Restricted earthfault
When the transformer grounded side is compensated with before mentioned zero sequence
compensation that side will be 1/3 (~33%) more insensitive for inside zone single phase faults. For this
reason it is advised that the restricted earthfault stage should be activated in the zero sequence
compensated side of the transformer and also because of normal phase differential protection cannot
be set to provide maximum sensitivity to inside area single phase (earth) faults due to transformer and
application depended properties described in previous chapter it is advised to be enabled in all cases
when the wye side starpoint is grounded. This differential stage looks the ingoing calculated residual
current and compares it to outgoing starpoint current. If the single phase (earth-fault) occurs outside of
the differential zone this function will not operate but if the fault should occur inside the differential zone
it will operate quickly, thus this de nition this protection is referred as “Restricted Earth Fault (REF)”
because it is sensitive to earth faults only inside the protection zone.
In the differential function for transformers two stages of low impedance restricted earth fault protection
are available.
Operation characteristics of REF HV and REF LV are similar than Idb> function presented percentage
characteristics, even though both sides are independent and freely settable. Differential and biasing
currents are calculated as follows per each side.
For HV side:
For LV side:
For both sides REF stages have average and maximum bias current calculation setting option as well
as the add or subtract differential current calculation options similarly to the phase differential stages
(depend of the installation directions of the CTs and desired sensitivity for bias calculation).
In differential stage the reference current for the REF protection for transformers is always the
protected side nominal current (HV, LV), which is calculated in the relay TRF module.
For transformer application the setting of the REF stage, whether it is located in the HV or LV side may
be set a lot more sensitive than phase differential. Basically the setting sensitivity should be de ned by
the fact if there is expected CT saturation or not (Transformer maximum single phase output compared
to neutral point CT ratings). The tripping characteristics may be set differently in case if the network is
directly grounded or through impedance and the fault current may be expected to saturate CTs in the
external fault also.
AQ-T216
Instruction manual
Version: 2.00
145
© Arcteq Relays Ltd