Overview of the development of new materials and technologies for power cable accessories

With the acceleration of urbanization in China and the improvement of urban and rural people's living standards, the proportion of cable applications in cities, especially large and medium-sized cities, is getting higher
and higher.
As a result, the strong demand for cable accessories products has also promoted the speed of
product development and improvement.

First, the role of power cable accessories

At the power cable terminal and joint, due to the disconnection of the cable metal sheath and shielding layer, the electric field distribution is much more complex than the cable body, and there is axial stress in the electric field at the cable termination, so it is necessary to use cable accessories to achieve continuous and connected cables, that is, a connection device
that can meet certain insulation and sealing requirements.

The cable has four main structural layers such as conductor, insulation, shielding and sheath, and the cable terminal and intermediate joints in the cable accessories as part of the cable line must be continued respectively, and the conductor connection is good, the insulation is reliable, the sealing is good and sufficient mechanical strength is achieved to ensure the quality of the cable terminal and cable joint, in order to ensure the power supply reliability
of the entire cable distribution network.

Second, the classification of medium voltage cable accessories of 35kV and below

At present, according to the main insulation molding process, the commonly used 35kV and below cable terminations and intermediate joints can be mainly divided into four common product types: wrapped type, heat shrinkable type, cold shrinkable type and prefabricated type
.
There are also cast and detachable cable accessories for specific product
ranges.

2.
1 Wrapped cable accessories Wrapped cable accessories refer to cable accessories made of insulation and shielding are made of strip wrapping, and the stress control mode adopts stress cone or high dielectric constant stress control belt, which has high requirements for the construction personnel's process level, and the construction process is more complicated, and has been gradually eliminated
by the site.

2.
2 Heat shrinkable cable accessories use the principle of "elastic memory" effect of polymer to develop various heat shrinkable pipes, branch sleeves, rain skirts and other heat shrinkable preforms, which are installed at the end or joint of the treated cable according to the program and heated to make them shrink and tighten in the desired position
.

Heat shrinkable cable accessories are simple to process, inexpensive and easy to maintain
.
However, there are also problems such as shortening the service life caused by the inevitable "respiration" of ambient temperature changes, which affects the reliability
of power supply.

2.
3 Cold shrinkable cable accessories The current engineering application of cold shrinkable tube and other cold shrinkable prefabricated parts, is silicone rubber or EPDM rubber as the main raw material, after special formula synthesis, pre-expansion on the spiral support core line formed, installation and use, without any external heat source, as long as the support core wire will shrink, and tighten in the
required position.

Cold-shrinkable cable accessories must be used within the specified period of use, and the price is high
.

2.
4 Prefabricated cable accessories use rubber materials to mold the reinforced insulation and semi-conductive shielding layer in the cable accessories into a whole or several components in the factory, and the cable accessories formed by the on-site set at the end or middle joint of the treated cable are called prefabricated cable accessories
.
Prefabricated cable accessories can be routinely tested at the factory, further improving the operational reliability of cable terminations and greatly reducing the on-site installation effort
.
In terms of structure, cold shrinkable cable accessories and prefabricated cable accessories are basically the same, but there are some differences
in material properties and handling.

2.
5 Cast cable accessories use thermosetting resin materials to be cast on site in the mold or box at the end of the treated cable or joint, and the cable accessories
are formed after curing.
It is especially suitable for the interconnection
of plastic extruded insulated cables and impregnated insulated cables.

2.
6 Detachable connectors allow cables and other equipment and fully insulated cable accessories to be connected or disconnected by cables, such as terminals, connectors and branch joints
.
The main objects of use of separable connectors are fully sealed switchgear, inflatable ring network units, cable branch boxes and other equipment, which can be regarded as a new type of cable termination
.
And more use of the structure
of prefabricated terminals.
It is not difficult to see from the above classification of cable accessories that prefabrication and cold shrinking technology that can simplify on-site installation requirements has occupied an important position
in the development of cable accessories.

Third, the new technology of cable accessories products

3.
1 The basic technical requirements for conductor connection are: good conductor connection: for the terminal, the cable conductive core and the outlet rod and terminal should be well connected; For intermediate connectors, the cable conductor should be well connected to the connecting pipe
.
That is, the contact resistance of the connection point is required to be small and stable
.
Compared with the resistance of the same length and section wire, the new installation ratio should not be greater than
that.

At present, the site mostly adopts crimping technology, which requires certain professional equipment
.
Using bolted connection technology, only a torque wrench is required during on-site construction to meet the requirements
of the basic technology of conductor connection.
This simplifies the professional requirements
for installing equipment.
Moreover, for aluminum cable connections that require the removal of non-conductive oxide layers, there is no need for special deoxide treatment
in advance.

3.
2 insulation material EPDM rubber and silicone rubber material have been widely used in prefabricated cable accessories, in the field of high-voltage cable accessories, comparatively speaking, can be divided into silicone rubber as the base of European structure and EPDM rubber as the base of Japanese structure
.
In the medium voltage sector, improved EPDM cable accessories offer better mechanical strength and tear resistance than their silicone rubber counterparts
.

3.
3 On-site pre-expansion On-site pre-expansion technology is also an installation method for cold-shrinkable cable accessory products, that is, within a few hours before installation, the cold-shrinkable joint is pre-expanded
on site using simple disposable tools.
It avoids some of the performance loss of conventional shrinkable products during transport and long-term storage, thus providing a more constant, durable and reliable radial pressure
.
It is particularly suitable as a spare part for systems
.

In short, in order to facilitate on-site installation and ensure installation quality, reduce the excessive requirements
for the technical level of on-site installers.
Prefabrication and simplification will become the main development direction of cable accessories technology in the future, and cold-shrinkable prefabricated cable accessories that can be tested before leaving the factory will become the mainstream of the site, so as to further ensure the safety and reliability of power system operation and achieve the best combination
of technology and economy.