GE T60 Instruction Manual page 476

GROUPED ELEMENTS
To provide higher security during CT saturation conditions and to ease CT requirements, the T60 incorporates a dedicated
CT saturation detection mechanism. The T60 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.
5
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).
PERCENT DIFFERENTIAL SLOPE 1
(BREAK 1). This setting determines the sensitivity of the relay for low current internal faults. The setting must be set high
enough to cope with the spurious differential current resulting from inaccuracy of the CTs operating in their linear mode,
that is, in load conditions and during distant external faults.
PERCENT DIFFERENTIAL BREAK 1
setting needs to be set above the maximum load current and can be moved to the AC current under which all the CTs are
guaranteed to transform without saturation. This includes the effect of DC components and/or residual magnetism. The
latter can be as high as 80% of the nominal flux, effectively reducing the CT capabilities by the factor of five. When
adjusting this setting, it must be kept in mind that the restraining signal is created as the maximum of all the input currents.
5-258
Figure 5-145: Percent differential operating characteristic
— This setting defines the minimum differential current required for operation. It is chosen,
— Defines the percentage bias for the restraining currents below the lower breakpoint
— Defines the lower breakpoint of the dual-slope differential operating characteristic. This
T60 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS