A2-202
21, rue d’Artois, F-75008 PARIS
http : //www.cigre.org
CIGRE 2012
Amorphous metal-based distribution transformers: evaluation of the current
technology situation and a proposed design for short-circuit withstand
G. LOIZOS1, T. SOUFLARIS1, P. LAGACHE2, I. GOKCEN3, M. SACOTTE2
1
Schneider Electric, Greece,
2
Schneider Electric, France,
3
Schneider Electric, Turkey
SUMMARY
Amorphous metal-based transformers (AMTs) have been around for more than 25 years but their
use has only been restricted to experimental level in the European utilities networks. This was due to
energy economics and to some technical disadvantages of amorphous metals when compared to grainoriented silicon iron steels.
AMTs currently find increasing use in the Asian and USA markets at single phase and low rated
powers < 250 kVA. If, however, AMTs are to penetrate the market of European utilities, they have to
withstand the stresses of the short-circuit test as described in IEC 60076. The effects of short circuit
currents can be tremendous both for the transformer (i.e. explosion) and for the stability of the
network. Since short circuits occur quite often, the short circuit withstand capability is regarded as
belonging to the main characteristics of a transformer installed in a European distribution network.
Short circuit withstand capability in amorphous transformers can be an insurmountable obstacle and a
major challenge for any transformer manufacturer.
This paper presents a range of amorphous transformers from 50 to 1250 kVA that passed the
short-circuit test in the ERDF laboratory.
KEYWORDS
Amorphous, distribution transformers, short circuit withstand test, wound core technology.
55th km Athens-Lamia N.R., GR 32011,
georgios.loizos@schneider-electric.com
Inofyta,
Greece,
Tel:
+30-210-6295315,
Fax:
+30-210-6295310,
Corresponding
0
author:
1. INTRODUCTION
Amorphous metal-based materials have been around for more than 25 years but their use has only
been restricted to experimental level in the European utilities networks. This was due to energy
economics and to some technical disadvantages of amorphous metals when compared to grainoriented silicon steels. The cost of amorphous metals was more expensive compared to conventional
silicon steels. In addition, the saturation magnetic inductions of amorphous metals are lower than those
of silicon steels, resulting in larger-sized transformers which in turn increase the transformer costs.
Moreover the amorphous alloys remain brittle creating scepticism regarding the transformer quality,
and have higher saturation magnetostriction that may result in noisier transformers when compared to
silicon steels at the same induction level.
In recent years the situation has been changed considerably, society needs sustainable and
environmentally friendly development. Energy costs increased. The saturation magnetic inductions of
Fe-based amorphous alloys have been increased, resulting in reduced transformer sizes requiring less
material. In spite of the increased flux density in the core, core loss has decreased and transformer
noise has been considerably reduced. These factors have recently resulted in amorphous metal-based
transformers (AMT), which are cost competitive with the conventional transformers [1] [2].
AMTs currently find increasing use in the Asian and USA markets at single phase and low rated
powers < 250 kVA. If, however, AMTs are to penetrate the market of European utilities, they have to
withstand the stresses of the short-circuit test as described in IEC 60076. The effects of short circuit
currents can be tremendous both for the transformer (i.e. danger of explosion) and for the stability of
the network. Since short circuits occur quite often, the short circuit withstand capability is regarded as
belonging to the main characteristics of a transformer installed in a European distribution network.
Short circuit withstand capability in amorphous transformers can be an insurmountable obstacle and a
major challenge for any transformer manufacturer.
This paper presents a range of amorphous metal-based transformers from 50 to 1250 kVA, that
passes the short circuit withstand test. A 400 kVA amorphous transformer of these series is currently
under trial in the ERDF network.
