Installation of High Voltage (HV) Cable Systems

Introduction and background

The recommendations adopted in TB 194 were prepared considering the conductor cross-sections available at that moment, mainly up to 1200 mm². The use of cables with extruded insulation and larger cross sections has become usual in the recent years and it appears that the effects of the thermo-mechanical forces on extruded insulation and conductor cross-sections considered 20 years ago need to be better considered.

With the purpose of studying in detail the above issue and give recommendations about the approach to be used when dealing with extruded insulation and large conductor cross-sections, a dedicated Working Group (B1.34) was established, which produced in 2016 the Technical Brochure TB 669 “Mechanical forces in large cross section cable systems”.

During their 2016 plenary Annual Meeting (August 23^rd, 2016), the members of SC B1, following a proposal from the Strategic Advisory Group, unanimously decided to launch a new Working Group, entitled WG B1.61 (Installation of HV Cable Systems) to update the TB 194, considering the work of several Working Groups/Task Forces and also considering the work done by WG B1.34.

Technical Brochure scope and document structure

The new Technical Brochure includes an update of the civil construction and cable installation techniques according to state-of-the-art methods, a collection of design rules for calculating thermo-mechanical forces and pulling forces and includes discussions regarding electrical system design. The revisions also took into consideration recent CIGRE SC B1 works such as TB 640, TB 669, TB 770, TB 714, TB 841, and TF B1.53. WG B1.61 also conducted a survey which covered the various construction and cable installation aspects adopted by the industry over the past 10 years.

TB 194 differentiates between “construction techniques” and “installation techniques” which were defined as follows:

• “Construction Techniques” are techniques used to create the cable route, mainly covering the civil works such as trenching; and
• “Installation Techniques” are aspects related to cable system design and cable installation methods.

WG B1.61 discussed the above definitions from TB 194 and the possibility of leaving them separated under different sections in the new TB or whether to combine them into a common chapter. With the aim of treating both the construction technique and the installation technique for each specific topic, it was decided that the structure of the new TB would combine them into one common chapter named “CIVIL CONSTRUCTION AND CABLE INSTALLATION TECHNIQUES”, separating the thermo-mechanical design and the pulling calculation topics into a separate chapter named “SYSTEM DESIGN PRINCIPLES”.

The new Technical Brochure is broken down into the following chapters:

• CHAPTER 1: Introduction
• CHAPTER 2: Materials description
• CHAPTER 3: Civil construction and cable installation techniques
• CHAPTER 4: System design principles
• CHAPTER 5: Appendixes

Below is a brief description of each chapter.

CHAPTER 1: INTRODUCTION, provides a brief description of the background for the new TB, the terms of reference used, outlines the structure of the new TB, briefly compares the new TB to the previous TB 194, and provides a high-level description of the market survey performed as part of the work. The results of the survey are outlined in Appendix D of the new TB.

CHAPTER 2: MATERIALS DESCRIPTION, provides an overview of the various components which make up a high-voltage underground cable system, how the components are related to one another, which civil construction and installation technique they are applicable to, and further discusses the importance of a high level of quality assurance and personnel training.

CHAPTER 3: CIVIL CONSTRUCTION AND CABLE INSTALLATION TECHNIQUES, discusses the various civil construction and cable installation methods used for high voltage cables systems and includes, with respect to TB 194, some new technologies as well as a list of hazards for high voltage cable installation, which may arise from poor design and/or installation techniques.

Chapter 3 is broken down into the following sub-sections: Cable Reel Handling, Direct Burial, Ducts & Duct Banks, Troughs, Tunnels / Culverts, Shafts, Horizontal Directional Drilling, Microtunneling/Pipe Jacking, Pipe Ramming, Mechanical Laying, Bridges and Water Crossing, Innovative Techniques, Temporary Installation, Interfaces, and Hazards caused by inappropriate civil works or installation design.

Each sub-section listed above starts with an overview of the specific method, providing a high-level description of the method, a list of the installation sequence, a description of typical application in which the method is used, and a table of pros and cons.

CHAPTER 4: SYSTEM DESIGN PRINCIPLES, provides some guidelines for the design of high voltage electrical systems, such as ratings and short circuits, metallic sheath bonding, EMF principles and shielding systems, corrosion, pulling force calculation and thermo-mechanical design, including cleating systems.

A section relevant to manholes and joint bay design has also been added.

A schematic block diagram of the design phases is included, to provide the readers with a comprehensive and quick guide of the necessary steps to be followed.

Conclusion

This new Technical Brochure is the result of an update of the civil construction and cable installation techniques according to state-of-the-art methods, a collection of design rules for calculating thermo-mechanical forces and pulling forces, with some general rules about HV cable system design.

Compared to TB 194, new open trench and pipe installation techniques have been included in the new TB.  Updated information on trenchless technologies derived from TB 770 has also been added, as well as a section on temporary installations.

A specific survey was conducted, and the results are included under the Appendices, showing the updated trend of the installation techniques in various countries.

The pulling calculation section has also been implemented with consideration given to pulling of multiple cables in one duct and also in long HDD sections.

Regarding thermo-mechanical forces, this new TB provides recommendations about design approaches when dealing with rigid and flexible cable systems, for duct installations, and for transitions between rigid and flexible installations. Thermo-mechanical forces involved with large conductor XLPE cable systems can generate high axial thrust and tension and/or significant cycling movements in the installed cable system. The new TB describes differences between installation designs and gives recommendations for each one, defining the main parameters to be considered and giving some formulas to help in the solution of the problem. Input cable data, such as the Linear Expansion Coefficient (α), the Effective Axial Stiffness (EAeff), and the Effective Bending Stiffness (EIeff), need to be obtained either by cable manufacturers or by measurements on full size cable samples. Nevertheless, due to the complexity of the physical phenomena involved, the suggested formulas, although valid, need to be carefully applied with the appropriate coefficients.