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5.6 S5 PROTECTION
phase source CTs, selected MTA angle, voltage and current phasors. Each overcurrent element can be individually pro-
grammed to operate for flow only in specific directions. For increased security, all overcurrent elements under directional
control add one power frequency cycle of intentional delay to prevent operational errors on current 'swings'. Some terms
commonly used in directional relaying are defined as:
•
Operating Current: the quantity whose directionality is to be tested.
•
Polarizing Voltage: a voltage whose phase will remain reasonably constant between a non-faulted and a faulted sys-
tem, used as a phase reference for the operating current.
•
Relay Connection: for phase directional relaying, the characteristic angle between the operating current and the
polarizing voltage in the non-faulted system.
•
Zero Torque Line: the boundary line between operating and blocking regions in the complex plane; in an electrome-
chanical directional relay, an operating current near this line generates minimum torque.
•
Maximum Torque Line: the line perpendicular, through the origin, to the Zero Torque Line in the complex plane; in an
electromechanical directional relay, an operating current near this line will generate a maximum amount of torque.
•
Maximum Torque Angle (MTA): the angle by which the Maximum Torque Line is rotated from the Polarizing Voltage.
The following diagram specifically shows the phasors involved for Phase A directional polarization, but the general princi-
ples can be applied to all directional elements.
5
The 750/760 uses the secure 90° or quadrature connection exclusively for phase directional polarization. An MTA setting of
90° represents a phase current in-phase with its phase voltage, which is leading the polarizing voltage by 90°. The table
below shows the operating currents and polarizing voltages used for phase directional control.
Table 5–11: PHASE DIRECTIONAL OPERATING CHARACTERISTICS
QUANTITY
OPERATING
CURRENT
Phase A
Ia
Phase B
Ib
Phase C
Ic
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 last measurement of the polarizing voltage which is
greater than the
MIN POLARIZING VOLTAGE
one second after the voltages have collapsed. Once the voltage memory has expired, after one second, the phase overcur-
5-40
Courtesy of NationalSwitchgear.com
Figure 5–9: PHASE A DIRECTIONAL OVERCURRENT POLARIZATION
POLARIZING VOLTAGE
ABC PHASE SEQUENCE
Vbc
Vca
Vab
value and uses it to determine direction. The voltage memory remains valid for
750/760 Feeder Management Relay
ACB PHASE SEQUENCE
Vcb
Vac
Vba
5 SETPOINTS
GE Multilin
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