The present invention relates generally to electrical power equipment, and particularly to high voltage electrical power equipment. More particularly, the invention relates to a method for providing an insulated electric cable or termination or joint. The purpose of the method is to increase the dielectric withstand strength of the cable/termination/joint. The method applies to both a (high voltage) DC cable or DC termination/joint and (high voltage) AC cable or AC termination/joint.
Extruded solid insulation based on a polyethylene, or a cross linked polyethylene (XLPE), has for almost 40 years been used for AC transmission and distribution cable insulation.
Therefore the possibility of the use of XLPE for cable insulation has been under investigation for many years. Cables with such insulations have the same advantage as the mass impregnated cable in that for transmission there are no restrictions on circuit length and they also have a potential for being operated at higher temperatures, thus offering a possibility to increase the transmission load.
It is well known that the level of crystallinity of extruded XLPE used for cable insulation is correlated to the electric withstand strength both for AC and impulse voltages and it is expected to improve the long term properties of extruded DC cables as well. A high level of crystallinity is also desirable as it will decrease the diffusivity of the material making the electric properties more stable over time. By applying high temperature after the crosslinking process the crystallinity can be increased as well as evenly distributed. However, the surface of a XLPE cable must be protected from oxygen to avoid degradation of the insulation material, which in turn affects the electric properties negatively.
It is known from document
It is an object of the present invention to provide a method for providing an insulated electric (high voltage) DC or AC cable as specified in the foregoing. It is also an object of the invention to provide a (high voltage) DC or AC termination or joint having similar properties.
The object mentioned above is accomplished by a method of providing an insulated electric cable or termination or joint, the method comprising :
The method according to the invention comprises after the heat treatment procedure a controlled cooling step while the outer surface of the polymer based insulation system is covered by the impermeable cover, thereby achieving a high crystallinity of the cross-linked polymer composition.
The cross-linked polymer composition is typically a cross-linked polyethylene, but could also be applied to other polyolefins, such as LDPE, HDPE, PP or copolymer of those.
The heat treatment can be performed at a heating temperature of between 80 and 120°C, and preferably at a temperature of between 85 and 105°C.
The heating temperature can be maintained for a time between 1 and 20 days, and preferably between 4 and 10 days.
The controlled cooling step can include a controlled decrease of the temperature of between 1 and 20 °C/hour.
The controlled decrease of the temperature can be performed from the heating temperature and until a cooling temperature of between 60 and 70°C, and preferably of between 63 and 67°C. Under this cooling temperature, no more or very limited crystallization takes place. Slow cooling is needed as not to freeze some morphological structures that have not yet crystallized at the heat treatment temperature. That is, some crystallization takes place during cooling, and these crystals should be as "perfect" as possible.
The substances present in the polymer based insulation system in a non-homogenous distribution can include at least one rest or byproduct from the cross-linking.
The substance present in the polymer based insulation system in a non-homogenous distribution can include at least one peroxide decomposition product.
The polymer based insulation system can comprise a first semi-conducting shield, the cross-linked polymer composition, and a second semi-conducting shield.
The method can be a method for production of insulated electric AC or DC cable and :
Said second substance is typically methane.
The impermeable cover can be a lead sheet, a metal cover such as a metal laminate and/or an outer covering or sheath provided that this layer is impermeable to the at least one substance present in the extruded insulation system.
Further characteristics of the invention and advantages thereof will be evident from the following detailed description of embodiments of the invention:
A method for production of an insulated electric high voltage cable according to an embodiment of the present invention will now be described with reference to
The cable can when found appropriate be further complemented in various ways with various functional layers or other features. It can for example be complemented with a reinforcement in form of metallic wires outside the outer extruded shield 13, a sealing compound or a water swelling powder introduced in metal/polymer interfaces or a system achieved by e.g. a corrosion resistant metal polyethylene laminate and longitudinal water sealing achieved by water swelling material, e.g. tape or powder beneath the sheath 15. The conductor need not be stranded but can be of any desired shape and constitution, such as a stranded multi-wire conductor, a solid conductor or a segmental conductor.
According to the method for production of the insulated electric high voltage cable, a polymer based insulation system is, in a step 21, extruded around the conductor 10, wherein the polymer based insulation system comprises the semi-conducting shield 11, the polyethylene based conductor insulation 12, and the second semi-conducting shield 13. In an alternative embodiment the polyethylene based conductor insulation 12 is exchanged for other polymer based conductor insulation.
Below is a short description of one exemplary method of performing the extrusion. A person skilled in the art realizes that there are other extrusion techniques that can be used with the present invention.
In the exemplary method the conductor is fed from a conductor pay-off through the extruder equipment and other processing and conditioning devices and is finally taken up on a cable core take-up. The conductor pay-off and cable core take-up may be reels or drums suitable for discrete lengths but can be of any suitable type including devices for essentially continuous handling of the supplied conductor and produced cable. The conductor is passed over a first wheel through a conductor preheater wherein it is preheated for a suitable temperature before the insulation system is applied by extrusion. The process is suitable for true triple extrusion where a triple head extruder is used. The inner and outer semi-conductive layers are applied using two separate extruders and a further third extruder is used for the main insulation.
After the extrusion operation the extruded polyethylene based conductor insulation 12 is advantageously, in a step 22, cross-linked. To this end the insulated DC cable is passed through a pressurized curing and cooling chamber, wherein the conditions is controlled to ensure the desired cross-linking degree and other structural characteristics that can be effected by this controlled conditioning and cooling of the extruded insulation system. Typically, the extruded polyethylene based conductor insulation 12 includes a number of additives, among them dicumylperoxide and additives. Thereafter the cable is hauled through a haul-off caterpillar and over a second wheel before being taken up for further processing.
The above approach offers the possibility of an essentially continuous or semi-continuous process for the application and processing of the extruded insulation system.
As a result of the process, however, methane may be created in the cross-linked polyethylene based conductor insulation 12. Any methane may be removed by exposing, in a step 23, the extruded DC cable to a heat treatment while the outer surface of the extruded polymer based insulation system is kept free from any covering to thereby allow the methane to leave.
Next, in a step 24, the outer surface of the extruded polymer based insulation system is covered by a cover 14 impermeable to one or more substances present in the extruded insulation system in a non-homogenous distribution.
The one or more substances include preferably one or more rest or byproducts from the cross-linking and/or one or more additives. The rest products include typically peroxide decomposition products, such as acetophenone and cumyl alcohol, and the additives include typically one or more antioxidants and scorch retarders. However, other additives may be of equal or even higher importance.
Next, after having covered the extruded DC cable with the impermeable cover, the extruded DC cable is, in a step 25, exposed for a heat treatment procedure in order to achieve a high crystallinity of the cross-linked polymer composition.
The heat treatment procedure is preferably performed at a temperature of between 80 and 120°C, and more preferably between 85 and 105°C, and for a time that is shorter the higher the temperature is, preferably between 4 and 10 days.
For instance, as shown in
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