GE D90 Plus Instruction Manual page 251

CHAPTER 7: PROTECTION
NOTE:
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
This element is intended to apply a block signal to an overcurrent element to prevent an
operation when current is flowing in a particular direction. The direction of current flow is
determined by measuring the phase angle between the current from the phase CTs and
the line-line voltage from the VTs, based on the 90° or quadrature connection. If there is a
requirement to supervise overcurrent elements for flows in opposite directions, such as
can happen through a bus-tie breaker, two phase directional elements should be
programmed with opposite element characteristic angle (ECA) settings.
To increase security for three phase faults very close to the VTs used to measure the
polarizing voltage, a voltage memory feature is incorporated. This feature stores the
polarizing voltage the moment before the voltage collapses, and uses it to determine
direction. The voltage memory remains valid for one second after the voltage has
collapsed.
The main component of the phase directional element is the phase angle comparator with
two inputs: the operating signal (phase current) and the polarizing signal (the line voltage,
shifted in the leading direction by the characteristic angle, ECA).
The table shows the operating and polarizing signals used for phase directional control.
Table 13: Operating and polarizing signals
Phase Operating signal
A angle of IA
B angle of IB
C angle of IC
When the phase directional overcurrent element is disabled or the operating current is
below 5% × CT nominal, the element output is logic 0.
When the phase directional overcurrent element is enabled, the operating current is above
5% × CT nominal, and the polarizing voltage is above the
value, the element output is dependent on the phase angle between the operating and
polarizing signals. The element output is logic 0 when the operating current is within
polarizing voltage ±90°; for all other angles, the element output is logic 1.
Once the voltage memory has expired, the phase overcurrent elements under directional
control can be set to block or trip on overcurrent as follows. When the
value is "Yes," the directional element blocks the operation of any phase
Memory Expires
overcurrent element under directional control when voltage memory expires. When the
Block When Voltage Memory Expires
phase overcurrent elements under directional control when voltage memory expires.
In all cases, directional blocking is permitted to resume when the polarizing voltage
becomes greater than the polarizing voltage threshold.
The phase directional element responds to the forward load current. In the case of a
following reverse fault, the element needs some time—in the order of 8 ms—to establish a
blocking signal. Some protection elements, such as instantaneous overcurrent, respond to
reverse faults before the blocking signal is established. Therefore, a coordination time of at
least 10 ms must be added to all the instantaneous protection elements under the
supervision of the phase directional element. If current reversal is of concern, a longer
delay—in the order of 20 ms—is needed.
Select the Settings > Protection > Elements > Group 1 > Current > Phase Directional OC
menu to open the phase directional overcurrent configuration window.
GROUPED PROTECTION ELEMENTS
Polarizing signal V
pol
ABC phase sequence
angle of VBC × (1∠ECA)
angle of VCA × (1∠ECA)
angle of VAB × (1∠ECA)
Voltage Cut Off Level
value is "No," the directional element allows tripping of
ACB phase sequence
angle of VCB × (1∠ECA)
angle of VAC × (1∠ECA)
angle of VBA × (1∠ECA)
setting
Block When Voltage
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