison So far, Fe-amorphous is considered by some SST designers as the best. is is because in addition to other merits, its saturation ux density reaches 1.56T. Transformer structure has in uence on power density and efficiency. erefore, the structure choice is very important aspect of the SST design. Of the two transformer structures (solenoidal and coaxial), solenoidal is the most used because of its following advantages [20].
(i) Easier to design (ii) Easier to manufacture (iii) Lower costs Two effects, skin and proximity effects must be put into consideration while designing an HFT. Skin effect is the tendency of an alternating electric current to distribute within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor.
is effect is demonstrated in Figure 25.  [20] At high frequencies, the skin depth is smaller and therefore, the skin effect causes increase in the effective resistance of the conductor, thus, reducing the effective cross sectional area of the conductor. Skin depth is the depth below the surface of the conductor at which the current density has fallen to I/e (about 37% of J) e current density 'J' and skin depth 'δ' are respectively given as follows [20]: Where J s is the current density on the surface, d is the distance from surface and 'δ' is the skin depth, ρ is the resistivity of the conductor and µ = µ 0 µ r is the permeability. Proximity effect occurs when more conductors carrying alternating current are nearby. e distribution of current within a conductor will be constrained to smaller regions if current is owing through one or more other nearby conductors. e resulting current crowding is referred to as the proximity effect. e crowding increases effective resistance of the circuit and it increases with the increase in frequency [20]. Proximity effect is illustrated in Figure 26. To a enuate the skin and proximity effects use of Litz wire is normally employed by the designers of HFT. Litz wire consists of many thin wire strands, individually insulated and twisted or woven together, o en in several levels. Figure 27 shows an example of Litz wire. Fig. 28. Example of Litz wire e HFT is the main component being used in the design of the SST to achieve reduction in size, volume and even cost. Many a empts have been made to optimize the HFT design for application in the SST such in [25][26][27][28][29][30].

Control mechanism
SST uses power converters in its implementation. is subsection will discuss brie y about power converters control mechanisms that are usually employed while designing SST. One of the control mechanisms for SST is a closed-loop controller. is controller in usually used in designing an SST to regulate the converter output voltage and compensate source or load disturbances. In this type of controller, the error between output voltage and its reference is minimized or eliminated. e error is the in accuracy between actual output and the interference. To design a desired power converter controller, the following methodologies are adapted.
(i) develop small signal model around the operating point of a steady-state (ii) get where the control-to-output transfer function takes place in the converter (iii) specify the loop gain expected depending on the design speci cations (iv) Design controller transfer function to match the desired loop gain Proportional-Integral (PI) controller is the most commonly used controller for power converters. Lead Compensator (LC) is an alternative to controller for power converter [31]. e compensator increases the loop-gain phase-margin. Disadvantage of the two controllers (PI and LC) which are mainly designed for a single operating point is that, they achieve only in nite gain at DC. erefore, it is difficult to reach zero steady-state error when the disturbance or reference is an AC signal at low frequency [32]. Gain-Scheduling Control (GSC) is an improved control technique compared to PI and LC. Alternative to GSC is Adaptive Control (AdC). e AdC adjusts control parameters. To achieve be er performance in GSC, on-line parameter is employed. Usually, adaptive control is employed on a system when its structure is known while its parameters are unknown or varying [32] . Other nonlinear methods of power converter controller implementation are [33] : i) Back-stepping based on Lyapunov function and ii) Fuzzy logic.

Modulation Technique
Modulation techniques for power converters are developed in the following form [34][35][36]. Although there are several space vector based algorithms, carrier based PWM is a dominant modulation technique in converters operations.
is is because, it requires a reference signal, carrier signals, and a simple comparator for the gating signals. On the other hand, Space vector based algorithms require at least three stages: a stage to select the vectors for modulation, a stage to compute the duty cycle and a stage where the sequence for the vectors is generated. is means that the space vector algorithms have higher hardware requirements than the PWM techniques.

Filter
Filters are used at input or output stage of SST to a enuate propagation of harmonics [37]. A typical input or output lter is shown in Figure 27. A lter for SST can either be composed of LC (Inductor, Capacitor) or LCR (Inductor, Capacitor and Resistor). In Figure 27, an LCC three order lter topology composed of LCR is shown. e LCR capability out-weighs that of the LC.
is is because, the LCR can be er restraint high frequency interference and can reduce gain in resonance peak point [37].

Applied Materials and Technology
Open loop transfer function of the lter in Figure 27 is given as follows [37]: Usually C 2 equals 1/3 or 1/2C 1 . Assuming, C 1 = C 2 = C, therefore, eqn. (21) can be simpli ed as follows [37]: Natural oscillation frequency and damping ration can be expressed as follows considering Figure 27 respectively [37].
where, ξ, R, are damping ratio and damping resistor respective. It can be deduced from eqn. (20) that, damping ratio is inversily proportional to damping resistor so long L and C are kept constant. Proper choice of the damping resistor is very important in the design of the lter. is is because the resistor as it comsumps power, it affects eff ciency of the SST.

OBSTACLE
OF OST TO UNIVERSAL ACCEPTANCE 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 delity 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. is 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. is change can be signi cantly reduced. (ii) Cost : Cost is a factor that is pu ing more con dence 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 : e 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.
9. FUTURE RESEARCH e 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 identi ed. (v) Possibility of enabling low loss high current MF interconnections opportunity (vi) Selection of propor insulation materials under MF voltage Stress.

CONCLUSION
e 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.