In a vacuum circuit breaker, the arc-quenching mechanism differs from that of an SF6 breaker. In an SF6 circuit breaker, deionized gas continues the arc. Whereas, in a vacuum circuit breaker, the metal vapor created on the contact surface carries the arc current. During the zero crossing of the current, the metal vapour gets condensed. As a result, the arc is queinched.
Small Contact Gap
If the contact gap is not enough, the arc column is short and thick. Also, under a small contact gap condition, the metal vapor density remains high. Therefore, during zero-current crossing, solidification of metal vapour may not be proper. Hence, the dielectric strength between the contacts may not be recovered properly. Therefore, the arc may restrike easily.
Large Contact Gap
If the contact gap is enough, the arc length increases. Hence, the metal vapor can easily spread and condense on shields. Therefore, the vacuum dielectric strength recovers properly. This prevents the chances of restrike. Hence, a larger contact gap offers better arc extinction. Again, a very large contact gap requires a stronger and larger operating mechanism. This causes an unnecessary increase in the cost and size of the vacuum circuit breaker. The optimized contact gap in the open condition of VCB is 8 – 12 mm, depending on the voltage level.
Low Contact Velocity
If the contacts separate slowly, the arc lasts longer. The longer arc sustains the metal vapor for a prolonged time. As a result, the dielectric recovery is delayed. Low contact velocity leads to repetitive arc restrikes and higher arc energy. Ultimately, this causes more contact erosion. It also causes a risk of interruption failure.
High Contact Velocity
If the contacts separate quickly, the arc duration reduces. It generates less metal vapor. So, during the zero crossing of the current, the metal vapour gets condensed properly. It results in a faster dielectric recovery. Hence, it results in proper arc quenching. Hence, there is lower contact wear and reliable interruption.
Again, a very high contact travel velocity has two disadvantages. Firstly, a very high traveling speed requires a more complex and stronger mechanism. Also, very high opening speed may cause arc interruption even before the actual zero crossing of the current. Consequently, a current chopping may occur. This may cause switching impulse overvoltage in the system. Also, too high contact travelling velocity can cause mechanical shock and wear. So, the optimized traveling speed of the contract is 1 to 2 m/s.
Practical Examples of the Effect of Contact Gap and Contact Velocity on Arc in VCB
Let us give a simple numerical example. The example shows how contact gap and contact velocity affect arc quenching.
Let us consider an 11 kV Vacuum Circuit Breaker. Suppose the contact travelling velocity during opening is 1.5 m/s. Say, the final gap after the completion of contact travel is 10 mm. Here, the power frequency is 50 Hz.
Obviously, the time required to complete the opening operation is
Now, the current crosses zero twice per cycle. Therefore, the 50 Hz current crosses zero 100 times per second. hence, the time between current zeros is
Therefore, the required contact gap is achieved at 5.33 ms, but the next current zero occurs in 10 ms. Therefore, at the next zero crossing of the current, the contact gap becomes sufficient. Hence, it can easily prevent restriking and quench the arc.
