Study on waveform parameters of impulse test voltage and on-site docking method for EHV/UHV GIS
Partial discharge detection and diagnosis in gas insulated switchgear: State of the art
Partial discharge characteristics of cavities with different appearances and positions in solid insulation
Dielectric Phenomena and Electrical Insulation: At the Turn from the First to the Second Century
The early history of our oldest conference, the Conference on Electrical Insulation and Dielectric Phenomena (a.k.a. CEIDP), had become a kind of mystery, as more and more details were forgotten over the years. With help from the Archivists of the National Academy of Sciences, however, it was possible to locate the meticulously written record of the first CEIDP years. Hidden under “Table of Contents” in the 1950 Annual Report of the Conference on Electrical Insulation (19th Annual Meeting) that had been organised by the National Research Council (NRC) of the National Academy of Sciences (NAS), we find a “History of the Insulation Conference to 1939” by William A. Del Mar, Vice-Chairman from 1923 until 1939, on pages XIV through XIX, as well as additional his- torical material. Some of the “Front Matter” was also collected separately in a booklet “30th Anniversary, 1920-1950, Conference on Electrical Insulation” by the NRC Division of Engineering and Industrial Research, National Academy of Sciences, and distributed to the “Founding Members of the Conference on Electrical Insulation” in recognition and appreciation of their membership and support.
According to the recovered documents, the history of the CEIDP – and of an organised Dielectrics and Electrical Insulation community that much later evolved into the IEEE Dielectrics and Electrical Insulation Society (DEIS) began in 1917 when Comfort A. Adams addressed the Wire and Cable Subcommittee of the AIEE Standards Committee with a proposal on “Comprehensive Cooperative Research in the field of Dielectric Phenomena”. Dr. Adams suggested “that a research committee be appointed, consisting of … men who are familiar with the general subject of dielectric phenomena and … who have facilities for carrying on research in this field.” Such a committee “would … lay out a plan of research, dividing it into sections, including experiments dealing first with the fundamental nature of dielectric phenomena and then proceeding to actual tests upon completed apparatus, including cables and electrical machinery.” When Dr. Adams became Chairman of the Engineering Division of the National Research Council, he proposed a committee based on institutional representation with Frank B. Jewett (who later was the founding president of Bell Laboratories) as chairman.
After a preliminary meeting in 1919, a committee of 38 members was appointed and held its first meeting at Western Electric Company’s Engineering Building in New York City on 20 September 1920. Subsequently, the so-called “Insulation Committee” seems to have been inactive until the appointment of John B. Whitehead as its chairman in 1922 and as Chairman of the AIEE Research Committee in 1923. As listed in the accompanying tables, the second meeting took place in 1923 under the auspices of the American Institute of Electrical Engineers (AIEE), one of the predecessors of the IEEE. On behalf of the committee, a paper entitled “The Problem of Insulation, Report of the Committee on Electrical Insulation, Division of Engineering, National Research Council” (which is still worth reading) was published in the Journal of the AIEE, Vol. 42, No. 6, pp. 618-622, June 1923 by Dr. Whitehead. Until 1927, the Insulation Committee acted only in cooperation with the AIEE Research Committee, met at the AIEE conventions and published reports in the Transactions of the AIEE. The year 1928 saw two meetings of the committee, but the first one in April was “devoted entirely to planning” so that it should not be counted as a Conference on Electrical Insulation.
With its November 1928 annual meeting, the Insulation Committee began its “independent life”, but continued to cooperate with the AIEE and with the NRC. Thereafter, the 1928 meeting was counted as the first “true” annual conference, and the numbering of the annual CEIDP meetings still adheres to the “conventional” NRC counting, even though William A. Del Mar already suggested a new numbering that begins in 1920. In the accompanying lists, we follow his suggestion – but do not include the “planning” meeting in April 1928 – so that we will hold the 95th annual conference at the CEIDP centennial in 2020, since the conference did not meet in 1921, 1922, as well as between 1942 and 1945 when the 2nd World War interrupted the conference series.
Over its first thirty years, the Conference on Electrical Insulation (CEI) was mainly held at industrial facilities, national laboratories, and universities. The custom changed in 1949 with the first conference in the Poconos which became a regular location for a large number of annual meetings up to the early 1980s and provided a special atmosphere far away from the big cities. The CEI(DP) with its increasing attendance thus started to follow the trend of holding conferences at hotels with sufficiently large meeting facilities. Right in the middle of its second thirty years, the words “Dielectric Phenomena” which had already appeared in the first proposal by Adams in the year 1917 (see above) were added to the conference title in 1966 – probably in order to stress the more fundamental and research-oriented nature of the CEIDP. After John B. Whitehead passed away in November 1954, the Whitehead Memorial Lecture was dedicated to his memory in 1955 and has since been awarded annually to a large number of eminent colleagues from all over the world.
