P44x/EN HW/E33 Relay Description
Page 40/44
MiCOM P441/P442 & P444
4.9 DEF Protection Against High Resistance Ground Faults
Protection against high-resistance ground faults, also called DEF (Directional Earth Fault), is
used to protect the network against highly resistive faults. High resistance faults may not be
detected by distance protection. DEF Protection can be applied in one of the two following
modes: faults using the following:
• The main operating mode, directional comparison protection uses the signalling
channel and is a communication-aided scheme.
• In backup-operating mode SBEF (Stand-By Earth Fault), an inverse/definite time
ground overcurrent element with 2 stages is selectable. A communication channel is
not used - OR – a zero sequence power with IDMT Time Delay (see section 5 in
chapter P44x/EN AP)
Both the main and backup mode can use different methods for fault detection and directional
determination (negative or zero sequence polarisation, RCA angle settable for backup SBEF
protection, etc.)
The use of Aided-Trip logic in conjunction with the DEF element allows faster trip times, and
can facilitate single-phase tripping if single-phase tripping is applied to the breaker.
The DEF directional comparison protection may be applied on the same signal channel as
the distance protection, or it may be applied on an independent channel (facility to use two
different aided-trip logic for distance or DEF element).
When used on the same signalling channel as the distance protection, if the distance
protection picks up, it has priority (the output from the DEF element is blocked from asserting
the Carrier Send common output).
The use of directional comparison protection with an independent signalling channel allows
the distance functions and DEF function to operate in parallel. Each function is routed to its
own Carrier Send output. If a ground fault is present where both the distance and DEF
elements pick up, the faster of the two functions will perform the trip.
4.9.1 High Resistance Ground Fault Detection
A high resistance fault is detected when residual or zero sequence voltage (3V0), and
current thresholds are exceeded or using the high speed algorithms:
• ∆I ≥ 0.05 IN
• ∆V ≥ 0.1 VN (L-G)
A fault is confirmed if these thresholds are exceeded for more then 1 ½ cycles
4.9.2 Directional determination
The fault direction is determined by measuring the angle between the residual voltage and
the residual current derivative. The fault is forward if the angle is between –14
° and +166°.
A negative or zero sequence polarisation is selectable in order to determinate the earth fault
direction.
4.9.3 Phase selection
The phase is selected in the same way as for distance protection except that the current
threshold is reduced (∆I ≥ 5% x IN and ∆V ≥ 10% x VN)
NOTE: If the phase has not been selected within one cycle, a three-phase
selection is made automatically.
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