Download COLDSHRINK DISCONNECTABLE JOINTS FOR POWER NETWORKS Harry G. Yaworksi, Senior Manager, Engineering

COLDSHRINK DISCONNECTABLE
JOINTS FOR POWER NETWORKS
Harry G. Yaworksi, Senior Manager, Engineering
ENERGY BUSINESS UNIT /// COLDSHRINK DISCONNECTABLE JOINTS FOR POWER
Coldshrink Disconnectable Joints
for Power Networks
DISCONNECTABLE JOINTS ARE WIDELY
DEPLOYED IN THE UNDERGROUND POWER
NETWORKS WITHIN THE UNITED STATES.
They provide electric utilities with the ability to quickly isolate and repair
failed power cable circuits saving time and cost. This paper provides a
synopsis of a new coldshrink disconnectable joint recently developed
for the utility market with higher reliability, innovative features and
smarter and faster connectivity.
Introduction
Figure 1. Congested Manhole
Electric utility power grids consist of thousands of circuit miles of underground medium voltage 15,000-35,000 volt distribution cables which carry
power to customers within their service area. In large urban networks the load density is very high and underground power cables supplying this load
are densely packed into subsurface conduits and manholes and are often continuously submerged. Manhole working space is congested and the
number of components and installation space required to assemble the joint must be minimized.
Disconnectable joints deployed in these congested manholes are often bent at severe angles during installation. Bent joints create stresses between
the cable and joint housing which can weaken the moisture seals and lead to premature failure. To further complicate this problem workmen need to
physically move cables and joints as part of their normal operating procedures which can further weaken the seals. This has produced an industry
failure rate for disconnectable joints which is much higher than non-disconnectable joints. A typical manhole is depicted in Figure 1.
Existing Technology
Existing disconnectable joints are constructed from a central disconnectable bus which is connected to a plurality of high voltage cables as depicted in
Figure 2. The central bus is constructed from an aluminum busbar which is overmolded with an ethylene propylene diene monomer (EPDM) rubber
primary insulation and conductive rubber shield.
Disconnected Bus
High Voltage Cables
Cable Adaptor
Figure 2. Busbar and High Voltage Cables
ENERGY BUSINESS UNIT /// COLDSHRINK DISCONNECTABLE JOINTS FOR POWER
EPDM rubber sleeves are then pushed over the high voltage cables, cable adaptors, and the insulated busbar to complete the installation. Figure 3
shows one of the rubber sleeves installed over the cables and onto the insulated bus. Figure 3 also illustrates the inherent problem with the existing
design. When the sleeve is slightly bent as often happens in congested manholes the seal is broken and water is allowed to migrate into the electrical
interfaces. Even a slight amount of moisture migrating into an electrical interface can lead to failure in a high voltage joint.
Broken Seal
Figure 3. EPDM Rubber Sleeve
Coldshrink Technology
Coldshrink jointing technology has been successfully deployed in the non- disconnectable joint market. Application of this technology was considered
to address the failure mode seen in existing disconnectable joint designs. Coldshrink joint sleeves are multi-stage injection molded from liquid silicone
rubber (LSR). The coldshrink sleeve is stretched diametrally in the factory and expanded onto a spiral holdout. During installation the holdout is
removed and the sleeve recovers tightly onto the cable substrate. Figure 4 illustrates the sleeves unexpanded (as molded), expanded, and installed.
It was theorized that the combination of a soft flexible sleeve with a high interference fit would provide a tight and resilient moisture seal that remains
secure even when the joint and cable is bent.
Spiral Holdout
Unexpanded
Expanded
Figure 4. Coldshrink Sleeves
ENERGY BUSINESS UNIT /// COLDSHRINK DISCONNECTABLE JOINTS FOR POWER
Installed
Coldshrink Disconnectable Joint Design
The new coldshrink disconnectable joint design addresses the primary customer issue of reliable performance on the bent cable found in congested
underground power networks. Additional benefits include a simplified installation process, reduced installation force, and a reduction in the number of
components.
The new coldshrink disconnectable joint is a hybrid of the existing non-disconnectable joint with design modifications that allow it to be installed on
disconnectable busbars.
A shielded high voltage disconnectable joint housing is multi-stage injection molded from highly elastic, high dielectric strength liquid silicone rubber
(LSR). A faraday cage is incorporated in the center of the joint to ensure that entrapped air over the connector region is not subjected to electrical
stress. A stress cone is incorporated at the end of the joint to minimize the electric stress at the cable shield terminus. The faraday cage and stress
cone are molded from semi-conductive LSR which has a volume resistivity of approximately 100 ohm-cm.
The insulation thickness and the shape and relative position of the faraday cage and stress cone are optimized through the use of electrical field
modeling software. The molded housing is shown in Figure 5.
Insulation
Faraday Cage
Stress Cone
Figure 5. Molded Disconnectable Joint Housing
The coldshrink disconnectable joint housing is designed to fit over existing industry standard disconnectable busbars. The joint housing is shown
installed over the disconnectable busbar and positioned over the high voltage cable in Figure 6. The joint housing is expanded on a spiral holdout so it
can be easily installed over the cable and onto the busbar.
Figure 6. Joint Housing during Installation
The disconnectable joint design incorporates all of the splicing functions into one joint body. The joint body, jacket seal and copper grounding mesh is
integrated into a single unit as depicted in Figure 7.
Figure 7. All-in-one Design
ENERGY BUSINESS UNIT /// COLDSHRINK DISCONNECTABLE JOINTS FOR POWER
Removal of the holdouts allows the pre-stretched disconnectable joint to recover with high sealing force during installation. The completed installation
is shown in Figure 8.
Figure 8. Completed Installation
The disconnectable joint design is currently being tested to the industry requirements of the medium voltage joint standard IEEE 404 and the separable
connector standard IEEE 386.
Conclusion
The faster and smarter connectivity of the new coldshrink disconnectable joint design has resulted in excellent customer acceptance and promises to
make TE Connectivity a market leader in this segment of the market.
About the Author
Harry Yaworski received his BS degree in physics in 1984 from Moravian College in Bethlehem, Pennsylvania. From 1984 to 1992 he worked as a
Product Development Engineer and Engineering Manager for Elastimold. He joined Raychem in 1992 as a Project Manager and is currently the
Senior Manager, Engineering in the TE Connectivity /Energy Division. His areas of expertise include the invention and design of power cable
accessories and computer analysis of electric fields. He currently holds 20 US patents for cable accessories and surge arrester products.
ENERGY BUSINESS UNIT /// COLDSHRINK DISCONNECTABLE JOINTS FOR POWER
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ENERGY BUSINESS UNIT /// COLDSHRINK DISCONNECTABLE JOINTS FOR POWER