5-136 G30 Generator Protection System GE Multilin5.6 GROUPED ELEMENTS 5 SETTINGS5The 2nd harmonic ratio, if low, causes problems in one phase only. This may be utilized as a mean to ensure securityby applying cross-phase blocking rather than lowering the inrush inhibit threshold.If set to “Per phase”, the relay performs inrush inhibit individually in each phase. If used on modern transformers, thissetting should be combined with adaptive 2nd harmonic function.If set to “2-out-of-3”, the relay checks 2nd harmonic level in all three phases individually. If any two phases establish ablocking condition, the remaining phase is restrained automatically.If set to “Average”, the relay first calculates the average 2nd harmonic ratio, then applies the inrush threshold to thecalculated average. This mode works only in conjunction with the traditional 2nd harmonic function.• INRUSH INHIBIT LEVEL: This setting specifies the level of 2nd harmonic component in the transformer magnetizinginrush current above which the percent differential element will be inhibited from operating. The value of the INRUSHINHIBIT MODE setting must be taken into account when programming this value. The INRUSH INHIBIT LEVEL is typicallyset to 20%.• OVEREXCITATION INHIBIT MODE: An overexcitation condition resulting from an increased volts/hertz ratio poses adanger to the protected transformer, hence the volts/hertz protection. A given transformer can, however, tolerate anoverfluxing condition for a limited time, as the danger is associated with thermal processes in the core. Instantaneoustripping of the transformer from the differential protection is not desirable. The relay uses a traditional 5th harmonicratio for inhibiting its differential function during overexcitation conditions.• OVEREXCITATION INHIBIT LEVEL: This setting is provided to block the differential protection during overexcitation.When the 5th harmonic level exceeds the specified setting (5th harmonic ratio) the differential element is blocked. Theoverexcitation inhibit works on a per-phase basis.The relay produces three FlexLogic operands that may be used for testing or for special applications such as building cus-tom logic (1-out-of-3) or supervising some protection functions (ground time overcurrent, for example) from the 2nd har-monic inhibit.
