Discussion Group C32D: Support of transmission cables in manholes

Transcription

1 Discussion Group C32D: Support of transmission cables in manholes IEEE Insulated Conductors Committee Meeting May 22-25, 2011 St. Petersburg Beach, Florida 1

2 Agenda This Meeting Introduction Review of Fall 2010 meeting minutes and past presentations Update on CIGRE activities Dennis Johnson Review of possible cable and joint support methods. Brainstorming of content for a guide Discussions on the Straw man guide Volunteers to help with adding to the Straw man guide and Bibliography Help with obtaining a copy of IEC Cable cleats for electrical installations Suggestions for future meetings 2

3 Introduction Scope: To provide a forum for discussion of factors to be considered when designing engineered constraining systems for single core XLPE transmission cable sytems joints in manholes. Mission: To consolidate information from suppliers, consultants, utilities and researchers on best practices for constraining single core transmission cables with joints in manholes. To explore related subjects on influencing factors, such as adjacent duct configurations and geometries. To eventually consider other cable and joint configurations, such as in tunnels, vertical shafts and transitions to direct burial. To eventually summarize the consolidated information in an IEEE Recommended Practice or Guide. 3

4 Agenda Background and Past Meetings (1) Background to Discussion Group This is the 4th formal meeting, following an initial ad hoc meeting Past discussions favored an approach based on Factors to consider, and Performance, rather than providing precise analytical solutions. An extensive bibliography will help those who want to learn more. We have been counseled to go slow before officially transitioning to a Working Group. Overview of presentations from previous meetings: Heating of Cable Supports Neal Parker, Puget Sound Energy Experience with cable support systems, Forest Rong, Black & Veatch Update on activities by Cigre B1.34, Mechanical forces in large cross section cable systems, Dennis Johnson, Power Engineers 4

5 Agenda Background and Past Meetings (2) Overview of presentations from previous meetings (continued): Experience with NSpan Vault Design in the Thermo-mechanical Design of XLPE Insulated Duct-manhole and Pipe Systems, Grian Gregory, Cable Consulting International Support of Transmission Cables within Manholes, Casey Spradlin, Southwire Support of Transmission Cables Within Manholes a personal view, Henk Geene, Prysmian Supporting Structures in Manholes, Pierre Argaut, Silec/General Cables Testing of TMB Models for Extruded Dielectric Cables, Steve Eckroad, EPRI. 5

6 Available methods of cable and joint constraint Methods Available 1. Straight cable and joint: rigidly cleated 2. Offset cable and joint: flexibly cleated + expansion loop 3. Offset cable and joint: rigidly cleated* Comments 1. Risk of movement of cable core through joint insulation 2. Risk of cyclic fatigue of sheath and catastrophic slipping of recoil cleat 3. No core movement in joint Minimal cyclic sheath strain * NSPan method 6 6

7 Method 1: Rigidly cleated straight cable and joint Plan view of manhole 7 7

8 Method1: Has risk of internal core movement in joint mold and external movement in cleats FR Void formation at interface Loss of shielding 8 8

9 Methods 2 and 3: Use a cable offset to lock the XLPE core: two S bends 9 9

10 Method 2. Flexible expansion loop: Uses a sprung recoil cleat Low cable thrust from duct Cable moves into vault High cable thrust from duct 10 10

11 Method 2: Flexibly cleated expansion loop: Risks Joint Duct Cable Risk: catastrophic axial slip of recoil cleat causing cable buckling Risk: cyclic fatigue of cable sheath in expansion loop 11 11

12 Method 3: Constraint system for a non-anchored joint - Rigidly cleated cable offset (NSpan method) FR: Absolute force, cable thrust from right FL: Absolute force, cable thrust from left FT = (FR FL) : Differential force to be withstood by cleats 12 12

13 Method 3: Constraint system for an anchor joint (NSpan method) 13 13

14 Joint Types Non-anchor Joint Pre-mold one-piece joint (OPJ) Molded rubber insulation Cast epoxy resin insulation Anchor Joint Prefabricated composite joint (PJ) 14 14

15 Cleat types 1. Cleat types: Clamp cleats normal Clamp cleats high strength 2. Cleat parameters Slip withstand strength Rubber liner axial stiffness 15 15

16 Need to calculate: For both conventional Non-Anchor joint (pre-molded one piece joint) and Anchor Joint (prefabricated) Minimum vault dimensions for optimum constraint strength Cable thermo-mechanical forces: FR, FL and FT S-bend dimensions: bend radius, offset and length Overall constraint system dimensions Cleats: number and spacing Stability: Whether the constraint system will slip en-masse The number of individual cleats that slip Whether the cable spans between cleats are within design limits of: Bending radius Cyclic fatigue Span length 16 16

17 Some factors to consider Manhole size (minimize civil costs versus cable comfort) Flexible versus fixed cables and joints Cyclic strain on sheath materials and fatigue life High thermal expansion of extruded insulations Permanent distortion of extruded insulation at clamps and bends Quantification of axial forces from adjacent ducts Control of axial forces from adjacent ducts Quantification and control of non-symmetric axial forces from adjacent ducts Tolerance of joints to axial and bending forces Tolerance of joints to internal core/stress cone movement Provisions for cables on inclines Testing of key mechanical parameters and standard test protocols Hardware considerations Modeling, calculations and simulations 17

18 More factors to consider (2) Implications of different sheath/shield designs. Requirements to prevent the inside of pre-moulded joints from migrating with respect to the outer casings. Situations requiring cable anchor joints (where the joint core is effectively bolted to a manhole wall). Main parameters provided for the manhole system: Bending radius limits not exceeded Sheath fatigue strain limits not exceeded (what are they for various sheath designs?) Sidewall force limits not exceeded Methods to calculate (or conservatively estimate) similar parameters for cables in the ducts, where they form tight helixes at high temperatures. 18

19 Table of Contents for Application Guide: Support of transmission cables in manholes (Straw Man) 1 Introduction & Scope Physical forces in vaults Duct and pipe systems Absolute Differential Tunnels and direct buried systems Methods for restricting cable movement inside vault (ductmanhole systems) Straight cable and joint: rigidly cleated Offset cable and joint: flexibly cleated + expansion loop Offset cable and joint: rigidly cleated Other 19

20 Table of Contents for Application Guide: Support of transmission cables in manholes (Straw Man) 2 Reducing forces inside the vault (duct-manhole systems) Duct diameter Route geometry & vault location (route planning) Cable design factors to consider Cyclic strain & fatigue life of insulation components Bending radius limits: in duct and in vault Operating temperature (above glass transition) Implications of different sheath/shield designs Joint design factors to consider Tolerance to axial and bending forces Tolerance to internal core/stress cone movement 20

21 Table of Contents for Application Guide: Support of transmission cables in manholes (Straw Man) 3 Cleat design factors to consider High thermal expansion of extruded insulations Non-linear increase of force along cable in vault Small slipping and resultant oversheath wear Vault design factors to consider Size and location Support structures Corrosion Modeling, calculations and simulations Standard test protocols & key mechanical parameters 21

22 Purpose of Document (as developed by May 24, 2011 meeting attendees) Scope: To provide guidelines for designing constraining systems for single core extruded transmission cable systems in ducts and manholes. Mission: To consolidate information on best practices for constraining single core extruded transmission cables with joints in manholes. To explore related subjects on influencing factors, such as adjacent duct configurations and geometries. 22