5 NEUTRAL CURRENT DIFFERENTIAL PROTECTION
In products that feature distance back-up protection, Neutral Current Differential protection is provided to clear
high-resistive earth faults which may be below the Phase Current Differential sensitivity limit.
If you wish to use the Neutral Current Differential protection you must enable it using the In Diff setting under the
NEUTRAL DIFF subheading of the CURRENT DIFF column.
The Neutral Current Differential protection can be very sensitive. Before it is allowed to operate, certain criteria
must be met:
● The operation of the Neutral Current Differential is time delayed compared with the phase current
differential protection to provide discrimination. Operation can be delayed further by setting the In Diff Time
setting. With the In Diff Time cell set to 0 (instantaneous), the operating time for the Neutral Current
Differential element will be at its minimum and typically will range between one-and-a-half and two-and-a-
half cycles of power system frequency.
● Operation of the Neutral Current Differential protection is blocked or inhibited for the following cases
○ A Phase Current Differential element has started.
○ A Distance protection element has started.
○ A Current Differential CTS element has blocked.
○ A Second Harmonic blocking detector has picked up
○ The device has detected that one or more of the circuit breaker (or isolator) poles are open.
● An External Inhibit condition using an opto-isolator mapped by the PSL is asserted.
The characteristic is determined by three protection settings. You can change the settings, but we strongly
recommend retaining the default values, which are as follows:
● In Diff Is1 = 0.1 pu
● In Diff Is2 = 2.0 pu
● In Diff k1 = 10%
This provides stability for small CT mismatches, while ensuring good sensitivity to resistive faults under heavy load
conditions.
Chapter 6 - Current Differential Protection P543i/P545i
108 P54x1i-TM-EN-1
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