"Cable sheath circulation"is a special current phenomenon in the operation of high-voltage cables, which belongs to the metal sheath (such as aluminum sheath, copper sheath) and the earth between the formation of a closed loop of induced current, and this phenomenon mainly occurs in the single-core high-voltage cable system. When a single-core cable passes alternating current, an alternating magnetic field is generated around the conductor. This magnetic field will induce an electromotive force on the metal sheath, and if the sheath is grounded at both ends (forming a loop), a loop current will be generated.

I. Three major risks from circulation

1, additional loss: ring current abnormality and long-term unprocessed, it will lead to continuous heating of the sheath, so that the cable temperature rise, measured cases show that when the ring current exceeds the permissible value of 30%, the XLPE insulation layer temperature gradient rises 8-12 ℃, a direct impact on the carrier flow.

2, accelerated aging: continuous circulation will trigger galvanic corrosion of the sheath material, shortening the life of the cable.

3, grounding system overload melting: large ring current state, may burn the grounding box, ring current concentration flow through the grounding wire, may trigger the connection point overheating. When the ring current up to 300A, copper grounding wire joint temperature can rise to 280 ℃ in 2 hours, eventually leading to melting, and this type of failure accounted for 23% of the failure of the cable intermediate joints.

II. Key role of sheath circulation monitoring devices

Shenzhen Dingxin Wisdom Technology Co., Ltd's Sheath Circulation Monitoring Device, with high-precision CT sensing, multi-parameter synchronous analysis, capable of 24-hourContinuous monitoring of cable parameters, to realize the ring current multi-parameter diagnosis of abnormalities. There are various versions of the equipment available, such as tower version, grounding box version, etc., in which the power extraction methods include CT inductive power extraction and photovoltaic power extraction, which can be selected according to the user's needs.

Past installation cases have shown that the sheath circulation monitoring device can not only reduce line loss by 3%~5%, but is also the key to safeguard the life of the cable. With the increase of cableization rate in the city, it will also become the standard configuration for the operation and maintenance of high-voltage cables.