Impact of Rate of Expansion on Electric Field Stress in Stress Cones - - PowerPoint PPT Presentation

Impact of Rate of Expansion on Electric Field Stress in Stress Cones & Joint Bodies

THOMAS KLEIN

INMR World Congress 2015 Munich

Impact of rate of expansion

• n electric field stress in

stress cones and joint bodies

Dr.-Ing. Thomas Klein

Introduction Requirements on cable accessories and field controlling components Challenges in designing field controlling parts Calculation of elastomer deformation Example for deformation Consequences of deformation Impact of reduced material thickness on the electrical field stress Impact of deflector deformation on the electrical field distribution Summary

Dr.-Ing. Thomas Klein 3

Agenda

Pre-moulded slip-on parts from silicone elastomers Geometric - capacitive field control for high voltage AC

accessories

Bore smaller than cable creates expansion and

pressure (dry type creates pressure by spring arrangement, too)

Defined range of expansion is essential

minimum: pressure to ensure electrical field strength maximum: mechanical strength of elastomer

Dr.-Ing. Thomas Klein 4

Introduction

Cost efficiency

materials size / volume of field control parts less number of sizes / moulds ⇒ maximization of range of expansion

Suitable for complete range of cable dimensions and cross sections

(overlapping between adjacent sizes)

Type test / long term test no problem Trouble free performance over lifetime Noncritical installation

Dr.-Ing. Thomas Klein 5

Requirements on modern cable accessories

Detailed and accurate electric field calculation by finite element analysis

software is needed Remedy: Minimized number of sizes leads to a wide range of expansion of stress cones/joint bodies

Significant deformation of field control system FEM field calculation is not accurate any more

Solution: Field control system has to be optimized by electric stress calculation in expanded state

Dr.-Ing. Thomas Klein 6

How to fulfil design requirements?

Assumptions:

• 1. Silicon elastomer ≅ incompressible material
• 2. Negligible changes in length on expanded stress cones

(verified by tests)

Volume and length are constant

Way of calculation:

divide body in layers calculate radius change at constant surface area build new geometric shape for electric field simulation

Dr.-Ing. Thomas Klein 7

Calculation of deformation

Cylindrical body:

Dr.-Ing. Thomas Klein 8

Example for deformation - Dimensions

[mm] ¡ 0% expansion ¡ 20% expansion ¡ 40% expansion ¡ inner diameter ¡ 100.0 ¡ 120.0 ¡ 140.0 ¡

• uter diameter ¡

200.0 ¡ 210.7 ¡ 222.7 ¡ wall thickness ¡ 50.0 ¡ 45.4 ¡ 41.4 ¡ diameter inside ¡ 150.0 ¡ 164.0 ¡ 179.2 ¡

Effects of deformation

• 1. Reduction of material

thickness (here: 50 mm to 41,3 mm)

• 2. Displacement of layers

inside the body (position of red line)

Dr.-Ing. Thomas Klein 9

Example for deformation - Results

• 1. Reduction of material thickness

increased electrical field stress on surface of insulating component

• Stress cone: field stress at interface to other insulating medium
• Joint body: field stress at grounded layer
• 2. Displacement of layers inside

Changes in electrical field distribution inside stress cone or joint body

Dr.-Ing. Thomas Klein 10

Consequences of deformation

Dr.-Ing. Thomas Klein 11

• 1. Reduction of material thickness - dimensions

Deformation after 30% expansion

20% increase of electrical field stress on surface of stress cone

(after 30% expansion)

Dr.-Ing. Thomas Klein 12

• 1. Reduction of material thickness – field calculation

Dr.-Ing. Thomas Klein 13

• 2a. Deformation of earth deflector - dimensions

Deformation after 30% expansion

No increase of electrical field stress, shift in position of maximum

Dr.-Ing. Thomas Klein 14

• 2a. Deformation of earth deflector – field calculation

Dr.-Ing. Thomas Klein 15

• 2b. Deformation of HV deflector - dimensions

Deformation after 30% expansion

10% increase of maximum electrical field stress

Dr.-Ing. Thomas Klein 16

• 2b. Deformation of HV deflector – field calculation

Consideration of deformation processes due to expansion of insulating parts is

necessary for an efficient design

Expansion causes a reduction of material thickness of stress cones and joint

bodies

Expansion leads to a deformation of deflector shapes inside stress cones and

joint bodies

Optimization with regard to reduced material thickness is simple

(optimization on smallest thickness = largest expansion)

Optimization on deflector deformation is an iteration process

(minimize deviations from optimized/best field distribution)

Dr.-Ing. Thomas Klein 17

Summary

Thank You for Your Attention

thomas.klein@strescon.de