Method and apparatus for power cable manufacturing

Abstract

The invention relates to a method and apparatus for power cable manufacturing, in which method conductor (12) is conveyed through at least one induction heater (10) for heating the conductor, and the power feed to the at least one induction heater is defined based on conveying speed, material, diameter and cross-sectional area of the conductor, wherein the said at least one induction heater (10) is controlled during the heating of the conductor (12) by controlling and adjusting the power feed to the induction heater based on monitored changes of calculated power loss in the induction heater for maintaining a predefined target value for the power feed for heating the conductor during the heating process, wherein the predefined target value for the power feed is calculated based on premeasured induction coil loss of the induction heater as a function of induction coil current, and on the ratio of induction coil loss to the conveying speed of the conductor.

Description

[0001] METHOD AND APPARATUS FOR POWER CABLE MANUFACTURING

[0002] Technical field

[0003] The present invention relates to manufacturing of power cables, such as medium voltage, high voltage and extra high voltage cables, with continuous vulcanization line.

[0004] Backqround

[0005] Continuous vulcanization lines (CV-lines) are used to simultaneously insulate and cross-link cross-linkable polyethylene (XLPE) insulated power cable cores. The most common CV-lines are continuous catenary vulcanization lines (CCV-lines) and continuous vertical vulcanization lines (VCV-lines).

[0006] In a CV-line the conductor of the cable comes from a pay-off reel, which conductor is coated with three insulation layers (a semiconductive layer, an insulation layer and second semiconductive layer). The insulation and semiconductive layers are cross-linked to improve the layers’ thermomechanical properties. As a final phase of the CV-line, the manufactured cable core is reeled on a take-up reel.

[0007] In a CV-line the cross-linking part of the line consist of a curing and cooling sections. In the curing section the insulation of the cable core is cross-linked in a nitrogen atmosphere with the radiant heat from the vulcanization tube (CV- tube). The curing section if followed by the cooling section, wherein the cable core is cooled either by cold nitrogen or water circulation.

[0008] The heating of the cable core in a CV-line is typically obtained with induction heating, the efficiency of which heating is a critical factor for the production speed of the CV-line. One generally used way to speed up the heating is to inductively heat the conductor before the insulation layers are extruded on the core (preheating) and then again during the curing inside the CV-tube (postheating).

[0009] In the prior art power adjustment process, the variables, such as line speed and temperatures of the heating pipes, are calculated with cross-linking calculation program based on physics model. In this program is input the required temperature of the conductor in degrees both after preheater and postheater. The program then optimizes other process variables so that highest possible line speed is obtained with sufficient cross-linking level, without exceeding the limits of other process variables, such as sufficient cooling for the cable. In the program is also input cable dimensions such as diameter, cross-sectional area and material of the conductor. With this input information it is possible to calculate the required power feed for the heating of the conductor in the cable manufacturing line at different line speed together with a power / speed factor. The obtained required power feed, which represents the heating power conveyed to the conduction, needs to be divided with efficiency ratio of the induction heater in order to obtain the required power feed for the induction heater for the manufacturing process of the cable.

[0010] Power of the inductive heating has an efficiency ratio which depends both on the conductor to be heated and the inductive heater used, which efficiency ratio can vary significantly with different conductors. The achieved heating of the conductor can be measured, and thus adjusted, after preheating, but the achieved inductive heating inside the CV-tube cannot be measured and adjusted by direct measurement.

[0011] The power adjustment of the inductive heating during postheating is typically done for each conductor-type by measuring the increase of temperature of the conductor at predefined speed and power when the measuring point of the conductor travels through the inductive heater coil. These measurements are done before the actual cable production is started, and the measurement results give the required ratio for power and speed in order to achieve the desired temperature increase for the conductor.

[0012] There are two main problems with this kind of power adjustment method: a) Measurements needs to be done individually for each conductor-type and often plurality of measurements are required for single conductortype. b) Changes in the efficiency ratio due to changes in the conductor cannot be compensated during production runs. This can lead to too low degree of cross-linking in the insulation layers and thus to rejection of the manufactured insulated power cable. Summary

[0013] For overcoming the above-mentioned problems the present invention provides a novel method for controlling the induction heater for heating of a conductor of or for cable, wherein the feed power to the induction heater during the heating process is controlled and adjusted based on comparison to predefined reference values. This allows real time heating adjustments to compensate the changes in conductors and thus guaranteeing proper temperatures of the conductor in the cable to provide sufficient cross-linking of the insulation layers. Alternatively the present invention can be used for quality control of conductors prior to their coating in the power cable production process.

[0014] When heating conductors for power cables, the efficiency ratio of the induction heater depends, in addition to the above-mentioned diameter and material of the conduction, also the structure of the conductor, i.e. how well the individual strands forming the conductor are in electrical connection to each other. Due to this the efficiency ratio of the induction heater is sensitive to the manufacturing process and quality of the conductor, and the efficiency ratio with a conductor produced with same manufacturing specifications can change between different batches, and even within the same batch. The present invention allows reacting to the quality changes in the conductor during cable manufacturing process by adjusting the heating of the conductor accordingly, which has not been possible with prior art solutions.

