GE D90 Plus Instruction Manual page 198

GROUPED PROTECTION ELEMENTS
188
The following settings apply to all phase and ground distance elements. There are five
ground distance and five phase distance zones of protection in each setting group.
Source
Range: LINE (SRC 1), BKR 1 (SRC 2), BKR 2 (SRC 3)
Default: LINE (SRC 1)
This setting identifies the signal source for all distance functions. The mho distance
functions use a dynamic characteristic; the positive-sequence voltage—either
memorized or actual—is used as a polarizing signal. The memory voltage is also used by
the built-in directional supervising functions applied for both the mho and quad
characteristics.
Memory Duration
Range: 5 to 25 cycles in steps of 1
Default: 10 cycles
This setting specifies the length of time that a memorized positive-sequence voltage is
used in the distance calculations. After this interval expires, the relay checks the
magnitude of the actual positive-sequence voltage. If it is higher than 10% of the
nominal, the actual voltage is used, and if lower the memory voltage continues to be
used.
The memory is established when the positive-sequence voltage stays above 80% of its
nominal value for five power system cycles. For this reason it is important to ensure that
the nominal secondary voltage of the VT is entered correctly (in the AC Inputs - Voltage
window).
Set Memory Duration long enough to ensure stability on close-in reverse three-phase
faults. For this purpose, consider the maximum fault clearing time (breaker fail time) in
the substation. On the other hand, the Memory Duration cannot be too long as the
power system can experience power swing conditions rotating the voltage and current
phasors slowly while the memory voltage is static, as frozen at the beginning of the fault.
Keeping the memory in effect for too long can eventually lead to incorrect operation of
the distance functions.
Force Self-Polarization
Range: any FlexLogic operand
Default: OFF
The distance zones can be forced to become self-polarized through this setting. Any
user-selected condition (any FlexLogic operand) can be configured to force self-
polarization. When the selected operand is asserted (logic 1), the distance functions
become self-polarized regardless of other memory voltage logic conditions. When the
selected operand is de-asserted (logic 0), the distance functions follow other conditions
of the memory voltage logic shown in the following logic diagram.
Figure 165: Shared distance settings
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION