4.8 Low temperature operation
4.8.2 Low temperature risks
Prospective low temperature risks are listed below in decreasing order of ranking:
Loss of radial contact of a rubber pre-moulded insulation component in a joint or termination with the cable XLPE insulation.
Disturbance of the interface with the rubber component by longitudinal retraction of the cable insulation.
Cracking of epoxy resin insulating components containing embedded metalwork.
Splitting of premoulded rubber components.
Failure of the conductor connectors in terminations and joints, due to increased thermomechanical retraction forces, noting that the large 2,500 mm2 conductor size will develop high retraction forces.
Failure of the watertight seals between the metallic sheath and the accessory casings due to thermomechanical retraction, resulting in water entry and/or loss of electrical continuity of the screen conductors.
Low temperature risks for joints
In prefabricated joints, both the XLPE cable insulation and the rubber premoulded joint insulation have ‘glass transition’ temperatures at which they become rigid, brittle materials of high stiffness (modulus). It is essential that the premoulded rubber insulation retains sufficient elasticity, softness and compressive deflection, such that it is able to continuously maintain intimate contact with the XLPE cable insulation when both cables cool to low ambient temperature. If intimate contact is lost a void between the cable and the accessory will form in which incipient electrical distress in the form of partial discharging will occur leading to complete electrical failure of the accessory. The commencement of the change from an elastomeric to a brittle state is not easily predictable from bench top tests on small material samples, as it depends upon the particular loading conditions. Having evaluated the material it is recommended that Proving Tests be performed on the complete accessory assembled on the cable whilst voltage energized under a range of operating conditions e.g.:
Sustained minimum ambient temperature
Cyclic temperatures
Transient temperatures,
Each of the above loading conditions should be performed both with and without axial thermomechanical loads applied by the adjacent cables to the joint.
The joint protection that surrounds the metallic casing is at risk of disturbance, cracking or loss of adhesion. The international test specification IEC 62067[1] gives tests on outer protection for buried joints. The test comprises the application of 20 temperature cycling whilst immersed under one metre head of water, followed by electrical tests on the cable jacket, joint protections, sheath interrupter insulation and bonding lead. The maximum temperature is within 15 to 20oC of the design temperature of 90oC (i.e. 70 to 75oC) and the minimum temperature is within 10oC above the ambient of the test house (unspecified). It is recommended that the method of test be amended to cool the joint down to -20oC during temperature cycling.
Low temperature risks for terminations
The low temperature operation risks are greater in an outdoor termination, because of the -50oC ambient temperature. The cables can either be terminated directly into an air insulated termination or into metalclad gas insulated switchgear terminations (GIS). (The abbreviation GIS should be taken to refer to metalclad gas filled terminations as these may or may not be connected to switchgear).
prevent swelling of the polymeric cable and accessory insulation a low viscosity hydrocarbon oil is not selected. Either silicone fluid or a PIB fluid is used. This design has the longest service experience at 500 kV. Low temperature risks are that a) the insulating fluid would become too viscous to maintain impregnation of the capacitor cone and b) that thermal contraction may disturb the insulating components and fluid seals.
‘Stress cone and insulator’. A premoulded rubber stress cone insulator is stretched and slid over the cable insulation. A porcelain or composite insulator is fitted and filled with insulating fluid. Low temperature risks are the same as those described for ‘capacitor cone and insulator’ termination and the prefabricated joints.
‘Prefabricated stress cone and insulator’, Figure 62. This is a variant of the
‘prefabricated and insulator’ type. Firstly, a premoulded rubber stress cone insulator, as described above is fitted. An epoxy resin casting with a conical bore is then fitted above the stress cone. The rubber stress cone is inserted and held into the conical bore by a bank of helical metallic springs. The purpose is a) to increase the contact pressure at the critical moulding/cable interface and b) to replace the fluid in the high electrically stressed zone around the stress cone with high strength epoxy resin. Low temperature risks are the same as those described for ‘capacitor cone and insulator’ termination and the prefabricated joints.
Figure 62. Outdoor termination with prefabricated composite, premoulded stress cone
The following three designs exist for 500 kV GIS terminations. The low temperature risks are the same
resin casting with a conical bore. The rubber stress cone is inserted into the conical bore and held in intimate contact with the interfaces by a bank of helical metallic springs. The design does not require to be filled with dielectric fluid and so is named a dry design. The advantage is that no monitoring or maintenance checks is required to check that the dielectric fluid is at the correct pressure and has not leaked either into the cable or into the environment.
Low temperature risks for the XLPE cable
The cable remote from the accessories is considered to have a good low temperature performance providing it is not subjected to movement when cold. The extruded XLPE insulation and MDPE or HDPE jackets are robust mechanical entities. Test specifications exists to test the cable insulation and the jacket for conditions likely to be experienced during installation in temperate climates, these being bend tests, abrasion tests and impact penetration tests. It is recommended for the 500 kV Study Project that these tests be performed at low temperature, for example at -20oC, to determine the minimum safe temperature at which the cable can be installed a) to determine the duration of the season available for installation and b) the ambient temperatures at which replacement cable could safely be installed following a service failure in winter. In the UK it is practice for the cable reel to be preheated to above -5oC for an MDPE or HDPE jacketed cable and to only install if the ambient temperature is above -5oC.
It is believed that other countries have lower limits. Standard practice for medium voltage XLPE cables is to store indoors overnight in a heated garage before installation in cold weather. Experience near Edmonton was that a medium voltage cable that was at -35°C temperature split the insulation to the conductor when an attempt was made to install it. The limiting minimum ambient temperature for installation of a 500 kV, 2,500mm², cable must be quantified for the Edmonton region of Alberta.
Whilst the HPT is not planning to install any cable during the winter months due to the risk of damage to a cable reel, a repair strategy would have to be prepared for all seasons.
In the cable factory, thermal contraction to ambient temperature occurs after a) the XLPE insulation
climates exists that excessive contraction will occur at the accessories. Specifications exist to test cable samples in the factory to ensure that the magnitude of the contraction does not exceed a prescribed limit.
For the 500 kV Study Project it is recommended that Proving Tests be formulated to ensure that damage due to excessive contraction does not occur to the cable or terminations when the cable is cooled to -50oC.