Valve Arrester

a discharger designed to protect electric equipment in AC networks from various voltage surges; it consists of a number of spark gaps connected in series with nonlinear resistances, that is, with resistances that vary in value with voltage. In order to equalize the voltage along the spark gaps, shunt resistances are connected in parallel with them. The spark gaps, the nonlinear resistances, and the shunt resistances are sealed within porcelain insulators; this eliminates the effect of atmospheric conditions on the characteristics of the discharger. Valve arresters provide a stable breakdown voltage and a volt-second characteristic that matches the volt-second characteristics of the insulation being protected. They are designed to extinguish the arc from the follow current.

When a rising voltage surge reaches the breakdown voltage of the arrester, the spark gaps break down and the voltage-surge current begins flowing to ground through the nonlinear resistances. When this happens, the voltage across the arrester depends on the voltage drop across these resistances; this voltage drop is lower than the breakdown voltage. This limits the voltage amplitude acting on the insulation. After the spark gaps break down, power-frequency (50-hertz) current also begins flowing across the arrester; this is the follow current, which must be cut off by extinguishing the arc in the spark gaps when the follow current first passes through the zero point. The lower the resistance value of the arrester, the lower the voltage across it and the better its protective effect; but the follow current increases with a decrease in resistance value, and it becomes more difficult to cut it off. In a Magne-valve arrester, the follow-current arc is extinguished by a magnetic blowout, which superimposes a magnetic field on the spark gaps.

The characteristics of modern valve arresters have been improved by using resistors with a high nonlinear coefficient.