A conducting particle attached to the surface of an insulator
may generate PD if located in a region of high electrical stress.
If the electrical stress persists for a long time, the decomposition
products of the PD may degrade the surface of the insulator and
cause the formation of discharge tracks. Some deterioration of
the surrounding insulating gas may also occur. When conducting
particles are present in gas-insulated switchgear, the dielectric
strength of the insulating gas and of the solid insulators tends todecrease with time following voltage application. The dielectric
strength-time characteristic of insulating gases satisfies the
empirical Inverse Power Model [11].
Decrease of mechanical strength is the main cause of all
failures. Switchgear ages because of material fatigue under cyclic
load. This may occur as a result of vibration or as a result of
very strong forces generated under fault conditions. Fatigue is
analyzed using an empirical model based on crack propagation,
similar to the electrical Inverse Power Model. Protection and
control systems incorporating electro-mechanical solid-state
components also deteriorate with age. In the case of microelectronic
components, thermal stress is expected to be a key
factor.
The lifetime t of circuit breakers is shortened by decrease of
dielectric, thermical, and mechanical strength. The failure rate
h(t) is well described by the aging model [12]