5.5 GROUPED ELEMENTS
To increase security for three phase faults very close to the location of the VTs used to measure the polarizing voltage, a
'voltage memory' feature is incorporated. This feature remembers the measurement of 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 following table shows the operating and polarizing signals used for phase directional control:
Angle of IA
Angle of IB
Angle of IC
MODE OF OPERATION:
When the Phase Directional function is "Disabled", or the operating current is below 5% × CT Nominal, the element
output is "0".
When the Phase Directional function is "Enabled", the operating current is above 5% × CT Nominal and the polarizing
voltage is above the set threshold, the element output depends on the phase angle between the operating and polariz-
ing signals as follows:
– 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:
BLOCK WHEN V MEM EXP
rent element under directional control when voltage memory expires. When set to "No", the directional element
allows tripping of phase overcurrent elements under directional control when voltage memory expires.
In all cases, directional blocking will be permitted to resume when the polarizing voltage becomes greater than the "polariz-
ing voltage threshold".
PHASE DIR 1 SIGNAL SOURCE:
This setting is used to select the source for the operating and polarizing signals.
The operating current for the phase directional element is the phase current for the selected current source. The polarizing
voltage is the line voltage from the phase VTs, based on the 90° or "quadrature" connection and shifted in the leading direc-
tion by the Element Characteristic Angle (ECA).
PHASE DIR 1 ECA:
This setting is used to select the Element Characteristic Angle, i.e. the angle by which the polarizing voltage is shifted in the
leading direction to achieve dependable operation. In the design of UR elements, a block is applied to an element by
asserting logic 1 at the blocking input. This element should be programmed via the ECA setting so that the output is logic 1
for current in the non-tripping direction.
PHASE DIR 1 POL V THRESHOLD:
This setting is used to establish the minimum level of voltage for which the phase angle measurement is reliable. The set-
ting is based on VT accuracy. The default value is 0.05 pu.
PHASE DIR 1 BLOCK WHEN V MEM EXP:
This setting is used to select the required operation upon expiration of voltage memory. When set to "Yes", the directional
element blocks the operation of any phase overcurrent element under directional control, when voltage memory expires;
when set to "No", the directional element allows tripping of phase overcurrent elements under directional control.
POLARIZING SIGNAL VPOL
ABC PHASE SEQUENCE
Angle of VBC × (1∠ECA)
Angle of VCA × (1∠ECA)
Angle of VAB × (1∠ECA)
is set to "Yes", the directional element will block the operation of any phase overcur-
L90 Line Differential Relay
ACB PHASE SEQUENCE
Angle of VCB × (1∠ECA)
Angle of VAC × 1∠ECA)
Angle of VBA × (1∠ECA)
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