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HTS Transformer Development
Presented by
S.W. Schwenterly, Oak Ridge National Laboratory
E.F. Pleva, Waukesha Electric Systems
For the DOE Peer Review
Washington, DC, Aug. 7, 2007
Team:
Oak Ridge National Laboratory– Bill Schwenterly, Randy James, Isidor Sauers,
Enis Tuncer, Alvin Ellis
Waukesha Electric Systems– Ed Pleva, Sam Mehta, Tom Golner, Jeff Nemec,
Bob Del Vecchio
U.S. Department of Energy
Project Purpose
To establish the technical and economic feasibility and benefits
of HTS Transformers of medium-to-large (>10 MVA) ratings.
Efficiency
Long lifetime at full load and “The Department of Energy
overload believes HTS technology is vitally
Smaller size and weight important to the modernization of
the nation’s power grid.”
Siting advantages, fire safety, low –Kevin Kolevar, Director, OEDER
environmental impact
To carry out materials studies in
support of this development.
Supports DOE Mission and
subprogram goal:
Develop revolutionary power
equipment using HTS wires
Characterize dielectric materials
and establish design rules
2
30-Year Life Cycle Cost Comparison
Conventional transformer @ 70% load factor and HTS transformer @ 100%
load factor, with $50/kA-m HTS conductor cost.
Intangibles such as reduced environmental impact and relative costs for
overload capacity also need to be considered.
Total ownership cost for HTS transformer looks comparable to a
conventional unit. Plus, HTS unit is still fully usable after 30 years.
Item 18 MVA HTS Trans. 24 MVA Conventional Trans.
Initial Cost $621K $550K
Core Loss $32,500 (13kW) $42,500 (17kW)
Load Loss $32K (refrigeration) $150K (winding)
Fire Suppression N/A $100K
Oil Containment Pit N/A $30K
Cryocooler Maint. $161K (3 Cryomech AL600’s) N/A
Monitoring $20K N/A
TOTALS $866.5K $872.5K
3
Fire is a constant hazard in
conventional transformers.
4
OUTLINE
Summary of previous program Pleva
1MVA Single-Phase Prototype
5 MVA Three-Phase Prototype
Follow-on program
Conceptual Design
Composite Dewars
Transf. design Spreadsheet & benchmarking Schwenterly
High Voltage Cryostats and Test Samples
High Voltage Test Results
Goals/Accomplishments
Technology Transfer/Collaborations
Future Plans
5
Program Overview
Phase 1: 1994 – 2000 (WES, IGC, ORNL and RG&E)
1 MVA single-phase prototype transformer design –
13.8kV HV/6.9kV LV, tested in 1998.
Materials testing– HV, vacuum, ac losses
Phase 2: 2000 – 2005 (WES, SuperPower, ORNL and Energy East)
5/10 MVA 3-phase prototype- 24.9kV HV/4.16kV LV; scaledown of 30
MVA concept design; completed and tested 2003 – 2004.
Materials testing– HV, ac losses
Most subsystems were tested successfully.
Transformer failed HV dielectric tests.
Coils cut apart and inspected; Root Cause Analysis completed in 2005.
Phase 3: 2005 – Present (WES, ORNL)
ORNL and WES continued collaboration in a Follow-On Project using
WES internal and DOE funding.
Project concentrated on cryogenic dielectrics and rework of conceptual
design.
Proposal for further development submitted in response to DOE SPE
solicitation, but not chosen by DOE.
6
1-MVA Prototype
Single phase,13.8/6.9 kV,
72.5/145 A.
LN
Bi-2212 coated conductor, Tank
1400 m, cryocooled to 25 K.
Core
Vacuum HV insulation.
1.68% Impedance. Shield
Stable operation limited to half
current rating by ac losses.
Tested to 11 kV and 150 A.
Cold test voltage limited by
arcing in superinsulation; unit
reached 13.8 kV warm in air.
Support
Leg
7
5/10-MVA Transformer
3-Phase 24.9/4.16 kV, 63/694 A.
Bi-2223 MF conductor, 12 km, cryocooled
to 30 K
Cast filled epoxy HV insulation.
0.84% Impedance.
Cryogenic, vacuum, & core cooling
systems ran well.
Short-circuit tested to 120 A with slow
heating; stable operation at 51 A.
High PD and failures at 8.2 kV and 13 kV
during HV testing.
Arcing evidence found at cracks in epoxy
when coils were cut open.
8
Lessons Learned
Readiness review methodology would have recognized risks
associated with aggressive program
To go from 1 MVA single phase to 5/10 MVA three phase
A new dielectric design
Dielectric system for cryogenic environment needs further
development
Dielectric material
Device specific design rules
Processing and fabrication techniques
Implications of large stressed volume
Technique to fabricate uniform quality large structures
All subsystems that were tested and debugged prior to assembly
performed as designed
LHe LN2 systems, core cooling, vacuum tank, HV mast/leads &
Bushings 9
2006-2007 follow-on project has
addressed:
Updates to conceptual design for best manufacturability,
marketability, reliability, cost, match to utility requirements.
Updates to ORNL transformer design spreadsheet.
Cryogenic dielectric studies for 30-MVA+ design ratings–
138 kV, 550 kV BIL
PD, ac and impulse breakdown strength
Thermal properties– shock, conductivity, heat capacity
Tests on scaled-down dummy coils
Composite dewar issues.
10
Conceptual Design Studies
Use YBCO in pressurized, subcooled nitrogen bath.
Avoids problems with potted coils.
Try to extrapolate conventional manufacturing
techniques to HTS design.
Pressurized bath is coupled to cryocooler by cooling
shell.
Composite dewar needed.
Metal dewar would form a shorted turn around core.
Individual phase dewars– simpler, but requires more leads &
bushings, external phase connections, individual coolers.
Common dewar for all 3 phases– complicated, but allows
internal phase connections and fewer leads; shared common
cryocoolers.
11
Transformer Schematic
Bushings
Cryocooler Winding Core Foam Removable
Top Plate
Cooling
Shell
Shield
Pressurized
Composite Coil Subcooled
Dewar Nitrogen
Unit is surrounded by weather enclosure in
place of oil tank.
12
Single-Phase Transformer Concept
138-kV Bushing
Existing 1-MVA Core
Existing AL-300 Cryocooler
Composite Dewar
3-phase version
13
Internal Details
Winding Pack–
Composite Dewar– 1.2 m OD, 0.6 m ID,1.2 m Ht.
1.6 m Diam. x 1.7 m Ht. 14
Composite Dewars
Cryogenic envelope around core limb cannot be a shorted
turn.
Non-conductive material needed that can take pressure and
mechanical loads– fiber/epoxy composite.
Visits to several composite vessel manufacturers identified
a promising vendor.
30-cm vessel has been
purchased for testing:
Vacuum outgassing
Leak checking
Impermeability to liquid nitrogen
Thermal cycling
Pressure tests
15
Spreadsheet aids conceptual design
Inputs– MVA, voltages, materials properties, costs
Outputs– Coil dimensions, weight, heat loads, losses,
costs
Periodically benchmarked to proprietary WES design
codes.
User inputs
Calculated results
Notes & Warnings
16
Transformer Specific Dielectric Issues
Issues
Materials properties
Processing Core Bushing
Non- linear impulse voltage Foam Removable
distribution and transferred Top Plate
surges
Lifetime PD free operation
Complex dielectric design
Complex mechanical design for
short circuit forces Composite
Winding
Large stressed insulation volume Dewar
Interfaces between solid-liquid-
vacuum; surface flashover Pressurized
Components and accessories Subcooled
Requirements Nitrogen
Meet standards & test
requirements Cooling
Shell
Cost effective
Robust/Manufacturable
30+ years life
17
High voltage test coil studies
Coil Construction–
Copper conductor; 1.8 mm x 11.4 mm
Standard WES winding procedures used
24 double pancakes
3 turns of 2-in-hand conductors
Each conductor has 8 layers of insulation; 0.8 mm build
2-mm & 4-mm radial spacers between pancakes
Individual HV leads to each pancake
No complex insulation processing– only simple evacuation
Tested in liquid nitrogen at 1 atm or 1.33 atm (5 psig).
First round of testing with one wire of each pair at HV; other wire
grounded (turn-turn simulation)
Second round of testing with pairs shorted together; HV on one
pancake; pancake below grounded (disc-disc simulation)
Voltages were 60-Hz ac, Full-wave Impulse, Chopped-wave Impulse
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HV Test Coil
Pressboard mandrel
and structure
4-mm spacers in
upper half
2-mm spacers in
lower half
19
HV Test Coil
Multiple HV contacts
in G10 top plate allow
easy sample selection
Avoids expense of
multiple HV bushings
20
Full Wave Neg. Impulse Data
•1% Values are
compared with
calculated 550-kV
impulse profile:
•Turn-turn–
28-30 kV;
5 kV required Turn-Turn
2-mm
Disc-Disc
•2-mm Disc-Disc–
60 kV;
58 kV required
21
Turn-Turn 60-Hz AC Data
Nominal turn-turn voltage is 100 Vac.
PD was less than 10 pC up to at least 5 kV.
Pressurizing bath to 5 psig gave much more stable
PD levels but did not change inception voltages
much.
Disc-Disc 60-Hz AC Data
Nominal disc-disc voltage is 1200 Vac.
With bath at 5 psig, PD was less than 2 pC at 5 kV.
One failure occurred at 15 kV on top 4-mm spacer.
Maximum voltages at ~200 pC ranged from 12 kV to
24 kV for 2-mm and 10 kV to 23 kV for 4-mm
spacers.
22
Inspection of coil showed only two
visible breakdowns.
Discs 16-17 Discs 23-24
23
Breakdown of copper tape (winding)
insulated with WES’s synthetic dielectric tape
Three 12” copper tapes wrapped
with synthetic dielectric tape (5
layers synthetic tape) crossed
with one 12” copper tape wrapped
with the same dielectric tape for
Bd measurements simulating
tape-to-tape breakdown
24
AC voltage tests on WES synthetic tape give
similar results in oil and 77-K LN2.
breakdown data for oil at room
temp and for LN2 at 77 K and
1 bar.
E (kV/mm) Number
Bath Stdev
Mean strength of samples
oil 27.8 3.3 14
LN2 29.2 3.8 46
25
76-cm Cryostat for Higher Voltages
Large diameter allows
tests on subscale coil
mockups and very high
voltage impulse tests in
pressurized nitrogen.
Can be pressurized to 1/2
atm.
Off-shelf epoxy
resin/paper bushing was
qualified in 77-K
nitrogen; tested to 500
kV impulse.
Cryostat supports both
generic base program
and transformer-related
studies in dielectrics.
26
FY 2007 Plans FY 2007 Performance
Participate in preparation Proposal submitted and DOE
of follow-on proposal. Funding received for FY 2007.
Submit SPE proposal on SPE proposal was submitted but
HTS transformer not chosen.
development.
Carry out further dielectric Tests performed on sample coil,
testing. small wrapped samples, and large
gaps in LN. Epoxy-resin/paper
bushing was successfully tested.
Generate a new con- New conceptual design completed.
ceptual design for a 24-
MVA transformer using
YBCO.
Investigate vendors for Several vendors contacted; 2
composite dewars. visits; vendor identified; test
vessel purchased. 27
Research Integration
ORNL/WES team possesses strong complementary abilities in
research, engineering, manufacturing, & utility operation.
Partners carried out frequent site visits of about a week each.
4 visits– ORNL to WES.
6 visits– WES to ORNL.
Team is working with vendors to develop composite dewars.
WES obtained epoxy resin/paper bushing for ORNL HV tests.
ORNL loaned a dewar vessel to WES for high-voltage testing of sample
coils.
Material developments in DOE-funded ORNL/WES Gridworks project
for conventional transformers (core steel, insulation) can also facilitate
HTS technology.
Communication several times a week by phone, E-mail, and fax.
28
Going Forward
Continue to seek funding options.
Study YBCO materials for the 24-
MVA reference design.
AC loss
Ic and overcurrent capability; operation in
liquid nitrogen
Fault current limiting
Build & test scaled-down single-
phase 138-kV transformer on
existing 1-MVA core.
Investigate electrical & mechanical
compatibility between all insulation
system components.
WES and ORNL continue to be committed to the development of
HTS transformers.
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