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Sunbelt Transformer
Transformer Design Comparisons
• General description of core/coil types• Analysis of short-circuit forces• Specific analysis of coil types and strengths• Overload capability• Cost comparisons• Weight / dimensional comparison
Sunbelt Transformer
Stacked Core Construction
• Also known as “core form”
• Utilizes variety of lamination thickness and quality
• 5-100 MVA typical
Sunbelt Transformer
Stacked Core Construction
Sunbelt Transformer
Wound Core Construction
• Also known as “distributed gap”
• Usually utilizes lowest cost core steel
• Below 5 MVA typical• Below 1 MVA standard• All pole-type
transformers
Sunbelt Transformer
Cylindrical Coil Construction
• Coils have cylindrical cross-section
• Concentrically wound with HV over LV (Lowest voltage nearest core)
• Requires stacked core
Sunbelt Transformer
Rectangular Coil Construction
• Coils have rectangular cross-section
• HV over LV winding• Core can be stacked
(core form) or wound (shell form)
Sunbelt Transformer
Short-Circuit Strength
• Winding Forces Repel– Force on conductor
equal to current times magnetic field
– High short-circuit current translates into high forces
• Horizontal Component– Present in ALL designs
• Vertical Component– Varies with design
Sunbelt Transformer
Short-Circuit Strength
• Horizontal Forces– Present in ALL designs– Cylindrical coil
movement limited by tensile strength
– Rectangular coils must be horizontally braced to restrict movement
Sunbelt Transformer
Short-Circuit Strength
• Vertical Forces– Present in all wire-wire
designs as current cannot redistribute from top to bottom of coil
– NO vertical forces in designs with full-height sheet winding (1 turn per layer)
Sunbelt Transformer
Short-Circuit Verification
• Anderson Program– Most widely used finite element program addressing short-
circuit strength and temperatures– Created by Odd W. Andersen
• ANSI Short-Circuit Requirements– Pass Standard Dielectric Tests After SC Test– No Mechanical Movement – No Abrupt Changes in the Voltage or Current Wave Shape– 2.0% Leakage Current Change (7.5% for non-circular coils)– 5% Excitation Current Change After SC Test (stacked cores)
Sunbelt Transformer
Transformer Design Comparisons
• Construction types to be compared:– Cylindrical, Disk Coils; Stacked Core– Cylindrical, Layer Coils; Stacked Core– Rectangular, Layer Coils; Stacked Core– Rectangular, Layer Coils; Wound Core
Sunbelt Transformer
Cylindrical-Disk Coil Construction
• No horizontal bracing– Only insulation separates
phases– Conductor tensile
strength limits horizontal movement
• Full-circumference vertical bracing– Known as “pressure
plates”, these distribute forces across entire coil
Sunbelt Transformer
Cylindrical-Disk Coil Construction
• Pressure plate functions– Support weight of coils
on bottom– Contain vertical forces
during short circuit– Distribute forces across
entire horizontal surface of the winding
– Keep coils from expanding when not oil-immersed
Sunbelt Transformer
Cylindrical-Disk Coil Construction
• Wound on keyed winding tube
• Pressed vertically to exact size
• Very little electrical stress disk-to-disk
• Good beyond 750 KV BIL
Sunbelt Transformer
Cylindrical Or Rectangular Layer Coils
• Sheet LV – Strap HV– Balanced Ampere Turns
Centerlines– No Vertical Forces - with
only one turn-per-layer
• Strap LV – Strap HV– Unbalanced Ampere
Turns Centerlines as taps changed
– Vertical Forces Present
Sunbelt Transformer
Layer Coil Construction
• Simple winding tube• Epoxy-coated paper as
layer insulation• If wound in single-
section can have high layer stress
• High amounts of paper require extra processing
• Limited to 350 KV BIL
Sunbelt Transformer
Rectangular Layer Construction
• Solid endplates required for smaller KVA designs
• Engineered endplates such as box beams for higher KVA
• Extensive bracing leads to increased weight for rectangular-layer
Sunbelt Transformer
Disk Coil Heat Transfer
• Best Heat Transfer– Open fluid flow with
keyed-spacers separating disks
– 4 degree winding rise easy to obtain
– The only design that should be used with two stage cooling
Sunbelt Transformer
Layer Coil Heat Transfer
• Poorest Heat Transfer– Oil only flows through
ducts– 10 degree winding rises
are typical; makes two stage cooling almost impossible
– Number of ducts limited by short-circuit strength
– Demand a heat-run if two stage cooling is expected
Sunbelt Transformer
Transformer Design Comparisons
• Description of core design differences • Short-circuit strength • Description of coil design differences • Overload capability • Cost comparisons• Weight / dimensional comparison
Sunbelt Transformer
$ - The bottom line.
• Evaluated by initial purchase price– Rectangular is lowest cost – weakest short-circuit design– Cylindrical-layer is 10-15% more – eliminates horizontal
bracing– 5% additional for Cylindrical-disk buys vertical bracing and
no need for horizontal support; strongest design possible
• Evaluated by total ownership cost– No-Load losses less in cylindrical design and load losses
equivalent– A $5/NL Watt evaluation can pay for cylindrical design– Layer designs have other problems such as gassing from
high moisture content in improperly cured insulation
Sunbelt Transformer
Weight and dimensional comparisons
• Space Utilization– Cylindrical will have smaller pad footprint, almost always
taller and thinner than rectangular design– Overall volume will always be smallest with rectangular
design – ideal network or vault application transformer
• Weight– Short-circuit bracing requirements make the rectangular unit
much heavier– Cylindrical disk will have 20-30% more oil content/KVA
• Standard 10/14 MVA cylindrical disk will weigh about 55000 lbs
• Standard 10/14 MVA rectangular layer will weigh 75000 lbs