Power transformer differential protection

The relay offers low-impedance differential protection for two

winding (two restraints) and three-winding (three restraints)

power transformers. The power transformer protection

application package includes the protection for a two-winding

power transformer. If support for three-winding power

transformer is needed, the corresponding protection add-on

package can be selected. Both low-impedance differential

functions feature three-phase multi-slope stabilized stages and

an instantaneous stage to provide fast and selective protection

against short circuits, winding interturn faults and bushing

flash-overs. A second harmonic restraint with advanced

waveform-based blocking ensures stability at transformer

energization. The fifth harmonic based blocking and unblocking

limits stabilize the protection performance in moderate

overexcitation situations. In case of three-winding differential

protection, the connection group phase shift matching can be

done with 0.1 degree resolution supporting cycloconverter

applications. If the tap-changer position information is

available, it is possible to further increase the protection

sensitivity by compensating the tap-changer position error

within the measured differential current.