GE D60 Instruction Manual page 684

DISTANCE ELEMENTS (STEPPED DISTANCE SCHEME)
Given the equivalent systems shown in the following figure, the angular difference between the zero-sequence or
negative-sequence currents at the relay, and the fault current can be calculated as follows:
where:
A is the local equivalent system
B is the remote equivalent system
L is the line
d is the per-unit (pu) intended reach of the zone (typically 0.7 to 0.9)
If abs(Θ ) < abs(Θ
0 2
fault current), and zero-sequence polarizing should be selected. Otherwise, negative-sequence is a better polarizing signal.
8
The calculated Θ angles reflect errors between the ideal and applied polarizing angles. Therefore, after selecting the
polarizing current, the corresponding angle is chosen as the non-homogeneity correction angle. For example, calculations
yielding Θ = 5° and Θ
0
homogeneity correction.
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.
If the system configuration can change significantly, consider all the relevant system topologies 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,
consider conservative reach (reduced) and/or correction angle (negative) settings.
8-4
Figure 8-1: Understanding system homogeneity
), the zero-sequence network is more homogenous (the zero-sequence current better approximates the
= –1° result in selecting the negative-sequence for polarization, and the value of –1° for non-
2
CHAPTER 8: APPLICATION OF SETTINGS
D60 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
Eq. 8-2
Eq. 8-3