3M QT-III 7640-T-HSG Cold Shrink Termination Kit

Overview

The 3M QT-III 7640-T-HSG series, together with the larger 7650-T-HSG series, are 3M Cold Shrink silicone rubber termination kits for terminating shielded solid dielectric power cables rated 5 through 35 kV indoors and in weather-protected enclosures. Each kit contains a one-piece, non-skirted, tracking resistant silicone rubber termination with High-K stress control, built-in top sealing compound and a high-ampacity ground assembly, and is qualified as an IEEE Standard 48 Class 1 termination. The 3M QT-III 7640-T-HSG design is chosen when the cable screen must be grounded through a braid capable of carrying high fault current.

Seven Star LLC supplies 3M QT-III 7640-T-HSG and 7650-T-HSG kits to substation contractors, switchgear and transformer installers, oil and gas and petrochemical plants, and industrial estates across the Sultanate of Oman and the GCC. We provide the 3M data sheet, check kit selection against your cable data, and offer engineering support from our Muscat office for projects in Sohar, Duqm, Salalah and the interior.

Working Principle

When the metallic screen of a shielded cable is cut back for termination, the electric field that was confined inside the insulation spreads out at the screen edge and concentrates there; without stress control, that concentration would cause partial discharge and eventual failure. The 3M QT-III 7640-T-HSG uses capacitive stress control based on High-K materials with a dielectric constant greater than 15 (typically 22 for the EPDM stress control tube and 25 for the stress controlling compound). These materials refract the electrical flux and spread the voltage stress along the termination length beyond the screen cut.

3M explains that stress in a continuous shielded cable is typically about 50 V/mil next to the shield and about 70 V/mil at the conductor, and that the termination reduces stresses to below those in the continuous cable while keeping stress on the insulator surface below 15 V/mil at rated voltage. Outside the stress control, the dark gray silicone rubber insulator provides tracking resistance and hydrophobic recovery, so surface contamination is less able to form conductive paths. The result is a compact termination with high power frequency and impulse performance.

Engineering Design of the 3M QT-III 7640-T-HSG

The complete assembly of insulator, High-K stress control tube, conformable stress controlling compound and environmental top sealing compound is prestretched at the factory onto a removable core. On site, the installer prepares the cable, places the termination and unwinds the core, and the silicone shrinks into place with no heat source and no force fit. The pre-formed ground braid, constant force spring, Scotch Mastic Strip 2230 and copper foil shielding tape 1181 strips in each kit connect and seal the cable screen. The kits are used with 3M Scotchlok compression lugs, stem connectors or QL2 mechanical shearbolt lugs, ordered separately.

The terminations accept cables with a maximum operating temperature of 105°C and an overload rating of 140°C, and their current rating meets or exceeds that of the cable. Maximum dimension and wet creepage distance are 330 mm for the 7640-T-HSG-110 kits and 419 mm for the 7650-T-HSG-150 kits. The ground braids are twin tail designs of 43,200 CMA on the smaller kits and 76,800 CMA on the 7645, 7646, 7655 and 7656 kits, which the data sheet charts against short circuit current and duration. Selecting the 3M QT-III 7640-T-HSG kit with the right braid is therefore part of the fault level check, not only the cable size check.

Key Features

3M lists the following benefits for the 3M QT-III 7640-T-HSG and 7650-T-HSG kits.

• Conforms to IEEE Standard 48 Class 1 requirements for 5, 8, 15, 25/28 and 35 kV terminations.
• One-piece design that covers a wide range of cable sizes with few kit numbers.
• Cold Shrink delivery: place the termination and unwind the core, with no heat and no force fit.
• High-K capacitive stress control spreads the electric field over the full insulator surface.
• High-ampacity pre-formed ground braid for demanding screen fault current duty.
• Compact length for installation in restricted switchgear and transformer spaces.
• Tracking resistant, hydrophobic dark gray silicone rubber insulator.
• Compatible with PE, XLPE and EPR insulations and tape, wire-over-tape and LC shields.

Industrial Applications

The data sheet lists switchgear, transformer, motor lead, bus and similar connections, in contaminated and non-contaminated indoor locations, free-hanging or bracket-mounted, upright or inverted. In Oman the 3M QT-III 7640-T-HSG fits typical medium voltage work such as terminating 11 kV and 33 kV feeder cables into indoor ring main units and switchboards, connecting large motors on pump and compressor drives, and terminating cables on dry-type and oil-filled transformers in desalination plants, oil fields and industrial estates. Terminations made with the 3M QT-III 7640-T-HSG can be field-tested by normal cable test procedures referenced to IEEE Standard 400.

As general guidance, the voltage class chosen must be equal to or greater than that of the cable, and outdoor exposed terminations need a Class 1 outdoor (skirted) design rather than this indoor tubular series. Seven Star LLC supplies 3M QT-III 7640-T-HSG kits alongside the equipment they connect to, including protection relays such as the ABB RET615 transformer protection relay, breakers like the ABB SACE Emax 2 air circuit breaker and MV components such as the Crompton Greaves 17.5 kV porcelain bushing.

Technical Specifications

The 3M QT-III 7640-T-HSG data below is taken from the 3M data sheet for the 7640-T-HSG and 7650-T-HSG series (October 2016); material values are typical.

How to Order

To order 3M QT-III 7640-T-HSG or 7650-T-HSG kits for a substation, switchgear or motor project in Oman or the GCC, send Seven Star LLC your cable voltage, conductor size, insulation diameter and quantity for a formal quotation. We will share the 3M QT-III 7640-T-HSG data sheet, confirm kit selection and arrange engineering support from our Muscat office.