In 1981, under the leadership of Eric O. Forster, against considerable opposition and after extensive negotiations between the NRC and the IEEE, the conference was taken under the umbrella of the IEEE and its Electrical Insulation Society (EIS) which in 1985 changed its name to Dielectrics and Electrical
Insulation Society (DEIS). Within the (D)EIS, the CEIDP initially maintained a unique and partially independent charter with an emphasis on “work in progress”, with an Annual Report and with a yearly Literature Review which had been issued as a separate Digest of Literature until 1978, but was continued almost annually as a special issue or section in the October issue of the Transactions. The latter service to the community of dielectricians had to be terminated after more than seven decades in 2009 (almost at the end of the third thirty-year period of the CEIDP) mainly due to (1) the development towards internet-based literature management, (2) the strong competition from commercial book publishers and (3) the lack of authors with the time and the expertise to contribute comprehensive bibliographical review articles. From a historical perspective, the rather difficult move from the NRC to the IEEE around 1980 was a return to its roots for the CEIDP, as most of the early meetings in the 1920s had been held in very close collaboration with the American Institute of Electrical Engineers (AIEE), one of the predecessors of the IEEE (cf. the accompanying list of early Annual Meetings). Only in the last decade before its centennial did the CEIDP board finally elect the first two chairwomen (Rajeswari Sundarajan for 2010-2011 and Nicola Bowler for 2018-2019), as well as the first female Whitehead Memorial Lecturer (Kaori Fukunaga in 2016), but the number of women in our community is still much too low.
During the second thirty-year period of the CEIDP, the selection of venues for the annual meetings included Canada for the first time (Varennes, Hydro Québec, 1973). More recently, the Canadian (Victoria, British Columbia, 1992 and 2000) and American (Millbrae, California, 1996) West Coast, Mexico (Cancun in 2002, 2011 and 2018) and even China (Shenzhen 2013) were chosen as conference locations. For the centennial in 2020, it is planned to return to the New York City area and to offer a visit to the historical site of the very first annual meeting in the former Bell Labs building complex on the lower West side of Manhattan. Since 1970, the complex is operated by the Westbeth Artists Community as a major NYC cultural center with artists’ housing and studios, event spaces, etc. The former office of
the Bell Labs president which had been used by Frank B. Jewett, the first chairman of our conference, in the 1920s and 1930s has been preserved and still offers a magnificent view across the Hudson river to the New Jersey shore and to the Statue of Liberty. At the CEIDP centennial during the week from 18 to 23 October 2020, we will of course not only look back on the first century, but will also invite perspective views towards the second century to be given by leading colleagues who are in the middle of their careers. You are all invited to attend the centennial of the CEIDP, to contribute to its technical program and its success, and to celebrate the first one hundred years of our thriving community of dielectricians.
Remark: When collecting the historical material and writing the above article, I received considerable support and useful advice from the former DEIS presidents Greg Stone, Bill McDermid and Frank Hegeler. Further historical and architectural details about the complex of buildings used by the Western Electric company (since 1898), by AT&T Bell Laboratories (from 1925 until 1966) and by the Westbeth Artists Community (since 1970) are found on the Wikipedia website “Bell Laboratories Building (Manhattan)” and from the New York City government website.
 Frank B. Jewett, “The Genesis of the National Research Council and Millikan’s World War I Work,” Reviews of Modern Physics, vol. 20, no. 1, pp. 1-6, January 1948.
 J. B. Whitehead, “Progress in Electrical Insulation Research – 1938,” Electrical Engineering, vol. 58, no. 1, pp. 23-31, January 1939.
 S. O. Morgan, “A Tribute to the Founders of the Conference on Electrical Insulation,” 1950 Annual Report, Conference on Electrical Insulation, NRC, Washington, DC, pp. XXIX-XXX, 1950 (Available on IEEExplore).
 A. von Hippel, “Trends and Tasks of the Conference on Electrical Insulation,” 1951 Annual Report, Conference on Electrical Insulation, NRC, Washington, DC, pp. 7-10, 1951.
 J. Tanaka, “History of the Dielectrics and Electrical Insulation Society,” IEEE Transactions on Electrical Insulation, vol. 25, no.1, pp. 3-16, February 1990.
Chairs of NRC CEI (1920– 1965), NRC CEIDP (1966– 1980) and IEEE CEIDP (1981–2019)
1920-1921 Dr. Frank B. Jewett
1922-1938 Prof. John B. Whitehead
1927 and 1938 Mr. William A. Del Mar
1939-1947 Dr. Ward F. Davidson
1948 Dr. Stanley O. Morgan
1949 Dr. John D. Piper
1950 Dr. Charles F. Hill
1951 Dr. Arnold H. Scott
1952 Prof. Arthur R. von Hippel
1953 Mr. Donald W. Kitchin
1954 Dr. Davis A. McLean
1955 Dr. Robert G. Breckenridge
1956 Dr. A. Harry Sharbaugh
1957 Dr. Thomas W. Dakin
1958 Mr. Earl R. Thomas
1959 Dr. John D. Hoffman
1960 Mr. William McMahon
1961 Mr. Stanley I. Reynolds
1962 Mr. Philip J. Franklin
1963 Mr. Joseph Sticher
1964-1965 Mr. Louis J. Frisco
1966-1967 Mr. Arthur J. Warner
1968-1969 Mr. Emmanuel L. Brancato
1970-1971 Dr. Daniel Berg
1972-1973 Dr. George S. Eager, Jr.
1974-1975 Prof. John J. O’Dwyer
1976-1977 Dr. Martin G. Broadhurst
1978-1979 Dr. P. Keith Watson
1980-1981 Dr. Eric O. Forster
1982-1983 Dr. Clive W. Reed
1984-1985 Dr. Arend van Roggen
1986-1987 Prof. Edward Sacher
1988-1989 Prof. J. Keith Nelson
1990-1991 Dr. G. Edward Johnson
1992-1993 Mr. Roy E. Wootton
1994-1995 Prof. Marshall O. Pace
1996-1997 Prof. Reuben Hackam
1998-1999 Prof. Lynn L. Hatfield
2000-2001 Dr. Soli S. Bamji
2002-2003 Prof. Vijendra K. Agarwal
2004-2005 Prof. Vishnu K. Lakdawala
2006-2007 Dr. Isidor Sauers
2008-2009 Prof. Huseyin R. Hiziroglu
2010-2011 Prof. Rajeswari Sundararajan
2012-2013 Dr. Mahmoud Abou-Dakka
2014-2015 Dr. Michel Fréchette
2016-2017 Dr. Enis Tuncer
2018-2019 Prof. Nicola Bowler
NRC/AIEE Conference on Electrical Insulation: Annual Meetings 1920–1950—The First 30 Years
Year, Location, New No.
1917 AIEE Standards Committee 0
1920 New York, NY (Western Electric) 1
No Meetings in 1921 and 1922
1923 Swampscott (AIEE Research Comm.) 2
1924 Philadelphia (AIEE Research Comm.) 3
1925 Saratoga Springs (AIEE Res. Comm.) 4
1926 White Sulfur Springs (AIEE Res. Comm.) 5
1927 Detroit, MI (AIEE) 6
1928 Baltimore, MD (Johns Hopkins U/AIEE) 7
1929 Cambridge, MA (MIT) 8
1930 Washington, DC (NBS) 9
1931 Boston, MA (Harvard University) 10
1932 Baltimore, MD (Johns Hopkins U/AIEE) 11
1933 Philadelphia, PA (Univ. of Pennsylvania) 12
1934 Chicago and Urbana, IL (Univ. of Illinois) 13
1935 Pittsfield, MA (General Electric) 14
1936 Cambridge, MA (MIT) 15
1937 New York, NY (Consolidated Edison) 16
1938 Pittsburgh, PA (Westinghouse) 17
1939 Cambridge, MA (MIT) 18
1940 Washington, DC (NBS and NRC) 19
1941 Williamsburg, VA (Williamsburg Inn) 20
No Meetings during 2nd World War
1946 Baltimore, MD (Johns Hopkins Univ.) 21
1947 Cambridge, MA (MIT) 22
1948 Washington, DC (NBS) 23
1949 Pocono Manor, PA (Pocono Manor Inn) 24
1950 Pocono Manor, PA (Pocono Manor Inn) 25
NRC Conference on Electrical Insulation (and Dielectric Phenomena): Meetings 1951–1980—The Second 30 Years
1951 Washington, DC (NBS) 26
1952 Lenox, MA (Curtis Hotel) 27
1953 Pocono Manor, PA (Pocono Manor Inn) 28
1954 Pocono Manor, PA (Pocono Manor Inn) 29
1955 Pocono Manor, PA (Pocono Manor Inn) 30
1956 Schenectady, NY (General Electric) 31
1957 Pocono Manor, PA (Pocono Manor Inn) 32
1958 Pittsburgh, PA (Westinghouse & Mellon) 33
1959 Pocono Manor, PA (Pocono Manor Inn) 34
1960 Washington, DC (NBS) 35
1961 Pocono Manor, PA (Pocono Manor Inn) 36
1962 Hershey, PA (Hershey Hotel) 37
1963 White Sulphur Springs, WV (Greenbrier) 38
