ZnO overvoltage limiters
Thanks to the use of voltage dependent resistors, mostly made of zinc oxide ZnO, the limiters are capable of limiting a follow-on current after an overvoltage has died away. This property allows to use such ZnO resistors as surge arresters without ignition spark gaps.
We design the limiters based of the following The parameters:
- The continuous operating voltage of the limiter UC - it represents the maximum value of voltage connected permanently to limiter terminals at a standard main frequency.
- The 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 main frequency. is maintained. Such voltage is defined as a voltage to which the limiter is exposed for 10 seconds following previous stress.
- The protective level of the limiter Up -it is the voltage on terminals for the given shape and peak value of current passing though.
- The nominal discharge current In - the peak value of an atmospheric current pulse that is used for the classification of overvoltage limiters.
- The 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 a discharge current is passing through it.
- The operating temperature ϑ - it represents the range of permissible ambient temperatures stated by the manufacturer for the limiter to work properly.
- The line discharge class - a number expressing the ability of the overvoltage limiter to absorb energy in the event of discharge in the line of long length.
Overvoltage resistance of ZnO limiters
Depending on the magnitude of permissible energy, the overvoltage limiters are divided into five classes. The higher the class, the higher the energy limiting capacity of the limiter. The energy that the limiter has to absorb when an overvoltage increases with the voltage of the grid in which it is used. Voltage rises slower than electrical energy. Limiters in networks with higher voltage must have a greater energy limiting capacity than limiters in grids 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 a given application.
An energy class expresses the ability of the limiter to absorb both atmospheric and switching overvoltages. It is given in 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 the energy capacity of the limiter.
The energy classes and examples of their use:
- Class I. - used in HV networks without being classified (5kA), or classified as class 1 or 2 (10 kA)
- Class II. - used in 110 kV networks
- Class III. - used in 110-400 kV networks, and in cable networks
- Class IV. - used with 400 kV long lines
- Class V. - used with 750 kV high voltage transmission lines