JPH0454447Y2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to an improvement of the terminal portion of a rubber/plastic cable for DC withstand voltage testing.

<Prior art and problems> When performing a DC withstand voltage test on rubber/plastic cables, it is essential to perform insulation reinforcement or electrolytic adjustment of the cable terminals in order to prevent dielectric breakdown at the cable terminals. , a method using a stress cone, or a method using a resistance grade method, etc., but all of these methods require a considerable amount of time for terminal treatment.

<Purpose of the invention> The object of the invention is to provide a terminal section for direct current withstand voltage testing of rubber/plastic cables that can be easily assembled without using stress cones or resistance grades.

<Structure of the invention> The terminal section of a rubber/plastic cable for direct current withstand voltage testing according to the invention is constructed by removing the cable sheath and cable external shielding layer from the end of the rubber/plastic cable to expose the cable insulation layer. A terminal part for a DC withstand voltage test of a rubber/plastic cable in which an insulating layer is immersed in insulating oil and electrolysis is relaxed by an interfacial charge generated by application of a DC voltage, the external shielding layer being The surface portion of the cable insulation layer in the vicinity is smoothed by a heat-resistant heat-shrinkable tape with a smooth surface, which is wrapped around the portion, heated, and removed after the heat treatment. It is the composition.

<Explanation of Examples> The present invention will be explained below with reference to the drawings.

In the figure, 1 indicates a rubber/plastic cable, 11 is a cable conductor, and 12 is a cable insulating layer. On the cable insulating layer 12, a semiconductive layer 13 (extruded semiconductive rubber/plastic coating) is formed. layer), a metal shielding layer 14 (copper tape winding layer) and a cable sheath 15 (rubber-plastic sheath).

2 indicates the rubber/plastic cable end;
The cable sheath, metal shielding layer and semiconducting layer have been removed to expose the rubber-plastic insulation layer. 120 is a rubber layer near the edge 131 of the semiconductive layer.
This is the surface of the plastic insulating layer, and has been subjected to surface smoothing treatment. This surface smoothing treatment involves wrapping a heat-resistant, smooth-surfaced heat-shrinkable tape (for example, mylar tape) and heating it from the outside to soften and smooth the surface of the rubber/plastic insulating layer where it is wrapped.
A method of unwinding the tape can then be used, and the working length l ₁ is usually about 10-20 cm.

Reference numeral 3 denotes an insulating tube, such as a vinyl chloride tube, which can surround the end of the cable, and a flange fitting 31 is attached to the lower end, and this flange fitting 31 is fixed to the cable sheath 15 with seal tape 4. 5 is insulating oil injected into the insulating cylinder 3. 61,62
7 is a grounding wire, and 7 is a winding tape (for example, vinyl tape) for preventing oil intrusion.

To perform a DC withstand voltage test on the rubber/plastic cable described above, a DC voltage is applied to the cable conductor. This electrification generates a charge in the insulating oil, and the charge with the opposite polarity to the applied voltage (that is, the charge with the same polarity as the induced charge induced in the cable shielding electrode) is directed toward the cable insulation layer by the electrostatic field. It once drifts and accumulates on the surface of the cable insulation layer, forming a so-called interfacial charge layer. When this interfacial charge is formed, the potential distribution V at the cable end under consideration is:
It is expressed as div grad V=-q/E (V: electric potential, q: interfacial charge density, E: dielectric constant), and is clearly different from the potential distribution div grad V=0 when no interfacial charge is generated. be. Here, the case where no interfacial charge is generated refers to an alternating current electric field in which the direction of the electric field is periodically reversed, or an impulse electric field in which the electric field is applied for a short time.

This interfacial charge has a higher density in a region where the electric field is stronger when no interfacial charge is formed, and has the effect of relaxing the electric field in that region. That is, under DC charging conditions, this interfacial charge relaxes the electric field concentration at the end of the cable shielding electrode, creating an electric field relaxation state similar to that of an artificially applied stress cone or resistance grade. Therefore, even if the insulator is simply exposed, if a DC withstand voltage test is performed with the terminal section immersed in oil, the breakdown voltage of the terminal section will increase significantly compared to alternating current or impulse voltage. In addition, in the case of AC and impulse voltages, the terminal destruction always occurs at the end of the external shielding electrode (section 131 in the figure), but in the case of DC voltage, it occurs within about several centimeters from the end of the electrode. Breakdown occurs at the terminal insulator portion of the electrode, and does not necessarily occur at the electrode end. This result indicates that there is an electric field relaxation effect due to the interfacial charge in the case of a DC withstand voltage test. The reason why the breakdown location is far away from the electrode end is that the terminal insulator near the electrode end is shielded by a high-density interfacial charge, and the electric field in the insulator in this area is transferred to the main pole of the cable. This is because the value is approximately the same as the electric field of an insulator. Therefore, if there are minute scratches or irregularities on the surface of the insulator in this part, the electric field will concentrate there and the breakdown strength will decrease.

However, if the insulator surface of this part (l ₁ part) is smoothed as in the present invention, the breakdown strength of this part can be significantly increased, and voltage breakdown at the cable terminal can be prevented. can.

Currently, 6.6KV class cross-linked polyethylene cable (3
When the terminal section of the present invention was used for a DC withstand voltage test (exposed insulating layer length l ₀ = 120 cm, smoothing length l ₁ = 10 cm), dielectric breakdown occurred on the cable side at DC -440 KV. However, in the case without smoothing, the breakdown occurs at the cable end, and the breakdown voltage is
It was DC-300 to 340KV. Furthermore, when the terminal of the present invention was applied to a 77 class cross-linked polyethylene cable (l ₀ = 3.5 m, l ₁ = 20 cm), results were obtained that the cable had a withstand voltage performance of DC-800 KV or higher.

<Effects of the invention> The terminal portion of the rubber/plastic cable for direct current withstand voltage testing according to the invention has the structure as described above, and the exposed rubber/plastic insulating layer near the end of the cable external shielding layer is smoothed, and,
The insulation in oil is sufficient, and the formation work of the conventional stress cone or resistance layer is unnecessary, and only smoothing treatment is required, so the formation work is simple.

[Brief explanation of the drawing]

The drawing is an explanatory diagram showing a terminal part for a DC withstand voltage test of a rubber/plastic cable according to the present invention. In the figure, 1 is a rubber/plastic cable, 12 is a cable insulation layer, 120 is a smoothed portion, 131 is an end of the cable outer shielding layer, and 5 is insulating oil.

Claims (1)

[Scope of utility model registration request] The cable sheath and external cable shielding layer are removed from the end of a rubber/plastic cable to expose the cable insulation layer, and the exposed cable insulation layer is immersed in insulating oil. Rubber made by relaxing the electric field
In the terminal section of a plastic cable for DC withstand voltage testing, a heat-resistant smooth surface is provided on the surface of the cable insulation layer near the external shielding layer, which is wrapped around the section, heated, and removed after heat treatment. A terminal section for DC withstand voltage testing of a rubber/plastic cable characterized by being smoothed with heat-shrinkable tape.