GE T60 Instruction Manual page 362

5.6 GROUPED ELEMENTS
5
• 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.
5-216
Figure 5–99: PERCENT DIFFERENTIAL OPERATING CHARACTERISTIC
T60 Transformer Protection System
5 SETTINGS
GE Multilin