Plasma Science and Technology, Vol.18, No.10, Oct. 2016
Thermal Dissipation Modelling and Design of ITER PF Converter
Alternating Current Busbar∗
GUO Bin (郭斌)1 , SONG Zhiquan (宋执权)1 , FU Peng (傅鹏)1 , JIANG Li (蒋力)1 ,
LI Jinchao (李金超)1 , WANG Min (王敏)2 , DONG Lin (董琳)2
1
2
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
China International Nuclear Fusion Energy Program Execution Center, Beijing 100862,
China
Abstract
Because the larger metallic surrounds are heated by the eddy current, which is
generated by the AC current flowing through the AC busbar in the International Thermonuclear
Experimental Reactor (ITER) poloidal field (PF) converter system, shielding of the AC busbar is
required to decrease the temperature rise of the surrounds to satisfy the design requirement. Three
special types of AC busbar with natural cooling, air cooling and water cooling busbar structure
have been proposed and investigated in this paper. For each cooling scheme, a 3D finite model
based on the proposed structure has been developed to perform the electromagnetic and thermal
analysis to predict their operation behavior. Comparing the analysis results of the three different
cooling patterns, water cooling has more advantages than the other patterns and it is selected to
be the thermal dissipation pattern for the AC busbar of ITER PF converter unit. The approach
to qualify the suitable cooling scheme in this paper can be provided as a reference on the thermal
dissipation design of AC busbar in the converter system.
Keywords: thermal dissipation, modelling and design, AC busbar, converter
PACS: 84.32.Vv
DOI: 10.1088/1009-0630/18/10/14
(Some figures may appear in colour only in the online journal)
1
Introduction
converter bridge. In Ref. [5], the eddy current loss
in the ferromagnetic wall has been investigated and
presented, and the results show that bulk conductivity,
material relative permeability, busbar phase sequence
and the air gap of building wall impact a lot on eddy
current loss of building wall. As a result, shielding of
the AC busbar is required to decrease the temperature
rise of the ferromagnetic wall and surroundings to
satisfy the thermal design requirement.
The International Thermonuclear Experimental
Reactor (ITER), which is an international nuclear
fusion project, is being developed to demonstrate
the feasibility of realizing the commercial using
of fusion power energy [1] . The poloidal field (PF)
converter unit, as an important part of ITER power
supply system, plays an essential role for the plasma
shape and position control in vertical and horizontal
direction [2−4] .
Fig. 1 presents the simplified circuit scheme of ITER
poloidal field (PF) converter system. The PF converter
system includes four six-pulse converter bridges, which
is supplied by two phase shifted rectifier transformers
providing four quadrant twelve pulse operations. The
system rated direct current (DC) current is 55 kA. As
Fig. 1 shows, AC busbar has the function to transfer
large AC current from the transformer to the converter
module. In the ITER site, a rectifier transformer will
be installed on the outside of the building, while the
converter module will be installed inside, as shown in
Fig. 2. In this way, there is a penetration for the AC
busbar on the building wall made by ferromagnetic
materials to connect the rectifier transformer and
∗ supported
Fig.1 Simplified circuit scheme of ITER PF converter
system
The Isolated Phase Busbar (IPB) continuousshielding type is presented in Fig. 2, the conductors
are shielded by three phase shielding and the three
enclosure shieldings are electrically connected by using
by National Natural Science Foundation of China (No. 51407179)
1049
Plasma Science and Technology, Vol.18, No.10, Oct. 2016
enclosed busbar [8] , temperature limit for the hotspot
on conductor is 105 o C. For the shielding limit, two
different requirements are defined on the basic of the
work condition. If the shielding is inaccessible to an
operator in the normal course of his duties, then 110 o C
is permissible. In contrast, if the shielding is accessible
to an operator in the normal course of his duties,
then the temperature limit is 70 o C. However, under
the operation guideline of the PF converter, the AC
busbar was required to be accessible during the normal
operation of the PF converter. Consequently, to be
conservative the maximum temperature at 70 o C was
chose to be the constraint for the cooling design of the
AC busbar shielding.
four aluminum bonding plates. When the busbar
conductor currents in three phases represented by
IMa , IMb and IMc flow, the circulating current on
the shielding is almost the same order represented by
IKa , IKb and IKc will be inducted on the shielding by
time-varying electromagnetic field [6,7] . However, while
large currents are flowing inside the busbar conductors
and enclosure shielding, thermal dissipation will be
the main problem in the AC busbar design. Some
types of cooling method have previously been proposed
and investigated to reject thermal load. In Ref. [8],
temperature rise of the IPB at rated current 12.5 kA
in natural cooling has been evaluated. However, the
approach depicted here is not suitable for the larger
current AC busbar installed in the limited space. In
Ref. [9], temperature rise of non-segregated phase air
cooling busbar has been simulated. But segregated
phase busbars are required to be used here to avoid the
larger electromagnetic force between adjacent phases.
In Ref. [10], a water cooled IPB for generators had been
investigated, but the cooling water inlet and outlet are
located at different sides of the busbar while the same
side is required by the ITER PF converter system.
Table 1.
Technology specification of the AC busbar
Phase
Phase angle
Frequency
AC peak current
Max conductor temperature
Max shielding temperature
Busbar length
3
A
0o
B
120o
50 Hz
30.34 kA
105 o C
70 o C
1m
C
−120o
Shielding current analysis
For the shielding and busbar conductor, the
equivalent circuit as shown in Fig. 3 can be used to
give the relationship between shielding and conductor.
It can be expressed as Eq. (1):
I˙R = I˙M + I˙K ,
Fig.2
where IM is the busbar conductor current, IK is the
shielding current and IR is the residual current. From
Eq. (1), the voltage equation can be found in Eq. (2).
Where rk is shielding resistance and Lk is shielding
inductance and LR is residual shielding inductance.
Scheme of the ITER PF converter AC busbar
In this paper, the induced current on the shielding is
first studied to solve the electromagnetic issue. Then,
three special types of busbar have been proposed and
sized to solve the limitation of the previous research.
The first type is the busbar under natural cooling
with cage type shielding which can solve the limited
space constraint. Second, a special isolated air cooling
busbar that can avoid the larger electromagnetic force
has been designed. Third, the twin cooling channel
conductor which has the inlet and outlet at the same
side has been investigated. For each type of busbar, an
electromagnetic and thermal analysis coupling model
has been built, and the FEM method has been applied
to predict the thermal behavior. Finally, from the
analysis results a suitable thermal dissipation pattern
for the AC busbar is presented.
2
(1)
Fig.3 Equivalent circuit diagram for the conduct and
shielding
zR z k
,
I˙R zR = −I˙k zk = I˙M
zR + zK
zk = rk + jvLk ,
zR = jvLR .
(2)
(3)
(4)
By substituting Eqs. (2), (3) and (4), the busbar
conductor current and shielding current can be
expressed as Eq. (5)
Thermal design requirement
−I˙K =
Table 1 presents the technical requirements of the
AC busbar. According to IEEE standard for the metal-
jwTR
I˙M ,
rk + jw(TR + TK )
TK =
1050
Lk
,
rk
(5)
(6)
GUO Bin et al.: Thermal Dissipation Modelling and Design of ITER PF Converter Alternating Current Busbar
TR =
LR
.
rk
(7)
Each phase of the busbar conductor is composed of
two groups of copper conductor which is in double U
shaped type. To minimize the leakage of the magnetic
flux on the surrounding, from the electromagnetic
analysis result, the cage gap width was chosen as
10 mm.
Where Tk is shielding leakage inductance constant and
TR is shielding circulation current mutual inductance
constant. If the shielding resistance is ignored, the
relation between IK and IM is determined by the
Lk /LR . In reality, LR À LK , so, −Ik = IM . In
this way, the same current as conductor can be used
to generate ohm thermal load on the shielding thermal
design.
4
4.2
When the rated current of the AC busbar is
30.34 kA, the power loss and busbar inductance
is calculated by electromagnetic analysis using the
software MAXWELL to generate the heat load input
for the thermal analysis by using ANSYS [11] . As the
thermal load and heat transfer mechanism for three
phase of busbar are almost the same, only one phase
of AC busbar in length of 1 m, is modeled to simulate
the heat transfer between busbar internal conductor,
shielding and environment heat sink. In this simulation,
heat loss by nature convection is 3.9 W/m2 ·o C which is
obtained by Eq. (8) when the air temperature is 31 o C.
