ZnO surge arresters are used in HVDC systems [121, 135] where, because of the converters
used in such systems, high frequency voltages are generated. As a result,
a mixed voltage in which an alternating voltage is superimposed upon a direct
voltage appears on the network. A comparison of both the current–voltage and
power-dissipation–voltage characteristic curves for direct and alternating voltages,
respectively, show a crossover point near the knee of high conduction (Figure 5.22).
This crossover is also frequency dependent, and its physical cause has not been
explained, although it appears to be associated with the dynamic response of the
material: