Solid State Transformers: Concepts, Classification, and Control

Round 1

Reviewer 1 Report

The article presents a wide, complete literature review of Solid State Transformers, with including concepts, topologies, classification and control.   The classification of Solid State Transformers is based on introducing the new methodology. The properties of individual properties are described and discussed. Selected applications of SSTs are presented.

The article is written in proper scientific form. But the length of the article seems to be too long and the reading of the article with the tracing of all detailed descriptions needs much attention and patience. The number of cited References is also quite great and includes 154 positions. The article consists of 9 main Sections, the principal Sections are divided into several Subsections.

The article is not ended with the notes about Author Contributions, Funding and Conflict of Interests.

There is a great number of abbreviations used in the article. The presentation of the list of abbreviations with the explanations is very helpful for study of the article. But this list is too long and some positions can be omitted, for instance the lines with precise descriptions of SST or common used abbreviations, as RMS, FLC, SVM, etc.

There are some editorial mistakes stated in the article:

Line 11: The last sentence of the Abstract should be ended with the dot.

Lines 49-52 - The sentence in Lines 49-52 is too long and contains more than 50 words. It is difficult to memorize and to understand such type of sentence.

Lines 432-437 - The sentence in Lines 432-437 is too long and contains more than 90 words !!. It is difficult to memorize and to understand such type of sentence.

Lines 446-449 - The sentence in Lines 446-449 is too long and contains more than 50 words. It is difficult to memorize and to understand such type of sentence.

The numbers and the titles of the tables are usually placed at the top.

Author Response

Dear Reviewer,

please find attached the reply to your concerns and the new version of the manuscript. 

Best Regards

Author Response File: Author Response.pdf

Reviewer 2 Report

To: energies

From: Anonymous reviewer

Subject: energies-760945 peer-review report

Date: 30.03.2020

Article: Solid State Transformers: Concepts, Classification and Control

A brief summary

The authors’ main goal is to present a complete overview on SST: concepts, topologies, classification, power converters, material selection and key aspects for design criteria and control schemes proposed in the literature. Also authors proposed a simple terminology to identify and homogenize the large number of definitions and structures currently reported in the literature. The article is based on an in-depth analysis of literature sources. It is worth noting the use of rich literature sources (154 items) for analysis.

The overall quality of the manuscript is good. The manuscript contains good-quality contents and results. Analysis results and conclusions presented in the paper can be used by designers and exploiters power systems. Furthermore, it constitutes suitable solutions in applications in energy conversion, traction, energy routing, grid interconnection and reactive power industry.

The proposal for a new terminology for a simple classification of SST deserves special recognition.

The topic of the manuscript is also interest to the readers of energies.

Broad comments

Introduction - section without comments.

Results and discussion – section without comments.

Conclusions  -  section can be slightly improved focusing on the its importance in the field of research and practical applications.

Specific comments

line

117 - in the Fig.3, extend the description of the drawing. The descriptions are only for parts 3a) and 3b);

134 - in the Fig.4, extend the description of the drawing. The descriptions are only for parts 4a), 4b), 4c),

4d) and 4e)

Author Response

Dear Reviewer,

please find attached the reply to your concerns and the new version of the manuscript. 

Best Regards

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper provides a comprehensive literature review from the solid-state transformers, from their applications, categorization, and construction. The paper provides a great summary from the state-of-the-art of the topic, however the English language (structure of the text, readability) of the text should be checked again and the following notes should be corrected:

Notes:

- Please, correct this information:
15 Passive transformers are an indispensable component in electrical power systems since William
16 Stanley demonstrated the first commercial transformer in 1886

"The first power transformer was shipped on September 16, 1884 by Ganz & Company. This first unit was a one-phase, shell-form transformer with the following specifications: 1400 W rated power, 42 Hz network frequency, 120/72 Volts, 11.6/19.4 Amperes, and 1.67:1 turn ratio ..." [1]

[1] Orosz, T. (2019) “Evolution and Modern Approaches of the Power Transformer Cost Optimization Methods”, Periodica Polytechnica Electrical Engineering and Computer Science, 63(1), pp. 37-50. https://doi.org/10.3311/PPee.13000.

- What does 'passive transformer' phenomena mean? This is not straightforward for me, because it can mean that the conventional transformers cannot be regulated, however, there are a lot of on-load tap-changing methods, which can change the voltage ratio on an 'active' component. But I can find out other meaning, it's not a clear terminology.

The forthcoming explanation is not clear and I guess not correct, because it does not consider the phase-shifting, insulating and etc. special transformers, which not contains semiconductor technology for reactive compensation or etc. [2] :
- "However, unlike passive transformers,
18 Solid State Transformers (SSTs), also known as Power Electronic Transformers (PETs), are capable
19 of compensating voltage sags and harmonic distortion, interconnecting asynchronous networks,
20 interfacing DC and AC port(s), compensating reactive power, regulating voltage magnitude, isolating
21 disturbances from source and load or vice versa and eliminating the use of mechanical actuators or
22 tap changers."

[2] Del Vecchio, R. M., Poulin, B., Feghali, P. T., Shah, D. M., Ahuja, R. "Transformer Design Principles: With Applications to Core-Form Power Transformers", 2nd ed., CRC Press, Hoboken, USA, 2010

- The reduced volume/weight ratio is mentioned several times, in the introduction and in the medium frequency transformer design section:
"Mediumfrequency transformers are key elements in SSTs. AMFT coupled with a power converter
504 achieves low footprint and higher power density while, enhancing its efficiency [46],[126]."
However, the conventional distribution transformers or power transformers are optimized for generally four different objectives:
mass, volume, efficiency and lifetime cost. This fourth criterion is very interesting and explained in [1,3] , which shows some interesting results. Some comparisons would like to be interesting to prove these concepts. The smaller footprint is very close to the term smaller carbon footprint, which should be proofed with some citation, or/and smaller size, etc terminology can be luckier.

[3] Georgilakis, P. S. "Spotlight on Modern Transformer Design", Springer-Verlag, London, UK, 2009.

- The economic viability of the Conclusions should be supported with more publications.
- The achieved watt/kg performances should be supported/illustrated with some examples from the literature.

- Galvanic insulation seems not an advantage, compared to the conventional technology.

- 6.2.2. Winding Material and Arrangement --- here the problem of the aluminum and copper selection is a typical problem below 4-10 MVA units, above 15-20 MVA, the short-circuit currents become larger than the aluminum materials are too soft for these large machines.

- Please, explain these sentences and correct the ideas behind them:
571 ... Air is preferred for
572 cooling. Epoxy is used if air insulation is insufficient to reduce the temperatures.

Epoxy or cast resin is solid insulations in the manufactured winding systems, they can increase the electrical withstand of the winding or the mechanical, but what do you mean that it can reduce the temperatures? Or please explain/introduce the winding gradient for the reader.

There are some water-cooled, novel high-frequency transformer applications to increase the watt/kg performance of the machines, please mention some of them [4].

[4] Grzesik, B., Stępień, M., Nowak, A. J., & Smołka, J. (2005). Influence of cooling system on the performance coaxial transformer.

- This insulation section should be organized better.

- Application of the biodegradable oils in transformers is a current problem in the industry. These, silicon oil or ester type materials can have a lot of benefits in the future and they can reduce the footprint of biodegradable oil-based transformers [5].

[5] Fofana, I. "50 years in the development of insulating liquids", IEEE Electrical Insulation Magazine, 29(5), pp. 13–25, 2013.

Author Response

Dear Reviewer,

please find attached the reply to your concerns and the new version of the manuscript. 

Best Regards

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The authors replied all of my points, however there are two points which should be corrected.

The first paragraph is still incorrect. It mixes the US and the world transformer history.

The first transformer made by the Zipernowsky - Bláthy - Déry, as the first commercial application, as well.  This device was the basis for alternating current power distribution networks and just such a network was installed at the National General Exhibition in Budapest in 1885.

In 1886 they installed the Rome-Cerchi steam power plant. This was the very first power plant which used, on the proposal of Bláthy, AC generators to supply a common network in parallel connection. The Ganz factory produced electrical equipment for the power network of the city of Rome for several decades.

You can found this information on Wikipedia, the recommended article or here: https://web.archive.org/web/20101206042832/http://www.iec.ch/cgi-bin/tl_to_htm.pl?section=technology&item=144

William Stanley made an alternative design (core-type) for Westinghouse company, one year later. Because, the patents of the original, shell-type was bought buy Edison's company.

The other false information is in the following sentence, which startes at line 604:

Also, utilization of SSTs in an application is decided by the Total Cost of Ownership (TCO), but, methodologies to implement environmental cost into the transformer TCO has not yet been developed

Here, the reference [2] contains more actual information and more deep review from these methods it should be cited here and this information is false/too old, because this recent paper published a research last year which considers the environmental factors:

Author Response

Dear Reviewer,

Thank you very much for your comments. Please find attached the response to your concerns and the corrected version of the paper.

Sincerely yours. 

Author Response File: Author Response.pdf