Cryogenic Dielectrics and HTS Power Apparatus: Research at
the University of Southampton
David Swaffield, Paul Lewin, George Chen and Jan Sykulski
Electrical Power Engineering Research Group, School of Electronics and Computer Science, University of
Southampton, Southampton, SO17 1BJ, UK
Introduction
Historically research into applications of low temperature superconductivity (LTS) (using multi-filament
wires of NbTi for example) has produced coils of both complex size and shape which have found
applications in such areas as nuclear magnetic resonance spectroscopy, magnetic resonance imaging and
proton cyclotrons. However, LTS has not been successfully applied to electrical power devices mainly due
to problems with reliability, high costs and complexities of cooling technology. On the other hand, high
temperature superconducting (HTS) materials offer better thermal stability, reduced cooling costs (cf LTS)
and improved reliability. HTS ceramic materials were discovered nearly twenty years ago and have
conductivities, when superconducting, more than a million times greater than copper at room temperature.
They operate reliably at liquid nitrogen (LN
2
) temperatures and offer practical current densities twenty times
greater than traditional copper windings. HTS materials present great potential for electrical power
applications including cables, transformers, generators, fault current limiters and flywheels. It is well
established that a major potential benefit of HTS power apparatus is that, compared to conventional designs,
there are savings in both size and weight for the same rated power. In addition, there are lower losses and
less environmental impact from nitrogen filled equipment compared with conventional mineral oil filled
equipment. The development of HTS technology has presented several research challenges in terms of
design, modelling and simulation as well as fundamental research into the behaviour of both solid and liquid
dielectric materials at LN
2
temperatures.
Southampton HTS projects
The University of Southampton has a long history of research into cryogenics and superconductivity. A
central focus for work in recent years has been the design and construction of demonstration power devices
as well as research into the behaviour of liquid nitrogen in the presence of electrical and thermal fields. For
the acceptance of HTS power apparatus designs, they will have to be both economically competitive with
conventional alternatives and reliable in operation. To achieve this, HTS designs must be optimised for
performance, to minimise cost of plant and cost of operation and to ensure reliable performance. It is
towards this goal that research has focussed at Southampton, both in the areas of electromagnetic design and
dielectric research. HTS applications present serious electromagnetic modelling challenges because of the
highly non-linear and anisotropic HTS tape material characteristics and because they are being applied in
unconventional designs. This paper considers some of the challenges of designing HTS power apparatus and
then discusses work to characterise solid dielectrics and LN
2
at temperatures in the range 64-77 K.
10 kVA demonstrator transformer design
A successful small 10 kVA demonstrator transformer design, manufacture and testing project was completed
at the University of Southampton in 1999, Figure 1 [1]. A single phase design with copper primary and
BSCCO-2223 superconducting tape pancake coil secondary was used to study the performance of tapes and
allow the comparison with electromagnetic models developed. The large separation between the two coils,
required to accommodate the cryostat, increases the radial flux densities and leakage reactance and so a three
limb core design was used with both windings on the centre limb. The current rating of the secondary coil at
a load of 10 kVA is 40A.
The flux component normal to the broad face of the HTS tape must be minimised as this is critical to the
current carrying capacity of the winding; for example to carry the peak current of 9.5 A per tape, flux density
must be less than 15 mT, compared with 110 mT for the parallel component. To achieve this, flux diverters
manufactured of powdered iron epoxy composite were placed next to each end coil. Figure 2 shows field
plots of the coils with and without the flux diverters in place.
(a) (b)
Figure 1, 10 kVA demonstrator transformer a) HTS pancake coils shown with cryostat open, b) copper coil
around closed cryostat, top yoke removed
Figure 2, Magnetic field plots around HTS coils with and without flux diverters
Figure 3 shows the modelled and measured losses in the cryogenic region plotted against secondary current
with and without the flux diverters. Tests were performed beyond the nominal design rating of the
secondary. These plots validate the modelling approach developed to estimate the AC losses described in [2].
The losses were reduced to a half of the value with no flux diverters, showing also the benefits of good
modelling as a design aid.
Figure 3, Measured and calculated losses with and without flux diverters
240 MVA grid auto-transformer design study
A design feasibility study was conducted to consider the technical and economic viability of a 240 MVA grid
autotransformer. [3,4] The key parameters of the design studied are listed below;
kVA: 240,000
Normal volts: 400/132 kV
Tappings: 132 kV ± 15% in 14 steps
Line current at normal volts: 346/1054 A
Diagram No: Yy0 Auto
Guaranteed reactance: 20%
Rated current densities:
Series winding* = 39.1 A/mm2
Common winding* = 36.9 A/mm2
Tap winding = 3.0 A/mm2 (conventional)
*average over composite conductor section, comprising both superconducting and matrix materials
The principal feature of the design is the removal of the copper windings and their replacement by HTS coils.
The HTS coils are less than 10% the size of copper equivalents. Thus an advantage is seen in the reduced
size and weight of the transformer design, Table 1. However, the inevitable result is windings of reduced
mechanical strength, thus additional bracing is necessary to withstand the radial bursting force and axial
compressive forces that occur during fault conditions. For the studied design the tap winding was chosen to
be located outside the cryostat to reduce the thermal in-leak which would result from the multiple
connections at ambient temperature. It is desirable to remove oil from the design to reduce the risk of fire
and prevent environmental impact from spillage. To remove oil from the design completely the tap windings
can be cooled by forced gas cooling.
The HTS tapes have a low thermal mass and are stable only over a small temperature range. The design is
therefore vulnerable to through faults, requiring disconnection and cooling for several minutes after
experiencing a fault. The transformer will survive the most severe fault if disconnected within 166 ms, see
Table 2. This is the major weakness of the HTS transformer design; however there is good overload
capability. The main savings are on ‘copper’ losses, which lead to the HTS design releasing only 23% of the
total losses when compared with a conventional transformer.
0
1
2
3
4
5
6
7
8
9
10
11
0 10 20 30 40 50 60
Secondary Current (A)
Loss (W)
Measured (no Flux Diverters)
Calculated (no Flux Diverters)
Calculated (with Flux Diverters)
Measured (with Flux Diverters)
Table 1. 240 MVA grid autotransformer size and weight comparison.
Parameter HTS Conventional
Core length *
height *
thickness *
Window, height * × width *
88.5
82.4
100
70 × 78.5
100
100
100
100 × 100
Core weight *
Winding weight *
Tap winding weight *
80
6.3
100
100
100
100
Cooling of core
and tap winding
Cooling of common
and series winding
Forced N2 gas
Liquid N2
(with refrigeration)
ONAN/OFAF
ONAN/OFAF
*shown as a percentage of conventional design equivalent.
Table 2. Technical features comparison.
Parameter HTS Conventional
Guaranteed % reactance
B in core, T
J rated, rms, A/mm2
20
1.67
38
20
1.67
2.83
Rated loss, total * 23 100
Overload capability
Through fault capability,
pu (+ doubling transient), recovery
time without disconnection
Survival time at 5 pu
(+ doubling transient)
2 pu, many hours
2 pu, 64 ms
166 ms
1.3 pu, 6 hrs
1.5 pu, 30 min
5 pu, 3 s
seconds (> 3)
Capital and operational expenditures were assessed and first-cost savings calculated, based on a discount
period of 10 years at 9.5% per year [4]. Savings of 36% as compared to a conventional copper winding
transformer were predicted. However, this was performed under the assumption that the transformer was
100% loaded all of the time. This assumption is unrealistic, indeed on the national grid in England and
Wales the load factor of transformers is typically only 23%. The application of HTS transformers therefore
requires careful consideration. One proposal put forward was to use a HTS unit to replace one of two
conventional transformers, of the size of that studied, running in parallel circuits on dual (main and reserve)
circuits. This would enable a higher load factor for the HTS transformer, thereby achieving the potential cost
savings. Alternative applications might be considered where transformers have a high load factor, for
example substations at the point of generation.
