Common insulation media for switchgear include SF₆ insulation, air insulation, and solid insulation. In transformers, insulating oil is commonly used as the insulating material. In most gas-insulated switchgear (GIS) equipment, partial discharge (PD) is a significant factor in the decomposition of SF₆ (SF₆).
The decomposition products are closely related to the type and severity of the discharge defect. SF₆ decomposition products vary depending on the type of discharge, such as arc discharge, spark discharge, and corona discharge. Under these three types of discharge, the primary gases produced by SF₆ decomposition are SO₂, SOF₂, SOF₄, and SO₂F₂. Within GIS equipment, the low-fluorinated sulfides produced by SF₆ decomposition quickly revert to SF₆ molecules. However, due to the presence of trace amounts of water and oxygen, a small amount of low-fluorinated sulfides will still form through a complex series of reactions, generating characteristic products such as SOF₄ and SO₂.
Air switchgear uses air as an insulating medium. During partial discharge, the gas components within the air decompose and recombine. By detecting characteristic gases not present in normal air, the status of electrical equipment using air as an insulating medium can be monitored. Domestic researchers have proposed using the decomposition components of air under partial discharge as a basis for monitoring and diagnosing the insulation performance of switchgear. By constructing a "needle-plate" model to simulate metal protrusion defects, they used a flue gas analyzer to measure the changes in CO and NO concentrations over discharge time. Other researchers constructed models of three switchgear faults and used electrochemical sensors to measure the changes in CO and NO concentrations under factors such as fault severity and duration, providing experimental data for reliable detection of switchgear faults.
When solid-state discharges occur, gas status monitoring primarily involves two scenarios: first, when free radicals generated by the solid insulating medium react with surrounding gas or liquid insulating medium to produce new characteristic gases; second, when the solid insulating medium generates gases with a distinctive odor.
When solid-state discharges occur within equipment such as GIS, discharges in solid insulating materials like epoxy resins produce various CHx fragments, such as C, CH, and CH₂. These active free radical fragments then react with other gas molecules to produce characteristic gases. Domestic researchers studying surface discharges on pot-type insulators in CIS have discovered the appearance of the characteristic CS₂ gas during surface discharges. Detecting CS₂ gas can reveal latent defects that could lead to insulation damage caused by surface discharges. COS gas is a key decomposition product associated with solid-state insulation discharges and is only produced when the discharge is intense.
