Design of 10 kA DC terminal connector for HTS cable

Design of 10 kA DC terminal connector for HTS cable

DESIGN OF 10 kA DC TERMINAL CONNECTOR FOR HTS CABLE
Abhay S. Gour, Pankaj S., Sudharshan H., R. Karunanithi, Centre for Cryogenic Technology,
Indian Institute of Science, Bangalore, India
V. V. Rao, Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur, India
Abstract
HTS cables are designed to carry bulk power with
high ampacity in superconducting state. These cables
require terminal connectors of same ampacity to transfer
power from power converter module at room
temperature to the HTS cable maintained at cryogenic
temperature. This paper deals with thermal analysis of
10 kA DC terminal connector for HTS cable. The ohmic
losses were computed which are used as input for steady
state thermal analysis of HTS terminal connector. Based
on the thermal gradient obtained from thermal analysis
length of conductor for terminal connector is calculated
for thermal stability of HTS cable.
conductor. An FEM based thermal analysis was carried
out for different heat loads and feedthough positions.
HTS DC CABLE DESCRIPTION
Fig. 1 shows the detailed view of HTS cable. It
comprises of HTS wire is wounded on supporting tube
and surrounded by electrical insulation material
enclosed inside a liquid nitrogen tube. The liquid
nitrogen tube is thermal insulated by vacuum tube and
non metallic jacket as shown in Fig. 1.
INTRODUCTION
HTS cables are used for transmission bulk power.
These cables are capable of carrying high current in
superconducting state. HTS cable specification varies
with the operating conditions like number of phases,
ampacity, operating voltage [1,2]. The designed cable
should be able to comply with electrical, mechanical,
structural, vacuum and cryogenic system.
One of the important components of HTS cable is its
end terminal. These terminals a intermediate links
which connects the power converters at room
temperature to the HTS cable maintained at cryogenic
temperature. These links maintains temperature gradient
across its length [3,4]. The terminal connectors have the
same ampacity as that of connected HTS cable. HTS
cable terminal design is complicated because of large
temperature variation (77 K to 300 K), operating
voltage, high ampacity and its integration and isolation
with vacuum and cryogenic system.
In this paper, design of HTS DC cable terminal is
discussed for operating voltage of 300 V and 10 kA.
This involves dimension optimization of cable terminal
conductor, location of LN2 and vacuum feedthroughs to
maintain the temperature gradient along the length of
Figure 1: The detailed view of HTS cable[]
DESIGN CONSIDERATION OF HTS
CABLE
There are two important design considerations for HTS
cable terminal namely, electrical and thermal. The
electrical specification includes ampacity (which
decides size of conductor) and operating voltage (size
and type of dielectrics and insulators). Thermal
specification includes ambient temperature (it helps in
calculating conduction heat flow) and maximum cable
temperature (it defines the maximum joule losses
permissible for the operating current) [5]. Fig. 2 shows
schematic of the conductor along with the heat loads.
𝐀
𝐓
𝐈𝐨𝐩𝐭 = 𝐋 ∫𝐓 𝐇
𝐤
𝐓
𝐋
√𝟐 ∫𝐓 𝐇 𝐤𝛒𝐝𝐓
𝐋
𝐝𝐓
(7)
When the transport current I is Iopt , the heat leakage
from the conductor to the cryostat is minimal.
Figure 2: Schematic of conduction cooled conductor
Eq. 1 defines heat flow in a conduction cooled
conductor.
𝐝
𝐝𝐓
𝛒(𝐓)𝐥𝟐
+
[𝐤(𝐓)𝐀
]
𝐝𝐱
𝐝𝐱
𝐀
=𝟎
(1)
The first term of Eq. 1 corresponds to conduction heat
load and second term corresponds to joule heat
produced by the current flowing through the conductor.
Heat conduction per unit length is given by Fourier’s
law of thermal conduction (Eq. 2).
𝐝𝐱
=
𝐤𝐀
𝐝𝐓
(2)
𝐐
Combining Eq. 1 and Eq. 2, the net heat load at a given
temperature (T) is given by
𝐓
𝐐(𝐓) = √𝐐𝟐𝐋 − 𝟐𝐥𝟐 ∫𝐓 𝐤𝛒𝐝𝐓 (3)
𝐋
The ratio of length to cross-section is given by Eq. 4.
𝐋
𝐀
𝐓
𝐤
𝐋
𝐓
√𝐐𝟐𝐋 −𝟐𝐥𝟐 ∫𝐓 𝐤𝛒𝐝𝐓
𝐋
= ∫𝐓 𝐇
𝐝𝐓
The analytical solution of the above equation
is not straight forward to calculate as the thermal
conductivity of material is a function of temperature as
well as material itself. This involves the use of FEM
based solution to determine the temperature rise for the
applied heat.
DESIGN SPECIFICATION AND
SIMULATION
An FEM simulation for the HTS cable terminal was
carried using Ansys thermal steady state simulation. The
terminal is designed with an ampacity of 10 kA and
operating voltage of 300 V. This specification was
selected because of its use in superconducting magnets
at J-PARC [6]. Fig. 3 shows the quarter sectional 3-D
drawing of HTS cable terminal.
Two type of heat transfer simulation were
carried out. In first, the thermal simulations were carried
out for fixed conductor length in LN2 with varied
feedthrough position to obtain the temperature gradient
profile for different heat loads. In second type of
analysis, feedthrough position were fixed with varied
conductor length in LN2 to obtain its effect on the
temperature gradient along the length of whole
conductor at different heat loads. The boundary
conditions and parameters used for the simulation are
listed in the table 1.
(4)
When the conductor is conducting a current I, the
minimum heat leakage QL to cryostat can be achieved
by taking the minimum of Eq. 3given by Eq. 5.
𝐓
(𝐐𝐋 )𝐦𝐢𝐧 = 𝐈√𝟐 ∫𝐓 𝐇 𝐤𝛒𝐝𝐓
𝐋
(5)
By substituting Eq. 5 in Eq. 4, an optimized
operating current Iopt can be obtained.
𝐈𝐋
(𝐀)
𝐨𝐩𝐭
𝐓
𝐤
𝐋
𝐓
√𝟐 ∫𝐓 𝐇 𝐤𝛒𝐝𝐓
𝐋
= ∫𝐓 𝐇
𝐝𝐓
(6)
Figure 3: The sectional 3-D drawing of HTS terminal
Table 1: The boundary conditions and parameters
Boundary Conditions &
Parameters
Values
Uniform Initial
Temperature
Liquid Nitrogen chamber
temperature
300 K
Vacuum Chamber,
Feedthrough
Perfectly Insulated
77 K
RESULTS AND DISCUSSIONS
Fig 4. shows the thermal steady state FEM
simulation for HTS cable terminal. The input heat
load was 600 W. The distance between the LN2 and
vacuum feedthrough was 250 mm. The maximum
temperature obtained at the end of conductor was
357 K. The end- joint temperature was found to be
263 K.
Figure 5: Maximum temperature with change in FT
distance for constant conductor length in LN2 300mm.
Figure 6: Maximum temperature with change conductor
length in LN2 for a fixed feedthorugh distance of 250
mm.
Figure 4: The FEM simulation results
Fig. 5 shows the variation of maximum
temperature along the conductor for different heat
loads. The distance between vacuum and LN2
feedthrough are varied. Conductor length inside LN2
is fixed to 300 mm.
Fig. 6 shows the variation of maximum
temperature along the conductor. The distance
between vacuum and LN2 feedthrough is kept
constant at 250mm. The conductor length in LN2 is
varied for different heat loads.
Fig. 7 shows the variation of end - joint temperature
along the conductor for different heat loads. The
distance between vacuum and LN2 feedthrough are
varied. Conductor length inside LN2 is fixed to 300
mm.
Fig. 8 shows the variation of end - joint temperature
along the conductor. The distance between vacuum and
LN2 feedthrough is kept constant of 250mm. The
conductor length in LN2 is varied for different heat
loads.
Figure 7: End-joint temperature with change in FT
distance with LN2 chamber length 300 mm
Figure 8: End-joint temperature change in LN2 chamber
conductor length with FT distance 250 mm
CONCLUSIONS
The thermal analysis 10 kA DC terminal connector for
HTS cable was carried out. The ohmic losses were
computed and were used as heat load input for the FEM
simulation. The position of the LN 2 and vacuum
feedthrough were optimized to have maximum
temperature and end joint temperature within the limits
of 360 K and 280 K respectively.
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[6] https://www.bnl.gov/magnets/JPARC_Correctors/construction.php