High-temperature superconducting transformer

1. HTS TRANSFORMER
ARGHA SAHA
IIT KHARAGPUR
CRYOGENIC ENGINEERING

2. INDE
X
TRANSFORMER
HTS TRANSFORMER
BENEFITS
COMPARISON
DESIGN TRADE OFF
CONSTRUCTION
COOLING DESIGN
CASE SYUDY

3. TRANSFORMER
 A transformer is an electrical device that
transfers electrical energy between two or
more circuits through electromagnetic
induction.
 A varying current in one coil of the
transformer produces a varying magnetic
field, which in turn induces a voltage in a
second coil.
 Transformers are used to increase or
decrease the alternating voltages in
electric power applications. 3- phase transformer

4. Basic principle
 By Faraday’s law of induction,
𝑣𝑠 = −𝑁𝑠
𝑑∅
𝑑𝑡
𝑣 𝑝 = −𝑁 𝑝
𝑑∅
𝑑𝑡
Now,
𝑣 𝑝
𝑣 𝑠
=
𝑁 𝑝
𝑁𝑠
=a
For step-down transformers, a > 1
For step-up transformers, a < 1
 We are considering the transformer is ideal .
 An ideal transformer is lossless and perfectly coupled.
(Perfect coupling implies infinitely high core magnetic
permeability and winding inductances and zero net magneto
motive force.)
 By the law of conservation, 𝑆 = 𝐼 𝑝 𝑣 𝑝 = 𝐼𝑠 𝑣𝑠
 By Ohm's law and the ideal transformer identity:
The secondary circuit load impedance can be expressed as,-
𝑍 𝐿 =
𝑣𝑠
𝐼𝑠

5. Basic principle
The apparent load impedance referred to
the primary circuit is derived to be equal
to the turns ratio squared times the
secondary circuit load impedance
𝑍 𝐿′ =
𝑣 𝑝
𝐼 𝑝
= 𝑎2 𝑍 𝐿
 Real transformers are having core losses, collectively called
magnetizing current losses, consisting of
• Hysteresis losses due to nonlinear application of the voltage
applied in the transformer core
• Joule losses due to resistance in the primary and secondary
windings
• Eddy current losses due to joule heating in the core that are
proportional to the square of the transformer's applied voltage.
• Leakage flux that escapes from the core and passes through one
winding only resulting in primary and secondary reactive
impedance.

6. HTS TRANSFORMER
 Greater efficiency
 Smaller, lighter and quieter
 Ability to run above rated power without
affecting transformer life
 Liquid nitrogen cooling
 Flexibility in siting
BENEFITS
 The conventional technology costs cheaper
when dealing with low power levels
 However, when talking about high power, the
cost SC is much lower and achievable

7.  In a conventional power transformer, load losses (LL) represent approximately 80% of
total losses. Of this load loss, 80% are 𝐼2 𝑅 losses. The remaining 20% consists of eddy
current losses. Even when the transformer is "idling," so-called "no-load losses" (NLL)
are generated in the core.
 Savings over conventional units were estimated to be greater than 35%, but the
unknown ac loss characteristics of the HTS materials made it difficult to assess viable
designs.
 A comprehensive study conducted for the U.S. Department of Energy found the life -
cycle costs of an HTS transformer, on average, to be half those of a comparable
conventional unit (Dirks 1993).
 National savings from the insertion of HTS transformers were estimated to be $25
billion through the year 2030.
 Over the size range of 30-1,500 MVA, Mumford (1994) estimated costs savings of HTS
transformers over conventional designs to be as great as 70% and transformer weight to
be 40% less.
BENEFITS

8. COMPARISON
ITEM 60 MVA HTS TRANS. 60 MVA CONVENTIONAL
TRANS.
Weight 16.6 tons (includes
cryostats, without liq 𝑁2 )
27.2 tons (Without
lubrication oil)
Core Size 2674 mm*2429 mm 3150 mm*2590 mm
Copper Loss N/A 100 KW
Core Loss 28.7 KW 33 KW
Refrigeration Loss 12 KW N/A
Total AC loss 40.7 KW 133 KW
Efficiency 99.93% 99.3%
Impedance 16.43% 19.83%

9. DESIGN TRADE OFF:
 In spite of having a lot of advantages some
limitations are also there in designing.
 Reduction in size will be limited by di-
electric design considerations. The
transformer must meet American National
Standard institute’s dielectric tests.
 Iron core mainly determines overall size and
mass also the overall performance. A lot of
Eddy current loss will be produced here (
order of 10 kw).
 So HTS transformers are consequently
designed to operate with cores near ambient
temperature and isolated thermally from the
windings. But we cannot have big cores to
reduce losses. So the core size to be reduced.
 But reducing core diameter adds to the no of
turns and so to the total length and cost to
the conductor
 Typically maximum flux density is about
0.1 – 0.3 tesla. Compared to conventional
all losses are less but as they are of low
temperature so it takes many times the
value of refrigeration.
 But another problem is at low
temperature the Current density will be
better but loss also will be high.

10. CONSTRUCTION
 HTS wires which are commonly used in high voltage power transformer can be
divided into two types: the 1st generation Bismuth Strontium Copper Oxides (BSCCO)
HTS wires
 The 2nd generation Yttrium-Barium- Copper Oxide (YBCO) HTS wires.
 Bi2223 has been more applied than Bi2212 since its critical temperature is 20 K
higher than Bi2212.
 HTS winding with YBCO wires begin to be considered because YBCO wires have
higher current density and better current magnetic field characteristics than BSCCO
wires.
WINDING:

11.  The cryostat must be nonmetallic and have good
low temperature resistance, especially in liquid
nitrogen.
 The basic material is epoxy resin, the curing agent
is low molecular weight polyamide, and the filler is
alkali- free glass fiber.
 The manufacture process consists four key steps:
1) mixing epoxy resin and polyamide with
appropriate matching
2) smearing the mixture onto alkali-free glass
fiber
3) rolling the smeared glass fiber
4) curing with appropriate time and
temperature.
 The cryostat consist warm shell, cold shell, vacuum
chamber and liquid nitrogen (LN2) chamber.
 In addition to the vacuum chamber, there is a
thermal insulation layer made from polyurethane
foam behind the lip for decreasing the thermal
CRYOSTAT:

12. CORE:
 A suitable magnetic core has also been studied to reduce the HTS transformer
losses.
 In China, the TBEA HTS power transformer project has developed the first HTS
transformer with amorphous alloy cores in the world.
 In Spain, a straight solenoidal geometry is now considered and studied in an air-
core transformer.
 In Korean, the Korean power company has considered the possibility of a HTS
transformer on a common magnetic core since it can be a solution for the
increment of capacity without new construction of substations.

13.  Japanese group led by Kyushu
University contained the HTS
windings in a main GFRP cryostat
filled with subcooled liquid nitrogen
at around 65 K, and located an iron
core through room-temperature
bore of the cryostat.
 The subcooled liquid is continuously
chilled by two sets of GM cryocooler
in a secondary cryostat and
circulated through transfer lines to
the main cryostat by a pump,
COOLING

14. COOLING:
 The US team under the Department of
Energy SPI (Superconductivity Partnership
Initiative) agreement has pursued a
completely different cooling design. In
order to avoid the expensive composite
cryostat, they placed both the windings and
the iron core in vacuum tank. The HTS
windings were maintained at around 30 K
by the circulation of helium gas chilled by a
GM cryocooler, and the radiation shields
were cooled at 77 K by liquid nitrogen and
another cryocooler.

15. COOLING:
Siemens in Germany has developed the HTS
transformers for railway applications. As
geometric constraint and compactness are
more significant than efficiency in the on-
board transformers, the whole core-and-coil
assembly was cooled at around 67 K with
subcooled liquid nitrogen. A huge capacity of
Stirling cooler was employed for a laboratory
test to supply the subcooled liquid through
transfer tubes

16.  In an advanced system the HTS pancake
windings are immersed in a liquid nitrogen bath
where the liquid is cooled simply by cold copper
sheets vertically extended from the coldhead of a
closed-cycle cryocooler located above.
 Liquid nitrogen in the gap between the windings
and the copper sheets will develop a circulating
flow by buoyancy force in subcooled state close
to the normal freezing point.
 Nitrogen functions as a heat transfer medium
and an electrical insulating fluid at the same
time. Since no circulating pump or transfer line
is necessary, the proposed cooling by natural
convection has great advantages in all aspects of
compactness, efficiency, and reliability, over the
forced-flow cooling of the previous systems
COOLING:

17. DEVELOPMENT OF A 630 KVA THREE-PHASE HTS TRANSFORMER
WITH AMORPHOUS ALLOY CORES
CASE STUDY

18. DEVELOPMENT OF A 630 KVA THREE-PHASE HTS TRANSFORMER
WITH AMORPHOUS ALLOY CORES

19. Overview of 630 kVA
three-phase
transformer
Schematic
associated view of
cryostats and the
amorphous alloy
core with 5 limbs.
The HTS transformer operation
field serving cable manufacture
plant
of TBEA

20. DEVELOPMENT OF A 630 KVA THREE-PHASE HTS TRANSFORMER
WITH AMORPHOUS ALLOY CORES

21. Thank You

22. WHAT IS AC LOSS ?
 Due to screening current
local power density is given
by E*J
 The energy is delivered by the
external magnetic field and is
supplied by the power source
of the magnet that generates
the field.
 The energy is converted into
heat that must be removed by
the cooling system.
 The screening currents give the sample a magnetic moment m, which
is calculated from the current distribution. Then the AC loss of the
sample is found by integrating either the product m×dB or B×dm over
a single magnetic-field cycle.
SCREENING CURRENT:

23. COUPLING CURRENT:
 A quite different sort of eddy currents is induced in a
superconductor consisting of separate filaments embedded in a
normal material.
 The currents in Figure are called coupling currents because they
couple the filaments together into a single large magnetic system.
The system has a magnetic moment higher than the sum of the
magnetic moments of the individual filaments. Then the AC loss
in alternating magnetic field is higher as well.