Cored generator design
Many conceptual HTS designs were proposed, and small demonstrator generators built with BSCCO tapes,
but most operate between 25 and 30 K. The advantage of lower temperature cooling is that the critical
currents and magnetic fields are an order of magnitude greater than at 78 K (i.e. LN
2
temperatures). Since
the magnetic field can be increased it is possible to have a core-less design. The disadvantage, as compared
to a LN
2
design, is that Liquid Neon (LNe) or Helium gas (GHe) has to be used as a cooling medium. This
leads to a greater complexity and higher cost of the refrigeration plant. Greater complexity results in worse
reliability, comparable to LTS applications, and higher maintenance costs. Moreover, the thermodynamic
efficiency is about eight times worse when working at 25–30K compared with 78K.
At Southampton these considerations have lead to the view that LN
2
temperature designs should be pursued.
The demonstrator generator designed is a 100 kVA 2 pole machine built to be operated under several cooling
methods, including 81K, 78K, 65K and 57K using LN
2
or liquid synthetic air and sub-cooled LN
2
or liquid
synthetic air [5]. A schematic diagram of the generator is shown in Figure 4. Liquid synthetic air (79%
nitrogen, 21% oxygen) has a melting point of 57 K and boiling point of 78 K, thus offering a wider liquid
range than with LN
2
. The use of a magnetic core rotor reduces the number of ampere-turns required by a
factor of ten and significantly reduces the fields within the coils. In the generator built the rotor has been
made of cryogenic steel (9% Nickel). The rotor has 10 identical pancake coils made of BSCCO (Ag clad Bi-
2223), the length of the wires is approximately 10 lengths of 40 m. Coils have been manufactured by a
process of winding the fragile HTS tapes onto a former interleaved with insulating tape and then resin
impregnated under vacuum. Each coil is then tested prior to rotor installation; this is to check that no
damage has occurred during manufacture, for example by in-plane bending of tapes. The modular design
allows any faulty coil to be replaced with ease.
Slip Rings
(DC current)
Coupling to
Driving Motor
&
Optical Link
A
View A
Stator
Stator Copper
Winding
Vacuum Vessel
& Superinsulation
Support Platform
B
View B
Detail
9% Ni Steel
Flux Diverter
HTS Coil
Copper Thermal
Shield
9% Ni Steel Rotor
Eight Fixing Postions
of Torque Spider
Six places to link Torque Spider
to the Rotor Body (See Text)
Torque Spider
Stiffening Cone
Cooling
Duct
Stainless Steel
Former
Damping Screen
Vapour Seal
Rotating
Liquid Seal
Clutch-Like
Liquid Coupling
Liquid Coupling
Junction
Superinsulation
Seal
Figure 4. Southampton HTS 100 kVA demonstrator generator design.
The pancake coils are placed onto two pole necks. The rotor shape is a hybrid design which uses flux
diverters made of 9% Nickel steel placed between the coils to reduce the normal component of the magnetic
field in the coil tapes by diverting flux around them and shaping the magnetic field profile in the gap,
Figure 5. In this manner the normal component of the field in the coils has been reduced to 0.038 T, while
maintaining 0.66 T in the air-gap. Cooling is provided by a purpose built closed circuit liquid cryogen
cooling system with pipe-network feeding liquid cryogen to the rotor body of the generator. To reduce
thermal heat leak into the cryogenic region the mechanical linkages, between rotor core and the steel shafts,
are made of fibreglass. A copper radiation screen is placed around the rotor core assembly to intercept heat
radiating from the vacuum region and to limit the time varying fields in the superconducting winding.
