JPH0365863B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention is a ground fault detection system that uses optical fibers to quickly detect ground faults and locate ground fault points in disaster prevention troughs that house power cables such as ultra-high voltage OF cables. It depends on the detection method.

There are various proposals to improve the disaster prevention function of troughs that house power cables, but for example, sealed disaster prevention troughs made of reinforced plastics containing flame-retardant additives such as aluminum hydroxide have been proposed to prevent cables from grounding. If the damage occurs due to the gas pressure that sometimes occurs, fresh air will flow in through the damage,
There was a risk that the fire would spread to the cable because the cable rupture hole heated by the arc would ignite. For this reason, improvements have been made to the disaster prevention trough itself. For example, pressure relief sections are provided at predetermined intervals on the lid of the disaster prevention trough, and the shapes of these pressure relief sections have also been improved in various ways.
Disaster prevention troughs have also been created that have pressure relief sections with slits that can withstand earth fault currents of ~60 KA.

However, when a ground fault occurs in a power cable, it is difficult to say that there is an appropriate method for detecting the fault and marking the ground fault point.

In other words, the Murray loop method, pulse radar method, discharge detection type pulse radar method, etc. are known as methods for locating this ground fault point, but the Murray loop method is used for locating parallel healthy phases, such as a three-phase simultaneous ground fault. If there is no ground fault, or if the ground fault resistance is high and the fault point does not discharge even if a high-voltage power source is used, the measurement accuracy will be slightly low, and the measurement requires skill. Furthermore, all of the pulse radar methods described above utilize reflection of electrical pulses, and cannot be said to have sufficient search sensitivity.

The present invention has been made under these circumstances, and it is designed to install optical fibers in the cable storage area of a trough in which power cables are laid, and to prevent the damage of optical fibers caused by high temperatures when a power cable ground fault occurs. A ground fault detection method characterized by detecting reflected light or backscattered light from a fault point where a break or local transmission loss changes to detect a ground fault accident and locate the ground fault point. be.

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a cross-sectional view showing a state in which optical fibers are arranged in a cable storage part for carrying out the present invention. A power cable 2 is housed and laid in the center of 1-1, and an optical fiber 3 is placed at an appropriate position in the remaining space. (In the figure, it is held along the upper edge of the cable storage section 1-1.) FIG. 2 is a simplified explanatory diagram for explaining the method of the present invention, in which a power cable is stored and laid in the trough 1, An optical fiber 3 is provided in parallel with this, and this optical fiber 3 is connected to a ground fault point locator 4. Here, when a pulse signal is sent from one end of the optical fiber 3, it returns as a reflected pulse at the broken point, and this broken point can be detected by the ground fault location device 4, resulting in the detection of the ground fault position.
Although not shown, if one end of the optical fiber 3 is connected to a ground fault point locator through a light emitting section and the other end is connected to a ground fault point locator through a light receiving section, a break in the optical fiber will be immediately transmitted to the ground fault point locator. , the occurrence of a ground fault can be detected.

The case where the optical fiber is disconnected due to a ground fault has been described above, but ground faults can also be detected based on changes in transmission loss and the ground fault point can be located for failures other than disconnections.

Regarding the optical fiber used in the present invention, for example, as shown in FIG. 3, the optical fiber 3 consists of an optical fiber strand 31, a primary coat 32 coated thereon, and a secondary coat 33 thereon.
The optical fiber 31 is a normal silica-based optical fiber having, for example, a core diameter of 50 .mu.m, a cladding diameter of 125 .mu.m, and a graded refractive index distribution with a relative refractive index difference of 1% between the core and cladding. The primary coat is not necessarily soft, but is preferably harder than normal. The secondary coating layer 33 is made of crosslinked polyethylene, silicone rubber, heat-shrinkable rubber such as ethylene vinyl acetate resin, or plastic.

When such an optical fiber is used, rapid thermal contraction occurs when the ambient temperature rises, and this strongly acts on the optical fiber strands, increasing transmission loss.

In addition, if the cladding material of the optical fiber is selected appropriately, the transmission loss will change due to the addition of thermal history, and the reflection of optical pulses can be increased. It is also possible to make it easier. In the event of an actual ground fault, the temperature near the ground fault will be about 5 seconds at 200℃ or higher, and about 2 seconds at 300℃ or higher. It is even more preferable to use optical fibers.

As described above, according to the present invention, the optical fibers installed in parallel to the power cable in the trough are used to prevent breakage in the event of a ground fault, as well as damage to the input end due to increased loss at high temperatures. By using changes in the intensity of the returning scattered light or reflected light, it is possible to measure the loss in the length direction of the optical fiber (optical pulse measurement) using a ground fault location device, which can prevent abnormal temperature rises due to ground faults. By detecting a sudden drop in the light reception level in that area when an abnormality occurs, it is possible to know the location of the abnormality, and to detect changes in the light reception level, a continuous monitoring device is configured using, for example, a differential circuit.
It can be seen as a waveform signal.

In addition, the present invention does not require any special operation to locate the ground fault point, so no skill is required, and since no cable is used, the ground fault point can be located regardless of the nature of the accident. Since the pulse response is better than that of the conventional method, highly accurate positioning can be achieved.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a trough equipped with optical fibers for carrying out the present invention, FIG. 2 is a simplified illustration of the method of the present invention, and FIG. 3 is an example of the optical fiber used in the present invention. FIG. 1...Trough, 1-1...Cable storage section, 1
-2...Lid, 2...Power cable, 3...Optical fiber, 4...Ground fault locator.

Claims (1)

[Claims] 1. An optical fiber is installed in the cable storage part of a trough with a power cable installed inside, and it is possible to prevent optical fiber breakage caused by high temperature when a power cable ground fault occurs or from a failure point where local transmission loss changes. A ground fault detection method comprising locating a ground fault and a ground fault point in a power cable by detecting reflected light or backscattered light to detect a fault point in an optical fiber.