Handbook of Electrical Installation Practice - BeKnowledge

620 Handbook of Electrical Installation Practice
Fig. 22.15 High impedance relay.
of transformer. The complete winding can be protected with solidly earthed neutral
but not when resistance earthed, and it will not detect phase faults (three-phase
protection), shorted turns or open-circuits.
Overall differential protection
The current balance principle can also be applied to cover both primary and secondary
windings. However, an overall scheme is affected by magnetising current,
although it remains balanced under normal load or through-faults providing CT
ratios are matched; mismatch in CTs causes spill current to flow in the relay circuit.
As most transformers are equipped with tap-changing the design of an overall
scheme for three-phase transformers must also take account of this mismatch under
through-fault conditions.
Thus the application of an overall scheme to three-phase transformers requires
a biased relay with characteristics as illustrated in Fig. 22.10 to maintain stability on
tap-changing and during magnetising inrush currents.
In both cases the out-of-balance current flowing through the relay circuit may be
several times the basic fault setting. A bias winding ensures that the relay remains
stable under these conditions, as explained on p. 611. Usual practice is to arrange
the bias characteristic with a slope of at least twice the slope of the expected steadystate
spill current characteristic.
During internal faults the whole of the available CT secondary current passes
through the relay operating circuit giving the rapid rise in operating current.
The second reason, already mentioned, for using a biased relay for overall transformer
protention is that spill current may flow during a magnetising surge. This
spill current contains a large percentage of second and higher harmonics and it is
convenient to convert these harmonics into bias current, thereby preventing the
relay from operating during magnetising inrush conditions.
One thing to be considered with ‘harmonic bias’ is that harmonics are also present
during internal faults due to CT saturation. To ensure that the relay will operate
under all internal fault conditions the harmonic bias unit should preferably be