GE D90 Plus Instruction Manual page 232

GROUPED PROTECTION ELEMENTS
Ground distance zone 1 guidelines for the stepped distance scheme
222
Figure 195: Understanding system homogeneity
Given the equivalent systems shown in the figure above, the angular difference between
the zero-sequence or negative-sequence current at the D90
be calculated as follows.
If abs (Θ_0) < abs (Θ_2), then the zero-sequence network is more homogenous (that is, the
zero-sequence current better approximates the fault current), and zero-sequence
polarizing should be selected. Otherwise, negative-sequence is a better polarizing signal.
The calculated Θ angles reflect errors between the ideal and applied polarizing angles.
Therefore, after selecting the polarizing current, the corresponding angle should be chosen
as the non-homogeneity correction angle. For example, calculations yielding Θ_0 = 5° and
Θ_2 = –1° should result in selecting the negative-sequence for polarization, and the value
of –1° for non-homogeneity correction.
Quite often the non-homogeneity correction angle is used to apply extra security rather
than correct the angle between the relay and the fault currents. For extra security the
angle should be set to negative values.
Ability to select an optimum polarizing signal and to correct for non-homogeneity should
be approached with care. The best polarizing signal and the correcting angle both depend
on system parameters and intended reach of the zone. If the system configuration is
static, the selection is straightforward.
However, if the system configuration may change significantly, all the relevant system
topologies shall be considered for calculations. Either a compromise choice is made, or
different settings are applied via switchable setting groups depending on system
conditions. In highly non-homogenous and dynamic systems where the correction for non-
homogeneity is not feasible, conservative reach (reduced) or correction angle (negative)
settings, or both, should be considered.
The zone 1 reach must be set so that nominally instantaneous operation does not extend
beyond the end of the protected line. However this may be somewhat more complicated
than for the phase elements, because of zero sequence mutual induction with an adjacent
parallel line, possibly carried on the same tower, which can be of service and grounded at
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION
Plus
and the fault current can
Eq. 9