EP0829883A2 - Damped high-voltage cable without oscillations - Google Patents

Abstract

Damped high tension cable for low-current supply to electrical equipment with conductor coaxially surrounded by insulation, new that the conductor has an electrical resistance between 10 k OMEGA /m and 1000 k OMEGA /m, preferably 20 k OMEGA /m and 100 k OMEGA /m. Preferably the conductor (2) is a conductive plastic. The conductor has a carrier (1) of non-conductive material, a wire or fibre, which it surrounds coaxially. Conductor and insulation (3) contain polyethylene plastics. There is an outer insulating casing (5). A screen may be included (4). The conductor is a carbon filled polymer, of which the resistance rises with increasing temperature. Also claimed is the corresponding manufacturing process.

Description

The invention relates to a high-voltage cable for the low-voltage power supply of electrical devices with a current conductor and an insulation coaxially surrounding the current conductor. The invention further relates to a method for producing such a high-voltage cable.

Such cables are used in particular for the high-voltage supply of electrostatic coating or flocking systems or the like. needed, which typically work with operating currents in the milliampere or microampere range and charging voltages in the order of 100 kV. One of the most important problems with these systems is the considerable energy that is stored in the often extended cable connections due to their large intrinsic capacity and is dangerous for several reasons if they are suddenly discharged.

In electrostatic coating or flocking systems there is a risk of explosion of the coating material-air mixture. For example, the energy required to ignite is about 0.2 mJ for solvent-based paint, about 5 mJ for plastic powder and about 500 mJ for flock fibers.

Furthermore, dangers for the operating personnel must be excluded. The hazard limit when touching parts of the system that are at high voltage potential is 300 mJ for humans.

In addition, the system itself is at risk. In the event of a short-circuit discharge from the high-voltage circuit, an extremely high current, which can be up to 10 kA, flows on the current return line, usually an earth potential line. Even with a relatively low contact resistance

Earth potential line can increase the earth potential by up to 1000 V or more, which leads directly to the destruction of electronic components of the system.

wobei C die Kapazität (in Farad) und U die Ladespannung (in V) bedeuten. Untersuchungen haben ergeben, daß hierbei das Hochspannungskabel, insbesondere wenn es zur elektrischen Abschirmung mit einem Metallgeflecht umhüllt ist, die Gesamtkapazität signifikant erhöht. Typische Kapazitätswerte einer elektrostatischen Beschichtungsanlage liegen zwischen 20 und 700 pF für das eigentliche Sprühsystem oder Flockfeld und 40 - 60 pF/m bzw. 200 - 600 pF/m für übliche ungeschirmte bzw. geschirmte Hochspannungskabel. Da in einer typischen elektrostatischen Beschichtungsanlage bis zu 100 m lange Hochspannungskabelverbindungen eingesezt werden, können sich z.B. Kapazitäten bis 60 nF und damit je nach Ladespannung Entladeenergien von mehreren J (Wattsekunden) ergeben.The discharge energy W (in mJ or mWs) is known to be calculated using the formula $${\text{W = 1/2 C x U}}^{\text{2nd}}\text{,}$$

where C is the capacitance (in farads) and U is the charging voltage (in V). Studies have shown that the high-voltage cable, in particular if it is covered with a metal braid for electrical shielding, significantly increases the overall capacity. Typical capacitance values of an electrostatic coating system are between 20 and 700 pF for the actual spraying system or flock field and 40 - 60 pF / m or 200 - 600 pF / m for usual unshielded or shielded high-voltage cables. Since high-voltage cable connections of up to 100 m in length are used in a typical electrostatic coating system, capacities of up to 60 nF and thus discharge energies of several J (watt seconds) can result, depending on the charging voltage.

Here it can be observed that e.g. in the event of a short circuit, the cable is not continuously discharged, but the discharge current executes a series of positive and negative oscillations with amplitudes that gradually decrease from a considerable maximum value. These vibrations can be particularly disruptive or dangerous.

