GE D90 Plus Instruction Manual page 216

GROUPED PROTECTION ELEMENTS
Ground directional supervision
206
Figure 182: Ground distance zone 2 scheme logic
A dual (zero-sequence and negative-sequence) memory-polarized directional supervision
applied to the ground distance protection elements has been shown to give good
directional integrity. However, a reverse double-line-to-ground fault can lead to a
maloperation of the ground element in a sound phase if the zone reach setting is increased
to cover high resistance faults.
Ground distance zones 2 through 5 use an additional ground directional supervision to
enhance directional integrity. The ground distance directional characteristic angle is used
as a maximum torque angle together with a 90° limit angle.
The supervision is biased toward operation in order to avoid compromising the sensitivity
of ground distance elements at low signal levels. Otherwise, the reverse fault condition
that generates concern will have high polarizing levels so that a correct reverse fault
decision can be reliably made. The supervision for zones 2, 3, and 5 is removed during
open pole conditions.
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION