Application of environmentally friendly and fire-safe transformer liquids
Submitting Institution
University of ManchesterUnit of Assessment
Electrical and Electronic Engineering, Metallurgy and MaterialsSummary Impact Type
TechnologicalResearch Subject Area(s)
Chemical Sciences: Inorganic Chemistry, Physical Chemistry (incl. Structural)
Engineering: Materials Engineering
Summary of the impact
Research on ester liquids (ELs) has proved they can be used in
high-voltage (HV) transformers,
bringing economic and safety advantages to the power industry and
environmental benefits to
society. Impact includes revisions to National Grid's oil policy
recommending ester-filled HV
transformers for use in London and the design and operation of the first
132kV "green" transformer
(valued between £1m and £2m). The research has led directly to the
creation of two international
standards for professionals in global power utilities specifying the use
of ELs in transformers.
These developments have contributed directly to Manchester SME M&I
Materials increasing sales
from £15m (2008) to £29m (2012).
Underpinning research
The impact arises from research undertaken in Manchester from 2004 to
date, with major
publications from 2007 onwards. The key researchers were:
- Professor Zhongdong Wang (Lecturer 2000-2007, SL 2007-2009, Professor
2009-date)
- Dr Qiang Liu (PDRA 2011-2012, Lecturer 2012-date).
PhD students included Dr Dan Martin (2004-2008), Dr Imad Khan
(2005-2009), Dr Jie Dai (2005-2009),
Dr Xin Wang (2008-2011) and Dr Xiao Yi (2008-2012).
The aim of the research was to develop the electrical design criteria for
ester-filled transformers;
formulate an efficient method for impregnating the winding structure with
ester liquids; and to
understand the fault-diagnostic techniques required to design, manufacture
and operate ester-filled
HV transformers (HV refers to ≥ 132kV). Ester liquids have advantages over
mineral oils in that
they are biodegradable, both within soil and water, and they have a high
flash-point, which makes
them inherently safer, allowing a higher operating temperature.
The key research findings are as follows:
- AC breakdown voltages of ester liquids under homogeneous fields were
proved to be similar to
those of mineral oil [1].
- Key electrical characteristics, such as partial discharge inception,
propagation and discharge
patterns, under extreme inhomogeneous AC fields were determined and
recommendations made
for new and amended IEC standards related to electrical testing of
insulating liquids [5].
- The breakdown voltage-gap distance equations for ester liquids were
formulated. This involved
the measurement of breakdown and pre-breakdown phenomena under standard
lightning
impulses, and established fast streamer-led breakdown theory as the main
cause of failure for
long liquid gaps [2] and complex insulation geometries involving
pressboard (solid insulation
barriers used in transformers) [6].
- Empirical equations were developed that linked liquid viscosity,
temperature and vacuum level
with the processing time required for ester liquids to impregnate solid
insulating materials [4].
- Dissolved gas analysis for condition assessment was demonstrated to be
appropriate for ester-filled
transformers, although the gas amounts and types were modified [3].
References to the research
The research was published in the three most prestigious journals in the
field of transformers and
electrical insulation: IEEE Transactions on Dielectrics and
Electrical Insulation, IET Electric Power
Applications and IEEE Electrical Insulation Magazine. Paper
[2] was cited by Dr Jan Hajek, Global
Manager Basic Technology Development of ABB in his keynote address at the
IEEE International
Conference on Dielectric Liquids, Trondheim, 2011, as one of the two
key liquid research
breakthroughs in recent years.
Key Publications
[2] Q. Liu and Z.D. Wang, "Streamer Characteristic and Breakdown in
Synthetic and Natural Ester
Transformer Liquids under Standard Lightning Impulse Voltage", IEEE
Transactions on
Dielectrics and Electrical Insulation, 2011, Vol.18, No.1, pp.
285-294 (Scopus citations = 18).
DoI 10.1109/TDEI.2011.5704520
Other Publications
[4] J. Dai and Z.D. Wang, "A Comparison of the Impregnation of Cellulose
Insulation by Ester and
Mineral Oil", IEEE Transactions on Dielectrics and Electrical
Insulation, Vol.15, No.2, pp.374-381,
April 2008. (Scopus citations = 8) DoI 10.1109/TDEI.2008.4483455
[5] Z.D. Wang, Q. Liu, X. Wang, P. Jarman and G. Wilson, "Discussion on
Possible Additions to
IEC 60897 and IEC 61294 for Insulating Liquid Tests". IET Electric
Power Applications 2011;
5(6): 486-493 (Scopus citations = 6). DoI 10.1049/iet-epa.2010.0209
[6] Q. Liu and Z.D. Wang, "Streamer Characteristic and Breakdown in
Synthetic and Natural Ester
Transformer Liquids with Pressboard Interface under Lightning Impulse
Voltage", IEEE
Transactions on Dielectrics and Electrical Insulation, 2011, Vol.18,
No.6, pp. 1908-1917
(Scopus citations = 4). DoI 10.1109/TDEI.2011.6118629
Details of the impact
Context
Fire and potential subsequent explosion is the major risk factor when
operating high-voltage
transformers in dense urban environments. Power transformers require
insulating and cooling
media, most commonly mineral oil an environmentally hazardous material
which is also a fire risk.
Ester liquids mitigate these hazards and risks, since they are
biodegradable and their flash point is
350 °C versus 150 °C for mineral oil. Ester liquids have been used in low
voltage (LV) transformers
(≤33 kV) for twenty years, but were not used in HV transformers before
2008. The Manchester
research established the performance of ester liquids under all the
critical stresses of HV
applications, which enabled the manufacture and operation of HV
ester-filled transformers with
environmental performance and fire-safety level expected by society.
Pathways to Impact
- The research team worked directly with sponsoring companies — M&I
Materials, Alstom Grid,
UK Power Networks (UKPN), National Grid, Scottish Power, Electricity
Northwest and TJH2B
(2005 to date).
- Technical papers were presented at leading international conferences
in the field — IEEE:
Conference on Electrical Insulation and Dielectric Phenomena (CEIDP),
International
Conference on Solid Dielectrics (ICSD), International Conference on
Dielectric Liquids· (ICDL)
and International Symposium on High Voltage Engineering (ISH).
- Presentations were regularly given to the Council on Large Electric
Systems, (CIGRE)
Session/Colloquium and working groups. CIGRE is the international
association that develops
professional practice guidance for engineers in the power system sector,
and is the relevant
technical council for the International Electrotechnical Commission
(IEC).
All these activities increased the awareness of HV performance of ester
liquids by the industry.
Reach and Significance of the Impact
1. Changes to international standards and professional practice.
International standards in the
electrical industry are established by the IEC. The research [5]
highlighted the need for updates to
existing IEC standards for the use of ester oils in HV transformers; these
are currently being
implemented by IEC Committee TC10 [a]. These standards are supported by
professional practice
guidance through CIGRE technical brochures which are the main practice
guides for power
engineers internationally. The results from the Manchester research are
the basis of several
sections in "Experiences in Service with Insulating Liquids" (2010) [f]
and "DGA in Non-Mineral Oils
and Load Tap Changers and Improved DGA Diagnosis Criteria" (2010) [g].
2. Ester liquid manufacturers: The research has enabled companies
producing ester oils to
expand their market from the small LV transformers that typically hold
<100 litres to larger HV
transformers that typically hold 40,000-80,000 litres of oil creating a
5.5m litre annual global market
[h]. In the UK M&I materials, a high growth SME, and a sponsor of the
research at Manchester, is
using the research results to support sales of Midel 7131. Their Technical
Manager stated "in doing
this, customers and potential customers are persuaded of the advantages of
using these
alternative fluids... M&I Materials' total sales increased from £15m
in 2008 to £29m in 2012" [a].
3. Transformer manufacturers: The equations developed in [4] and
[6] enabled manufacturers to
design and manufacture ester-filled transformers. In 2008 Alstom Grid, the
third largest transformer
manufacturer in the world, became the first manufacturer in the UK to
design and manufacture an
ester liquid-filled 132kV/33kV/90MVA transformer. Such transformers are
large capital items
(typically valued at £1-2m) that are only manufactured to order and this
first transformer was
installed in EDF Energy's network (see 4 below). Alstom Grid can now
manufacture a sustainable
range of green "eco-efficient" power transformers (10 to 200 MVA and up to
245kV) [e], and
continue to use the Manchester research to help design a 400kV
ester-filled transformer insulation
model to validate this technology for the highest UK transmission voltages
[b].
4. Distribution Utilities: Based on the statistical evidence in
research output [1], EDF Energy
(now UK Power Networks, the distribution utility for London) made a
strategic decision to become
the first utility to own and operate a "green" 132kV transformer using
natural ester oils from
renewable resources that are biodegradable and more fire resistant [d].
5. Transmission Utilities: National Grid, the UK power
transmission system operator, has used
the Manchester research to revise their oil policy to enable the use of
ester oils to meet fire-safety
concerns. National Grid has also ordered the first fire-safe 400kV
transformer prototype with
20,000 litres of M&I Material Midel 7131, the liquid analysed at
Manchester. The cost of each
transformer is £1m-£2m [c].
The improved fire safety and environmental benefits of ester oils are
driving change internationally
and we are aware of anecdotal reports of US based utilities changing the
oil type in existing
transformers in high risk locations such as Disneyland.
Sources to corroborate the impact
a. Letter from Technical Manager, M&I Materials (CIGRE SC A2 WG A2.34
convenor) —
demonstrating the impact of the research on the company's sales and the
impact on
international best practice.
b. Letter from Unit Managing Director, ALSTOM Grid Research &
Technology Centre, Alstom
Grid — supporting the technological impact on new products of manufacturer
based in the
UK
c. Letter from Technical Lead on Transformers, National Grid (IEC
transformer chair) —
confirming the safety impact of ester liquids on the UK transmission
operator.
d. EDF Energy (now UKPN) on "green transformers" in 2008 — press release
supporting the
Manchester R&D involvement in the installation of the 1st
green transformer.
e. Alstom Green Power Transformer Brochure — product brochure describing
the range of
"green" transformers incorporating ester oils.
f. "Experiences in Service with Insulating Liquids" (2010) — CIGRE
brochure defining best
practice using Manchester research results
g. "DGA in Non-Mineral Oils and Load Tap Changers and Improved DGA
Diagnosis Criteria"
(2010) — CIGRE brochure defining best practice using Manchester research
results
h. Transforming the Transformer Industry, inform August 2008 — a paper
informing the size of
global market for ester liquids