2. CORE LOSSES AND CHALLENGES OF TODAY
As it is well known, the material mainly used in the magnetic cores of transformers is the grain
oriented electrical steel. It is divided into various grades according to the level of the material’s
specific losses. During the last decades, electrical steel industry made a considerable progress in the
improvement of its magnetic properties and the reduction of its losses. Hence the thickness of the
sheet from 0.30 mm and 0.27 mm was reduced to 0.23 mm achieving a reduction of losses. Thickness
of 0.18 mm is considered the following step and some suppliers already offer the new material for trial
purposes. Nevertheless the expected benefits from it are limited indicating that this technology has
reached its limits. A technological breakthrough is therefore needed in order to have the no load losses
of a transformer reduced by a factor of two, three or more. Only amorphous metal based cores can
achieve such a reduction and they are in existence the last thirty years.
The satisfaction of all technical and quality specifications of European electricity authorities is a
fundamental requirement if AMTs are to be widely used in Europe. European companies having
accumulated great experience and knowledge in energy distribution have their own stringent
specifications in order to ensure service continuity under the most difficult conditions. The current
transformer technology using electrical steel has proved its worthiness and reliability something that
has to be repeated by the new amorphous core technology.
At first, it has to be cleared that the use of amorphous metal-based cores has no effect in the
electrical constraints of the transformer design with the exception of the coil windings shape. Possible
contamination of the windings from broken flakes, due to the brittleness of the material, during the
active part assembly may pose a problem but with appropriate care during manufacturing this problem
can be eliminated. It has also no effect in the high voltage and dielectric strength considerations of the
transformer. The thermal performance of the transformer is better because the losses are reduced.
55th km Athens-Lamia N.R., GR 32011,
georgios.loizos@schneider-electric.com
Inofyta,
Greece,
Tel:
+30-210-6295315,
Fax:
+30-210-6295310,
Corresponding
1
author:
Sound power levels are higher at the same induction, but fall within the current requirements of
outdoors distribution transformers. In other words, the increase of noise levels is not proportional to
the greater reduction of losses and therefore is not a constraint in the design of the transformer. The
only concern that remains is to mitigate the effects of the electromagnetic forces during a short circuit
event by taking sound design choices and preventative countermeasures. Until recently, many
transformer manufacturers and electricity companies considered the short circuit withstand of the
AMTs an insurmountable obstacle.
Schneider Electric combining the efforts of a dedicated R&D team from Greece, France and
Turkey with the forty years experience of the Greek plant in producing transformers of wound core
technology developed a range of amorphous core transformers with extremely low losses from 50 to
1250 kVA able to succeed in the short circuit test.
3. MAGNETIC PROPERTIES OF THE NEW AMORPHOUS MATERIAL
The AMTs series make use of the most recently developed amorphous iron-based material there is
in the market, Metglas® 2605 HB1. It is an alloy of Iron (85-95 %), Boron (1-10%) and Silicon (110%) [3].
Core losses and A-weighted sound power levels of the material are depicted in Fig. 1 and they are
compared with those of commercially available of Metglas® 2605 SA1 and Μ2 grain oriented steel.
The M2 grade was selected as one of the most advanced grades of grain oriented SiFe steel.
(a)
(b)
Fig. 1. (a) Core losses at 50 Hz as a function of operating induction. Core losses in the materials SA1 and HB1
are approximately the same at 50 Hz; the small difference is not visible in this figure (b) A-weighted sound
power level as a function of operating induction [1].
Table 1 shows more characteristics of 2605HB1 compared to 2605SA1 and M2 [4].
Table 1
Saturation
flux
density
Bs(T)
Si-Fe
2.03
2605SA1
1.56
2605HB1
1.64
Comments
Higher
values
allow
smaller
size
Material
Coercive
force Hc
(A/m)
Electric
resistivity ρ
(μΩ·m)
Core losses
P13/50(W/kg)
Magnetostriction
λs (ppm)
Sheet
thickness
(mm)
Stacking
factor (%)
45
2.0
1.5
Lower
values
produce
lower
losses
0.5
1.3
1.3
Higher
values
produce
lower losses
0.440
0.070
0.063
Smaller
better
-1
27
27
Lower values mean
quieter operation
0.23
0.025
0.025
Thicker is
easier to
work
>95
>84
>84
Higher
values allow
smaller size
55th km Athens-Lamia N.R., GR 32011,
georgios.loizos@schneider-electric.com
Inofyta,
Greece,
Tel:
is
+30-210-6295315,
Fax:
+30-210-6295310,
Corresponding
2
author:
4. AMORPHOUS DISTRIBUTED GAP CORES
The high quality amorphous cores are made from 2605ΗΒ1 amorphous ribbon. The cores are
distributed gap (wound) type and are partially epoxy coated to give the finished core mechanical
rigidity, while sealing the edges. Electrical insulation between the sheets is achieved by a naturally
occurring coating of silicon dioxide. The cores are preassembled and annealed at 350°C with a one
hour soak.
