Under normal operating conditions and voltage, the ZnO blocks act like a capacitor. The voltage distribution along the block column then depends on the capacitance of the ZnO blocks and the influence of stray capacitances. The stray capacitances are strongly dependent on the height of the block column. Short arresters - up to about one meter in height - usually have a sufficient linear voltage distribution along the block column, as the self-capacitance of the ZnO blocks is relatively high and thus no need of grading rings. For taller arresters, the influence of stray capacitances makes the voltage distribution less linear. If no measures are taken to prevent an uneven voltage distribution on a tall arrester, the local voltage stress at the top may reach (or even exceed) the knee-point of the voltage-current characteristic of the zinc oxide material. This leads to a localized increase in the power losses, with high temperatures in the block column as a consequence. Above the knee-point of the current-voltage characteristics, the blocks start to conduct large currents, which would ultimately lead to the failure of the arrester. Tall arresters therefore must be equipped with some type of voltage grading. This can be achieved by additional grading capacitors and/or grading rings. Provision of suspended grading rings is the most common way of improving the voltage distribution.
High Voltage Products
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