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 on electric field stress in stress cones and joint bodies Dr.-Ing. Thomas Klein

Agenda � 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

Introduction � 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

Requirements on modern cable accessories � 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

How to fulfil design requirements? � 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

Calculation of deformation 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

Example for deformation - Dimensions Cylindrical body: [mm] ¡ 0% expansion ¡ 20% expansion ¡ 40% expansion ¡ inner diameter ¡ 100.0 ¡ 120.0 ¡ 140.0 ¡ outer diameter ¡ 200.0 ¡ 210.7 ¡ 222.7 ¡ wall thickness ¡ 50.0 ¡ 45.4 ¡ 41.4 ¡ diameter inside ¡ 150.0 ¡ 164.0 ¡ 179.2 ¡ Dr.-Ing. Thomas Klein 8

Example for deformation - Results 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

Consequences of deformation 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

1. Reduction of material thickness - dimensions Deformation after 30% expansion Dr.-Ing. Thomas Klein 11

1. Reduction of material thickness – field calculation � 20% increase of electrical field stress on surface of stress cone (after 30% expansion) Dr.-Ing. Thomas Klein 12

2a. Deformation of earth deflector - dimensions Deformation after 30% expansion Dr.-Ing. Thomas Klein 13

2a. Deformation of earth deflector – field calculation � No increase of electrical field stress, shift in position of maximum Dr.-Ing. Thomas Klein 14

2b. Deformation of HV deflector - dimensions Deformation after 30% expansion Dr.-Ing. Thomas Klein 15

2b. Deformation of HV deflector – field calculation � 10% increase of maximum electrical field stress Dr.-Ing. Thomas Klein 16

Summary � 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

Thank You for Your Attention thomas.klein@strescon.de