In recent years, due to the rapid economic development, the power load has also increased substantially. In order to adapt to the economic development and improve the power supply capacity, a new power project has been put forward by a ThermoElectron Corporation, and some new projects have selected single core power cables. But before and after 1997, single core power cables were out of order, which made people doubt the application of single core power cables. Therefore, a clear understanding of the cause of the accident, and correctly understand the single core cable will play a beneficial role in the rapid development of the current and future electricity.
1.accident brief example
Old east city substation main change capacity of 31.5 MVA, rated current 1734 A. At the 10.5 kV side of a small load, the YJV32-8.7/15-1 * 240 type mm2 cables per phase with two parallel cables, cable laying the main changes from the 10.5 kV frame to the distribution room, in cable trench, but in the frame near the cable spare length is greater than 10 m, the phase plate circle after buried underground, shield ends ground. In the summer of 1997, a system accident occurred. After searching, the cable was buried in the underground part of the damage, while the load current was only about 600~800 A. The 35 kV outlet frame of old city substation is connected by cable, the cable is YJLV32-26/35-1 * 240 mm2, the cable is buried directly, and the spare length is embedded in the split coil. Shield grounding at both ends. In the summer of 1996, the system single-phase grounding, after inspection, the cable has a root end of 20 cm long, has been burnt paste, while the armored ground wire welding is not good. West thermal power plant 35 kV outlet knife to overhead line end rod with YJLV32-26/35-1 * 185 mm2 cable connection. The installation is the same as above. In the summer of 1997, a system accident occurred, and the cause of the accident was checked as above.
2.accident cause analysis
The above examples can be summarized as follows: from the cable model YJV32 and YJLV32, it can be seen that the cables are XLPE insulated PE sheath, fine steel wire armored copper core or aluminum core power cable. The cables are buried in the soil and are laid separately. Shield grounding at both ends. For the convenience of analysis, look at the city (including farm) meteorological conditions: extreme maximum temperature +43.1, the extreme minimum temperature -42.8 degrees Celsius, average evaporation capacity: 1550.6 mm, an average annual rainfall of 338.2 mm, and 6 to September rainfall is only 17% of evaporation. YJV32 and YJLV32 single core cable, its wire sheath is woven with 4 copper wires. The role of copper wire is to isolate the steel wire, so that the steel wire can not be formed into the magnetic circuit. It has been proved by experiments that the effect is limited. However, the load carrying capacity of armored cable will be reduced by 30% - 40% compared with that of non armoured cable. The decrease of small section is large, and the decrease of large section is small. For the 240 mm2 section, the decrease is 40%.
Due to severe shortage of rainfall in this region. The soil is very dry, in the calculation of cable ampacity, soil resistance coefficient should be 3 Km/W, while the general information provided in the load flow is in the soil resistance system with 1 Km/W conditions, so that the soil resistance coefficient is 3 Km/W compared with 1 Km/W capacity reduced 20% ~ 25%. Embedded single core cable in soil take triangle array, and a three-phase gapless contact load flow; planar arrangement, cable spacing is 2 times of the diameter of the cable, cut the flow of small, the latter than the former reduces from 23% to 26%. But the horizontal arrangement in the air is opposite, larger than the triangular arrangement, and the carrying capacity is increased by 20% to 30%, which can not be ignored. As mentioned earlier, we are disorderly and buried in the soil, rather than adopting a favorable triangle arrangement, which inevitably loses the carrying capacity.
The shielding and armoring of cables are either one end or both ends, and the ampacity will also be affected. The size and arrangement of cables on the way to the end, triangle, grounding and grounding ends of load flow changes little, about 3%, but the level of arrangement, two end grounded than the end of the ground load flow decreased by 13%. The steel wire armored ground wire can not be firmly welded and the contact resistance is increased. When the circulating current passes through, the heat is concentrated at a certain point, so that the temperature is increased, and the insulation is damaged because of overheating, and is also worth considering. The temperature change in our district can reach -42.8 degrees centigrade from +43.1 centigrade. Such a great change is undoubtedly a great test for the material and construction technology of the cable terminal. Because of the convenience of construction and the low cost, few prefabricated terminals with good safety are used at the moment, and heat shrinkable and cold shrinkable terminals are preferred. And a variety of manufacturers, can not help but mixed, the products with poor sexual ability may be used. If the product has poor weatherability, high and low temperature after the change, loss of elasticity and tightness, easily absorbed by moisture, water treeing phenomenon of cable insulation, the destruction of the accident. To prevent the occurrence of such accidents, regular products should be used by qualified manufacturers to produce qualified products. Under the condition of better topography and geology in the substation, the choice of no steel strip or without steel wire armour can meet the requirement. If using steel wire armored single-phase power cable, in several cases, the cable carrying capacity will be reduced by 40%, so it is inadvisable to choose steel wire armored single-phase power cable here.
3.conclusion
In order to avoid cable accidents, should be carried out in strict accordance with the design and construction of the "norm" and "engineering design of cable power cable line construction and acceptance specification" requirements, and adopted the latest scientific and technological achievements is the key, the special points are summarized as follows.
In the distribution panel, the load current is large, the single core cable is optional, even is required. Under normal circumstances, if the use of three core cable, the terminal will inevitably lead to intensive wiring, it is difficult to ensure the safety of electrical distance, the use of large cross-section of single core cable can improve the situation