Fig. 2 shows a photo of the amorphous core and magnetic circuit diagrams used in amorphous
cores transformers.
(b)
(a)
Fig.2. (a) Amorphous core made from Metglas® 2605 HB1, (b) The two types of magnetic circuits for the
AMTs, five legged (left) and three legged or Evans (right).
The lower operating induction combined with the lower stacking factor result in a greater core size
and consequently to greater transformer dimensions and cost of the transformer. The issue of
transformer overall dimensions becomes even more difficult due to the fact that the transformer design
engineer has to select from a limited range of core widths (i.e. 142, 170 and 213 mm). This is not
happening in the conventional technology of grain oriented steel where the roll width is approximately
1000 mm and can be slit to any width offering great design flexibility.
Another point that needs special attention is the brittleness of the amorphous material. The material
is fragile and breaks into little flakes when it is stressed beyond its bending limit. Tiny iron pieces are
unavoidably produced during the assembly of the active part where the core is opened and closed after
the insertion of the coils. It is therefore extremely important to handle with caution and ensure their
removal during the production process.
Furthermore the amorphous core shows little withstand to external stresses in contrast to SiFe steel
cores where the magnetic circuit is rigid and acts as a mechanical support to the windings. In AMTs
the core cannot support its own weight and protecting measures against the great external stresses are
needed in order to sustain the mechanical strength and rigidity of the active part.
5. WINDINGS
The low voltage windings are made of copper foil conductor. Copper bars are welded at the ends of
the foil, maintaining conductivity, in order to connect the coils to the bushings. The high voltage
windings are of layered construction and made of one or two round or rectangular copper wires.
The insulation between the layers consists of a paper with a special diamond pattern of epoxy
adhesive coated on it which cures during the drying process and bonds the windings into a single
structure.
Distributed gap wound cores require the use of rectangular windings due to the rectangular shape
of the core section. Since the radial repellent forces during the short circuit are higher in the
rectangular windings than in the round ones (used in stacked core transformers), specially designed
metallic frames and vertical supports have been placed in order to ensure that the radial and axial
55th km Athens-Lamia N.R., GR 32011,
georgios.loizos@schneider-electric.com
Inofyta,
Greece,
Tel:
+30-210-6295315,
Fax:
+30-210-6295310,
Corresponding
3
author:
forces during the short circuit test do not exceed the limits of the winding materials mechanical
strength.
The short circuit electromagnetic stresses were simulated and the results are shown in Fig. 3.
(a)
(b)
Fig. 3. (a) Simulated distribution of stresses during a short circuit event, (b) radial forces during a short circuit
[5].
6. ACTIVE PART ASSEMBLY
The cores needed special handling in order to minimize the manufacturing stresses. The use of
lifting magnets was therefore necessary for transporting and handling the cores during the assembly
process. A tilting table was used to position the core for the core/coil assembly. The procedure is the
following:
-the coils are positioned on the table
-the opened cores slide into the coils
-cores close and core joints get laced
-frames and vertical supports are placed
-the table is tilted in the upright position and the active part assembly is completed.
The overall design of the coil and frame structure proved to be able to withstand short-circuit strength
without overstressing the core.
7. SHORT CIRCUIT WITHSTAND CAPABILITY AND TRIAL TESTS IN
ÉLECTRICITÉ RÉSEAU DISTRIBUTION FRANCE (ERDF)
A range of amorphous core transformers of rated power 50 to 1250 kVA were constructed by the
industrial unit in Greece. They satisfied all requirements and tests of ERDF and most importantly the
short circuit withstand test. It has to be noted that the very stringent ERDF specifications require a
maximum change in the short circuit reactance less than 4% (7.5% according to IEC 60076-5). Table 2
shows the technical characteristics of three types of the new range. In parallel, three amorphous core
transformer prototypes of rated power 400 kVA, 20/0.41 kV were constructed. These units are
currently under trial and observed closely in the ERDF network. The noise level for the 400 kVA was
measured to be 52 dB(A) with 58 dB(A) required by ERDF.
Table 2
kVA
kV/kV
P0 (W)
Pk (W)
Uk (%)
400
630
1250
20/0.41
20/0.41
20/0.41
185 (A0=430)
340 (A0=600)
440 (A0=950)
4600 (Ck)
5400 (Bk)
11000 (Bk)
4
4
6
55th km Athens-Lamia N.R., GR 32011,
georgios.loizos@schneider-electric.com
Inofyta,
Greece,
Tel:
+30-210-6295315,
Fax:
+30-210-6295310,
Change of the
short circuit
reactance
0.7 %
1.03 %
1.03 %
Corresponding
4
author:
8. INCENTIVES FOR THE USE OF AMORPHOUS TRANSFORMERS
In the last years, the small drop of the amorphous material cost combined with the increase of
energy and environmental costs (cost of losses and cost of CO2 emissions) has resulted to comparative
annual total owning costs of the AMTs and the conventional silicon steel transformers. Especially in
the case of grid tied renewable energy parks where the produced kWh is subsidized, the payback time
of an AMT due to the energy savings cost is in the order of a few years.
An incentive of more significance might be the increasing awareness of the presence of voltage
harmonics on power systems due to the increased use of power electronics. Supply voltage harmonics
lead to increased iron losses approximately proportional to the square of the harmonic number and the
magnitude of the harmonic component. AMTs are uniquely suited to providing low loss performance
under low loading and at higher frequencies due to their lower thickness and higher electrical
resistivity [6].
9. CONCLUSIONS
Schneider Electric presented a range of amorphous iron based distribution transformers
from 50 to 1250 kVA at a competitive cost. These transformers not only can withstand the
short circuit test but they can satisfy the stringent specifications of ERDF. Furthermore this
advanced manufacturing technology has been successfully applied in three windings
transformers used in grid tied photovoltaic parks. Such a transformer of rated power 1600
kVA, 0.36-0.36/20 kV, P0= 510 W (A0=1200 W), Pk=17000 W was recently sold to SMA, a solar
technology company.
BIBLIOGRAPHY
[1]
[2]
[3]
[4]
[5]
[6]
R. Hasegawa, D. Azuma, “Impact of amorphous metal-based transformers on energy efficiency
and environment”, Journal of Magnetism and Magnetic Materials, 320, 2008, pages 2451-2456.
A. Sato, H. Terada, T. Nagata, S. Kurita, Y. Matsuda, K. Fukui, D. Azuma, R. Hasegawa
“Development of distribution transformer based on new amorphous metal”
Metglas, Materials Safety Data Sheet.
K. Inagaki, M. Kuwabara, K. Sato, K. Fukui, S. Nakajima, D. Azuma, “Amorphous transformer
contributing to global environmental protection” Hitachi Review Vol. 60, 2011, No. 5, pages
250-256.
B. Jarry, P. Lauzevis, P. Lagache, M. Sacotte “Amorphous sheet core transformer under
experimentation on the ERDF network” 20th CIRED, paper 90, 2009.
A.J. Moses “Iron-based amorphous ribbon-challenges and opportunity for power applications”,
Journal of Optoelectronics and advanced materials, vol.4, No. 2, 2002, pages 231-236.
55th km Athens-Lamia N.R., GR 32011,
georgios.loizos@schneider-electric.com
Inofyta,
Greece,
Tel:
+30-210-6295315,
Fax:
+30-210-6295310,
Corresponding
5
author: