Download TEMPUS: PROTECTION AGAINST OVERVOLTAGES 1: The use of

TEMPUS: PROTECTION AGAINST OVERVOLTAGES
1: The use of arcing horns
Electrical grids and high voltage installations are sometimes subject to transient overvoltages.
These overvoltages originate from:
-
the grid itself (e.g. due to switching operations),
external phenomena (e.g. lightning).
The overvoltages due to e.g. switching operations in the grid are rather limited. In general, the
insulation/insulators can withstand these limited overvoltages (e.g. 5 times the normal voltage
level). Overvoltages due to e.g. lightning can be very large. The insulation/insulators are not
able to withstand this high overvoltage.
By using arcing horns, the excess of charges is drained to the ground (see Figure 1) which
reduces/eliminates the overvoltage. Due to the high voltage, an electrical breakdown of the air
occurs. Charges are drained to the ground and the insulator is protected against damage.
Figure 1: Arcing horns
Arcing horns are cheap in comparison with other overvoltage protection devices but it takes a
time of 1 μs to 10 μs to form the arc. Due to this time delay, the installation must still
withstand a peak voltage. Moreover, sometimes the arc does not extinguish. Even when the
overvoltage has disappeared, the arc remains and a 50 Hz current is flowing which causes a
single phase earth fault.
Figure 2: Arcing horns
Overvoltage protection is not always obtained by using arcing horns in the free air. An
encapsulated or gas filled arcing horn has the same working principle but the horns or
electrodes are encapsulated (sometimes they contain a noble gas).
2: Voltage Dependent Resistors
Instead of using arcing horns, overvoltages can be avoided/limited by using a VDR (Voltage
Dependent Resistor) which is a nonlinear resistor. SiC and ZnO are commonly used as VDR.
For normal working voltages, their resistances are high and almost no current is flowing. The
VDR has no influence on the operation of the installation.
As the voltage across the VDR increases, its resistance decreases. This implies the current
increases significantly which limits the overvoltage. Due to this current through the VDR
(mounted between the installation and the ground), the excess of charges is drained to the
ground.
Figure 3 visualizes the relationship between the applied voltage and the current in case of a
SiC (Silicon Carbide) and a ZnO (Zinc Oxide) Voltage Dependent Resistor. Especially in case
of the ZnO (newer) Voltage Dependent Resistor, the voltage is limited. By placing multiple
Voltage Dependent Resistors in series, a limitation at higher voltages is possible.
Figure 3: Overvoltage protection using a VDR
References
D. Van Dommelen, Productie, transport en distributie van elektriciteit, Acco, Leuven, 2001.