Power Outages If There Is Thunder In Rural Areas? Maybe The Transformer Is Not Ready For Lightning Protection
In a Miao village in Guizhou, whenever a thunderstorm cloud swept over the footstools, the rusty transformer at the entrance of the village will be on time "strike". Power repair records show that the village experienced 27 lightning strikes during the three years of power outages, the shortest failure interval of only 8 days. This is not an isolated case - the national grid statistics show that the rural power grid lightning failure rate is 4.3 times that of the urban power grid, and the core of the problem is often hidden in the unassuming transformer.
I.Lightning: the invisible killer of the rural power grid
Lightning strikes instantly generate shock waves comparable to small missiles. 10kA lightning current injected into the transformer windings in 0.1 seconds, the coil turn-to-turn voltage up to 20 times the normal operating voltage. This overvoltage as current tsunami, can instantly break through the aging insulation paper, resulting in a short circuit between turns. Anatomy of a mountain transformer shows that the winding insulation carbonization depth of 3mm after being struck by lightning, equivalent to the equipment life shortened by 15 years.
Grounding system failure is the amplifier of lightning damage. Detection found that some rural transformer grounding resistance as high as 48Ω, far more than 10Ω safety standards. This is equivalent to the construction of lightning "highway", lightning current can not be effectively leaked into the earth, the counter-attack over-voltage distribution lines into a charged "fire snake", triggering a chain of tripping.
II.The lightning protection system of the "triple loophole"
Lightning arrester aging as a failure of the "lightning rod". A village using the valve-type lightning arrester has been in service for 12 years, the test found that its DC reference voltage decreased by 35%, the residual voltage value rose to 4.8kV. this state of the lightning arrester response time delay of 0.5μs, can not be in the 8μs of the thunder voltage wave head time within the effective action, resulting in 70% of the direct intrusion of lightning current into the equipment.
Insulation fit disorders exacerbate equipment damage. Transformer casing creepage distance if less than 18mm/kV, rain surface leakage current will increase three times. A lightning accident, due to the casing of the outer insulation strength mismatch, resulting in discharge channels along the surface, the industrial frequency renewal current lasted 0.8 seconds, and ultimately triggered a fire caused by the surge in oil temperature.
Lack of line protection to form a "lightning channel". When the overhead line stall distance exceeds 150 meters, the induction overvoltage strength increased by 60%. A village in Guangxi 10kV line due to the lack of installation of overhead ground lines, lightning tripping rate is 2.7 times the standard line. Conductor and tree safety distance is less than 3 meters, tree flash will trigger a second lightning strike.
III.The construction of lightning protection "Golden Bell"
New lightning protection equipment to bring technological innovation. Zinc oxide lightning arrester using multiple parallel structure, the residual voltage will be controlled below 2.3kV. Intelligent detuning device can recognize the characteristics of lightning strike traveling wave and put in current limiting reactor within 3ms. After installing a lightning protection integrated management device in a pilot village, the lightning strike tripping rate dropped by 82%.
Grounding network transformation is the cornerstone of lightning protection. The use of copper-coated steel grounding body with resistance-reducing agent, can make the grounding resistance stabilized at 4Ω or less. The three-dimensional grounding network design combines the horizontal grounding electrode and vertical grounding electrode, and utilizes the soil layering characteristics to establish a three-dimensional drainage channel. After the transformation of a mountainous substation, the impact impedance of the ground network is reduced to 7Ω, and the lightning current discharge efficiency is increased by 65%.
Routine maintenance builds the last line of defense. Regular scanning of the infrared thermal imaging camera can detect a contact point temperature rise of 0.5℃, and ultrasonic testing can capture localized discharge signals at the 30pC level. A power supply center restored the flashover voltage to 85% of the new product by cleaning the insulators of accumulated dirt. Anti-corrosion treatment of grounding lead wires extends the life of the grounding system to 25 years.
In an island village in Fujian, the transformer after lightning protection renovation has been in safe operation for 1400 consecutive days. When we see the transformer on the rainy night lightning arrester flashing blue glow, that is not only the release of the surge, but also the modern lightning protection technology weaving the guardian knot. The Chinese Academy of Electric Power predicts that with the popularization of graphene grounding materials and intelligent lightning arrester, the lightning failure rate of rural power grids will drop to urban levels within five years. The next time a thunderstorm strikes, those once-fragile transformers, or will be transformed into a guardian of the light of the steel guards.