ABB REL 356 Current Differential Protection
3-14
Settings and Application
Ground Directional Unit Polarization (DIRU)
The ground directional unit can be zero sequence or negative sequence polarized. When zero sequence polarized, it
uses all zero sequence quantities to determine the power flow direction and is sensitive to zero sequence mutuals
between parallel lines. When negative sequence polarized it uses all neagtive sequence quantities to determine the
power flow direction and its operation is not influence by mutual effects. Set DIRU = ZSEQ if mutuals are not a
consideration and DIRU = NSEQ if strong zero sequence mutuals are present for the transmission line.
Ground Backup Unit Settings – Versions with Distance Option only
Medium Set Ground Overcurrent Unit (IOM)
This overcurrent unit supervises the trips of the ground units in the impedance backup system. If TOG is set, it is also
used for tripping after a time delay. The IOM unit measures 3I0 and should be set above maximum unbalance during
load conditions.
The recommended setting is IOM = 0.5 A.
Ground Overcurrent Timer (TOG)
This timer starts timing after IOM has operated. The relay system will trip when the timer has operated. It is used as
complement to the ground distance units for high resistive ground faults. This unit needs to be coordinated with down-
stream devices.
As with the distance units, the ground time overcurrent function is only enabled during channel failure or when the
differential protection is blocked, as selected by the BKUP setting.
Zone 2 and Zone 3 Settings - Versions with Distance Option only
Settings for Zone 2 and Zone 3 protective systems are similar. Application of the distance units follow the standard
application for a conventional step distance, non-pilot relaying system.
Even though they are called Zone 2 and Zone 3, functionally they can serve as one underreaching instantaneous zone
and one overreaching time delayed zone as a conventional Zone 1 and Zone 2 stepped system.
If used as an instantaneous underreaching zone, Zone 2 should be set to never overreach the remote end. Recom-
mended setting is 80% of the line impedance. Zone 3 should be set to underreach any Zone 1 covering the adjacent lines
coming out of the remote terminal. Zone 3 should also cover at least 100% of the protected line plus 20% of the shortest
adjacent line under all conditions.
Zone 3 timers, for phase and ground, should be set to coordinate with the forward and reverse adjacent high speed trips.
The timer should include the breaking time of the slowest adjacent breaker plus a tolerance of two to three cycles.
The characteristic of the distance measuring elements are determined by the settings PANG, GANG, ZR specified
above and the reach settings ZGF, ZGR, and ZP.
Ground units in both zones are self polarized and have a forward reach (ZGF) and a reverse reach (ZGR). The FDOG
(forward ground directional unit) is used to supervise the forward direction while the reverse reach is used to define the
overall size of the characteristic and thus the reach along the R-axis.
The phase to phase and three phase units in REL 356 have only a forward reach, ZP. These units are inherently
directional.
Содержание REL 356
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