The impact of high-altitude environments on high voltage testing

The impact of high-altitude environments on high-voltage products is multifaceted, with the core reasons being the reduction in air density and atmospheric pressure, which directly affects the dielectric strength (insulating capability) and heat dissipation efficiency of air.
The following is a detailed analysis of the main effects, principles, and countermeasures:

1. Core Impacts and Principles

1). Core Impacts and Principles1. Decrease in External Insulation Strength (The most significant impact)

· Principle: Low atmospheric pressure at high altitudes results in thin air and increased spacing between gas molecules. This allows electrons to gain longer mean free paths and greater kinetic energy before colliding with the next molecule, making collision ionization easier to occur, leading to a reduction in the breakdown voltage. Products become more prone to arcing, flashover (discharge along the insulation surface), and breakdown.

· Specific Manifestations:

o Air Gap Breakdown Voltage: The breakdown voltage for the same air gap decreases significantly with increasing altitude.

o Insulator Flashover Voltage: The flashover voltage along the surface of insulators, whether support insulators or bushings, also decreases.

· Engineering Specifications: During design, altitude correction must be applied to the insulation distance based on the product's operational altitude. Correction factors (>1) are typically used to increase the required insulation distance or insulation level. International standards (such as IEC 60071) and national standards (such as GB 311.1) provide detailed regulations on this.

2). Decreased Heat Dissipation Capacity (Temperature Rise Issue)

· Principle: Heat dissipation occurs primarily through three methods: conduction, convection, and radiation. In high-altitude areas, the efficiency of convective heat dissipation is greatly reduced. Because the air density is low, its heat-carrying capacity is poorer, making it harder to dissipate the heat generated during product operation.

· Specific Manifestations:

o The temperature rise of transformers, reactors, and other equipment will exceed design values.

o The junction temperature of power devices (such as IGBTs in inverters, SVGs) increases, reducing reliability and shortening lifespan.

o All heat-generating equipment, such as motors and circuit breakers, may face overheating risks.

· Engineering Specifications: The cooling system of the product needs to be redesigned or the product must be derated. For example, increasing the heat dissipation area, adopting forced air or liquid cooling, or directly reducing its rated capacity (i.e., "derating") to ensure the temperature rise remains within allowable limits.

3). Reduction in Corona Inception Voltage

· Principle: Corona is a luminous discharge phenomenon that occurs when the air undergoes local ionization due to an extremely high electric field strength near an electrode. As altitude increases and pressure decreases, corona discharge becomes more likely.

· Specific Manifestations:

o Corona is more likely to occur where the electric field is concentrated, such as on high-voltage conductors, busbar connections, and the ends of transformer bushings.

o Corona generates ozone (O₃), which corrodes metals and organic materials; produces audible noise (buzzing) and radio interference (RF noise), affecting communications; and also causes energy loss.

· Countermeasures: Use larger diameter conductors, employ corona rings (grading rings), optimize electrode shape (e.g., using spherical ends) to smooth the electric field distribution and reduce local field strength.

4). Increased Internal Pressure in Sealed Products

· Principle: For sealed electrical equipment (such as gas chambers in GIS (Gas Insulated Switchgear), high-voltage capacitors, cable terminations, etc.), the internal pressure is sealed at standard atmospheric pressure. When transported to a high-altitude, low-pressure environment, the external pressure decreases, leading to an increase in the pressure difference between the inside and outside of the equipment.

· Specific Manifestations:

o Places enormous stress on the equipment enclosure and sealing structure, potentially causing deformation, leakage, or even rupture.

o For oil-filled equipment, it may cause expansion and deformation of the conservator or tank.

· Countermeasures: Strengthen the mechanical strength of the enclosure and improve the sealing design. For gas-filled equipment like GIS, it is common to fill them with higher pressure insulating gas (like SF₆) at the factory located in a lower altitude area to compensate for the low external pressure at high altitudes.

5). Enhanced Ultraviolet (UV) Radiation

· Principle: At high altitudes, the air is thin, and the atmosphere's absorption and scattering of ultraviolet radiation are weakened, leading to a significant increase in UV intensity at ground level.

· Specific Manifestations: Accelerates the aging, cracking, and degradation of organic insulating materials, seals, paint coatings, etc.

· Countermeasures: Select materials with strong weather resistance and UV resistance (such as specific types of silicone rubber, EPDM, etc.).

2. Summary and Response Strategies

To ensure the safe and reliable operation of high-voltage products in high-altitude areas, manufacturers and designers must take the following measures:

1. Insulation Reinforcement and Correction: According to the altitude, increase the creepage distance and electrical clearance based on the correction factors specified in the standards, or increase the product's rated insulation level.

2. Thermal Design Optimization: Enhance cooling solutions, such as increasing heat sink size, adopting forced cooling, or clearly marking derating curves for different altitudes (i.e., the higher the altitude, the lower the maximum allowable power or current).

3. Corona Protection Design: Conduct strict electric field simulation calculations for all high-voltage terminals and connection parts, using grading and shielding methods to suppress corona.

4. Mechanical Structure Reinforcement: For sealed equipment, verify the mechanical strength of the enclosure to ensure it can withstand the significant internal-external pressure difference.

5. Material Selection: Use special materials that are resistant to high temperatures, UV radiation, and aging for key components.

6. Standards and Testing: Product design, manufacturing, and testing must comply with relevant high-altitude technical standards, and type tests should be conducted, where possible, in simulated high-altitude low-pressure chambers.

In conclusion, the high-altitude environment is not simply a change of location for high-voltage equipment; it is a severe challenge that requires systematic redesign and evaluation from four aspects: electrical insulation, heat dissipation, materials, and mechanical structure.