Neutral Current Unbalance (Ansi 60N) - GE C70 Instruction Manual

OVERVIEW CHAPTER 9: THEORY OF OPERATION
Note that under external fault conditions, sensitivity can be much different from the non-fault sensitivity.
9.1.5.4 Auto-setting
As a convenient alternative to manually determining inherent unbalance factor settings, the relay can automatically
calculate these settings from its own measurements while the capacitor is in-service, as described in the Commands
chapter. The technique that the relay uses is to set the operate signal variable to zero in equation 9.43 and solve for the
inherent unbalance factor k using the average of several successive measurements of the currents. However, the
A
assumption made here is that when the auto-set command is executed, the capacitor is in an acceptably balanced state,
wherein the operating signal ought to be zero. Following the auto-set command, the protection measures changes from
the state that existed at the time the auto-set command executed.

9.1.6 Neutral current unbalance (ANSI 60N)

9.1.6.1 Operating principle
The neutral current unbalance function is based on the balance between interconnected neutral currents of two parallel
banks, and is applicable to both grounded and ungrounded installations. This section deals first with grounded
applications, with a section at the end that discusses the differences with ungrounded applications. A window CT
measuring the vector difference between the two neutral currents is required for grounded-wye configurations.
If the two banks were identical, then the inter-neutral current is zero. The failure of an element in either bank then results in
inter-neutral current, which is sensed by the relay. However, with the two banks slightly mismatched due to manufacturing
tolerances, a circulating zero-sequence current can be present. Compensation for this inherent unbalance current is
available to increase the sensitivity of the function.
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9-14 C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL