Comprehensive Review of Solid State Transformers in the Distribution System: From High Voltage Power Components to the Field Application

Abstract

This paper presents a systematical and progressive appraisal of the technology since the inception of AC-AC conversion, which is seen to be an indispensable and vital for the advancement of the Solid State Transformer (SST) in a distribution system. Special attention is given to the concepts/topologies and architecture of the SST, the DC-DC conversion devices in the isolation facet, the inversion gadget in the bridge arrangement and their integration in the development of an economically viable and efficient SST design. For the purpose of this article, a number of research papers, research proposals and research dissertations/studies have been accessed that mostly cover work related to this device and brief discussions about several aspects. The various sections of this article are correspondingly devoted to the review of SST design and its various configurations, the significant stages of the DC-DC converter from a transformer-specific perspective and lastly the high-frequency inverter. In critical appraisal, the SST in a three-stage perspective, the dual half-bridge converter and high-frequency half-bridge inverter are the most viable and promising means, offering tremendous advantages over other configurations with emphasis on economy, high flexibility and control performance; thereby weighed as the most workable, practical and realistic solutions. The foremost potential application of this expedient includes a vital component of the power grid.

1. Introduction

Over the past two decades, the SST has evolved rapidly, and it is now considered to be the most suitable and appropriate conversion device to replace the traditional prevailing transformer. In this device, the weight/volume savings, extensive efficiency enhancement and above all, the cost economization are taken to be the hallmarks. Several topologies, arrangements and uses of SST technology have been, and continue to be, anticipated and evaluated [1]. Conceptually, Figure 1 illustrates the basic concept diagram of the SST, which involves a combination of high-frequency transformer and power electronics components.

Figure 1. Concept diagram of SST (or PET) [2,3].

From a practical aspect, the SST configuration comprises of three main stages (i.e., a rectifier, isolation through the HF transformer and finally the DC-AC converter to reproduce line frequency AC). Figure 2 illustrates such an arrangement from a practical perspective.

Figure 2. Practical Topology of SST [4].

The most promising and practical three-stage structure contributing to the anticipated solution is shown in Figure 3. This circuit contains low-voltage and medium-voltage DC buses and a completely decoupled arrangement between two AC sources with reactive power compensation capability.

Figure 3. Elaborative Applied Topology of SST [5].

To date, the fabrication of SSTs is in the research and development phase. In fact, the invention of the SST comprises a practical semiconductor scheme, adopted to achieve a high power density at high frequencies, resulting in a reduction of size and cost. The weight and size may be up to three times smaller than that of the equivalent traditional transformer. Moreover, it is more environmentally responsible as no fluid/liquid dielectric is incorporated for cooling and other related purposes [6]. SSTs can be made operational/functional with a refined state-of-the-art communication interface incorporating elements such as smart metering, diagnostics and space control. SSTs are also considered for use in single-wire earth return transmission systems [7]. As the SST is the combination of a powered electronic circuit and a HF transformer, the voltage regulation, voltage dip/sag protection, fault segregation and DC output are some of its main features [8]. Consequently, SSTs will be used as energy routers/drives incorporating both AC and DC interfaces in impending distribution systems. The advantages of SST comprise their reduced weight, compact volume, active controllable devices, voltage sag and outage compensation, direct regulation of voltage, isolation of fault, power factor correction, harmonic isolation, cost-effectiveness and environmental friendliness [9].

SSTs are not yet commercialized and are still in the exploration stage [10]. Many firms and research organizations/establishments are undertaking research in this area and are committed to testing prototypes to suit them in particular applications. It is estimated that the SST market will be commercialized in a year or so and exhibit a remarkable growth rate in the coming years [11]. The concept of SST was first presented by McMurray [12] in 1968; this introduction was generally based on solid state switches using high-frequency isolation. SSTs were noted in the National Science Foundation (NSF) Generation-III Engineering Research Centre (ERC) as “Future Electric Energy Delivery & Management (FREEDM) Systems” which came into existence in 2008 and again in 2010; the proposal of SSTs was acknowledged as one of the ten most promising technologies in the appraisal of the Massachusetts Institute of Technology (MIT) [13]. The selection of the most suitable topology for SST execution is the main challenge, which can be tackled by analyzing several of the existing latent, possible and promising topologies which can provide the unidirectional power course. In this scenario, a number of available topologies for SSTs (including a general AC-AC power converter) have been deliberated and studied [14]. Some of these topologies and configurations are not in support of the parameters of bi-directional power flow. Effort has been undertaken to categorize SST topologies and to earmark the most befitting one as per obvious and explicit needs [15]. Figure 4 illustrates the general classification of SST topologies [16,17].

Figure 4. General Classification of SST [16,17].

1.1. Single-Stage SST

There are several single-stage topologies (direct AC-AC conversion) which facilitate a unidirectional power flow; however, the lack of a DC link is still taken as the major drawback of these topologies, which is evident in Figure 5. Here, the combination of storage elements and power flow improvement would definitely require additional devices/procedures, resulting in an increase in system complexity and size as well as cost of the overall system. Nevertheless, due to its simple and brief configuration, it currently does present a promising cost-efficient and lightweight solution [18,19].

Figure 5. A single-stage topology [18,19].

1.2. Two-Stage SST

The direct current link of this design may not be considered viable for distributed energy storage (DES) as well as distributed energy resource (DER) integration, due to the high energy voltage and the absence of arrangement for isolation from the grid [19,20]; however, the fabrication under this arrangement may not be practically viable in SST applications. This arrangement also possesses two-stage conversions, where galvanic isolation along with a voltage step-down process is addressed in the DC-DC converter stage. In one of these two arrangements, a low voltage DC link does not exist. Figure 6 illustrates a two-stage SST topology [21].

Figure 6. A Two-stage SST (a) with MVDC Link. (b) With LVDC Link [21].

1.3. Three-Stage SST

A three-stage structural design with two DC links, is the most workable, practical and realistic solution due to its high flexibility and control performance. It ensures numerous functions that are enviable when compared to SST functions. This configuration encompasses the conversion of AC voltage at the input to a corresponding DC voltage, resulting in a medium voltage direct current (MVDC) link. Then, this MVDC is processed to change into HF AC voltage, which is routed through to the medium-frequency transformer. At this stage, the voltage level is condensed, rectified once more to a low-voltage DC level and finally constitutes a low-voltage DC link. At a further stage, this obtained low-voltage direct current (LVDC) is transformed once more in order to achieve 50 Hz AC voltage. The diagrammatic representation of this topology with three distinct stages is illustrated in Figure 7. The MVDC is considered to be a viable one for renewable energy sources connected with SSTs [22].

Figure 7. Three-stage SST topology [22].

Figure 8 exhibits the typical arrangement of a SST incorporating a medium voltage AC to DC converter, a DC to DC converter with the capability of galvanic isolation and lastly the low voltage DC to AC converter. A SST with such a three-stage topology is measured and taken as the most accepted transformer in the distribution system; that is why it is also referred to as a three-port power exchanger and energy router [23].

Figure 8. Block scheme of SST [23].

It is acknowledged that from all perspectives, a three-stage SST topology is the most appropriate execution. This three-stage topology contains five blocks, and is equipped with an MVDC link to connect the power sources and a LVDC link to connect the energy storage devices as one system. It is the most acceptable deliberation because of the absolute separation/decoupling of the two AC sources with the proficiency of reactive power regulation. Being energy efficient, their power capability is ten (10) times greater than that of existing traditional transformers [23]. Table 1 gives an overview of the comparisons of SST topology. SST technology is anticipated to progress in the near future; however, certain facets still require in-depth study and research. For example: (1) The non-establishment of real-time communication within a network among SSTs. (2) The nonexistence of requisite literature on the implementation of SST technology in four-wire distribution schemes. (3) The protection/safety, consistency and lifetime cost analysis of SSTs, etc. However, the advancement/growth in SST commerce is incredible and auspicious enough to allow hope for wide-scale application in the near future.

Table 1. SST Topology Comparison (Functional Capabilities supported) [23].

Parameter	Single-Stage (AC to AC Conversion)	Two-Stage Using LVDC Link	Two-Stage Using HVDC Link	Three-Stage with Both HVDC and LVDC Links
Input Power Factor Correction	No	Yes	Yes	Yes
DC Link Provision	No	Yes	Yes	Yes
Galvanic Isolation from Grid	Yes	Yes	Yes	Yes
DES and DER Management	No	Yes	Yes	Yes
Feasibility in SST applications	No	No	No	Yes
Cost and Weight	Lowest	Lower	Lower	Low
Input Voltage Regulation	No	Good	Good	Very Good
Reactive Power Compensation	None	Sure	Sure	Sure
Circuit Implementation	Easy	Easy	Easy	Easy
Ride-Through Capability of the SST	No	No	No	Yes
Power Quality Improvement	No	Yes	Yes	Yes
Input Current Limiting	No	Yes	Yes	Yes

In this manuscript, an effort is made to comprehensively deliberate/review the issues with the emergence of a high-voltage and high-power SST and related state-of-the-art investigations, mainly focusing on high-voltage power devices, high-power and high-frequency transformers, AC-AC topologies, and the application of SSTs in distribution systems. The rest of the review is structured into six sections. Section 2 gives an overview of the previous work (Literature Appraisal). Section 3 clinches the critical review. A transformer as a galvanic isolator of the DAB is discussed in Section 4. Section 5 deliberates on the various functions and applications of SSTs. Finally, Section 6 deals with the conclusion. It also deliberates on the three-phase high-frequency inverter in a half-bridge topology perspective where the efficiency performance parameter is measured by incorporating various but equal loads (i.e., 100 Ω–5000 Ω) in each phase. Figure 9 represents a simulated efficiency model of the three phase inverter. Various measurements for requisite parameters in the execution of the simulation have been charted out. Table 2 illustrates the various data received from the simulation model from an independent three-phase perspective.

Figure 9. Efficiency vs. output power (inverter with three independent phases) [23].

Table 2. Efficiency Valuation-DC-AC Inverter with Three Independent Phases [23].

Load (Ω) (Each Phase)	Input Voltage (V) (Each Phase)	Input Current (A) (Each Phase)	Input Power (W) (Each Phase)	Output Voltage (V) (Each Phase)	Load Current (A) (Each Phase)	Output Power (W) (in Each Phase)	Efficiency Each Phase) %
100	5000	12.47	62,320	2494	24.94	62,180	99.77
200	5000	6.242	31,210	2497	12.48	31,170	99.88
500	5000	2.499	12,500	2499	4.997	12,490	99.94
1000	5000	1.25	6251	2499	2.499	6247	99.94
2000	5000	0.6254	3127	2500	1.25	3124	99.91
3000	5000	0.4171	2086	2500	0.833	2083	99.87
4000	5000	0.313	1585	2500	0.625	1582	99.83
5000	5000	0.2505	1252	2500	0.5	1250	99.79

It is apparent from Figure 9 that higher efficiency (99.94%) exists at a heavier (100 Ω) load and lower efficiency (99.77%) at a lighter (5000 Ω) load. The reason for this variation is that at lighter load, the processed power is lesser, whereas the snubber losses are same; accordingly, at a lighter load, the efficiency is lower. Correspondingly, at a heavier load, the processed power is larger and the snubber losses are same; therefore, at heavier load, the efficiency is greater [24].

2. Review of Related Works/Literature Appraisal

In order to better understand and comprehend the work conducted in the field of SSTs, DC-DC converters and high-frequency inverters, an extensive and detailed literature review has been carried out. First, the study was carried out to weigh up the advantages and disadvantages of the conventional transformer in contrast to the SST. Various topologies regarding the design/development of SSTs favorable to our environment were deliberated in depth and finally the three-stage topology of SSTs was given due significance. In the subsequent sections, the literature review of the SST and its significant constituents (i.e., isolated DC-DC converter and high frequency inverter design) is highlighted: Table 3, Table 4 and Table 5 illustrate the summarized literature review of the solid state transformer (SST), DC-DC converter and high frequency inverters, respectively [25].

Table 3. Review Summary-Solid State Transformer [26,27,28].

