Can hundreds of thousands of volts of high voltage break down optical fibers? Does the metal casing need to be grounded?
In high-voltage transmission lines, substations, and UHV power engineering, fiber optic sensing technologies (such as Fiber Bragg Grating FBG, Distributed Optical Fiber Sensing, etc.) have been widely applied. The technical principles for the insulation characteristics of optical fibers in a tens of thousands of volts high-voltage environment and the potential treatment of metal encapsulation are analyzed as follows:
I. Will tens of thousands of volts high voltage break down optical fibers?
Conclusion: The optical fiber material itself will not be penetrated and broken down by the electric field directly, but “surface discharge (flashover) burnout” must be strictly prevented in engineering.
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Material Physical Insulation Strength (Bulk Breakdown):
- The base material of communication and sensing optical fibers is high-purity silica (\text{SiO}_2) fused quartz glass, which is an insulating medium.
- The dielectric strength (dielectric strength) of quartz glass is extremely high, with typical values reaching 10 \sim 100\ \text{kV/mm}, which is far superior to the dielectric strength of dry air at standard atmospheric pressure (about 3\ \text{kV/mm}).
- Optical fibers are non-conductive and non-magnetic, and there is no closed loop for electromagnetic induction. The electric field itself cannot directly cause breakdown within the bulk medium of the optical fiber.
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Main Failure Mechanism: Surface Discharge and High-Temperature Arc Melting (Flashover):
- The tens of thousands of volts high voltage cannot penetrate the optical fiber itself. However, when bridging a high voltage with a large potential difference (e.g., from a tens of thousands of volts high-voltage conductor to the ground potential side), the high field strength will be distributed in the external space and surface of the optical fiber.
- If the electrical creepage distance of the optical fiber surface is insufficient, or if moisture, dust, and conductive pollutants are adsorbed on the surface, corona discharge or even through-air arc breakdown (flashover) will occur in the air medium on the outer surface of the optical fiber.
- The local temperature of an air arc can reach thousands of degrees Celsius, while the softening and melting point of quartz glass is about 1200 \sim 1700\ ^\circ\text{C}. Once a surface arc discharge occurs, the coating and quartz core of the optical fiber will be instantly burned and vaporized, causing breakage.
- Engineering Protection Standards: Optical fibers introduced from a high-voltage potential to the ground potential side must ensure sufficient insulation distance to the ground and be equipped with high-voltage composite insulators (such as silicone rubber umbrella skirts) to provide sufficient creepage distance against pollution flashover.
II. Does metal encapsulation need to be grounded?
Conclusion: Metal encapsulation sensors installed on high-voltage power lines are strictly prohibited from being grounded and must be installed with “equipotential installation”; continuous metal casings are strictly prohibited in areas spanning potential differences.
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High-Voltage Conductor Side: Equipotential Installation (Strictly Prohibited Grounding):
- When a metal-encapsulated sensor (such as a fiber Bragg grating temperature sensor encapsulated in a stainless steel micro-tube) is directly fixed to a tens of thousands of volts high-voltage conductor, disconnector, or busbar joint, the metal casing and the measured high-voltage conductor are in a state of complete equipotential.
- In an equipotential state, no current flows through the metal casing, and there is no risk of power frequency short circuit.
- If the metal encapsulation is grounded at this time, it is equivalent to creating a direct metallic short-circuit fault from the high-voltage phase line to the ground, which will instantly trigger a massive UHV ground short-circuit arc and an explosion.
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Principle of Potential Transfer Isolation:
- The metal encapsulation protective layer is only allowed to exist locally in the high-voltage equipotential zone.
- In the optical path from the tens of thousands of volts high-voltage conductor to the low-potential monitoring end (demodulator), the metal protective casing must be disconnected and terminated at an insulated transition section, all conductive metal components must be stripped, and it must be converted to a fully dielectric insulated optical cable (without any metal strength member or metal sheath) for downward routing, cutting off the potential conduction path.
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Specifications for Metal Components on the Low-Voltage/Ground Potential Side:
- If the metal casing or metal armor layer is located in a low-voltage ground potential area such as a substation ground cabinet, according to power protection specifications, it must be reliably single-point grounded to dissipate static induction charges generated by the coupling of strong spatial electromagnetic fields.
III. Selection of Sensing Encapsulation in High-Voltage Strong Electric Field Environment
To completely eliminate local electric field distortion, suspended potential discharge, or induced eddy current heating caused by metal casings in high-strength alternating electric fields, ceramic encapsulation or all-dielectric polymer encapsulation is usually preferred for direct temperature and strain measurement on primary high-voltage power equipment:
1. Ceramic Insulated Encapsulated Sensor
Encapsulates fiber Bragg gratings using high-insulation ceramic materials. It has no metal components and naturally possesses high-voltage resistance, high-temperature resistance, and high insulation impedance characteristics.
2. Polymer Encapsulated Sensor
Encapsulates the grating section with non-metallic polymer materials, combining flexibility and insulation properties to avoid metal electric field concentration effects.