[0015] In the method of the invention for power cable manufacturing a conductor is conveyed through at least one induction heater for heating the conductor, and the power feed to the at least one induction heater is defined based on conveying speed, material, diameter and cross-sectional area of the conductor, wherein the said at least one induction heater is controlled during the heating process by controlling and adjusting the power feed to the induction heater based on monitored changes of calculated power loss in the induction heater for maintaining a target value for the power feed for heating the conductor during the heating process.

[0016] In the method of the invention the target value for the power feed is calculated based on premeasured induction coil loss of the induction heater as a function of induction coil current, and on the ratio of induction coil loss to the conveying speed of the conductor. In the context of the invention the feature “power feed” to the at least one induction heater is the electrical power fed to the induction heater for heating the conductor.

[0017] In the context of the invention the feature “calculated power loss in the induction heater” is the power loss in the induction heater, i.e. the power lost from the heating of the conductor. This power loss changes in accordance with the quality changes in the conductor during the heating process. This power loss can be calculated and monitored from measured induction coil current of the induction heater during the heating process, for example. This calculation of the power loss, however, generally requires calibration process for the induction heater in order to provide useful information for the present invention.

[0018] In the context of the invention the feature “predefined target value for the power feed” is the power that needs to be used for the actual heating of the conductor during the heating process in order to obtain the required temperature in the conductor.

[0019] In an embodiment of the invention the premeasured induction coil loss of the induction heater is measured by raising stepwise the power input to the induction heater and measuring both the induction coil current and the power input without the conductor, and by determining mapping between the power input and the measured induction coil current.

[0020] In an embodiment of the method of the invention the method is part of a quality verification of a conductor for a power cable.

[0021] In an alternative embodiment of the invention the method is part of a power cable manufacturing process wherein the cable is manufactured with a continuous vulcanization line by coating a conductor with at least one layer of cross-linkable insulation material and by applying cross-linking reaction to the insulation material layer after the coating.

[0022] In this embodiment the method the controlled induction heater is preferably preheater and / or postheater of the continuous vulcanization line. The present invention also provides an apparatus for power cable manufacturing, which comprises a continuous vulcanization line with at least one induction heater for heating a conductor of the cable, wherein the apparatus comprises devices for implementing the method of any of the preceding claims.

[0023] In an embodiment of the apparatus of the invention the said devices comprise automatic control system for controlling the at least one induction heater.

[0024] More precisely the features defining a method in accordance with the present invention are presented in claim 1 , and the features defining an apparatus of the invention are presented in claim 6. Dependent claims present advantageous features and embodiments of the invention.

[0025] Brief description of the drawings

[0026] Exemplifying embodiment of the invention and its advantages are explained in greater detail below in the sense of example and with reference to accompanying drawings, where

[0027] Figure 1 shows schematically a principle layout of a continuous vulcanization line, and

[0028] Figure 2 shows schematically a principle layout of a continuous vulcanization line in which the present invention is used.

[0029] Detailed description of an embodiment

[0030] Figure 1 shows a principle layout of a continuous vulcanizing extrusion line. The vulcanizing extrusion line comprises a payoff 1 , a metering capstan or metering caterpillar 2, a preheater 3 for a conductor element, an extrusion group with an extruder head 4, a postheater 5, a vulcanization tube 6, a cooling tube 7, a caterpillar or a capstan 8 and a take-up 9.

[0031] The operation and construction of the extrusion line is well-known to a person skilled in the art, and therefore operation or / and construction of the extrusion line is not described in detail here. Referring to said matter it is noted here that for example preheater 3 can be placed upstream of the metering capstan 2 etc. In the cable manufacturing process described above, the insulation has to be extruded onto the conductor at a temperature sufficiently low as to avoid premature cross-linking in the extrusion equipment, as this would lead to defects of the insulation.

[0032] After the extrusion step, the material has to be heated up at a temperature that is sufficiently high to start and complete the chemical reaction within the shortest possible time.

[0033] The crosslinking reaction is carried out in the vulcanization tube 6, i.e. a tube surrounding the extruded electrical cable located downstream of the extruder head 4, inside which the cable is heated up by radiant and / or convective heat transfer.

[0034] The heat diffusion in the insulation material is however low, and the polymer layers close to the conductor will take the longest time to increase temperature and undergo the chemical reaction desired.

[0035] A well-known method to improve the heating / crosslinking process is to heat the conductor internally so that heat diffuses into the insulation material extruded also form inside and therefore accelerates the crosslinking reaction. This internal heating is materialized by using inductive heating procedure.

[0036] Figure 2 shows a principle layout of a continuous vulcanizing line in which the present invention is used. In Figure 2 same reference numbers are used to indicate corresponding parts when compared to Figure 1.

[0037] Figure 2 shows in fact a part of a continuous vulcanizing extrusion line, i.e. only the parts needed to understand the invention are shown. For example pay-off and take-up is not shown on Figure 2. A person skilled in the art however instantly understands the structure and operation of the line shown in Figure 2 when looking for example Figure 2 together with figure 1 .

[0038] The embodiment shown in Figure 2 has an extrusion group with extruder head 4 and telescopic tube and, vulcanizing tube 6 and cooling tube 7 which in this embodiment is a device with closed-circuit gas cooling. Conductor guided to the extruder head 4 is shown with reference number 12.

[0039] In this embodiment there is also an inductive heating device 10 for heating the coated conductor 12 placed after the vulcanization tube 6 and before the cooling tube 7, i.e. the coated conductor is heated after the vulcanization tube 6 by using inductive heating. At this location of the inductive heating device 10, the cross-linking reaction has progressed sufficiently so that the insulation is partially cross-linked close to the conductor 12, and the conductor can be heated up by means of inductive heating and thus accelerate completion of the cross-linking of the insulation material.

[0040] In the embodiment of the present invention the inductive heater 10, which operates in this case as a postheater in the continuous vulcanization line, is controlled by control system 14.

[0041] Before power cable production with the continuous vulcanization line is started, the inductive heater 10 together with the control system 14 is calibrated. In this calibration process the coil resistance factors of the inductive heater 10 are determined, which factors are then used for calculating net power Pnet value. And since the cable conductor is not conveyed through the inductive heater 10 during the calibration process, the Pnet is zero because power feed Pteed to the inductive heater equals loss power Pioss.

[0042] Preferably the calibration process is done by raising stepwise the power input to the induction heater 10 and measuring both the induction coil current and the power input, and then determining mapping between the power input and the measured induction coil current. This mapping can result in calibration table or a calibration function can be adapted to the achieved results. The calibration table and / or calibration function is then used by the control system 14 to define required target net power Pnet values for the power feed to the induction heater 10 to be used in the control.

[0043] In relation to the calibration process it is to be noted, that it does not comprise only the actual induction coil losses but also all losses to the surrounding environment due to installation, and is therefore unique for each individual equipment and / or installation. Thus, whenever a change in the heating of the conductor 12 during cable manufacturing process is starting to occur, it is reflected in the coil current of the inductive heater 12, such as for example as increased current, leading to increased calculated power losses Pioss, which in turn leads to decreased calculated net power Pnet. This decrease can then be automatically compensated during the cable manufacturing process by increasing the feed power Pfeed set point until the required net power Pnet is reached. This way sufficient heating for the cable conductor 12 can be guaranteed in the cable manufacturing process.

[0044] The specific exemplifying embodiment of the invention shown in the figures and discussed above should not be construed as limiting. A person skilled in the art can amend and modify the embodiment described in many evident ways within the scope of the attached claims. Thus, the invention is not limited merely to the embodiment described above.

Claims

Claims:1 . Method for power cable manufacturing, in which method conductor (12) is conveyed through at least one induction heater (10) for heating the conductor, and the power feed to the at least one induction heater is defined based on conveying speed, material, diameter and cross-sectional area of the conductor, characterized in that the said at least one induction heater (10) is controlled during the heating of the conductor (12) by controlling and adjusting the power feed to the induction heater based on monitored changes of calculated power loss in the induction heater for maintaining a predefined target value for the power feed for heating the conductor during the heating process, wherein the predefined target value for the power feed is calculated based on premeasured induction coil loss of the induction heater as a function of induction coil current, and on the ratio of induction coil loss to the conveying speed of the conductor.

2. Method of claim 1 , wherein the premeasured induction coil loss of the induction heater (10) is measured by raising stepwise the power feed to the induction heater and measuring both the induction coil current and the power feed without the conductor (12), and by determining mapping between the power feed and the measured induction coil current.

3. Method of claim 1 or 2, wherein the method is part of a quality verification of a conductor (12) for a power cable.

4. Method of claim 1 or 2, wherein the method is part of a power cable manufacturing process wherein the cable is manufactured with a continuous vulcanization line by coating a conductor (12) with at least one layer of crosslinkable insulation material and by applying cross-linking reaction to the insulation material layer after the coating.

5. Method of claim 4, wherein the controlled induction heater (10) is preheater and / or postheater of the continuous vulcanization line.

6. Apparatus for power cable manufacturing, which comprises a continuous vulcanization line with at least one induction heater (10) for heating a conductor (12) of the cable, characterized in that the apparatus comprises devices (14) for implementing the method of any of the preceding claims.

7. Apparatus of claim 6, wherein the said devices comprise automatic control system (14) for controlling the at least one induction heater (10).

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