Writer/Year	Rating/ Specification	Topology/Configuration	Operational Effects/Results	Observation/Attainments
Brooks et al. (1980)	-	Single-stage, AC-AC (Step Down Conversion)	Reduce the input/applied voltage to a lower value at the output	Lesser voltage level Devoid/lack of magnetic isolation High stress factor
Resischi et al. (1995)	-	Single-stage, AC-AC(Step Down Conversion)	Reduce the applied voltage to a lesser value at the output Lower stress factors	The voltage level is lower Lack of transformer isolation Lack of DC bus
Harada. (1996)	-	Single-stage, (HF AC link)	Reduction in transformer size and weight Rational stress factor	No arrangement for instantaneous voltage regulation as well as for voltage sag compensation
Kang et al. (1999)	-	Three-stage (High Frequency AC link)	Reduction in size and losses Improved voltage regulation	No arrangement for P.F improvement Overall efficiency is up to 80%
J. Edward et al. (2000) Ronan et al. (2002)	10 kVA, 7.2 kV/240 V	Three-stage Cascaded H-Bridge (High Frequency AC link)	Enhancement in control and P.F. Energy storage system protection	Reliable The efficiency—90%
Jih-Sheng et al. (2005)	-	Three-stage (High Frequency AC link)	Provision of PQ functions, galvanic isolation and isolation of a disturbance from either source or load	Different voltage Rising cost but with appreciable efficiency
J. Ai-Juan et al. (2006)	5 kVA, 220 V/380 V	Two-stage, (AC/AC HF link)	Unbalanced linear load/input voltage	Lower THD
J. S. Lai et al. (2006)	50 kVA, 2.4 kV/240 V/120 V	Three-stage (3 level NPC in MV side)	Voltage Sag and load variation Unbalanced load	Voltage sag compensation Fault isolation
Iman et al. (2006)		High Frequency DC link	PF improvement, protection of critical loads and energy storage system	Use of numerous PE converter and DC link electrolytic capacitors
Iman-Eini et al. (2008)		Single-phase, three-stage, (cascaded H-bridge)	Power factor improve-ment, protection of critical loads and energy storage system	Low efficiency and reliability
H. Iman-Eini et al. (2009)	1.5 kW, 230 V/39 V	Three-stage, (Cascaded H-bridge)	Voltage sag Nonlinear load	Harmonic voltage and reactive Power compensation.
Maitra et al. (2009)		High Frequency DC link	Maintain the source voltage balance among different modules	Lesser efficiency due to a number of components
M. Sabahi et al. (2010)	2 kW, 110 V/20 V	Single-stage (AC to AC Conver-sion)	Steady state response	Bi-directional power flow Maximum PPT
S. B.Yu Du et al. (2010)	20 kVA, 7.2 kV/240 V	Three-stage, (Cascaded H-Bridge)	Steady state response	Bi-directional power flow Maximum power point tracking
Ling et al. (2011)		High Frequency DC link	Reduction in voltage stress Rating of components	Lower efficiency
Jie Zeng et al. (2011)		High Frequency AC link	Core loss calculated as 81 W	Efficiency of the SST up to 96.6% (from half to full load)
C. Zhao et al. (2011)	1.2 MVA, 15 kV/16.7 Hz	Two-stage, (Cascaded H-bridge)	Steady state response	Bi-directional power flow
X. Liu et al. (2012)	1 kW, 208 V/120 V	Three-stage (Two Level)	Steady state response	Bi-directional power flow
Subramanya (2012).	-	Three-stage AC-AC (Matrix converter based SST HF DC link)	Reduction in number of components	Escalating efficiency, with reduction in PQ with the voltage stress factor
Xinyu et al. (2012)	-	3Φ, single-stage (Matrix converter for PET HF AC link)	Voltage regulation is 0.5–1	Higher THD
Z. Li et al. (2013)	110 kV AC-750 V DC	Two-stage, (AC/DC/DC, MMC)	Steady state response	Bi-directional power flow
Xu She et al. (2014)	3.6 kV-120 V/10 kVA	Three-stage (FB, HF AC link)	Accessibility for ac as well as DC outputs	Efficiency ranges from 84% to 88%
M. Morawiec et al. (2015)	600 kVA, 3.3 kV DC/ 3.3 kV DC	Three-stage, (Cascaded H-bridge in MV side)	Steady state response	Bi-directional power flow
H. J. Yun et al. (2015)	5 kW, 300 Vac /380 V DC	Two-stage, (Cascaded H-bridge)	Steady state response	Bi-directional power flow
J. Ge et al. (2015)	2 kW, 300 V/60 V	Three-stage (two-level)	Steady state response Load variation	Bi-directional power flow
B. Zhao et al. (2015)	2 kVA, 380 V/120 V	Three-stage, (cascaded)	Steady state Power flow reversal	Bi-directional power flow
Madhusoodha-nan et al. (2015)	5.8 KVA, 5 kV DC/800 V DC	Three-stage, (NPC)	Steady state response	Bi-directional power flow
Swapni et al. (2016).	-	Three-stage (High Frequency AC link)	Capability to substitute the old version conventional transformer	CT is at demerits of heavy weight Poor voltage regulation Saturation of core
H. Chen et al. (2016)	50 kVA, 480 V/480 V	Single-stage, Dyna-C (AC-AC)	Steady state response	Bi-directional power flow
N. Nila et al. (2016)	3-kVA, 2.4 kV/127 V	Three-stage (two-level)	Steady state response, nonlinear load	Unidirectional
H. Chen et al. (2016)	10 kVA,208 V	Single-stage AC-AC, (Two-level)	Steady state response	Bi-directional power flow
Y. Liu et al. (2017)	2 kW, 400 V/208 V	Single-stage, AC-AC, (Matrix Based)	Steady state, load variation and unbalanced voltage and current	Bi-directional power flow
Aleksandar et al. (2018)		High Frequency AC link (3 phase modular)	Possibility/opportunity of SST implementation in marine electrical power systems	Greater complexity/ higher price
N. Verma et al. (2019)		Technical appraisal of SST in electrical system has been presented	SST will attain a key role to address the drawbacks of conventional transformer	-
M.zharuddin et al. (2020)	-	Main advantages of SSTs have been extensively analyzed and their performance has been compared with the existing ones	Additional developments in SST field will be explored in the nearby future	-

Table 4. Review Summary-Isolated DC-DC Converter [26,27,28].

