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Through a case study on the handling of a fault in which a current transformer was damaged multiple times, this post shares with you the differences between the selection and application of electrical equipment for high-altitude areas and those used in normal environments. Faulty equipment: GBC-35 type high-voltage switchgear in a certain 35kV substation. : The current transformers in the incoming cable cabinet and the main transformer outgoing cable cabinet have been damaged. Eight typical applications of high-value and low-value selectors: yunrun.com.cn/tech/1974.html ① After replacing the current transformer, it was used normally for over a year; however, the fault of the current transformer (denoted by TA) being damaged occurred again in the incoming cable cabinet. The arc generated by the short circuit radiates to the adjacent busbars, triggering another inter-phase short circuit and causing the longitudinal differential protection to activate. ②Located at an altitude of 2,600 meters, the conductive and insulating components in the high-voltage switchgear in this substation must be of the high-altitude type. Inspect the components inside the cabinet; the ZL-35/400Y type bus pole insulators, LCZ-35 type current transformers, CWB-35/400 type bushings, etc., are not designed for high-altitude areas and can only be used in regions below an altitude of 1000 meters. ③Furthermore, in accordance with the standards of the power industry, in high-altitude areas, the requirements for the air insulation gaps within high-voltage cabinets are different as well; they must be greater than those in low-altitude areas. For example, at an altitude of 2200m, the clear distance between the conductor and the grounding electrode, as well as the clear distance between conductors of different phases, is required to be 336 mm; at an altitude of 1000m, this value is only 330 mm. Moreover, the air insulation gaps in these high-voltage cabinets are generally quite small as well. yunrun.com.cn/tech/1973.html ① Replace components that are not suitable for high-altitude areas: a) Replace busbar post insulators with ZL-35/400GY type; b) Replace current transformers with LCZ-35Q type; c) Replace wall-mounted bushings with CWB-35/400GY type. ②At locations where the air gap is small, a solid insulating plate is inserted as a barrier, or heat-shrinkable insulating sleeves are used to cover the copper busbars. After the modification, an insulation withstand test was conducted, and the results met all the regulatory requirements; since then, no further faults have occurred. In high-altitude areas, air density, pressure, temperature, and humidity all decrease accordingly, leading to a reduction in the insulating properties of both air and insulating materials. Therefore, in high-altitude areas with an elevation of over 1000 mm, the external insulation strength of high-voltage electrical equipment must be compensated for. Power industry regulations stipulate that at altitudes of 2000–3000 meters, the electrical strength of high-voltage electrical equipment below 110 kV must be increased by one level. High-voltage switchgear must be manufactured in accordance with the standards of the power industry; otherwise, it will pose safety risks. The impact of temperature and pressure in high-altitude areas on electrical switching equipment 1. Selection based on conditions: At altitudes ranging from 1000 to 5000 meters, for every increase of 100 meters, the pressure decreases by approximately 0.8–1 kPa ； A decrease in air pressure reduces the dielectric strength and cooling efficiency of the air, leading to difficulties in arc extinguishment in switches and an increase in electrical temperatures. At the same time, excessively low air temperatures can cause certain materials within electrical equipment to become hard and brittle, and can increase the viscosity of some oils or cause them to solidify, thereby affecting the proper functioning of the equipment. Large temperature differences between day and night can lead to condensation, causing components to deform, crack, or have their ceramic parts break. Therefore, the selection of electrical equipment in high-altitude areas has specific requirements; it is necessary to verify its electrical parameters or opt for electrical equipment designed for high altitudes. 2. Insulation performance: The climate in high-altitude areas has a significant impact on high-voltage switchgear. For products installed in high-altitude areas, such as electrical appliances selected for testing at altitudes below 1000 m, the power-frequency and impulse test voltages shall be appropriately corrected in accordance with IEC Publication 694. 3. Temperature rise: In high-altitude environments, the air density decreases, which weakens the convective effect for heat dissipation; as a result, the temperature rise increases. In high-altitude areas, the temperature drops by 0.5°C for every 100 meters of elevation increase, and it can be considered that this decrease in temperature partially compensates for the above effect. The low air density during low pressure reduces the dielectric strength of the air, thereby causing a decrease in the insulation voltage tolerance of electrical equipment ； Due to the increase in the temperature of components in air-cooled electrical equipment, as well as the difficulty in extinguishing arcs in air, it is necessary to pay attention to temperature rise compensation for such electrical equipment and take appropriate measures. 4. Making-and-breaking capacity and sectionalizing capacity: A decrease in air pressure and air density in high-altitude areas leads to longer arc extinguishing times for switchgear, as well as severe damage to the contacts, which in turn reduces their making-and-breaking capacity. Therefore, it is necessary to take into account the making-and-breaking capacity of electrical equipment used in high-altitude areas. 5. Electrical life: Due to the low extreme temperatures in high-altitude areas, as well as the product’s temperature rise and arc extinguishment time, the high-altitude environment has a certain impact on the lifespan of electrical equipment. In high-altitude areas, the temperature difference between day and night is large; as a result of thermal expansion and contraction, organic insulating coatings and materials develop cracks. In the presence of oxygen and dust, this accelerates their aging process and shortens their service life. Suggestions for the selection and application of high-voltage switchgear in high-altitude areas: 1. Currently, medium-mounted switchgear is designed for use at normal altitudes ； Trolley-type switchgear cannot resolve corona or discharge issues in the busbar bushings and contact boxes inside the cabinet in high-altitude areas. The switchgear suitable for high-altitude areas is limited to fixed-type switchgear or gas-insulated switchgear. A fixed cabinet is one option, but it’s not the best solution; strictly following the standards, the height can only reach 3800m. To fully meet the standards, it is best to use an inflatable cabinet. 2. When a fixed cabinet is applicable, the model of the circuit breaker inside the cabinet also requires the altitude of the location where it will be used to be specified. And a **formal GY-type test report is required. In addition, the switchgear (including the devices inside it) meets the requirements for high-altitude areas in strict accordance with standards.