1964 Cleveland, OH (Union Carbide, Parma) 39
1965 Buck Hill Falls, PA (The Inn) 40 Dielectric Phenomena Added to Title
1966 Pocono Manor, PA (Pocono Manor Inn) 41
1967 Pocono Manor, PA (PoconoManor Inn) 42
1968 Buck Hill Falls, PA (The Inn) 43
1969 Buck Hill Falls, PA (The Inn) 44
1970 Pocono Manor, PA (Pocono Manor Inn) 45
1971 Williamsburg, VA (Cascades Center) 46
1972 Buck Hill Falls, PA (The Inn) 47
1973 Varennes, QC (Hydro Québec, IREQ) 48
1974 Downingtown, PA (Downingtown Inn) 49
1975 Gaithersburg, MD (NBS) 50
1976 Buck Hill Falls, PA (Buck Hill Inn) 51
1977 Colonie, NY (Americana Inn) 52
1978 Pocono Manor, PA (Pocono Manor Inn) 53
1979 Whitehaven, PA (Pocono Hershey Res.) 54
1980 Boston, MA (Parker House) 55
IEEE Conference on Electrical Insulation and Dielectric Phenomena: Meetings 1981–2020—The Last 40 Years with DEIS
1981 Whitehaven, PA (Pocono Hershey Res.) 56
1982 Amherst, MA (University of Mass.) 57
1983 Buck Hill Falls, PA (Buck Hill Inn) 58
1984 Claymont, DE (Wilmington Hilton) 59
1985 Amherst, NY (Buffalo Marriott) 60
1986 Claymont, DE (Wilmington Hilton) 61
1987 Gaithersburg, MD (NBS) 62
1988 Ottawa, ON (Skyline Hotel) 63
1989 Leesburg, VA (Xerox Conference Center) 64
1990 Pocono Manor, PA (Pocono Manor Inn) 65
1991 Knoxville, TN (Hyatt Regency) 66
1992 Victoria, BC (The Empress Hotel) 67
1993 Pocono Manor, PA (Pocono Manor Inn) 68
1994 Arlington, TX (Arlington Marriott) 69
1995 Virginia Beach, VA (Virginia Beach Resort) 70
1996 Millbrae, CA (Westin SF Airport) 71
1997 Minneapolis, MN (Radisson Metrodome) 72
1998 Atlanta, GA (Sheraton Colony Square) 73
1999 Austin, TX (Renaissance Austin) 74
2000 Victoria, BC (The Empress Hotel) 75
2001 Kitchener, ON (Four Points Hotel) 76
2002 Cancun, Mexico (Hyatt Regency) 77
2003 Albuquerque, NM (Hyatt Regency) 78
2004 Boulder, CO (Millennium Harvest House) 79
2005 Nashville, TN (Sheraton Downtown) 80
2006 Kansas City, MO (Hyatt Regency) 81
2007 Vancouver, BC (The Fairmount Hotel) 82
2008 Quebec City, QC (Hotel Delta Quebec) 83
2009 Virginia Beach, VA (Virginia Beach Resort) 84
2010 West Lafayette, IN (Purdue University) 85
2011 Cancun, Mexico (Hyatt Regency) 86
2012 Montréal, QC (Delta Centre-Ville) 87
2013 Shenzhen, China (Ming Wa Conf. Center) 88
2014 Des Moines, IA (Marriott Downtown) 89
2015 Ann Arbor, MI (Sheraton Ann Arbor) 90
2016 Toronto, ON (Chelsea Hotel) 91
2017 Fort Worth, TX (Hilton Fort Worth) 92
2018 Cancun, Mexico (Iberostar Hotel) 93
2019 Richland, WA (Pacific Northwest Ntl Lab) 94
2020 to be determined (near New York City)
Whitehead Memorial Lecturers 1955–2018
1955 Prof. John G. Kirkwood
1956 Prof. Peter Debye
1957 Mr. William A. Del Mar
1958 Prof. Charles P. Smyth
1959 Dr. Stanley O. Morgan
1960 Prof. Arthur R. von Hippel
1961 Prof. Raymond M. Fuoss
1962 Prof. Robert H. Cole
1963 Prof. Lars Onsager
1964 Dr. Kenneth S. Cole
1965 Prof. Leonard B. Loeb
1966 Dr. Davis A. McLean
1967 Prof. C. J. F. Böttcher
1968 Prof. Heinz Raether
1969 Dr. Thomas W. Dakin
1970 Dr. Frederick Seitz
1971 Dr. J. Franklin Hyde
1972 Dr. A. Harry Sharbaugh
1973 Mr. Kenneth N. Mathes
1974 Prof. Herbert Fröhlich
1975 Dr. John D. Hoffman
1976 Prof. T. John Lewis
1977 Prof. John G. Trump
1978 Prof. Bernhard Gross
1979 Prof. Yoshio Inuishi
1980 Prof. J. Ross MacDonald
1981 Prof. James H. Calderwood
1982 Prof. Graham Williams
1983 Prof. John J. O’Dwyer
1984 Dr. John C. Devins
1985 Dr. Eric O. Forster
1986 Prof. Masayuke Ieda
1987 Dr. Ray Bartnikas
1988 Dr. Rodney V. Latham
1989 Mr. Aage Pedersen
1990 Prof. Andrew K. Jonscher
1991 Dr. Hans-Rudolf Zeller
1992 Prof. Gerhard M. Sessler
1993 Prof. J. Keith Nelson
1994 Dr. Richard J. van Brunt
1995 Dr. P. Keith Watson
1996 Dr. Dilip K. Das-Gupta
1997 Dr. Roland Coelho
1998 Prof. Markus Zahn
1999 Prof. Tatsuo Takada
2000 Prof. Christian Mayoux
2001 Dr. Toshikatsu Tanaka
2002 Prof. Len A. Dissado
2003 Prof. Howard J. Wintle
2004 Dr. Jean-Pierre Crine
2005 Dr. Edward A. Cherney
2006 Prof. Teruyoshi Mizutani
2007 Prof. Yoshimichi Ohki
2008 Dr. Soli S. Bamji
2009 Prof. Friedrich Kremer
2010 Prof. Gian Carlo Montanari
2011 Prof. Hitoshi Okubo
2012 Dr. Christian Laurent
2013 Prof. Masoud Farzaneh
2014 Prof. Reimund Gerhard
2015 Prof. Stanislaw Gubanski
2016 Dr. Kaori Fukunaga
2017 Prof. Steven Boggs
2018 Prof. Herbert Kliem
From The Editors
From the Editors’ Desk
In this issue, our Young Professionals column brings us an article from Xiangen Zhao who describes his on-going research into the application of high-speed Schlieren photography on investigating the long spark discharge in air. Other researchers who would like to contribute articles related to their work should contact the Editors with an abstract.
The present issue of the Magazine brings three articles related to testing insulation systems of gas insulated switchgear (GIS) installations.
The first article entitled “Study of Waveform Parameters of Impulse Voltage Test Method for EHV/UHV GIS” is jointly authored by a team of researchers representing academic and industrial institutions in China. These are Xuandong Liu, Tao Wen and Qiaogen Zhang from Xi’an Jiaotong University, Lingli Zhang from State Grid Taizhou Power Supply Company and Xiangyu Tan from Electric Power Research Institute of Yunnan Power Grid Corporation. The article first reports on laboratory investigations on the influence of wave front time on 50% breakdown impulse voltage of a domed rod-plane gap. The results indicate that this dependence appears in the form of a ‘U’-shaped curve with the waveform front time of the minimum breakdown value at about 400 – 600 ns. It is thus concluded that this type of steep-fronted impulses should be used instead of lightning impulses for testing GIS insulation systems. For producing such steep-fronted impulse voltage waveforms in EHV/ UHC substation environment and to verify the universality of the experimental results on site, the authors present in the second part of the article two different docking methods for connecting impulse voltage generator and GIS installation. These tests were performed at a 1000 kV substation in Nanjing and a 500 kV substation in Kunming. The presented results of field investigations show the difficulties one faces in realizing the steep-fronted impulses or even standard lightning impulses by connecting compact impulse generator and GIS bushing with an external wire lead, being a consequence of relatively high inductance of such a test circuit. On the other hand, when the impulse voltage generator was directly connected to GIS via a variable impedance transmission section, the wave front time of impulse voltage acting on the GIS bus was about 700 ns, which is close enough to the optimal range obtained through the laboratory experiments.
