The ADSS cable subjected to tension during operation is in the strong electromagnetic field existing in the space around the wire, and the capacitance coupling between the cable and the earth makes it in a space potential position. Under the action of space potential, the wet or dirty (inevitable) surface of the optical cable will produce a ground leakage current and heat, the heat will make the optical cable surface water evaporation, random (uncontrollable) to form a dry zone, blocking the surface leakage current. When the potential at both ends of the strip is high enough, a discharge is generated to form an arc (called a "dry strip arc"). The basic conditions for the occurrence of electrical corrosion are certain leakage current and high enough space potential.
Ground leakage current:
According to west Coast data of Hanterstor, Scotland: no arcing occurs when ground leakage current is less than 0.3mA. Therefore, 0.3mA is recognized as the threshold value for non-arc occurrence; When the ground leakage current exceeds the threshold of about 0.5mA, an arc will be generated; As the ground leakage current exceeds 1mA, the current isolation becomes serious. However, when the ground leakage current is larger (about more than 5mA), the arc activity will stop, that is, the high current does not produce an arc. The test results also show that the degree of electrical corrosion is related to the tension of the cable.
Standard for optical cable to withstand space potential:
According to the corresponding standards and specifications, the space potential of ADSS cable sheath can be divided into two levels.
Grade A: PE sheath ≤12KV;
Grade B: AT sheath ≥12KV; The upper limit of class B sheath is not stipulated in relevant standards and specifications, and is usually referred to as 20 -- 25KV.
The ground leakage current is driven by space potential. In transmission lines on a distance, assumes that the cable and wire spacing is constant (sag) and with the spacing is constant (in fact, impossible, is only assumption), with the various points and grounding on the surface of the fiber optic cable end distance change is very big, although the induced voltage in the middle of the span is very high, but the constant charging is very big, charging current, cable surface without leakage current; With the approach to the end of the metalware installed in the tower, the induced voltage tends to zero sharply, while the grounding leakage current is large, and the grounding leakage current at the end of the grounded metalware reaches the maximum value. If the arcing condition is satisfied, electrical corrosion occurs. The distance from the point at which the ground leakage current begins to increase to the end of the fixture is called the "active length", which is the most dangerous area for electrical corrosion. Thus, there are two active lengths within a pitch.
Influence of dual circuit phase:
The spatial potential of cable suspension point is affected by the phase arrangement of three-phase conductor, the phase change of single circuit has little effect, and the phase change of double circuit has great effect. In the dual-circuit system, the abC-ABC phase is the smallest in terms of the installation "space" of ADSS, while the other phases are enlarged to varying degrees, especially the "space" of ABC-CBA phase is the largest. For a certain suspension point, the single circuit operation of double circuit line is complicated due to power failure on one side, and the space potential may decrease or increase. Other effects in addition to phase, when the suspension point of a cable is certain, the space potential will also be affected by the diameter of wire and ground wire, wire splitting, fittings and cable swing, etc.
Control of electrical corrosion:
As far as is known, all corrosion faults occur in the "active length" region, so the range to be controlled is also concentrated in the active length region.
Under static condition, the space potential of the hanging point of AT sheath ADSS cable working in 220KV system should be controlled not more than 20KV (double loop and multi-loop common frame line should be lower); For ADSS cable with PE sheath working in 110KV and below system, the space potential of its hanging point should be controlled at less than 8KV. The spatial potential design of static hanging points should take into account:
(1) System voltage and phase arrangement (double-loop and multi-loop are very important).
(2) tower shape (including tower head and call height).
(3) The length of the insulator string (the length varies according to the grade of contamination).
(4) diameter of guide/ground wire and wire splitting.
(5) The safety distance between the wire and the ground and the crossing object.
(6), tension/sag/span control (in no wind, no ice and under the condition of annual average temperature, the load is not greater than the cable ES is 25%RTS; In the design of meteorological conditions, the load is not greater than the cable MAT 40%RTS).
(7) jumper (tensioning tower) and grounding body (such as cement rod cable) should be studied and considered.
Under dynamic conditions, the space potential of AT sheath ADSS cable working in 220KV system should be controlled not more than 25KV; For ADSS cable with PE sheath working in 110KV and below system, the space potential of its hanging point should be controlled not more than 12KV. Dynamic conditions should at least take into account:
(1) System voltage is nominal voltage, in some cases there will be +/- (10-15) % error, take the positive tolerance;
(2), gold string (mainly hanging string) and cable swing;
(3) the possibility of transposition of the original phase;
(4) The possibility of single loop operation of double loop system;
(5) Actual situation of pollution removal in the region;
(6) There may be new crossing lines and objects;
(7) Status of municipal construction and development plan along the line (it is possible to raise the ground);
(8) other circumstances that will affect the optical cable.
The electrical corrosion of ADSS cable sheath subjected to tension in operation is caused by ground leakage current and dry band arc of approximately 0.5 -- 5mA caused by space potential (or electric field intensity) through capacitive coupling. If measures are taken to control the ground leakage current below 0.3mA, no continuous arc is formed, the electrical corrosion of the sheath will not occur in principle, the most realistic and effective method is still to control the cable tension and space potential. The static space potential design of ADSS cable with AT or PE sheath should not be greater than 20KV or 8KV respectively, and should not be greater than 25KV or 12KV respectively under the worst dynamic conditions. Fiber optic cables are safe to operate. The static space potential of 20KV (mostly 220KV system) or 8KV (mostly 110KV system) is not less than (1 -- 3) m or 0.5m at the end of the anti-vibration whip from the metal tool to the twisted wire respectively, which is one of the effective measures to improve the electric corrosion of ADSS cable. At the same time, the vibration damage of ADSS cable and other anti-vibration methods (such as applicable anti-vibration hammer) should be studied.
The installation position of the cable cannot be determined empirically simply by the system voltage level and/or the distance of the out-of-phase conductor (often referred to as the "hanging point"). The spatial potential of the hanging point should be calculated according to the specific situation of each tower type. Although in recent years, ADSS cable electrical corrosion failure occurred repeatedly, but a lot of practice has proved that ADSS cable can continue to be popularized and applied in 110KV system; The ADSS cable used in 220KV system can also be extended after considering static and dynamic working conditions. On the premise of ensuring the quality of ADSS cable and standardizing the engineering design, construction and operation conditions, the electrical corrosion of ADSS cable can be controlled. It is suggested to formulate and implement corresponding norms/procedures.