Writer/Year	Topology/Configuration	Operational Effects/Results	Observation/Attainments
Zhong et al. (2013)	Dual Half Bridge (DHB) DC-DC converter [1 KW (385–48 V)]	Usage of nominal 50% duty cycle by each switch to function Attainment of ZVS over an extensively varying load	Efficiency = 96 + % (light and heavy weight both)
M. Narimani et al. (2014)	Dual Active Bridge DC-DC converter	3-level DC-DC converters in FB arrangement possess capability for enhancing light load efficiency as compared to 2-level DC-DC converters	Enhanced efficiency at light load
Kai Zhang et al. (2015)	Dual Active Bridge DC-DC converter (FB)	Two equivalent resistances are added in primary of HF transformer to increase the efficiency	Enhanced power accuracy and efficiency
Arsalan et al. (2016)	Dual Half Bridge DC-DC Converters (Half Bridge)	Better anti-imbalance capability in transformer Can function in step up (29 V–380 V) as well as in step down (380 V–29 V)	During step down mode, the efficiency = 90 + % During step up mode; the efficiency is 82%.
Soban et al. (2017)	Three-level DC-DC converter	Maintaining galvanic isolation with high voltage conversion ratio using medium frequency transformer (400 Hz–20 k Hz)	THD reduction of input voltage (of HF inverter) is from 48% to 37%, whereas in its primary current the THD is from 32% to 28% The efficiency is increased from 81% to 92%
Srinithi et al. (2017)	1.7 kW isolated DC-DC converter (bi-directional)	Very compact in size Possesses reduced power loss coupled with reduced heat sink requirement	In step down mode operation (for DC input) the peak efficiency is 80% In step up mode operation, the peak efficiency is 80%
Su-Han et al. (2017)	3-kW experimental and trial DC-DC converter prototype	High efficiency Reduced voltage stress Operation at a varied ZVS range in lowered switching losses	Enhanced efficiency
Mojtaba. et al. (2017)	DC-DC converter (Isolated/Non isolated)	In an evaluation concerning isolated and non-isolated DC-DC converters, the only isolated converters meet the following parameters as: Standards of utility grid Simple in accomplishment of multiple output voltages Suitability for high power levels	Use of magnetic coupling (i.e., transformer) proved to be beneficial in achieving high voltage gain Reduction in noise
Levy Costa et al. (2018)	20 kW quadruple active bridge (QAB) converter	To augment the efficiency in the light of expenditure, a multi-objective enhancement algorithm established which combined these parameters to have prime point for expenses	Efficiency is 97.5% Converter’s loss is around 57%, in contrast to IGBTs; this reduction is because of the usage of SiC-MOSFETs

Table 5. Review Summary-High Frequency Inverter [26,27,28].

Authors/Year	Converter Configuration	Operational Effects/Results	Observation/Attainments
Haifeng et al. (2011)	Dual Half Bridge DC-DC converter (1 KW)	In an appraisal of the performances between DAB and DHB, the DHB being more economical with less number of switches is preferred and supported	Achieving efficiency of 97.2% at 50 KHz operation
Rishi et al. (2015)	Multi-level PWM inverter (Cascaded H Bridge)	Favored for cascaded H Bridge MLI	Better performance in MLIs than conventional ones
Harati et al. (2016)	Three phase inverter	Load impedance can be adjusted while matching the source impedance	Improved efficiency in real time
Venkatesa et al. (2016)	DC-DC Converter (with H6 Bridge)	Six level solutions for single phase grid connected converters	5 levels inverter attained efficiency up to 95%
Hyun-Jun et al. (2016)	Dual Active Bridge (DAB)	For improvement of overall efficiency, a systematic approach was followed for coupling inductance and dead time duration	Improved efficiency
Harish et al. (2016)	Multilevel inverters	MLI possesses 3 major topologies such as, capacitor clamped, diode camped and cascaded	Improved performance in MLIs
Charai et al. (2017)	Multilevel inverters	With increased number of levels, the inverter gives better performance in the system	Eleven levels inverter offers more efficient performance in terms of the P.F., THD and its efficiency as compared to 7–9 level MLI
Akash et al. (2017)	Three-phase PWM Inverter	Three phase ac power of 0.8 V, displaced by 120 phase shift provided to the inverter resulted into an almost sinusoidal current wave	The amplitude MI was ≤ 1.
Kathar et al. (2017)	Multilevel inverters	Topology comparison of Diode-Clamp inverter, Capacitor–Clamped inverter, Cascaded and generalized multilevel cells was deliberated	H-Bridge possesses its easy extensibility to a high number of level as well as implementation
Tomasz et al. (2017)	Three-phase DAB (with 3 L-NPC half bridge)	Three level topologies are promising solution for LVC	HB solution is preferred at MV side of DC-DC converter
Ambula.et al (2017)	Single-phase 7 level inverter	Efficient and effective substitute for the conventional one	Enhanced efficiency. Lesser THD
Levy et al. (2018)	Multiple-Active-Bridge DC-DC Converter (20 kW prototype)	To meet the set goals (low cost/high efficiency), a development of a multi-objective optimization algorithm initiated	Efficiency is 97.5% Use of SiC-MOSFET reduces the losses to 57% as compared to the IGBT
Tomasz et al. (2017)	Review of MLI (DC-DC Converter)	Half bridge produced half the output voltage as compared to full-bridge, resulting in reduction of transformer turn ratio by a factor of two	Lowest part number Very high DC-gain
Chih et al. (2012)	Half-Bridge PV Inverter System	Active switches are reduced to half in comparison to FB topology	System simplification Reduction in cost
Gavaras et al. (2016)	Half Bridge Inverter (isolated)	Up to 15th harmonics there exists 44.999% THD in a single half bridge inverter	With LC low pass filter, the THD is reduced to 0.0183%
Sathisha et al. (2017)	Unipolar and Bipolar PWM for single phase VSI	Unipolar modulation has lesser THD as compared to bipolar with better PQ	Unipolar has 0.368% THD lesser than bipolar modulation
Zhigang Gao et al. (2016)	Half Bridge Inverter (isolated)	It measures the output powers and efficiencies of cascaded HB (CHB) rectifier, isolated DC-DC converter and low frequency DC-AC inverter	The efficiency of HF inverter approximates to 97.25%
Soban et al. (2017)	40 V/120 V, 5 KW Three-level isolated DC-DC converter.	Reduction in THD of primary voltage (HF inverter) is from 48% to 37% Reduction of THD of primary current (HF inverter) is from 32% to 28%	Efficiency enhanced from 81% to 92%
Shiladri et al. (2018)	Half-Bridge (HB) Inverter (Transformer Isolated)	Operation at optimal point and satisfactory	Constant/adequate vibrant response Propose substantial efficiency gains over conventional square-wave control

