Optimizing the Opening Velocity for a Vacuum Circuit Breaker With Cup-Type Axial Magnetic Field Contacts

This article extends the quantitative method of optimizing the opening velocities for a vacuum circuit breaker (VCB) with cup-type axial magnetic field (AMF) contacts. The material of the AMF contact is Cu
₅₀
Cr
₅₀
. We define v
₁
as the initial opening velocity over half the full contact gap and v
₂
as the average velocity over the full contact gap of 20 mm. We optimize v
₁
and v
₂
using the transition of anode-discharging modes, of which we combine the footpoint mode and anode spot mode into a large-gap high-current (LGHC) mode. The experiment shows that v
₁
should be higher than a threshold velocity v
_{1_th}
, which corresponds to a peak critical contact gap d
_{1_th}
for the transitions of an intense arc mode into a diffuse arc mode, in high-current interruption. The higher v
₁
is than v
_{1_th}
, the faster the intense arc mode evolves into the diffuse arc mode. This helps successfully interrupt the short-circuit current in a short arcing time. Another threshold velocity, v
_{2_th}
, corresponds to the maximum critical contact d
_{II_th}
for the formation of an LGHC mode in the second arc current half-wave. A high v
₁
design needs v
₂
to be lower than v
_{2_th}
. The lower v
₂
is than v
_{2_th}
, the better the VCB avoids the LGHC mode in high-current interruption. This helps interruption over a long arcing time. We propose an intelligent opening and breaking operation for VCBs with cup-type AMF contacts, whereby the VCB opens with different opening velocities according to the short-circuit current level.