ZnO overvoltage limiters
Thanks to the use of voltage-dependent resistors, mostly zinc oxide ZnO, the limiters are capable of limiting follow-on current after overvoltage has died away. Their property has allowed using such ZnO resistors as surge arresters without ignition spark gaps.
The parameters on the basis of which we design the limiters are as follows:
- Continuous operating voltage of the limiter UC – it represents the maximum value of voltage connected permanently to limiter terminals at mains frequency.
- Rated voltage of the limiter UR – it represents the maximum effective value of voltage for which the limiter is designed while the correct function under conditions of temporary overvoltage at mains frequency is maintained. Such voltage is defined as voltage to which the limiter is exposed for 10 seconds following previous stress.
- Protective level of the limiter UP – is voltage on terminals at a given shape and peak value of current passing though.
- Nominal discharge current IN – the peak value of an atmospheric current pulse that is used for the classification of overvoltage limiters.
- Residual voltage URES – it represents residual voltage on the overvoltage limiter. It is actually the peak value of voltage that appears between terminals of the overvoltage limiter when discharge current is passing through it.
- Working temperature ϑ – it represents the range of permissible ambient temperatures stated by the manufacturer for the limiter to work properly.
- Line discharge class – a number expressing the ability of the overvoltage limiter to absorb energy in the event that long lines are discharged.
Overvoltage resistance of ZnO limiters
By the magnitude of permissible energy, the overvoltage limiters are divided into five classes. The higher the class, the higher is the energy capacity of the limiter. The energy that the limiter has to absorb during overvoltage rises with the voltage of the network in which it is used. Voltage rises slower than energy. Limiters in networks with higher voltage must have a greater energy capacity than limiters in networks with lower voltage. The selection of the energy class and of the rated discharge current is based on the frequency at which the energy capacity is exceeded in the given application.
An energy class expresses the ability of the limiter to absorb both atmospheric and switching overvoltage. It is stated in the units kJ/KV of the limiter’s voltage and is independent of nominal voltage.
Energy classes 1 to 5 divide the limiters into groups according to the magnitude of the permissible energy of overvoltage that they are able to absorb without being degraded or without losing heat stability at operating voltage. The higher the class, the greater is the energy capacity of the limiter.
The energy classes and an example of their use
- Class I. – use in HV networks without classification of class (5kA) or class 1 or 2 (10 kA)
- Class II. – use in 110 kV networks
- Class III. – use for 110-400 kV networks and for cable networks
- Class IV. – 400 kV long lines
- Class V. – extremely extensive 750 kV cable networks