Of course, there has been no lack of attempts to avoid the dangers that arise when using conventional copper conductor cables as far as possible by means of various protective measures.

For example, it has been proposed to reduce the conductor cross section of the cable so that the capacitor formed by the cable only has a small surface area and a correspondingly low capacitance. A thin metal wire was used as the conductor (DE-GM 19 93 972). However, the production of such a cable is very complex, and the achievable cable lengths are limited for manufacturing reasons. In addition, the thin wire tends to break.

Furthermore, it is known and common practice to dampen the high-energy discharges from a conventional high-voltage cable with a copper conductor by means of downstream high-resistance resistors of several 100 MOhms. However, these high-impedance resistors cause a considerable voltage loss during operation of the system, which can be up to 50 kV. This leads to a reduction in the efficiency and to a considerable loss of performance of the coating system. In addition, the installation of the resistors, which are quite voluminous due to the necessary high-voltage strength, requires considerable design effort (cf. DE-GM 73 07 686).

A high-voltage cable has also become known, the conductor of which consists of a resistance mass. The resistance value of this cable is around 100 MOhm / m at 20 ° C. As a result, dangers from high-energy discharge are largely avoided, but the disadvantage of high voltage losses is even more pronounced here than with the damping resistors mentioned above. Another disadvantage of this known high-voltage cable is its relatively large diameter of approximately 12 mm, so that it is unsuitable, for example, for movable spray devices. In addition, the known cable has such a large negative temperature coefficient (the resistance value can drop in the range between 5 and 40 ° C from 800 to 50 MOhm / m) that a safety risk can arise.

Furthermore, a high-voltage cable has become known, the conductor of which consists of a conductive organic liquid which is filled into a tube made of insulating material. The resistance value of these cables can be several 100 MOhm / m. This cable, too, consequently leads to considerable voltage losses and also has an undesirably large diameter. In addition, an electrolytic decomposition process can be observed with prolonged loading, which increases the cable resistance in the direction infinitely, so that the cable becomes unusable.

High-voltage cables with carbon fiber conductors are also known as ignition cables for motor vehicle engines. These cables are not shielded and extremely flexible, but have practically no damping effect, since the cable resistance is only 0.15 to 0.40 Ohm / m depending on the temperature. The use in electrostatic coating systems is therefore not sensible.

High-voltage cables, which are not provided with a braided shield, were also used in coating systems because the capacity can be reduced to about 50%. However, the problem of high-energy discharges cannot be solved to a significant extent. In addition, in many cases, unscreened cables have to be subject to considerable installation restrictions in order to avoid high voltage breakdowns and to ensure the necessary protection against accidental contact. A conventional high-voltage cable without shielding often does not meet the increasingly important requirements for electromagnetic compatibility (EMC).

The aim of the invention is therefore one that is particularly suitable for electrostatic coating and flocking systems High-voltage cable that safely prevents high-energy discharges and in particular the strong current vibrations mentioned above, even without a high-resistance resistor and with negligible voltage losses.

According to a first aspect of the invention, this object is achieved in that the current conductor has an electrical resistance which is on the one hand considerably larger than in the known copper or carbon fiber conductor cables, but on the other hand considerably lower than in the known high-resistance high-voltage cables. The resistance value (here the DC resistance is always meant) should be between about 10 and 1000 kOhm / m, preferably between 20 kOhm / m and about 100 kOhm / m. The resistance value is generally constant over the entire length of the cable.

It was surprisingly found that the discharge of such a cable does not take place in the form of strong current vibrations as in the known case mentioned. Rather, when the cable is suddenly discharged, the current drops quickly and without significant vibrations after only one current peak, which is at a typical resistance value according to the invention almost 200 times smaller than the maximum amplitude in the case of a copper cable.

A major advantage here is that the cable capacity no longer plays a practical role. The cable according to the invention can therefore be designed in any way without taking into account the capacity to take account of other desired properties. In particular, there is no reason in this regard to forego the shielding that is usually desired, for example by means of a copper braid. The cable can therefore also meet all EMC requirements.

