GE T35 Instruction Manual page 403

CHAPTER 5: SETTINGS
The T35 percent differential element is based on a configurable dual-breakpoint / dual-slope differential restraint
characteristic. The purpose of this characteristic is to define the differential/restraint ratio for the transformer winding
currents at different loading and fault conditions to securely distinguish between external and internal faults. Increased
differential current can occur under load or external fault conditions due to current unbalance between transformer
windings and can be caused by the following:
• Inherent CT inaccuracies
• Onload tap changer operation: it adjusts the transformer ratio and consequently the winding currents
• CT saturation
To provide higher security during CT saturation conditions and to ease CT requirements, the T35 incorporates a dedicated
CT saturation detection mechanism. The T35 CT saturation detector can detect saturation happening after approximately
3 ms from fault inception. In case CT saturation is detected, the percent differential is blocked instantly and relies further
on the directional element in case fault evolved from external to internal to operate without any delays. Directional
element supervision is not applied if no CT saturation is detected.
The saturation detector is an integral part of the transformer percent differential element. It has no settings but uses some
of the differential characteristic parameters. The flags indicating CT saturation are available on a per-phase basis as
FlexLogic operands.
The directional principle is also an integral part of the transformer percent differential element and has no associated
settings. The directional element dynamically identifies what appears to be the faulted circuit and compares its current
angle with that of the sum of the remaining currents of the protected zone. The element declares a transformer internal
fault if the angle is less than 90°. Directional indicating flags signal operation on a per-phase basis and are available as
FlexLogic operands.
For more information on CT saturation detection and directional principle, see the Theory of Operation chapter.
Settings are defined as follows.
PERCENT DIFFERENTIAL PICKUP
based on the amount of differential current that can be seen under normal operating conditions. Two factors can create
differential 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 recommended (the factory default is 0.1 pu).
T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
Figure 5-102: Percent differential operating characteristic
— This setting defines the minimum differential current required for operation. It is chosen,
GROUPED ELEMENTS
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