HTS Transformer Development

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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

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.
   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

         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
Fire is a constant hazard in
conventional transformers.

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
Technology Transfer/Collaborations
Future Plans
                   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
       Proposal for further development submitted in response to DOE SPE
       solicitation, but not chosen by DOE.
                    1-MVA Prototype
Single phase,13.8/6.9 kV,
72.5/145 A.
Bi-2212 coated conductor,          Tank
1400 m, cryocooled to 25 K.
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.

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.

                 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
   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
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.

        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
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

                     Transformer Schematic

   Cryocooler   Winding                Core                 Foam         Removable
                                                                         Top Plate


    Composite Coil                                                 Subcooled
    Dewar                                                          Nitrogen
                      Unit is surrounded by weather enclosure in
                      place of oil tank.
Single-Phase Transformer Concept
138-kV Bushing

Existing 1-MVA Core

Existing AL-300 Cryocooler

Composite Dewar

                  3-phase version

                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
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

Spreadsheet aids conceptual design
Inputs– MVA, voltages, materials properties, costs
Outputs– Coil dimensions, weight, heat loads, losses,
Periodically benchmarked to proprietary WES design

      User inputs
      Calculated results
      Notes & Warnings

   Transformer Specific Dielectric Issues
   Materials properties
   Processing                         Core   Bushing
   Non- linear impulse voltage                  Foam      Removable
   distribution and transferred                           Top Plate
   Lifetime PD free operation
   Complex dielectric design
   Complex mechanical design for
   short circuit forces                                     Composite
   Large stressed insulation volume                         Dewar
   Interfaces between solid-liquid-
   vacuum; surface flashover                                Pressurized
   Components and accessories                               Subcooled
Requirements                                                Nitrogen
   Meet standards & test
   requirements                                              Cooling
   Cost effective
   30+ years life
        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

               HV Test Coil
Pressboard mandrel
and structure
4-mm spacers in
upper half
2-mm spacers in
lower half

                HV Test Coil
                        Multiple HV contacts
                        in G10 top plate allow
                        easy sample selection

Avoids expense of
multiple HV bushings

         Full Wave Neg. Impulse Data

•1% Values are
 compared with
 calculated 550-kV
 impulse profile:
 28-30 kV;
 5 kV required        Turn-Turn
•2-mm Disc-Disc–
 60 kV;
 58 kV required

         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
         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
Inspection of coil showed only two
       visible breakdowns.
 Discs 16-17       Discs 23-24

     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

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

  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
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.
   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.
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
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
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.
   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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