According to another aspect of the invention, the current conductor is made of electrically conductive plastic.

On the one hand, this allows a resistance value in the desired range to be easily achieved. Another important advantage is that such a conductor has no negative temperature coefficient. The cable according to the invention also has the advantage of good flexibility, which is important in practice, and can have a small outer diameter. In addition, the cable can be manufactured easily and with little effort, that is, correspondingly economically.

Fig. 1

den Querschnitt eines Hochspannungskabels der hier beschriebenen Art;

Fig. 2

den elektrischen Widerstand des beschriebenen Kabels in Abhängigkeit von der Temperatur;

Fig. 3

den elektrischen Widerstand eines bekannten Hochspannungskabels in Abhängigkeit von der Temperatur;

Fig. 4

den Stromverlauf bei der Entladung eines konventionellen Hochspannungskabels; und

Fig. 5

den Stromverlauf bei der Entladung eines erfindungsgemäßen Kabels.

Further features of the invention result from the following explanation with reference to the drawing. It shows:

Fig. 1

the cross section of a high voltage cable of the type described here;

Fig. 2

the electrical resistance of the cable described depending on the temperature;

Fig. 3

the electrical resistance of a known high-voltage cable as a function of temperature;

Fig. 4

the current flow when a conventional high-voltage cable is discharged; and

Fig. 5

the current profile when discharging a cable according to the invention.

The high-voltage cable shown in cross section in FIG. 1 consists of a central electrically non-conductive wire or thread 1 with a circular cross-section, a cylindrical current-conducting layer 2 coaxially surrounding this thread, and a cylindrical cylindrical layer surrounding the current-conducting layer Insulation 3, a conductive shield surrounding the insulation 3 in the form of a thin copper braid 4 and an outer jacket 5 coaxially surrounding the arrangement described so far.

The wire-like thread 1 consists of an electrically non-conductive, tensile and flexible plastic material such as polyester, polyamide or the like. This thread serves as a carrier for the current-conducting layer 2, which preferably consists of flexible plastic which has been made conductive in a known manner, e.g. made of polyethylene (PE) filled with soot particles. The insulation 3 expediently consists of a similar, but electrically non-conductive plastic, that is also of PE material. The outer jacket 5 can e.g. consist of polyurethane (PUR).

For the dielectric strength of the cable, field distribution within the cable is also important. The field strength on the conductor layer 2 should be as small as possible. When the braid 4 is used as the outer conductor, it depends on the ratio of the inside width D of the outer conductor to the outer diameter d of the layer 2 and has its minimum value at D / d of about 2.7. In practice, it is therefore advisable to maintain values for D / d between more than 2 and less than 3.4.

In a practically implemented embodiment, the diameter of the thread 1 serving as a carrier can be approximately 0.8 mm, that of the conductive layer 1.9 mm and that of the insulation 5.2 mm. In this case, the diameter of the sheath 5 enveloping the copper braid 4 and thus of the entire shielded cable is 7.5 mm.

For special cases in which shielding is not necessary, a cable corresponding to the example described above can be used, in which only that Copper braid 4 is missing and the jacket 5 is applied directly to the insulation 3. The cable diameter is reduced to 6.7 mm.

However, other suitable materials can also be used within the scope of the invention. For the conductive layer, e.g. conductive elastomer material conceivable. Static shields can also be made from conductive elastomer material.

The cable described above is characterized in particular by its small dimensions, low weight (about 66 g / m or 37 g / m with or without shielding) and good flexibility and flexibility. The minimum bending radius is approximately 75 or 70 mm. The cable can work in continuous operation in a temperature range between -20 and +80 ° C without any problems.

Known extrusion processes can be used to manufacture the cable. In this case, the conductive layer 2 is first applied to the thread 1 and the composite thus formed is then surrounded or extrusion-coated with the plastic of the insulation 3. The plastics and process conditions are chosen so that there is a permanent gapless connection between the individual cable components. It is particularly expedient if the plastics of the conductive layer 2 and the insulation 3 fit well together. Furthermore, the plastics should have the same possible expansion coefficients.