The second article in this issue presents an extensive literature review summarizing the state-of-art in “Partial Discharge Detection and Diagnosis in Gas Insulated Switchgear”. It is authored by Qasim Khan, Shady S Refaat, Haitham Abu-Rub and Hamid Toliyat from the Department of Electrical Engineering of Texas A&M University in Doha, Qatar and Collage Station, Texas, USA. After a brief introduction of the latest market scenarios and benefits provided by the use of GIS and GITL over any other switchgear and transmission systems, different approaches to their condition monitoring are presented. The article finally concentrates on techniques developed to detect and quantify partial discharge activity in GIS. It includes advancement in detection and localization techniques of different PD sources with conventional and non-conventional schemes. It also summarizes the research work done on various types of analyses adopted for signal denoising, feature extraction and classification as well as points to limitations still being open for future research.
The third article reports on “Partial Discharge Characteristics of Cavities with Different Appearances and Positions in Solid Insulation” and is authored by Qian Zhang, Junhao Li, Xutao Han, Xuanrui Zhang and Cong He from Xi’an
Jiaotong University and by Xiu Yao from State University of New York at Buffalo, USA. The investigations reported in this article concentrated on determining partial discharge initiation voltage and amount of initial charge released during the tests in realistic defects that may appear in epoxy insulators for applications in GIS installations. Automatic pressure gelation technique was used to manufacture model insulators, which contained cavities of various sizes, quantities, positions and shapes. The presented results indicate that the inception voltage is lower and the released charge higher in cavities of larger diameter, of curved shapes, or located in the vicinity of electrodes. At the same time, axial length of cavities has a complex influence on PD characteristics. When the cavity extends long in the direction of applied electric field (larger than 50 mm), the inception voltage drops rapidly while the released charge remains at a high level, which create a dangerous condition for insulator service. It is claimed that the type of measurements presented in the article well reflect the severity of cavity defects and can thus be used as diagnostic parameters in manufacturing of GIS insulators as well as in service conditions.
News From Japan
Development of a Highly Durable SiC Power Device for Electric Vehicles
As a part of the UN’s Sustainable Development Goals (SDGs) and those agreed to at the 2015 UN Climate Change Conference (COP21) held in Paris, energy consumption needs to be reduced. One of the effective ways in achieving this goal is to reduce electrical energy consumption by replacing fixed speed drive systems with variable speed ones, so that less energy is used when loads are reduced. Modern variable speed motors are fed from electronic inverters, which modulate the widths, amplitudes, or frequencies of pulse voltages that are fed to them. The heart of such a pulse modulation system, consisting of converters and inverters, is power transistors.
There are two types of power transistors that are currently widely used. These are bipolar power transistors and power metal oxide semiconductor field-effect transistors (MOSFETs). The bipolar transistors use relatively slow minority carriers in their switching operations, have relatively low voltage withstand levels and require relatively long times for switching operations. A major advantage of MOSFETs over bipolar transistors is that MOSFETs use majority carriers for their switching operations and are therefore much faster. Although the voltage withstand levels of power MOSFETs in early days were not too high, this level was gradually increased by improvements of their design, such as the adoption of a double diffusion MOS (DMOS) structure, as shown in Figure 1(a).
One of the most important industrial goods that can significantly benefit from the availability of high-speed high-voltage power transistors are electric vehicles (EVs), in which electric motors controlled by power electronics are used instead of combustion engines. Furthermore, the reduction of the electric power consumption in EVs is essential from the above-mentioned viewpoint of global energy saving as the use of EVs is expected to spread explosively in many countries.
Until recently, power electronic devices have mainly been based on silicon (Si) as the principal semiconductor material, regardless of whether in bipolar transistors or MOSFETs. A new generation of power electronic devices has recently emerged based on silicon carbide (SiC) as the principal semi-conductor material and is attracting attention of researchers and potential users. They can operate at much higher voltage levels than those using Si and therefore, can offer significant saving of
energy consumption in converter-inverter systems.
As mentioned above, the structure of conventional power MOSFET is of DMOS type, in which the on-resistance is not low enough for effectively securing the high power flow. Another issue common in SiC power devices is that their on-resistance may vary greatly, unlike in Si based ones, depending on the positioning of SiC crystal plane. To resolve this issue, a trench SiC MOSFET, shown in Figure 1(b), has been introduced to pass the electric current along the crystal planes of a lower resistance. However, because of the complex structure of the trench MOSFET, electric field concentrated at the trench edge on the base plane is high and strongly contributes to a decreased durability of the device.
To solve this concern, Hitachi has recently developed a new structure, called original fin-formed trench DMOS-FET or, for short, TED-MOS. In this new type of field effect transistor, the reduction in on-resistance is achieved because of the smaller trench pitch. It can also provide a higher durability, since the maximum electric field is reduced in the device and therefore the new DMOS-FET can be applied to various industrial applications that require high-rated voltage levels, up to 3.3 kV .
Very recently, Hitachi has further advanced this TED-MOS structure, so that it becomes suitable for converter systems of EVs that require high current density at a relatively low voltage of 1.2 kV. As shown in Figure 1(c) by its schematic illustration and cross-sectional photograph along the line A-A’, the new TED-MOS has a field relaxation layer (FRL) on the top of a PN junction formed at the center of the device. It allows to significantly reduce the electric field strength on the PN junction, which is formed as a part of an n-type junction FET (JFET). The newly proposed TED-MOS has also a current spreading
layer (CSL) to reduce the resistance in the n-JFET region, by which the path of the flowing electric current connects the n-JFET to the two sides of the fin-like trenches composed of low-resistance SiC crystal .
Results of electric field simulations at the cross-sections of a DMOS and those of the conventional and newly proposed TED-MOSs, without and with the field relaxation layer, are shown in Figure 2. Since the channel is formed in the two TED-MOSs at a certain depth from the substrate surface, the insulator thickness above the JFET region can be easily increased. Therefore, the electric field intensity Eox above the JFET can be reduced in the insulating oxide to a negligibly low level. The maximum Eox should appear at the gate oxide on the trench. The lower parts of Figure 2 show that the conventional TED-MOS can suppress the highest value of Eox by 25% from 2.4 MV/cm to 1.8 MV/cm compared to the DMOS. However, the doses of implanted ions in the CSL and JFET are extremely high and the depth of CSL (Wd) is limited, as shallow as 170 nm (Figure 2).
The proposed TED-MOS with the field relaxation layer (FRL) can reduce Eox even more effectively, reaching, with a deeper Wd, 42% from 2.4 MV/cm to 1.4 MV/cm, as compared to that of DMOS. Figure 3 shows the dependence of Eox on Wd. It is clearly indicated that Eox remains low in the proposed struc-
ture in a wide range of Wd. Therefore, a good balance can be achieved between Wd and the trench depth to reduce the channel resistance Rch.
Thanks to this innovation, the newly proposed SiC based TED-MOS successfully realizes, simultaneously, both requirements, i.e. lower electric field and lower on-resistance as com-
pared with conventional TED-MOS devices. It is shown in Figure 3 that in the new TED-MOS the highest electric field is reduced by 40% compared to that of the conventional DMOS-FET, while maintaining the rated voltage of 1.2 kV. The highest resistance is also reduced by 25%. The letter A in Figure 3 rep-
resents the values of Eox and Wd employed in fabrication of the new device. Furthermore, the modified structure allows for further shortening the on/off switching operation times and, as a result, the energy loss of the switching operation becomes also reduced by 50%. Several key device parameters of the proposed TED-MOS with the FRL are summarized in Table I. Hitachi actually claims that the simultaneous realization of low loss and high reliability of this new TED-MOS can better meet the
stringent requirements of various converter systems for EVs. Hitachi intends to put the device into practical use very soon and it will also be applied in other types of electric transducers
within societal infrastructure systems, not only for EVs, and its use will contribute to the prevention of global warming and to the realization of a low-carbon society.
This article was completed with the help of Dr. Akio Shima of Hitachi, Ltd.
 N. Tega, K. Tani, D. Hisamoto and A. Shima, IEEE 30th Int. Symp. Power Semicon. Devices ICs (ISPSD), pp. 439 – 442, 2018.
 T. Suto et al., Euro. Conf. SiC Related Mater. (ECSCRM 2018), TU.02a.03, 2018.
Applying Schlieren Techniques to Investigate Long Sparks in Air
My name is Xiangen Zhao, born in Hubei, China in 1989. I received the BSc degree and the Ph.D. degree from Huazhong University of Science and Technology (HUST) in 2011 and 2017 respectively. I have been working on the application of high-speed Schlieren photography on investigating the long spark discharge in air for nearly 4 years. During these years, two Toepler’s lens-type Schlieren systems shown in Figure 1 have been built. One was established in 2015 when I was a Ph.D. student at HUST. The other was completed in 2018 when I was a postdoctoral researcher at China Southern Power Grid (CSG).
