GE C70 Instruction Manual page 469

CHAPTER 6: ACTUAL VALUES METERING
For display and oscillography purposes, all phasor angles in a given relay are referred to an AC input channel pre-selected
by the setting. This setting defines a
SETTINGS  SYSTEM SETUP  POWER SYSTEM  FREQUENCY AND PHASE REFERENCE
particular AC signal source to be used as the reference.
The relay first determines if any "Phase VT" bank is indicated in the source. If it is, voltage channel VA of that bank is used
as the angle reference. Otherwise, the relay determines if any "Aux VT" bank is indicated; if it is, the auxiliary voltage
channel of that bank is used as the angle reference. If neither of the two conditions is satisfied, then two more steps of this
hierarchical procedure to determine the reference signal include "Phase CT" bank and "Ground CT" bank.
If the AC signal pre-selected by the relay upon configuration is not measurable, the phase angles are not referenced. The
phase angles are assigned as positive in the leading direction, and are presented as negative in the lagging direction, to
more closely align with power system metering conventions. The figure illustrates this.
Figure 6-3: UR phase angle measurement convention
6
6.4.1.3 UR convention for measuring symmetrical components
The URs calculate voltage symmetrical components for the power system phase A line-to-neutral voltage, and
symmetrical components of the currents for the power system phase A current. Owing to the above definition, phase
angle relations between the symmetrical currents and voltages stay the same irrespective of the connection of instrument
transformers. This is important for setting directional protection elements that use symmetrical voltages.
For display and oscillography purposes the phase angles of symmetrical components are referenced to a common
reference as described in the previous sub-section.
WYE-connected instrument transformers
• ABC phase rotation: • ACB phase rotation:
The above equations apply to currents as well.
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 6-13