GE D90 Plus Instruction Manual page 236

GROUPED PROTECTION ELEMENTS
226
Given the equivalent systems shown in the figure, the angular difference between the
zero-sequence or negative-sequence current at the D90
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 is 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, choose the corresponding angle as the
non-homogeneity correction angle. For example, calculations yielding Θ_0 = 5° and Θ_2 =
–1° 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, set the
angle to negative values.
Approach with care the ability to select an optimum polarizing signal and to correct for
non-homogeneity. 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.
When the system configuration changes significantly, consider all relevant system
topologies for calculations. Either make a compromise choice, or apply different settings
via switchable setting groups depending on system conditions. In highly non-homogenous
and dynamic systems where the correction for non-homogeneity is not feasible, consider
conservative reach (reduced) or correction angle (negative) settings, or both.
Ground distance zone 1 guidelines for stepped distance
The zone 1 reach must be set so that nominally instantaneous operation does not extend
beyond the end of the protected line. However this can 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
multiple points. A fault beyond 100% of the protected line can cause overreach unless the
reach is reduced significantly, sometimes as low as 65% of the line length. If the line being
protected does not have a significant interaction with an adjacent circuit, then the typical
80% setting can be used. If there is significant mutual coupling between the parallel lines,
then the mutual compensation feature of the ground distance elements can be used
instead of a drastic reduction in the reach.
However, even in this case, there is more uncertainty as compared with the phase distance
elements because the zero-sequence impedance of the line and thus the zero-sequence
compensating factors can vary significantly due to weather and other conditions.
Ground distance zone 2 guidelines for stepped distance
To ensure that the zone 2 can see 100% of the line, inter-circuit mutual effects must be
considered, as they can contribute to a significant under-reach. Typically this occurs on
double circuit lines, when both lines carry the same current. Conduct an analytical study to
determine the appropriate reach setting.
The main purpose of this element is to operate for faults beyond the reach of the local zone
1 element, and therefore a time delay must be used similar to the phase fault case.
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION
Plus
and the fault current can be
Eq. 9