Figure. 5.4.10. - 64. Cable end differential with natural unbalance in the phase current measurement.
When calculating the residual current from phase currents the natural unbalance may be in total
around 10% and still the used CT:s are in the promised 5P class (probably most common CT accuracy
class). When the current natural unbalance is compensated in this same situation the differential
settings may be set more sensitive and the natural unbalance does not affect into the calculation.
Figure. 5.4.10. - 65. Cable end differential when fault happens.
If in the cable end should occur any starting faults the cable end differential catches the difference in
between of the ingoing and outgoing residual currents and the resulting signal can be used for
alarming or tripping purpose for the feeder with failing cable end. The sensitivity of the algorithm and
settings are freely user settable.
Restricted earth fault protection is normally used in Y winding of a power transformer. This function is
needed to prevent the main differential protection from tripping in outside protection area faults in some
cases it has disabled or limited sensitivity to catch inside area earth faults. For this purpose, restricted
earth fault function is stabile since it monitors only the side it is wired to and compares the calculated
and measured residual currents. In case of outside earth fault the circulating residual current in the
faulty phase winding is not causing tripping because the comparison of measured starpoint current
and calculated residual current differential is close to zero.
AQ-F255
Instruction manual
Version: 2.00
119
© Arcteq Relays Ltd
