Double circuit lines must be taken into account in the operating principle of the protection scheme to avoid unwanted tripping of «sound» phases, which could be the result of an excessively general phase selection.
Phase selection for an inter-circuit fault
During a two-phase fault selection, for example on loop AB, the P44x checks direction on the two adjacent ground loops, (A to Neutral and B to Neutral). The direction is determined using either the conventional algorithm or the high-speed algorithm (using superimposed quantities), depending on fault severity. If superimposed components are used, the transient (fault) energy is summated phase by phase.
( )
Trip single pole Trip single pole
The directions of the two adjacent ground loops are compared, as follows:
• If the two directions are forward, the fault is a two-phase fault on the protected line.
• If only one of the directions is forward, for instance Sa, the fault is single-phase (A to Neutral) on the protected line.
• If the two directions are reverse, the fault is not on the protected line.
Protection against Current Reversal (Transient Blocking)
When a fault occurs on a line, which is parallel to the protected line, the pilot schemes on the protected line may be subjected current reversals from sequential clearing on the parallel line. A fault on the parallel line may start by appearing external to the protected line in the reverse direction, and then, after a sequential operation of one of the parallel line breakers, the fault appears forward. This situation can affect security of certain pilot schemes on the protected line. Relay 3 senses reverse current
1 2 Relay 3 senses forward current Forward
FIGURE 16 - DIRECTION REVERSAL FROM SEQUENTIAL CLEARING OF PARALLEL LINES The P44x provides protection against the effects of this phenomenon by employing transient blocking. An adjustable timer is available that will block direct and permissive transfer trip signals from being used in the P44x logic, and will also block the P44x from sending direct or permissive transfer trip signals. This timer is designated as «Reverse Guard Timer».
This provides protection against fault current reversal and will still allow fast tripping in the event of faults occurring in zone 1, if zone 1 is independent (not used as overreach zone).
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.
4.9.4 Tripping Logic
Legend For Tripping Logic Diagrams (DEF) Abbreviation Definition
Vr> Threshold of residual or zero sequence voltage (3Vo) Ied Threshold of residual current for forward fault
Forward Forward directional with zero/negative sequence polarisation Reverse Reverse directional with zero/negative sequence polarisation DEF blocking Blocking of DEF element
Carrier Receive DEF Carrier received for the principal line protected (same channel as distance protection)
Iev Threshold of residual current (0.6 x Ied) Tripping mode Single or three-phase tripping (selectable)
Z< starting Convergence at least 1 of 6 loops within the tripping characteristic (internal starting of the distance element) t_cycle Additional time delay (150ms) of 1 pole AR cycle
t_delay Tripping time delay
t_trans Carrier Send delay settable
& &
2 Pole or 3 Pole Selection 1 pole dead
FIGURE 17 - DIRECTIONAL COMPARISON PROTECTION PERMISSIVE SCHEME
& &
2 Pole or 3 Pole Selection 1 pole dead
FIGURE 18 - DIRECTIONAL COMPARISON PROTECTION BLOCKING SCHEME
If the DEF directional comparison transmission is selected on the same channel that is used to transmit distance aided-trip messages, the DEF will have the same tripping logic as the main protection (permissive or blocking).
4.9.5 SBEF – Stand-By earth fault (not communication-aided)
This protection trips the local breaker directly, without a aided-trip signal, if a high resistance fault remains after a time delay. The time delay varies inversely with the value of the fault current. The selectable inverse time curves comply with the ANSI and IEC standards (see Appendix A).
This protection three-pole trips and can block autoreclosing.
Directional
FIGURE 19 - SBEF – STAND-BY EARTH FAULT