Based on my experiences, there are three main advantages of high-speed Schlieren photography comparing with traditional photography on the long spark discharge observation. First, Schlieren techniques detect the refraction index variation, which can be used to observe the thermal characteristics of spark channels, such as the air density, air temperature, and thermal radius. Second, luminosity of spark channel has little influence on the Schlieren images due to an auxiliary luminous source. So even though the discharge luminosity is so strong that it covers the channel, or too weak to be detected, the detailed characteristics of spark channels still can be distinguished. Third, much higher spatial resolution can be achieved due to the amplification effect of the de-collimating lens, which can be used to study the fine structure of discharge channel. These advantages may help us get some more interesting findings in the long spark discharge. My main achievements are described as follows.
Temperature is one of the most important parameters for spark discharge in air and very few experimental results have been reported. Therefore, my primary goal was to reconstruct the temperature profile of spark channel from a Schlieren image. It was not an easy task because the image noise and axis offset have great effects on inversion precision. With the specific arithmetic to solve these two problems, the temperature profile of discharge channel can be calculated, as shown in Figure 2. Note that the measured temperature profile is averaged both in space and time because of the limitation of spatial and temporal resolutions of the Schlieren system. This is very important when comparing simulations with measurements and when studying the critical temperature for leader initiation. Until
now, my research group continues working on the improvement of spatial and temporal resolutions of the Schlieren system.
At the same time, I was also very interested in the positive leader branching phenomenon. In my opinion, there would be more than one stem at the leader tip before branching. If stems at the leader tip can be observed, positive leader branching phenomenon may be explained to some extent. To do this, the traditional high-speed photography seems impossible as the channel luminosity has covered the stems, while the Schlieren technique may be a better choice. However, it is also very difficult to capture the Schlieren image of leader tip due to the random of leader path, the high leader velocity and the limited observation scope of Schlieren system. As the leader tip is always regarded as an electrode tip, I carried out some experiments to observe the stems around the electrodes at the beginning of spark discharge. Typical results are plotted in Figure 3, which show that, in most cases, more than one stem will initiate from the electrode tip. Although the branching mechanism of positive leader still remains unclear, these experimental results help me find the conditions to create only one stem, which is very useful when studying the relaxation phase of discharge channel. Moreover, the results also remind us that the current partition should be considered when simulating one of the discharge channels with the measured current as an input.
After I finished my Ph.D. research project in 2017 at HUST, I became a postdoctoral researcher at CSG. In the operation of a practical power grid, the reclosing time is an important parameter after the trips due to lightning strikes, et al. The reclosing operation should be done after the recovery of air insulation. Therefore, part of my postdoctoral project is aimed at the relaxation process of spark channel. Schlieren techniques are used again in this project as the channel is not luminous at that phase, but the air density in the discharge channel is different from that of the surrounding air. Figure 4 shows two typical results. As shown in Figure 4(a), the channel evolution around the anode in withstanding cases is similar to the growing process of a ‘mushroom’. After breakdown occurs in the air gap, the channel which I call a ‘caterpillar’ that is putting on weight is shown in Figure 4(b). With the optical flow method, the movement of the heated gas in the channel can be calculated during the recovery process, as shown in Figure 4(b). The recovery time can also be estimated roughly if the moment when there is no disturbance in the Schlieren image is considered as the end of relaxation process.
As the transmission lines in CSG are struck frequently by lightning, my postdoctoral research is also related to the physical mechanism of lightning discharge, especially on the behaviors of space stem or leader during the leader stepping process, which is believed to exist in both positive leader and negative leader. However, there seems no direct evidence for the existence of positive space stem or leader in the laboratory. Then, my work tries to find direct evidence of them. Recent simulations have shown that the positive space stem is very close to the leader tip, and is too small to be observed probably for the traditional high-speed photography, but not necessarily for the high-speed Schlieren photography. Therefore, I conducted some experiments in a 1.27 m air gap under switching impulses in our laboratory and the typical results are shown in Figure 5. It seems that the positive space stem does exist under the positive switching impulse in Figure 5(a). Compared with the negative space stem which appeared under the negative switching impulse in Figure 5(b), the positive one is indeed closer to the leader tip. This study is still ongoing, and the above conclusion needs to be further confirmed.
Although the observations of long sparks using Schlieren technique are so fascinating, experiments are no mean feat and even boring sometimes. Failure is a common part. Thanks to the help from my teams at HUST and CSG, I can focus on this research field during these years. I’d like to pay them my special
thanks here. At last, I really appreciate all my family in Figure 6. Their endless love and support will make me go further on the road of all the research work I am interested in.
State Key Laboratory of HVDC Transmission Technology,