GE C70 Instruction Manual page 299

5 SETTINGS
The neutral current unbalance element responds to a differential current signal measured as a vectorial current difference
between neutral currents of parallel banks. In case of ungrounded banks, regular, preferably low-ratio CT can be used. In
case of grounded banks, a window or core-balance CT shall be used for increased accuracy/sensitivity.
The function applies simplified compensation for the inherent bank unbalance. Accurate compensation requires three bal-
ancing factors that are difficult to measure in practical situations. Instead, a simple and practical compensation responding
to both the positive-sequence current and negative-sequence current is applied. This method is easy to apply and remains
accurate as long as the zero-sequence bank currents are relatively small. Under large zero-sequence bank currents, the
applied method of compensation for the inherent bank unbalance is less accurate. The solution to this problem is to use a
small portion of the zero-sequence current at the bank terminals to produce extra restraint and preserve high sensitivity of
this protection method.
By setting both the slope and the inherent unbalance compensating factor to zero one could apply the function as a plain
overcurrent element responding to the split-phase neutral current.
The relay allows manual setting of the inherent unbalance factor or automatic setting via a dedicated command. For addi-
tional information, refer to the Theory of operation and Application of settings chapters.
Figure 5–69: NEUTRAL CURRENT UNBALANCE CONNECTIONS
The following settings are available. The settings shown below are for the neutral current unbalance 1 element. The same
descriptions apply to the neutral current unbalance 2 and 3 elements.
• NTRL CUR UNBALCE 1 BANK SOURCE: This setting indicates the signal source that signifies the split-phase neutral
current in its ground current channel. At minimum the ground channel of this source must be set. If the factor compen-
sating for inherent bank unbalance is set above zero signalling the intent of using the compensation, the phase cur-
rents of this source must be configured as well. Without the phase currents configured under this source, the function
would run only if the magnitude of the inherent unbalance factor is set to zero.
GE Multilin
Regular, preferably
low-ratio CT for
ungrounded banks
Window / core-
balance CTs for
grounded banks
C70 Capacitor Bank Protection and Control System
5.6 GROUPED ELEMENTS
C70 CAPACITOR BANK RELAY
Neutral
CT bank
BANK
60N
SOURCE
Ground
CT
834735A1.CDR
5
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