GE C70 Instruction Manual page 327

5 SETTINGS
Figure 5–87: NEUTRAL VOLTAGE UNBALANCE BASIC CONNECTION
The following settings are available for all three neutral voltage unbalance elements.
• NTRL VOL UNBAL 1 NTRL SOURCE: Indicate signal source that signifies the neutral-point voltage of the protected
bank. This single-phase voltage is to be configured under the auxiliary VT channel. The VT ratio and secondary nomi-
nal voltage are set under separate menu. Nominal voltage of this auxiliary channel becomes a unit (1 pu) for the pickup
settings of this function.
• NTRL VOL UNBAL 1 BUS SOURCE: Indicate signal source that signifies the bus voltages of the bank. This source
must have the phase voltages configured in order to facilitate compensation for the inherent unbalance of the bank.
The zero-sequence voltage required for system unbalance compensation can be derived from the phase voltages, or
delivered to the relay from an external source, typically a broken-delta VT (the selection is controlled by the next set-
ting of this function). In the latter case, the 3V0 signal is expected and must be wired and configured under the auxiliary
voltage channel of this source. This 3V0 voltage must be labelled as "Vn" under the VT configuration. The VT ratio and
secondary nominal voltage are set under separate menu. Phase voltages of this source must be connected in wye for
the function to operate. The function uses several voltages potentially produced by VTs of different ratios and nominal
secondary values. The relay scales all the signals automatically for meaningful calculations.
• NTRL VOL UNBAL 1 BUS 3V0: This setting specifies the source of the bus zero-sequence voltage required for sys-
tem unbalance compensation. If set to "Calculated", the relay would derive the zero-sequence voltage as a vector sum
of the three phase voltages and will run the function if voltage in all three phases is greater than 0.25 pu. If set to "Mea-
sured", the relay expects the 3V0 signal (not a V0) to be wired to the relay under the auxiliary channel of the bus
source, and labelled as "Vn". In this case, the voltage check to run this function is not performed.
• NTRL VOL UNBAL 1 GROUND: This setting selects the grounding arrangement of the bank. For ungrounded banks it
should be set to "VT (ungrnd)". When the bank neutral is solidly grounded through the CT with a resistor in the CT sec-
ondary, then this setting should be chosen as "CTxR (grnd)".
When the "CTxR (grnd)" value is selected, the measured bank neutral point voltage is shifted by 90° lagging to com-
pensate for the angular difference between the bus neutral voltage and the voltage derived from the bank neutral point
current. The value of resistor R should be chosen so the voltage measured by the auxiliary input of the relay matches
bus V0 during system faults (for example, an SLG fault at the bus). This is accomplished by the following condition:
GE Multilin C70 Capacitor Bank Protection and Control System
5.6 GROUPED ELEMENTS
5-203
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