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1VLM000651, Rev.3, en 2014.01.21
1. Description
VT Guard Pro is an advanced security device which protects MV inductive voltage transformers (VTs) against
ferroresonant oscillations. It is designed to be used in open-delta connection of three single-phase VTs.
Ferroresonant oscillations may be initiated by transient events (such as switching operations) in power networks. The
undamped ferroresonant oscillations are dangerous to the equipment installed due to large overcurrents and
overvoltages which may lead to permanent damage of the equipment. VT Guard Pro belongs to a new family of
advanced protection devices which protect VTs against ferroresonant phenomena by proper and prompt damping
action (Fig. 1), without creating excessive power consumption (as in case of using ordinary damping resistors) during
normal operation of the network.
Two versions of the VT Guard Pro are available:
a) VT Guard Pro
b) VT Guard Pro–D
Both versions are identical from functionality point of view (dumping ability), but the VT Guard Pro–D has extended
functions which allows self-diagnostic. This diagnostic feature allows the device to check itself (functionality of its
circuits) and is able to send information about its state into a superior system.
Fig. 1 – Demonstration of VT Guard Pro effectiveness – dumping action