In addition to the good physical properties of the cable, its electrical properties and here in particular the electrical resistance are of particular importance to the invention.

With the dimensions and materials mentioned above, the electrical resistance of the cable can typically be around 40 kOhm / m. As shown in FIG. 2, the resistance value changes in the range from 5 to 40 ° C. with increasing temperature only between approximately 35 and 47 kOhm / m. In principle, the recognizable positive temperature coefficient of the resistance value should be preferred.

For comparison, FIG. 3 shows the resistance curve of the known cable mentioned at the beginning with a current conductor consisting of a high-resistance resistor. There resistance values between 800 and 50 MOhm are shown in the same temperature range. The strong negative temperature coefficient can mean that at low temperatures due to the voltage drop due to the high resistance a satisfactory operation of the device powered by the cable is no longer possible, while on the other hand the falling damping effect must be taken into account for safety reasons at higher temperatures.

As has already been explained above, the invention is based on the surprising finding that, when the cable is discharged, with the correct choice of its electrical intrinsic resistance, no large current peak values and, above all, practically no current oscillations occur, without the acceptance of significant voltage losses and without the need for expensive external devices Damping measures.

To explain this effect, the typical temporal current curve is initially shown in FIG. 4, as measured when a 3.8 m long sample of a conventional shielded high-voltage cable with copper conductors was discharged, the cable voltage being 10 kV and the cable capacitance being 220 pF. The current initially reached the very high value of 75 A and considering the relatively short cable length decayed to zero after a period of 2000 nS only after numerous oscillations with initially large amplitudes.

In contrast to this, a completely different course was determined by measuring the discharge process with a likewise 3.8 m long piece of cable according to the invention, which was also at 10 kV and had a capacitance of 290 pF. As shown in FIG. 5 with a correspondingly enlarged current scale and extended time scale, only a single current peak of approximately 0.5 A was shown, which dropped from less than 0.1 μs in the comparable time period from approximately 2 μs.

In this experiment, the shielded cable shown in Fig. 1 was used. As already mentioned, in contrast to conventional high-voltage cables, the shielding and, in general, the cable capacity in the invention have no significant influence on the discharge behavior.

Claims (10)

Damped high-voltage cable for the low-voltage supply of electrical devices
with a conductor
and an insulation coaxially surrounding the current conductor, characterized in that the current conductor has an electrical resistance between approximately 10 kOhm / m and approximately 1000 kOhm / m. High-voltage cable according to claim 1, characterized in that the current conductor has an electrical resistance between 20 and 100 kOhm / m. High-voltage cable according to claim 1 or 2, characterized in that the current conductor consists of electrically conductive plastic. High-voltage cable according to one of the preceding claims, characterized in that the current conductor consists of a layer which coaxially surrounds a wire or thread serving as a support and made of electrically non-conductive material. High-voltage cable according to one of the preceding claims, characterized in that the current conductor and / or the insulation consist of polyethylene plastics. High-voltage lamp according to one of the preceding claims, characterized in that the insulation is coaxially surrounded by a shield and / or an outer jacket made of insulating material. High-voltage cable according to one of the preceding claims, characterized in that the current conductor consists of polymer material filled with carbon black, the resistance value of which increases with increasing temperature. High-voltage cable according to one of the preceding claims, characterized in that the ratio of the outer diameter of the current conductor and the outer diameter of the insulation (D / d) surrounding it is more than 2 and less than 3.4. Method for producing a damped high-voltage cable for the low-voltage supply of electrical devices, characterized in that
a wire or thread serving as a support and consisting of electrically non-conductive material is surrounded with a layer of conductive plastic material which has an electrical resistance between approximately 10 and 1000 kOhm / m,
and that the conductive layer is then surrounded with insulating material. Use of a high-voltage cable according to one of claims 1 to 8 for electrostatic coating or flocking systems.

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