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Protection of voltage transformers against ferroresonance

KPB Intra s.r.o.
Ing. Josef Stejskal

The phenomenon of „ferroresonnance “ can occur in single –pole voltage transformers in HV inadequately earthed cable
networks. The impedance of transformer and the earthing capacity of the cable create a potentially oscillating RLC circuit.
When an overvoltage come by the case of earth connection or some switching, the phenomenon of ferroresonnance can
appear. The consequences are overheating of magnetic core and coil, damage of insulation and burst.

To prevent HV equipment we recommend to use connection of dump resistor in open delta of additional secondary windings
(see picture 12 below). Mounting dimensions are in the picture 13. The value of resistance is 68 Ohm/ 200W (type
of TR305 producer is „ Tesla Blatná“). Resistor is available to order in our firm and we can deliver it single or with other

We recommed to use it in new installations as prevention

Obr. 13
Obr. 14

We hope to contribute to understanding of issues and prevention of consequent potential damages by giving the explanation above.