Discussion Group C32D:
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 61914 ‘Cable cleats for electrical installations’
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
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
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.
Available methods of cable and joint constraint
Methods Available Comments
1. Straight cable and joint: rigidly
cleated 1. Risk of movement of cable core through joint insulation 2. Offset cable and joint: flexibly
cleated + expansion loop
2. Risk of cyclic fatigue of sheath and catastrophic slipping of ‘recoil’ cleat
3. Offset cable and joint:
rigidly cleated* 3. No core movement in joint Minimal cyclic sheath strain
Method 1:
Rigidly cleated straight cable and joint
Method1: Has risk of internal core movement in
joint mold and external movement in cleats
Loss of shielding Void formation at interface
Methods 2 and 3: Use a cable offset to lock the
XLPE core: two ‘S bends’
Method 2. Flexible expansion loop:
Uses a sprung ‘recoil’ cleat
Low cable thrust
Cable moves into vault
from duct
from duct
Method 2: Flexibly cleated expansion loop:
Risks
Joint
Duct Cable
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
Method 3: Constraint system for an anchor joint
(NSpan method)
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Joint Types
Pre-mold one-piece
joint (OPJ)
Prefabricated composite
joint (PJ)
Cast epoxy resin insulation Molded rubber insulation
Non-anchor Joint
Cleat types
• 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
Need to calculate:
For both conventional Non-Anchor joint (pre-molded one piece joint) and Anchor Joint (prefabricated)
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
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.
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
(duct-manhole systems)
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
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
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
