GE T60 Instruction Manual page 420

GROUPED ELEMENTS
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.
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.
PERCENT DIFFERENTIAL BREAK 2
setting below the fault current that is likely to saturate the weakest CT feeding the relay. When adjusting this setting, keep
in mind that the restraining signal is created as the maximum of all the input currents.
PERCENT DIFFERENTIAL SLOPE 2
(BREAK 2). This setting affects stability of the relay for heavy external faults. Traditionally, the value chosen for this setting
needs to be high enough to accommodate the spurious differential current resulting from saturation of the CTs during
heavy external faults. This requirement can be considerably relaxed in favor of sensitivity and speed of operation as the
relay detects CT saturation and upon detection applies the directional principle to prevent maloperation. When adjusting
this setting, keep in mind that the restraining signal is created as the maximum of all the input currents.
5-238
Figure 5-133: 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
— Defines the higher breakpoint of the dual-slope operating characteristic. Set BREAK 2
— Defines the percentage bias for the restraining currents above the higher breakpoint
T60 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS