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Siemens Transformer Technology SeminarInsulation & Thermal Design

Customer Technical Meeting Pomona, CA May 24 – 25, 2016

Siemens AG – Transformers

© Siemens AG 2016 All rights reserved.

Insulation & Thermal DesignWinding Selection

Kurt Kaineder - EM TR MPT GTCPage 2

Windings:



Conductors: Paper Insulated Flat Conductors

If higher mechanical stability is required:Multiple conductors with Epoxy bonding.In this case, the individual strips are insulatedfrom one another with enamel.




Conductors: Continuously Transposed Conductors (CTC)

CTC Axial twin CTC

If higher mechanical stability is required:• Strands with Epoxy bonding• CTC with intermediate paper layer




Conductors: Conductor Insulation• High electrical breakdown withstand• Thermal stability• Kraft Paper – made of long wood

fibers, purified from resin materialand lignin through a chemical process.

• The Degree of Polymerization (DP)measures the length of the fibers and is a measure for aging


Density Thermal Class

Cellulose Paper 0.7-0.85 g/cm³ 105

Cellulose Paper, Thermallyupgraded

0.7-0.85 g/cm³ 120

Calendered paper 0.95-1.1 g/cm³ 105

Calendered Paper,Thermally upgraded

0.95-1.1 g/cm³ 120

Meta Aramid (Nomex 410) 1 g/cm³ 220



Example: Net Covered CTC for lower winding gradients



Insulation & Thermal DesignInsulation Materials

Solid Insulation MaterialsBased on natural cellulose fibers – all materials can be impregnated with oil

Papervery thin material (50 ... 200 mm)electrical +++, mechanical –

Pressboardthickness 0,5 ... 8mmSheet material, angle ringssnout segmentselectrical ++, mechanical +




Solid Insulation Materials

Laminated PressboardGlued pressboard withCasein or Polyesterthickness 9 .... 120mmelectrical +, mechanical ++

Laminated Woodglued veneer from beech (or birch)with Phenolharzdifferent direction of layersthickness 10 ... 120mmelectrical -, mechanical +++




Solid Insulation Materials

Synthetic materials – cannot be oil impregnated

• Insulating varnish• Nomex• Glass fiber



Insulation & Thermal DesignTypical Insulation Structure

Example



Insulation & Thermal DesignTypical Insulation Structure

Field plot of a 123 kV transformer

Test: one phase induced

The green lines areequipotentials – they connectpoints of equal voltage. Theelectric field gradient cutsthese at 90 degrees.



Insulation & Thermal DesignPD and breakdown considerations

General cause of partial discharge and/or a breakdown is high electricalstress according the following situations:

Causes can be• Cavity not filled with oil• Contamination (metal)• Design (selection of material, failure in design)• Mechanical damage• Manufacturing outside of tolerance• Sharp edges




Cavity not filled with oil (inside glue, glass fiber, …)

• Electrical field strength higher than inside the materialEcavity ~ 2 … 5x Ematerial

• Breakdown voltage of cavity < Breakdown voltage of material• Start of partial discharge inside the cavity• Breakdown


Material with Er > 1

Er ~ 1 air, vacuum



Contamination (metal) inside material (pressboard, laminatedpressboard, KP-wood, paper)

• High field strength at the edges of the metal part, local overload• Partial discharge• Breakdown possible (dependent on the size of the metal part)


Metal


Insulation & Thermal DesignPD considerations

Causes of Partial Discharge and Breakdown – ExampleManufacturing outside of tolerance


Cylinder

High electrical field strength§ partial discharge§ breakdown

≥ 2 mm

HV-

disc

win

ding



Causes of Partial Discharge and Breakdown - Examples


Deformed angle ring

Soft paper insulation



Causes of Partial Discharge and Breakdown - Examples


Contamination with metal parts Glue in the insulation arrangement


Insulation & Thermal DesignThermal Design Considerations

Source of conductor and core heatingLoss contribution:

ContributionLoad currentOhmic resistance of winding conductor: I²·R 60-90 %eddy current losses in all metallic parts due

to the magnetic stray flux of the windings < 20 %

No-load voltageMagnetic flux Φ in core 10-30 %core steel ≈ 1 W/kgexciting current < 1 % ofrated current




Cooling types

e.g. ONANor one transformer with several conditions,

e.g.: ONAN, ONAF, OFAFformer ANSI code: OA, FA , FOA since 2006: ANSI = IEC


Oil Loop Symbol

Oil Natural ON

Oil Forced (pump) OF

Oil Directed (pump, directedflow into main windings) OD

Cooler Type Symbol

Air Natural (radiator) ANAir Forced (radiatorwith fans, or cooler)

AF

Water Forced(cooling by water)

WF



Oil flow in core type transformers:


OF: Oil Forcedoil pumps: flow into tank

ON: Oil Naturaloil flow by heated parts(buoyant force)

OD: Oil Directedoil pumps: flow directedinto windings

pumppumptop

LV HV

Cor

e

Rad

iato

r

bottom



Examples


ONAN / ONAFradiators with fans

ODAF5 Coolers



Conventional Limits as per IEEE

Thermally upgraded paper shall be used for insulation components thatdetermine the minimum life expectancy, such as: winding insulation, layer tolayer insulation, lead insulation.• The allowed Hotspot Temperature of thermally upgraded paper insulation is

30+80=110°C.• IEEE C57.91-2012 suggests that the expected lifetime at 110°C could be 15-

20 years.• The lifetime depends strongly on the moisture and oxygen content of the oil

and insulation, therefore an oil preservation system is recommended.


Limits as per IEEE C57.12.00-2010Top Oil rise above ambient 65 KWinding rise above ambient 65 KHotspot rise above ambient 80 K

Ambient Limit IEEE C57.12.00-2010Range …+40°C

oil: ≥ -20 °C

Daily average 30 °C



Standard Model


A is the top-oil temperature derived as the average ofthe tank outlet oil temperature and the tank oil pockettemperature

B is the mixed oil temperature in the tank at the top ofthe winding (often assumed to be the sametemperature as A)

C is the temperature of the average oil in the tankD is the oil temperature at the bottom of the windingE represents the bottom of the tankgr is the average winding to average oil (in tank)

temperature gradient at rated currentH is the hot-spot factorP is the hot-spot temperatureQ is the average winding temperature determined by

resistance measurementX axis indicates temperature riseY axis indicates relative vertical positions■ measured point; ● calculated point

Win

ding



Detailed simulationLoss Distribution in individual partsof the windings can be calculatedwith very high precision

On this basis calculation of theHotspot-Temperature is done, basedon the calculated winding gradients, oilgradients and outer cooling systemsimulation.




Hotspot distribution

Test


Mineral oil Synthetic Ester Natural Ester

Hot-spot rise 69.8 K Hot-spot rise 77.4 K Hot-spot rise 80.2 K



Winding to oil gradient of a CTC


14,2 K

Interface paper-oil 8,9 K

Paper Insulation3,0 K

Oilfilm 1,5 KEnamel 0,8 K

Loss W/m²

Temp

Distance



Oil Flow Concepts for the Windingssuitable for natural or directed oil flow

Test


Layer Windings withStripsDisc Windings withoutradial cooling ductsbut with axial coolingducts

Disc Windings withradial and axial coolingductsIncreased coolingsurface

Disc Windings withradial cooling ductsThe oil is guided by oilguide washers

Disc Windings withradial and axial coolingductsThe oil is guided by oilguide washers

Very efficient for ON and OD cooling


ODRecommendable for largertransformersLimit: oil velocity to avoidelectrostatical charging


Examples of temperature rises in oil and windings(same windings, losses and radiators)


7855

5739

2323

55

39 78

63

55!

45!

3535

39

37

ONHigh longitudinal rise

59

52

41

38

35

35

39

37

OFCooler top oil: low,but hotspot unchanged

Not recommendable


Siemens AG – MPT TransformersContact

Kurt KainederHead of EngineeringHead of Electrical Design GlobalTechnology Center MPTSiemens AG Oesterreich

Transformers Linz

Kraussstraße 74020 LinzPhone: +43 (5) 1707-71098Fax: +43 (5) 1707-55391

Mobile: +43 (664) 615 4946Mobile: +43 (664) 80117 71098

E-mail: [email protected]

Siemens.com/transformers

mailto:[email protected]

mailto:[email protected]

© Siemens AG 2016 All rights reserved.Page 30

Siemens US – TransformersContact

James McIverPrincipal Application EngineerE T TR US6860 Bermuda Road, STE 100Las Vegas NV 89119USA

Mobile: (702) 241-0157

E-mail:[email protected]

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