GE T35 Instruction Manual page 249

5 SETTINGS
• PERCENT DIFFERENTIAL PICKUP: This setting defines the minimum differential current required for operation. It is
chosen, based on the amount of differential current that might be seen under normal operating conditions. Two factors
may create differential current during the normal transformer operation: errors due to CT inaccuracies and current vari-
ation due to onload tap changer operation.
A setting of 0.1 to 0.3 is generally recommended (the factory default is 0.1 pu).
• PERCENT DIFFERENTIAL SLOPE 1: This setting defines the differential restraint during normal operating conditions
to assure sensitivity to internal faults. The setting must be high enough, however, to cope with CT saturation errors dur-
ing saturation under small current magnitudes but significant and long lasting DC components (such as during distant
external faults in vicinity of generators).
• PERCENT DIFFERENTIAL BREAK 1 and PERCENT DIFFERENTIAL BREAK 2: The settings for break 1 and break
2 depend very much on the capability of CTs to correctly transform primary into secondary currents during external
faults. Break 2 should be set below the fault current that is most likely to saturate some CTs due to an AC component
alone. Break 1 should be set below a current that would cause CT saturation due to DC components and/or residual
magnetism. The latter may be as high as 80% of the nominal flux, effectively reducing the CT capabilities by the factor
of 5.
• PERCENT DIFFERENTIAL SLOPE 2: The slope 2 setting ensures stability during heavy through fault conditions,
where CT saturation results in high differential current. Slope 2 should be set high to cater for the worst case where
one set of CTs saturates but the other set doesn't. In such a case the ratio of the differential current to restraint current
can be as high as 95 to 98%.
• INRUSH INHIBIT FUNCTION: This setting provides a choice for 2nd harmonic differential protection blocking during
magnetizing inrush conditions. Two choices are available: "Adapt. 2nd" – adaptive 2nd harmonic, and "Trad. 2nd" – tra-
ditional 2nd harmonic blocking. The adaptive 2nd harmonic restraint responds to magnitudes and phase angles of the
2nd harmonic and the fundamental frequency component. The traditional 2nd harmonic restraint responds to the ratio
of magnitudes of the 2nd harmonic and fundamental frequency components. If low second harmonic ratios during
magnetizing inrush conditions are not expected, the relay should be set to traditional way of restraining.
• INRUSH INHIBIT MODE: This setting specifies mode of blocking on magnetizing inrush conditions. Modern transform-
ers may produce small 2nd harmonic ratios during inrush conditions. This may result undesired tripping of the pro-
tected transformer. Reducing the 2nd harmonic inhibit threshold may jeopardize dependability and speed of protection.
The 2nd harmonic ratio, if low, causes problems in one phase only. This may be utilized as a mean to ensure security
by 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, this
setting 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 a
blocking 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 the
calculated 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 magnetizing
inrush current above which the percent differential element will be inhibited from operating. The value of the
setting must be taken into account when programming this value. The
INHIBIT MODE
set to 20%.
• OVEREXCITATION INHIBIT MODE: An overexcitation condition resulting from an increased volts/hertz ratio poses a
danger to the protected transformer, hence the volts/hertz protection. A given transformer can, however, tolerate an
overfluxing condition for a limited time, as the danger is associated with thermal processes in the core. Instantaneous
tripping of the transformer from the differential protection is not desirable. The relay uses a traditional 5th harmonic
ratio 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. The
overexcitation inhibit works on a per-phase basis.
GE Multilin T35 Transformer Protection System
5.6 GROUPED ELEMENTS
INRUSH
is typically
INRUSH INHIBIT LEVEL
5-115
5