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Chinese Materials Research Society
Progress in Natural Science: Materials International
Progress in Natural Science: Materials International 2012;22(3):244–249
1002-0071 & 2012 Ch
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ORIGINAL RESEARCH
Reducing the core loss of amorphous cores for
distribution transformers
Deren Lin, Liang Zhang, Guangmin Li, Zhichao Lu, Shaoxiong Zhou
China Iron & Steel Research Institute Group, Advanced Technology and Materials Co., Ltd., No. 76, Xueyuan Nan Rd., Haidian
District, Beijing 100081, China
Received 7 March 2012; accepted 22 March 2012
KEYWORDS
Amorphous core;
Annealing technique;
Reducing core loss;
Loss deterioration;
Distribution
transformer
inese Materials R
ier Ltd. All rights
sponsibility of Chi
016/j.pnsc.2012.04
thor. Tel.: þ86 10
derenli@atmcn.com
Abstract Although amorphous alloy ribbons have lower core loss, serious loss deterioration
occurs in amorphous cores constructed from amorphous alloy ribbon for distribution transformer.
In the present study, the loss deterioration mechanism and influencing factors of amorphous cores
for distribution transformers were investigated. The influence of distributed gaps on core loss of
transformer core was calculated by simulation software and validated by experiments. The boron
fluctuation of amorphous alloy ribbons affects the annealing temperature of amorphous
transformer cores. A superior core annealing technique with annealing parameters was developed,
and appreciable decrease in core loss was obtained.
& 2012 Chinese Materials Research Society. Production and hosting by Elsevier Ltd. All rights reserved.
1. Introduction
Distribution transformers are a significant source of energy
loss in an electric power networks, despite they are very
efficient electrical machines. The losses of transformer consist
of no-load losses and load losses. No-load losses are constant
esearch Society. Production
reserved.
nese Materials Research
.005
5874 2811.
(D. Li).
and appear so long as a transformer is in service, while load
losses vary with the square of the load current carried by the
transformer and are only significant under higher load condi-
tions. The dominant no-load loss is core loss which is
associated with the time-varying nature of the magnetization
and results from hysteresis loss, eddy current loss and
anomalous loss [1]. Therefore, the need to reduce core loss
has become an increasingly important design and manufacture
consideration in all size of distribution transformer. Amor-
phous alloy exhibits low hysteresis loss resulting from a small
magnetic anisotropy and low eddy current loss resulting from
a large resistivity and thinner thickness [2]. Amorphous
ribbons are widely used in electrical power system such as
distribution transformers and inductive devices [3,4] because
of low core loss. Comparing with Si-steel in the application of
distribution transformer, no-load core loss of amorphous core
distribution transformer decreases by 60–70% respecting to
different capacity of transformers. The use of amorphous alloy
ribbon to distribution transformer cores is gradually gaining
widespread acceptance due to lower core loss.
Reducing the core loss of amorphous cores for distribution transformers 245
The core loss of amorphous transformer cores was effec-
tively reduced by optimizing winding tension, core annealing
technique [5], field annealing [6], laser scribing [7], mechanical
scribing [8] and building factor of amorphous cores [9]. Although
low core loss is expected for amorphous transformer cores, large
loss deterioration occurs in amorphous cores even after field
annealing [9]. In order to further reduce the core loss, the aim of
this paper is to systematically study the influence of core
structure, fluctuation of composition and field annealing techni-
que on core loss of large quantity transformer-size cores.
Fig. 2 The structure of a 30 kV A transformer cores constructed
from 38 groups of strips with 40 strip layers per group.
2. Experimental
The amorphous ribbons for preparing transformer cores were
produced by the planar-flow casting method, the nominal
compositions are Fe94.5�xSi5.5Bx (wt%), where x¼2.35–2.55,
the ribbon width is 14270.5 mm and the ribbon thickness is
2570.5 mm. The magnetization and core loss of annealed
sample were measured by IWATSU SY-8232 BH ANALY-
ZER. Examination by X-ray diffraction of the as-cast ribbons
revealed that all samples were amorphous. The core losses of
transformer cores were measured under a controlled sinusoi-
dal induction waveform at 50 Hz. The crystallization tem-
perature of the as-cast ribbons was measured by NETZSCH
DSC 404C, the heating rate was 10 K/s. The boron content
in the as-cast ribbons was measured by iCAP 6300 ICP
SPECTROMETER.
3. Results and discussion
3.1. Structure-related core loss
Over the past years, significant improvements have been made
in amorphous metal transformer core to reduce the core size,
manufacturing costs and the losses introduced into the
distribution transformer by the transformer cores. The no-
cut core has the advantage of lowest core loss comparing to
cut-cores including the step-lap core, the lapped joint core and
Fig. 1 The lapped joints and distributed gaps of a step-lap core.
A group of strips is composed of 40 layers of amorphous strip, the
overlapped length is 14 mm and the distance of each gap is 4 mm.
the butt joint core, but the disadvantage of no-cut core is the
cost resulting from the necessity of coiling around cores. As to
the cut core, the step-lap cores [10,11] are widely adopted in
the present amorphous metal transformer because of the
superiority to the lapped joint core and butt joint core [12].
In the step-lap core, a group of strips with 10–40 strip layers
overlaps 14 mm between the opposite ends of the group and
the distance of each gap is 4 mm. A plurality of this type of
lapped joints is angularly staggered, repeating in a stair-step
fashion as one proceeds from the window to the outer
periphery of the core. Fig. 1 show the lapped joints and
distributed gaps of a step-lap core, where a group of strips is
composed of 40 layers of amorphous strip cut from contin-
uous spool of amorphous ribbon, the overlapped length is
14 mm and the distance of each gap is 4 mm.
The different numbers of strip layers of a group result in
different sizes of gap. The sizes of the distributed gaps have
large influence on the core loss provided that the total strip
layers of a core are identical. To evaluate the influence of
distributed gaps on the core loss, four 30 kV A transformer
cores were made with strip layers of 10, 20, 30 and 40 per
group. The gap size varies with the number of strip layers of a
group and the gap number consequently varies with gap size
due to the same total strip layers of the four cores. For all of
the cores, the weight of each core is 30 kg, the total strip layers
of each constructed core are 1520, the lamination factor is 0.86
and the size of windows A, B and C are 220 mm, 120 mm and
41.5 mm, respectively. Fig. 2 shows the structure of a 30 kV A
transformer core constructed from 38 groups of strips with 40
strip layers per group. The core losses of the four cores were
calculated by using software INFOLYTICA MAGNET 7.2.
In order to simplify the model for calculation, only the gap
number and gap size are considered as variants and the other
parameters are identical. The material parameters of magneti-
zation and core loss at 50 Hz were used during the calculation
0 10 20 30 40 50 60
1.0
1.1
1.2
1.3
1.4
1.5
B (T
)
H(A/m)
1.0 1.1 1.2 1.3 1.4 1.50.04
0.08
0.12
0.16
0.20
Loss
(W/k
g)
B (T)
Fig. 3 (a) The measured static B–H curves of annealed amorphous
ribbon. (b) The dependence of measured core loss of annealed
amorphous ribbon on flux density at 50 Hz.
Fig. 4 (a) The simulated flux density distribution in the lapped
joints of a 30 kV A transformer core constructed from groups of
strips with 10 strip layers per group. (b) The simulated flux density
distribution in the lapped joints of a 30 kV A core constructed
from groups of strips with 40 strip layers per group.
D.R. Li et al.246
process. Fig. 3a shows the measured static B–H curve of
annealed amorphous ribbon and Fig. 3b shows the dependence
of measured core loss of annealed amorphous ribbon on flux
density at 50 Hz. The calculated total core loss of each 30 kV A
transformer core is obtained by adding components of the
building loss of each group to the nominal loss. Fig. 4a and b
show the simulated flux distribution in the lapped joints of
cores constructed from groups of strips with 10 strip layers per
group and 40 strip layers per group, respectively. The config-
uration parameters and calculated results of core loss are listed
in Table 1. Extra losses were introduced to both of the cores in
joint regions due to the distributed gaps which result in extreme
variation of in-plane flux and deviation of flux from long-
itudinal direction of amorphous ribbon. These mechanisms
combined raise the nominal loss by 18.8% and 29.3% for cores
with 10 strip layers per group and 40 strip layers per group,
respectively. The increase of nominal loss resulting from
deviation of flux was also seen in grain-oriented silicon steel
transformer core [13]. Air gaps in joint region reduce the
effective permeability of a core and cause localized variable
flux distribution which in turn increases the losses according to
the gap size [14]. For amorphous transformer cores, the
calculated results also show that the core loss increases with
the increment of gap size. It can be seen from Fig. 4a that a
large magnetization vector occurs near the joint region and
passes through the joint area smoothly at a small angle in core
with 10 strip layers per group. This indicates that the flux
deviation of the core with 10 strip layers per group is small
compared with that of the core with 40 strip layers per group.
Fig. 5 shows the dependence of core loss on the sizes of
distributed gaps in term of strip layers per group. The results
of the core losses were experimentally measured for the
30 kV A transformer cores constructed from different strip
layers per group provided that the total layers of each core are
identical. It can be seen that the core loss increases with the
increment of gap size in term of strip layers of a group. The
experimentally measured results of core loss dependence on
the sizes of distributed gaps are qualitatively in good agree-
ment with the calculated results of core loss dependence on the
sizes of distributed gaps.
3.2. Field annealing
With the growing acceptance of amorphous alloy ribbons by
transformer manufacturers and users, the annealing of large
quantities transformer-size cores can be further optimized to
shorten annealing time and to reduce core loss. The magnetic
properties of amorphous transformer cores are generally
optimized by annealing in an applied magnetic field. The
purpose of the annealing is to minimize the internal stress
which is caused by the rapidly quenched casting and core
making processes. The combination of temperature and time is
chosen such that it is sufficient to relieve as much stress as
possible without causing crystallization of the amorphous
Table 1 The configuration parameters and calculated results of 30 kV A transformer cores.
Gap number One gap size (mm2) Total gap size (mm2) Materials loss (W/kg)a Calculated loss (W/kg)
10 layers core 125 1.08 135.00 0.133 0.158
40 layers core 31 4.32 133.92 0.133 0.172
aMaterials loss was obtained from Fig. 3b by polynomial interpolation.
10 15 20 25 30 35 400.15
0.16
0.17
0.18
0.19
0.20
0.21
Cor
e lo
ss (W
/kg)
Number of strip layers of a group
Fig. 5 The dependence of core losses on the sizes of distributed
gaps in term of strip layers per group.
0 60 120 180 240 3000
50
100
150
200
250
300
350
400
Oven soak temperature Core center temperature Core skin temperature
Tem
pera
ture
(°C
)
Time (min.)
Heat-up Heat preservation Cool-down
Fig. 6 A typical annealing profile of oven soak temperature and
core temperature for a batch of 630 kV A transformer cores.
350 360 370 380 390 4000.16
0.18
0.20
0.22
0.24
0.26
0.5
1.0
1.5
Core loss
Cor
e lo
ss (W
/kg)
Temperature (°C)
Exc
iting
pow
er (V
A/k
g)
Exciting power
Fig. 7 The dependence of core loss and exciting power on
annealing temperature of heat-preservation.
30 60 90 120 1500.17
0.18
0.19
0.20
0.21
0.22
Core loss
Exc
iting
pow
er (V
A/k
g)
Cor
e lo
ss (W
/kg)
Time (min.)
0.2
0.4
0.6
0.8
Exciting power
Fig. 8 The dependence of core loss and exciting power on the
annealing time of heat-preservation.
Reducing the core loss of amorphous cores for distribution transformers 247
alloy. The magnetic field is used for inducing longitudinal
uniaxial anisotropy to reduce the core loss and exciting power.
An annealing technique was developed for large quantities
transformer-size cores. Fig. 6 shows a typical annealing profile
of oven soak temperature and core temperature for a batch of
630 kV A transformer cores, the total number of cores in the
oven soak was 12 and the weight of each core was 102 kg. The
soak temperature was program controlled by a computer to
ensure that the cores in the oven soak were annealed at a
certain temperature and duration. An annealing process
consists of heat-up, heat-preservation and cool-down, the
total annealing cycle was around 5 h. The magnetic field was
longitudinally applied throughout the annealing process to
induce uniaxial magnetic anisotropy.
The combination of annealing temperature, annealing time
and applied magnetic field is optimized to reduce the core loss
and exciting power. Fig. 7 shows the dependence of core loss and
exciting power on the annealing temperature of heat-preservation
ranging from 355 1C to 395 1C for duration of 90 min under
longitudinally applied magnetic field of 46 Oe, the core loss and
exciting power of a batch of 630 kV A cores were measured at
50 Hz and 1.35 T. With the increase of annealing temperature of
heat-preservation, the core loss decreases drastically and then
increases slightly, exhibiting a minimum value around 375 1C.
The exciting power monotonously decreases with the increase of
the annealing temperature of heat-preservation. Fig. 8 shows the
dependence of core loss and exciting power on the annealing time
of heat-preservation ranging from 30 min to 150 min at 375 1C
20 30 40 50 60 700.16
0.18
0.20
0.22
0.24
0.26
0.2
0.4
0.6
0.8
Core loss
Cor
e lo
ss (W
/kg)
Applied field (Oe)
Exciting power
Exc
iting
pow
er (V
A/k
g)
Fig. 9 The dependence of core loss and exciting power on the
longitudinally applied magnetic field.
2.38 2.40 2.42 2.44 2.46 2.48 2.50 2.52 2.54 2.56
510
515
520
525
530
Cry
stal
lizat
ion
tem
pera
ture
(°C
)
B Content (wt.%)
Fig. 10 The dependence of crystallization temperature of amor-
phous ribbons on the boron content in amorphous ribbons.
2.40 2.42 2.44 2.46 2.48 2.50350
355
360
365
370
375
380
Opt
imiz
ed a
nnea
ling
tem
pera
ture
(°C
)
B Content (wt.%)
Fig. 11 The dependence of optimized annealing temperature of
transformer cores on the boron content in amorphous ribbons.
D.R. Li et al.248
under longitudinally applied magnetic field of 46 Oe, the core
loss and exciting power of a batch of 630 kV A cores were also
measured at 50 Hz and 1.35 T. The core loss exhibits a minimum
value around the annealing time of 60 min and the exciting
power also monotonously decreases with the increase of anneal-
ing time of heat-preservation. Fig. 9 illustrates the dependence of
core loss and exciting power on the longitudinally applied
magnetic field ranging from 25 to 67 Oe for cores annealed at
375 1C for 90 min. With the augment of longitudinally applied
magnetic field, both the core loss and exciting power decrease to
minimum values and then increase, indicating that there exists
an optimal magnitude of longitudinally applied magnetic field.
To obtain low core loss and exciting power, it is necessary to
use an optimized longitudinally applied magnetic field during
the annealing process. For amorphous ribbon with longitudin-
ally induced anisotropy, the magnetization process is domi-
nated by domain nucleation and motion of longitudinal domain
wall with concomitance of large Barkhausen jumps. The total
losses of an amorphous core rise with increasing strength of
longitudinally induced anisotropy [15], the observed large total
losses result from domain nucleation and wall pinning [16].
3.3. Boron content and optimal annealing temperature
Typical compositions of amorphous ribbons for transformer
cores are FeSiB ternary alloy [17,18] whereby the metalloids Si
and B are necessary for glass formation and amorphous
structure stabilization. This alloy system has a good glass
forming ability [19] and is easily accessible by planar-flow
casting in large scale production. Although the range of
compositions which can be prepared in the glassy state by
planar flow casting from the melt is wide, the detailed
composition has large influence on the soft magnetic proper-
ties of amorphous ribbons. For Fe94.5�xSi5.5Bx (wt%) alloys,
the boron content has large influence on the crystallization
temperature and the soft magnetic properties of amorphous
ribbon. Fig. 10 shows the dependence of crystallization
temperature of amorphous ribbons on the boron content. It
can be seen from Fig. 10 that the crystallization temperature
of amorphous ribbons increase with the increment of boron
content. As mentioned above, the core loss and exciting power
can be effectively reduced by properly elevating the annealing
temperature. Fig. 11 shows the dependence of optimized
annealing temperature of transformer cores on the boron
content in amorphous ribbons. It is obvious that the optimal
annealing temperature of transformer cores increases with the
content of boron, which is beneficial for the core losses.
4. Conclusions
The loss deterioration factors including the core structure, the
fluctuation of boron content in amorphous ribbons and field
annealing parameters were optimized. The core losses of
amorphous transformer cores increased with an increase of
the sizes of distributed gaps, and the experimental results are
in qualitatively good agreement with the calculated results.
Increasing the boron content in amorphous ribbon resulted in
an increase of the crystallization temperature and of the
optimal annealing temperature of transformer cores. A sophis-
ticated field annealing technique was developed, which effec-
tively reduced the core losses of transformer cores.
Reducing the core loss of amorphous cores for distribution transformers 249
Acknowledgment
This work was supported by the National High-Tech Research
and Development Programme under Grant No. 2012AA030301.
References
[1] Y. Okazaki, H. Kanno, S. Kousaka, E. Sakuma, Magnetic
properties of surface-treated Fe–Si–B amorphous alloy, IEEE
Transactions on Magnetics 23 (1987) 3515–3517.
[2] M.E. McHenry, M.A. Willard, D.E. Laughlin, Amorphous and
nanocrystalline materials for applications as soft magnets, Pro-
gress in Materials Science 44 (1999) 291–433.
[3] R. Hasegawa, Applications of amorphous magnetic alloys, Material
Science and Engineering A375–377 (2004) 90–97.
[4] R. Hasegawa, Applications of amorphous magnetic alloys in
electronic devices, Journal of Non-Crystalline Solids 287 (2001)
405–412.
[5] D.M. Nathasingh, H.H. Liebermann, Transformer application of
amorphous alloys in power distribution system, IEEE Transac-
tions on Power Delivery PWRD 2 (1987) 843–850.
[6] C. Beatrice, C. Appino, E. Ferrara, F. Fiorillo, Loss in transverse
field annealed amorphous ribbon, Journal of Magnetism and
Magnetic Materials 160 (1996) 302–304.
[7] D. Rayboulf, M. Meola, R. Bye, S.K. Das, Reducing the magnetic
losses of amorphous ribbon, Materials Science and Engineering
A241 (1998) 191–201.
[8] K.S. Tan, Amorphous ribbon grain refinement by scribing, IEEE
Transactions on Magnetics 22 (1986) 188–191.
[9] Y. Okazaki, Loss deterioration in amorphous cores for distribu-
tion transformer, Journal of Magnetism and Magnetic Materials
160 (1996) 217–222.
[10] D.E. Ballard, W. Klappert, Method of Manufacturing an
Amorphous Metal Transformer Core and Coil Assembly, US
Patent no. 4,734,957, 1988.
[11] D.E. Ballard, W. Klappert, Method for Making a Transformer
Core Comprising Amorphous Metal Strips Surrounding the Core
Windows, US Patent no. 5,093,981, 1992.
[12] F. G. Grimes, E. Hammack, Butt-lap-step Core Joint, US Patent
no. 4,761,630, 1988.
[13] A.J. Moses, Comparison of transformer loss prediction from
computed and measured flux density distribution, IEEE Transac-
tions on Magnetics 34 (1998) 1186–1188.
[14] A.J. Moses, Prediction of core losses of three phase transformers
from estimation of the components contributing to the building
factor, Journal of Magnetism and Magnetic Materials 254–255
(2003) 615–617.
[15] V. Ungemach, W. Kunz, R. Hilzinger, Influence of the induced
anisotropy on the magnetic properties of amorphous Co66Fe4(-
Mo,Si,B)30 alloys, Journal of Magnetism and Magnetic Materials
41 (1984) 363–365.
[16] J.L. Porteseil, O. Geoffroy, Wall pinning by nanocrystals in an
amorphous matrix, Journal of Magnetism and Magnetic Materi-
als 140–144 (1995) 1855–1856.
[17] F. Luborsky, J. Becker, J. Walter, D. Martin, The Fe–Si–B
ternary amorphous alloys, IEEE Transactions on Magnetics 16
(1980) 521–525.
[18] K. Hoselitz, Magnetic properties of iron–boron–silicon metallic
glasses, Journal of Magnetism and Magnetic Materials 20 (1980)
201–206.
[19] B.S. Dong, S.X. Zhou, D.R. Li, C.W. Lu, F. Guo, X.J. Ni, Z.C.
Lu, A new criterion for predicting glass forming ability of bulk
metallic glasses and some critical discussions, Progress in Natural
Science: Materials International 21 (2011) 164–172.