Natural cooling busbar
4.1
Structure design
For the case of the AC busbar under nature cooling,
both the busbar conductor and shielding are under
natural cooling. Fig. 4 shows the general arrangement
of the AC busbar in natural cooling and Table 2 lists the
main design parameters for the busbar under natural
cooling.
Fig.4
λ gρ2 l3 ∂v ∆t
P r)0.25 ,
(8)
ha = 0.59 (
L
u2
where, L is the characteristic length, g for gravity
acceleration, ∂v for cubic expansion coefficient, ∆t is
for the difference in temperature between busbar and
air, u is for the dynamic viscosity, λ is the thermal
conductivity and Pr is prandtl number.
Under these parameter definitions, the temperature
contour of the AC busbar under natural cooling can
be obtained from this thermal fluid analysis shown in
Fig. 5, and the main result is listed in Table 3.
From Fig. 5 it can be seen that the maximum
temperature of the busbar conductor is 75.7 o C, which
is much lower than the system requirement. For the
shielding, the maximum temperature of 65 o C is also
lower than the IEC standard requirement 70 o C.
Natural cooling busbar structure
Table 2. Design parameters for the busbar under
natural cooling
Conductor
Material
Copper
Thickness
20 mm
Dimension 300 mm×100 mm
Length
1000 mm
Shielding
Material
Aluminum
Thickness
10 mm
Dimension 500 mm×500 mm
Length
1000 mm
Fig.5
Table 3.
Cross-section
Electromagnetic thermal analysis
Temperature contour of the busbar under natural cooling
Natural cooling electromagnetic and thermal analysis results
Weight (kg)
Inductance
Resistance
Max T (o C)
(m2 )
Conductor
Shielding
(µH)
(µΩ)
Conductor
Shielding
0.013
106
70
0.09
2.56
75.7
65.0
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Plasma Science and Technology, Vol.18, No.10, Oct. 2016
5
Air cooling busbar
5.1
In boundary condition definition, the force
air cooling convection coefficient is calculated as
18.5 W/m2 ·o C by Eq. (9) and the nature convection
cooling coefficient for the external shielding is
sized to be 5.532 W/m2 ·o C by Eq. (8). Where
f =(1.82 lgRe−1.64)−2 , Re is Reynolds, d is the cooling
water channel diameter.
The electromagnetic and thermal analysis result for
one group of AC busbar in air cooling is demonstrated
in Fig. 7 and Table 5. When the ambient temperature
is 31 o C in the simulation, the maximum busbar
conductor temperature is 59.4 o C and the maximum
temperature of the external shielding is 41.9 o C.
Structure design
In the case of air cooling AC busbar, both the
conductor and shielding are designed to be cooled by
forced air. The main design parameters for this kind of
busbar are shown in Table 4.
Table 4.
Design parameters for the air cooling busbar
Conductor
Material
Aluminum
Thickness
20 mm
Dimension 200 mm×200 mm
Length
1000 mm
Shielding
Material
Aluminum
Thickness
20 mm
Dimension 300 mm×300 mm
Length
1000 mm
As shown in Fig. 6, two conductors in a square shape
are grouped into one phase in parallel to transfer AC
current. The force air flows through the gap between
busbar and shielding at one conductor inlet and turns
around at the end of conductor to get out from another
gap. The inlet temperature of the circulating air is
31 o C at the flow rate of 496.8 m3 /h for each phase. To
decrease the pressure drop of circulation air, velocity
which takes account for the main factor of the pressure
loss is kept at 5 m/s inside the air cooling AC busbar.
Forced air supplying cooling power to the AC busbar
feeds the three phase of the AC busbar in one PF
converter unit, which are in parallel to avoid the worst
case occurred at the end of the flow route phase if in
series.
Fig.6
5.2
Fig.7 Temperature distribution of the AC busbar in air
cooling
6
6.1
Structure design
The design of the AC busbar in water cooling is
shown in Fig. 8 and the main design parameters are
listed in Table 6.
Air cooling busbar structure
Electromagnetic thermal analysis
Fig.8
The thermal analysis of the air cooing AC busbar
is an electromagnetic and thermal coupled problem.
Electromagnetic analysis by MAXWELL was firstly
performed on the AC busbar at the rated AC current
flowing into the conductor to generated thermal load
for thermal analysis. ANSYS has been used to simulate
the air cooling AC busbar thermal behavior under its
normal operation.
hfa =
Water cooling busbar
For the AC busbar in water cooling, busbar
conductor is cooled by cooling water and the shielding
was cooled by air. The busbar conductor rejects
heat load to cooling water flowing through the twin
cooling channel inside conductor. The cooling water
is designed to supply three phase conductors in series.
For each phase, the cooling water flows through one
cooling channel to another cooling channel by changing
direction on the other side of the conductor.
d
l (f /8)(Re − 1000)P rt
[1 + ( )2/3 ]Ct . (9)
p
2/3
L 1 + 12.7 f /8(P r
L
− 1)
f
Table 5.
Cross-section
(m2 )
0.03
Air cooling electromagnetic and thermal analysis results
Weight (kg)
Conductor Shielding
80
AC busbar structure in water cooling
70
Inductance
(µH)
Resistance
(µΩ)
0.03
2.08
1052
Max T (o C)
Conductor Shielding
59.4
41.9
GUO Bin et al.: Thermal Dissipation Modelling and Design of ITER PF Converter Alternating Current Busbar
Table 6.
Design parameters for the water cooling busbar
Conductor
Material
Cooling channel diameter
Dimension
Length
6.2
Electromagnetic thermal analysis
and 45.88 o C. The natural convection coefficient for the
shielding was calculated by Eq. (8) as 5.532 W/m2 ·o C
under the ambient temperature 31 o C.
Thermal analysis result for three phases of AC
busbar with shielding is presented in Table 7 and
Fig. 10. The maximum temperature of the busbar
conductor is 55.9 o C and the maximum temperature of
the shielding is 50 o C, both of which are lower than the
thermal design requirements.
Busbar power loss, inductance, and resistance
in three phases are calculated by performing
electromagnetic analysis at the rated AC current. Two
parts are composed in the thermal analysis. The first
part is thermal fluid analysis on the busbar conductor
that used Fluent to calculate the force convection
equation between interior conductor and cooling
water [12] . The second part is thermal mechanical
analysis on the overall busbar model used ANSYS to
predict the overall temperature distribution.
6.2.1
Thermal fluid analysis
As the thermal load and convection heat transfer
mechanisms of the conductors in three phases are
totally the same, only one phase of the conductor has
been modeled into Fluent. Cooling water flow rate was
set as 1.65 m3 /h at the inlet temperature of 31 o C.
Fig. 9 shows the temperature distribution of
conductor with the circulation cooling water. From
the analysis result, after absorbing the heat load from
conductor, the cooling water temperature rise for one
phase of the busbar conductor is 7.44 o C and the
maximum temperature of the conductor is 47 o C.
Fig.10
cooling
7
Thermal mechanical analysis
Conductor and shielding thermal mechanical
analysis has been performed in the second part.
The convection coefficient between cooling water and
conductor is extracted from the thermal fluid analysis.
As the cooling water manifold for the three phase
conductors is connected in series, the coolant inlet
temperature at each phase is specified as 31 o C, 38.44 o C
Table 7.
Cross-section
(m2 )
0.01
Temperature contour of the AC busbar in water
Comparison and discussion
Electromagnetic and thermal analysis results of three
cooling types of AC busbar can be summarized as
Table 8.
From Table 8, it can be seen that the inductance
of the water cooling case is the largest of the three.
However, when the busbar is enclosed by shielding,
the inductance will decrease substantially due to the
isolated magnetic field for each busbar phase. So
the electromagnetic parameter for the AC busbar type
selection is not the key factor. The determining factor
is whether the busbar cooling scheme is in accordance
with IEEE standard requirement and has high cost
performance. As listed in Table 8, the busbar cooled by
water not only has the lowest maximum temperature
during normal operation but also consumes a smaller
amount of coolant than force air cooling.
From
the busbar structure and cost aspect, the busbar
under water cooling occupies the smallest space of the
three. So the busbar under the water cooling thermal
dissipation method has more advantages than the other
two, which should be the best solution for the AC
busbar in the ITER PF AC/DC converter system.
Fig.9 Temperature contour of the AC busbar conductor
in water cooling
6.2.2
Shielding
Material
Aluminum
Thickness
20 mm
Diameter
950 mm
Length
1000 mm
Aluminum
20 mm
100 mm×120 mm
1000 mm
Water cooling electromagnetic and thermal analysis results
Weight (kg)
Conductor Shielding
31.5
100
Inductance
(µH)
Resistance
(µΩ)
0.33
6.63
1053
Max T (o C)
Conductor Shielding
55.9
50.0
Plasma Science and Technology, Vol.18, No.10, Oct. 2016
Table 8.
Analysis result in natural, air, water cooling method
Cooling type
Nature
0.09
0.03
0.33
Busbar resistance (µΩ)
2.56
2.08
6.63
Max temperature-conductor (o C)
75.7
59.4
55.9
Max temperature-shielding (o C)
65.0
41.9
50.0
Copper
Aluminum
Aluminum
Shielding material
Aluminum
Aluminum
Aluminum
Busbar weight (kg)
176
150
131.5
Coolant flow rate
3
–
Conclusion
41734 m /h
3
This paper has focused on the thermal dissipation
modeling and design for the cooling of AC busbar,
which has been used in the ITER PF converter
system. Three special busbar structures under natural,
air, and water cooling have been proposed, and
coupled electromagnetic and thermal analysis has been
performed to qualify the design. Water cooling has
many advantages over the other cooling schemes, which
has been selected as the suitable thermal dissipation
pattern for AC busbar in the PF converter unit. The
proposed AC busbar structure and qualify methodology
used in this paper can provide a good reference for the
AC busbar cooling design in the high power converter
system.
4
5
6
7
8
9
Disclaimer
9
The view and opinion expressed herein does not
necessarily reflect those of the ITER organization.
Acknowledgment
10
The authors would like to express gratitude to
Ministry of Science and Technology of China for the
foundation and staff of Institute of Plasma Physics,
Chinese Academy of Sciences (ASIPP) for helpful
discussions and suggestions.
11
12
References
1
2
Water
Busbar inductance (µH)
Conductor material
8
Air
47.25 m3 /h
Fu P, Gao G, Song Z Q, et al. 2013, Fusion Science
Technology, 64: 741
Fu P, Gao G, Xu L W, et al. 2010, Review and analysis
of the AC/DC converter of ITER coil power supply. In
Proceedings of Applied Power Electronics Conference,
IEEE, Palm Spring, CA, USA, p.1810
Jiang L, Gao G, Xu L W, et al. 2015, Journal of Fusion
Energy, 34: 49
Choi Seung-Kil, Kim Jin-Soo, Chang Hong-Soon,
et al. 2001, Analysis on the magnetic properties of
an isolated phase bus system. Electrical Machines and
Systems. 2001. ICEMS 2001. Proceedings of the Fifth
International Conference on, Shenyang, 2001, Vol. 2,
p.1166–1169
Skeats W F, Swerdlow N. 1962, Power Apparatus
and Systems, Part III, Transactions of the American
Institute of Electrical Engineers, 81: 655
IEEE Standard for Metal-Enclosed Bus, in IEEE
Std C37.23-2003 (Revision of IEEE Std C37.23-1987),
p.01-48, 2004
You J, Liang H, Ma G, et al. 2012, Steady Temperature
Rise Analysis of Non Segregated Phase Bus Based on
Finite Element Method, Electrical Contacts (Holm).
2012 IEEE 58th Holm Conference on, Portland, OR,
2012, p.1–5
Abram L, Goodall J, Wentworth T G. 1969, The
Proceedings of the Institution of Electrical Engineers,
116: 1185
Zhao Bo, Zhang Hongliang. 2010, The application
of Ansoft 12 in the Electromagnetic Engineer Fields.
China Waterpower Press, Beijing (in Chinese)
Yu Yong. 2008, Introductory and Advanced Course
Book for Fluent. Beijing Institute of Technology Press,
Beijing (in Chinese)
(Manuscript received 23 November 2015)
(Manuscript accepted 23 March 2016)
E-mail address of GUO Bin: [email protected]
Rebut P H. 1995, Fusion Engineering and Design, 30:
85
Benfatto I, Mondino P L, Roshal A, et al. 1995,
AC/DC converters for the ITER poloidal field system.
16th IEEE/NPSS Symposium, 1: 658
1054