REF protection being a current balance scheme applied to only one side of the transformer, it would appear that no second harmonic restraint is required as in the case of differential relay.
However, REF relay, usually being set very sensitive (about 10-15% of CT rating) and without any intentional time delay, it is imperative that CT secondary current balance is maintained fairly accurately for all through faults, in zone phase faults, system recovery transients (after external fault has been cleared by another relay) and transformer ‘inrush’ currents. Of the above, transformer ‘inrush’ imposes the most severe duty on CTs and can cause saturation. The inrush current contains high magnitude of both odd and even harmonics which are not symmetrically placed across all phases (resulting in neutral harmonic currents other than triple n.) and DC offset of long decay times of about 100 msec to several seconds with different magnitudes in each phase. If fast reclosing is employed in feeder circuits connected to the same side of the transformer, the remnant flux in CT core can aggravate the situation further. Under these conditions, there will be mismatch in secondary currents and if suitable precautions are not taken the relay may maloperate.
The well known High Impedance REF scheme, if properly set, is quite secure under the above system conditions. Here again the relay is made immune to DC and tuned to system frequency with the further advantage that the setting voltage can be made only on the basis fundamental and therefore, CTs of lesser knee point can be used. But the High Impedance scheme has a disadvantage that dedicated special protection class CTs are normally required. Also, voltage limiting devices are required across relays in each phase to limit excessive voltages during in-zone faults fed directly by the system.All these add to the cost.
With numerical relays, manufacturers claim that dedicated CTs are not necessary because of the processing capability of these relays. Most of the present-day numerical relays use low impedance measurement principle with the following security measures:
(a) Current bias (static or a combination of static and dynamic)
(b) Directional check (using fundamental phasor) of residual current with neutral current as polarizing signal.
(c) Coincidence of residual and neutral current.( In case of actual faults they are almost coincident.)
(d) Operation only on the fundamental component of the differential current.
With the above security measures, especially the directional check, I understand these relays offer good security against maloperation in the above mentioned scenarios. As Mr.Hamid has mentioned, some numerical relays still use second harmonic restraint and block the relay if it is more than a certain percentage ( fixed setting). I am not sure why they do this. May be they place more onus on the security of the protection. Other reason I think would be the inability of the algorithm to make directional check in the event of too low neutral current and thus use the restraint function for security. I am not sure about this. Please post you comments.