3. Critical Evaluation and Discussion

A critical analysis of the literature significantly highlights a comparison of the three topologies and those most viable for the SST applications; a transformer-isolated DC-DC converter with performance comparative to that of DAB and DHB, and lastly, a DC-AC converter in the bridge configuration.

3.1. Solid State Transformer Architecture

The most suitable and appropriate configuration, competent enough to support additional/supplementary missions/functionalities as opposed to a regular transformer, was identified. These functionalities included: (1) on-demand reactive power support to the grid, (2) voltage regulation, (3) power quality and (4) the current limiting, restraining and provision of DC bus. The configurations considered were bi-directional, dictating a minimum requirement of substituting a regular existing transformer. A critical, influential and significant analysis of three main topologies for the implementation of a SST was deliberated, and the following conclusions were reached:

A single-stage SST topology is a configuration without a DC link, which restricts its operation of the integration of renewable energy sources, as well as energy storage devices. The deficiency of such a DC link exhibits no planning in terms of voltage regulation coupled with reactive power compensation [29]. Therefore, no arrangement existed for input power factor correction. In these circumstances a single-stage topology is not given due deliberation for the contestants of SST applications for future grid requirements [30,31,32].

A two-stage SST with a high HVDC link is considered to be least suitable arrangement, for the reason that it is lacking an isolation arrangement from the grid. Therefore such a topology does not comply with or conform to smart grid requirements. Moreover, this DC link is not in place for DES and DER. Furthermore, the necessity of the huge size of filters for eradicating large ripple currents is considered to be the foremost and key shortcoming for attaining the voltage regulation and henceforth, practically not possible in SST applications. The DC-AC converter part of such arrangement is a double phase inverter [33,34].

The most feasible, appealing and practical topology of the SST is a three-stage configuration (i.e., topology with high voltage (HV) as well as with low voltage (LV) DC links). These links are meant to enhance the ride-through competency of SSTs, thereby allowing the improvement of power quality at the input as well as output ends. This three-stage topology offers all the preferred SST functionalities, simplifying the control design. The LVDC link contributes to all of the anticipated SST functionalities, including the interfacing of DER and DES. The structural design of this topology comprises a distinct rectifier, an isolated DC-DC converter (i.e., a high frequency inverter, high frequency medium power transformer and a rectifier) and DC-AC converter stages. This topology offers tremendous advantages over the other topologies, including high flexibility and control performance, and it is weighed as the most workable, practical and realistic solution [35,36,37,38].

3.2. Transformer-Isolated DC-DC Converter

The transformer-isolated DC-DC converter as an entity and the SST as a scheme; both of these arrangements make use of HF inverter at the input side of the MPHF transformer in order to produce AC voltage. Transformer-isolated converters are incorporated primarily for the establishment of galvanic isolation, to enhance safety, to raise noise immunity and to avoid the transmission of voltage transients to the output. With fragmentary technological progression in power electronics, exploration in this arena is very vigorous and rationalized, and numerous research articles/manuscripts are circulated, which is evident from the manuscripts referred to in this study [39,40,41,42].

Power conversion using high-frequency inverters are achieving improved consideration in scheming high-frequency power distribution arrangements. Researchers (academic/industrial) and investigators are carrying out extensive research on the design of HF inverters in different configurations/topologies. In a bridge arrangement, the vital structures proposed are the full bridge (FB) and half bridge (HB) inverter in step down mode and step up mode; performance concerning efficiency and total harmonic distortion (THD) were deliberated through experimental results [43,44].

Dual half bridge (DHB) and dual active bridge (DAB) are two popular, prevalent and well-structured arrangements among the PS dual bridge DC-DC converter. Figure 10 and Table 6, Table 7 and Table 8 simplify the appraisal of the operational situations, advantages and disadvantages of these two types of converter (i.e., DAB and DHB) which note that: (1) in DHB organization, the transformer flux swing is just half of that of the DAB arrangement, provided the same switching frequency and transformer effective cross-sectional area are used; (2) in comparison to DAB, a DHB uses half of the number of switching devices. Eventually, such uniqueness requires deliberation for the purposes of finding a more economical and inexpensive solution [45,46,47,48].

Figure 10. Comparison between DAB and DHB [45,46,47,48].

Table 6. Evaluation of DHB and DAB [45,46,47,48].

Parameter	DHB	DAB
Transformer Turn Ratio (n)	Vin/Vout	Vin/Vout
Duty Cycle (D)	½	½
Transformer flux swing (▲B)	(Vin/2)D/nAf	VinD/nAef
Switching Devices	4	8

Table 7. Advantages and disadvantages of DAB [45,46,47,48].

Advantages	Disadvantages
Use of soft switching for all power switches	Higher number of switches than DHB
Identical control procedure in both energy flow directions	Soft switching facility does not exist at light loads
Possibility of modular structure	-
Nominal voltage and current stress	-

Table 8. Advantages and disadvantages of DHB [45,46,47,48].

Advantages	Disadvantages
Incorporates half as many switches than DAB	Bulky capacitors
Use of soft switching for all switches	Current in switches is twice as high
Simple structure and control	No soft switches at light loads
Modulator structure possible	-

3.3. High Frequency DC-AC Inverter

The literature survey emphasizes the importance of HB configuration in terms of output voltage, number of bridges, number of switches, etc. The advantages of the HB arrangement are as follows: (1) the output voltage is halved compared to the full bridge (FB) voltage requirement (i.e., step down), (2) the number of bridges required is halved as compared to the FB topology (3) the sufficient simplification of a power scheme layout, (4) the reduction in the complexity of control and protection circuitry, coupled with the reduced converter price [49,50,51,52,53].

A number of researchers have worked on the design of inverter in the FB configuration, whereas very few have based their studies on HB topology [54,55,56].

3.4. Embryonic Development—HFHP Isolated DC-DC Module

A transformer-isolated DC-DC converter stage in high frequency perspective is considered to be the most significant and vital element of the SST system [57,58,59]. Conventionally, the frequently used practical devices in practice are the high voltage IGBTs, based upon three-level NPC topology; however, the 1.7 kV IGBT with low cost can also be incorporated for certain applications [60,61,62,63]. With the emergence of wide-band gap devices, the DC-DC stage working frequency is escalating, even up to 50 kHz. The DAB converter undergoes the most frequently used topology with high performance [64,65,66,67].

The LLC unregulated resonant converter (DCX) is already in practice by ABB. System efficiency has improved gradually. From the magnetic aspect, the nano-crystalline and MnZn ferrite are the most tremendous and frequently used core materials, because of their enlarged working frequency [68,69,70]. The transformer structures most commonly adopted are still the UU core shape and EE core shape [71,72,73]. Litz wire is used in almost all the transformer windings, in order to reduce the frequency current conduction loss. Most transformers use dry-type insulation; whereas ABB is already switching to oil-immersed insulation owing to its 15 kV medium voltage application [74,75,76,77].

4. Transformer as Galvanic Isolator of the DAB/DHB

In the present electronics arena, the medium-voltage high-frequency transformer (MVHFT) design has attracted global interest; it is considered to be a decisive and performance-driven component in the field of existing conventional 50/60 Hz transformers, the heaviest high-power converters [78,79,80]. By virtue of its expected and review performance, the MVHFT is likely to supersede and replace the present weighty entity within future MV/HV conversion structures. Such a structure is the magnetic circuit, which must be incorporated to minimize the mass/size/volume of the magnetic circuit to obtain a commendable reduction in the overall volume/weight of high power converters, in accordance with the prevailing technology advancements and interests [81,82,83]. To comprehend, for the mission of magnetic mass reduction, the most universally implemented procedure is to increase the frequency so as to cut down the volume (as the frequency is inversely proportional to the area of the product). The transformer low-frequency transformer performs unsatisfactorily in certain electrical applications, therefore in this context a novel strategy has to be devised [84,85,86,87,88]. To start with, the first and foremost issue to be deliberated with the MVHFT is the type of magnetic core to be implemented [89,90,91]. The characteristics to be expected from the core include a high saturation flux density (B_sat) and reduced core loss, as well as stable operation at high temperatures [92,93,94,95].

In comparison to conventional transformers, the SST incorporates power electronic converters using a HF transformer. Power switches such as MOSFET, IGBT, etc., are widely used [96,97,98,99]. The HF transformer plays a vital role in design and functionality. It deals mainly with the efficiency aspect, depending on the operating condition, and wire/core selection [100]. Although the high operating frequency contributes to the compactness and density of the transformer, there are many more limitations/restrictions which must be taken into consideration, such as insulation, power loss and costs [101,102,103,104].

There are two kinds of losses which contribute to total transformer losses; these include the core losses (no load loss) and the winding/copper losses (load loss) [105,106,107,108]. HF transformers in SSTs primarily cater to the performance and overall efficiency; that is why the selection of suitable materials, along with the optimization of the design, is imperative in meeting all of the requirements for the operating conditions [107,108,109]. Various types of core material characteristics are also momentarily abridged [110,111,112,113]:

• Nano-crystalline (FT-3M): Possesses saturation flux density, B_max,1.23 (T), Curie temperature T_c 570 (°C) and maximum operation temperature.150 (°C).
• Ferrite (3F3): Possesses saturation flux density, B_max, 0.45 (T), Curie temperature T_c 200 (°C) and maximum operation temperature. 120 (°C).
• Super-alloy: Possesses saturation flux density, B_max 0.79~0.87 (T), Curie temperature T_c 430 (°C) and maximum operation temperature. 125 (°C).
• Amorphous (2605SA): Possesses saturation flux density, B_max 1.57(T), Curie temperature T_c 392 (°C) and maximum operation temperature. 150 (°C).

5. Applications of SST and DC-DC Converter

Transformers are installed at the ends of generating stations. Distribution substations are used in the transportation of electric power at long distances in order to lower the voltages required by homes, businesses and other utilities (i.e., with the key function of reducing the high voltages) [111,112,113,114]. Currently existing transformers operate only in one direction, and some of the services provided by the SST structure comprise the safety of load and the power system from power supply disorders, the sag compensations, the load transients and harmonic regulations, the unity input power factor (PF) in reactive load, the sinusoidal input current in the case of non-linear loads, the safety against output short circuit, the operation on distributed voltage level, the amalgamation of energy storage and the medium frequency isolation [115,116,117,118,119,120,121,122].

To implement this technology in a befitting manner, tremendous efforts have been carried out to design and structure the SST and observe its impending application in the distribution system [123,124,125]. On one hand, Figure 11 depicts the customary distribution system, incorporating the transformer for integrating renewable energy resources & energy storage devices, operating the traction/locomotive system and interfacing the FACTs devices (e.g., the active power filters and reactive power compensator). On the other hand, this figure also highlights the anticipated impending SST-based distribution system. Therefore, it is evident that the SST can replace the traditional transformer when coupled with some power electronics with appropriate cohesive and dense system potentials. In addition, the efficiency and cost concerns of the SST may be tackled prudently. However, the consistency and lifetime of the SST-interfaced distribution system are of a great concern for the utilities [126,127,128,129,130,131,132,133,134,135,136,137,138].

Figure 11. SST applications in distribution system [126,127,128,129,130,131,132,133,134,135,136,137,138].

The applications and uses of the SST in certain spheres are much more striking and appealing [139]. Some examples of these applications include: (1) The locomotive and related traction system as a significant and momentous weight reduction mechanism; this results in the enhancement of efficiency and a reduction in EMC/harmonics (2) The desired energy generation as a means for cheaper offshore platforms. (3) Smart grid applications for the dynamic adjustment of energy distribution. (4) Integration with other energy systems. (5) Applications between generation sources and load/distribution grids to attain a unity power factor from energy transportation. (6) The controlling of active power between two distribution grids and action as a reactive power compensator for both grids. (7) Linkage between the MV and LV grid to control the amount of reactive power flow. (8) Action as an interface for distributed generation and smart grids [140,141,142,143]. Diverse applications/usages lead to different requirements. Figure 12 exhibits a schematic overview of SST applications [144,145,146,147,148].

Figure 12. Schematic overview of SST applications [144,145,146,147,148].

In a high-power conversion scenario, the DAB DC-DC converter is found to be the most viable and promising elucidation, which is in-fact a galvanically isolated DC-DC conversion device [149,150]. The various characteristics and applications of the isolated DC-DC converter include: (1) Being a modular, symmetric structure with high power density for multiport operations. (2) Being extensively used to interface with the distribution grid on a population level (i.e., with the 220 VAC, 50/60 Hz utility grid). (3) Energy/power storage schemes. (4) Fuel cells and interfaces for multiple renewable energy sources (e.g., photovoltaic (PV) modules). (5) Chargers for plug-in hybrid electric vehicles (PHEVs)/battery electric vehicles (BEVs). (6) Bi-directional conversion capability. which supports the growth of smart interactive power NWs, where energy arrangements compose an energetic role for the provision of numerous kinds of support to the grid (e.g., vehicle-to-grid (V2G) perceptions). (7) AC microgrids and inhabited DC distribution systems (DC nano-grids) [151,152,153,154,155]. Solar power technologies, wind power for homes/businesses, etc., are the aspects for renewable energy [156].

6. Conclusions

A comprehensive, technological and critical literature review on concepts/developments in the field of a smart distribution transformer for the smart grid, known as SST, was conducted. Different topologies and designs for the fabrication of the SST, isolated DC-DC converter and high-frequency inverter have been concisely reviewed and summarized. Summarizing the significance of the SST, it can be concluded that the SST could be the next big thing in power electronics. When SSTs are implemented and incorporated practically into the system in the future, they will definitely modify the method of power delivery and distribution. Because of their significant peculiarities and functionalities, SSTs will also become essential constituents in the future smart grid, thereby allowing them to direct power from any source to any terminus by exploiting the most efficient route possible.

In conclusion, the topology and configuration of SSTs are determined by each particular application, and dependent upon distinctive features and limitations such as power levels/voltage levels, performance, reliability, costs, etc.