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 transformers Requirements: mechanical solidity (weight – tons) climatic-resistant (lifetime – 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 winding LW winding Magnetic core Separation between windings Rulers / bars Bracing bars Isolation 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 ( °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 transformers Testing 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