Switching in Electrical Transmission and Distribution Systems

6 Current Interruption in Gaseous Media

6.1 Introduction

When a HV circuit-breaker receives a command to separate its contacts and to interrupt a short-circuit current, the contact separation can generally take place anywhere within the sine wave of the alternating current. After that, regardless of the arc-extinction medium, the current will flow through an arc between the contacts. The arc core comprises extremely hot gas (temperature above 15 000 K), which is fully ionized and has an electrical conductivity comparable to graphite.

As long as the current is high, the arc has negligible influence on the current flow, and the arc voltage is usually only a few hundred volts. Cooling the arc by a gas flow at this stage will only result in a slight increase in the arc voltage, just to compensate for the increased arc cooling.

To interrupt the current, the circuit-breaker has to wait for a natural current zero. As the current drops towards zero, the arc cross-section decreases. When the current reaches zero, the channel has reduced to a thin thread of ionized gas. At the very moment of current zero, no energy is put into the arc. If it were possible for the arc to disappear at this instant, the current would be successfully interrupted. The arc, however, has a thermal inertia quantified by the arc time constant. Success of interruption depends on how much thermal energy is stored in the arc and how quickly this energy can be removed by cooling. This means that, immediate after ...