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