Magnetic circuits of transformers

Magnetic circuits of transformers - assembly

Obtuse layout (no-overlapping)

• easy assembly

• for small-sized transf.

• disadvantages:

• short circuits

• power losses

• air-gap

Fixing of magnetic circuits:

• screw (out of magnetic circuits!!!)

• laminate skeleton

• welding (only small-sized transf.)

Overlapping layout

• hard to assembly

• for big-sized transf.

• advantages:

• lower power losses

• better magnetic properties

• better solidity

Cross-section of magnetic circuits:

(different efficiency of filling winding)

Arrangement of windings and magnetic cores:

Questions to solve:

• 1/3 phases?

• HV/low-voltage?

• cooling?

Possible arrangement and layout of sheets

Procedure of assembly (video)

• 1. Creating of bases layout• 2. placing of boundary sheets (plates)• 3. adjustment by rulers (sliding bar)• 4. placing and assembly of all sheets• 5. pressing – controlling of height• 6. isolation / impregnation• 7. check-in of cooling channels• 8. final pressing, binding• 9. drying (110 °C) and final pressing (screws)• 10. inspection of isolation and magnetic proper.

• old design – screws • today – popular is bandage with laminate strips/binding

Testing of magnetic circuits

Targets/aims:• isolation of sheets• influence of shaving• additional power-losses in

assembly/binding structure (vibrations etc.)

How to do this?• Auxiliary windings (one loop)• Measuring of local temperature

• After 20 min• Increase after 70 min.

• Noise (50 Hz), vibrations

Big-sized transformers:

• important is binding (bandaging) after each manufacturing step!

Containers for transformersRequirements:• mechanical solidity (weight – tons)• climatic-resistant (lifetime – 15-20 years)• oil-resistant (check-in important!)• inner-tension less (without internal cracks,

stresses etc. • temperature resistivity (-30 °C 100 °C)

important is quality of welding process

Type of transformer containers

• smooth/plain surface – independent cooling by oil• wavy surface – air-flow cooling + air fans

(waves, radiators, pipe systems, harps, etc.)

testing of containers leakages - by hot oil (90°C-100°C)

Other testing techniques:

• water based testing – not convenient, possible corrosion,

• compressed air/nitrogen,

Containers for Trans.

Wavy surface and cross-section of container’s wall

Principle of cooling channels, harps etc.

Conservation = cca. 10 % of volume of container contains preservation solution or chemicals

Radiators and harps for cooling pipes of containers:

Winding of transformers (coils)Types of windings

1. Concentric (cylinder) windings• „One-stage“ or „multiple-stage“ windings• Desk/boards winding• Other types

Issues:• HW/low-voltage• cooling• Capacity of winding (minimizing)• efficiency of filling („empty window“ !!!)

Basic layouts of windings

Possible cross-section of windings

Double layer, separation by paper/plastic foil

Single layer, easy to manufacture

Permutations of layers in HV winding

Arrangement of cylinder winding

HV windingLW windingMagnetic coreSeparation between windingsRulers / barsBracing barsIsolation cylinder

Principle of „coil-winding“

Wires are turned as a multiple beam of wires

Isolation of windings

• Standard operation:– isolation layer on wires (organic/an-

organic), or– isolation based on position of wires

(Air-gap)

• Over-voltage isolation: – typically atmospheric transient

effects (10-6 s)– most critical parts – input bushing– protection – based on HV rings:

against discharges and used for forming of electrical field (not displayed on the picture)

Production of isolation components

1. Isolation binding/bandage:• hard bandage (isolation textile material is wind-up on metal

rings, cylinders etc.),• soft bandage (just strips of soft textile isolation material),• processing – covering with epoxy varnish/paint, dried and than:

– hardening in furnaces (100-200°C).

2. Wedges, insertion liners, cylinders:

• made from glass, wood, ceramics, etc.

3. Bushings:• oil-proof (also for hot oil up to 130°C)• typical material – ceramics – porcelain (up to 45 kV)

Finalization of big transformers

• binding/bandaging – very important procedure,• drying of isolation – with continuous pressing and forming of

magnetic circuits,• impregnation by varnish (epoxy, polyimide, polyester etc.),• impregnation processed by plunging into solution, drying in

vacuum:– small-sized transformers – only 1x, whole part,– big-sized transf. – only winding, 2x,– drying in temperature 130 °C for 30 hours)

• assembly of bottom chassis – mechanical support which serves as a carrier (bases),

• assembly of upper chassis – mechanical support for connections, HW bushing, outlets etc.

Testing of transformersTesting procedure follows two basic types:• Piece test – on EACH device (100 %); isolation is only

tested!; typical for mass production of small-sized transformers.

• Verifying test – more difficult; typically focused on heat transport etc.; at the beginning of production – during a development process.

Important tested parameters:• short and open-circuit voltage;• isolation protection – 4 kV between winding and magnetic

circuit (ferromagnetic core);• isolation protection – 7 kV for SAFETY transformers