GE T35 Instruction Manual page 157

5 SETTINGS
Id (Ir)
Figure 5–41: PERCENT DIFFERENTIAL OPERATING CHARACTERISTIC
• MINIMUM 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 differ-
ential current during the normal transformer operation: errors due to CT inaccuracies and current variation due to
onload tap changer operation.
A setting of 0.1 to 0.3 is generally recommended (the factory default is 0.1 pu).
• SLOPE 1: This setting defines the differential restraint during normal operating conditions to assure sensitivity to inter-
nal faults. The setting must be high enough, however, to cope with CT saturation errors during saturation under small
current magnitudes but significant and long lasting DC components (such as during distant external faults in vicinity of
generators).
• BREAK 1 and 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 nom-
inal flux, effectively reducing the CT capabilities by the factor of 5.
• 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" –
traditional 2nd harmonic blocking. The adaptive 2nd harmonic restraint responds to both 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 har-
monic 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.
GE Multilin
Operating Characteristic (Id vs. Ir)
Transition Region
(cubic spline)
BREAK 1
PICKUP
Ir
T35 Transformer Management Relay
5.5 GROUPED ELEMENTS
BREAK 2
SLOPE 2
Region
5
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