GE 845 Instruction Manual page 297

CHAPTER 4: SETPOINTS
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
and the measured ground current, and applies a restraining current defined as the
maximum measured line current (Imax) to produce a percent slope value. The slope setting
allows the user to determine the sensitivity of the element based on the class and quality
of the CTs used.The figure below shows typical wiring between the winding and ground
CTs and the 845 CT terminals, to assure correct performance of the protection. Restricted
Ground (Earth) Fault protection is also available for delta windings with an in-zone
grounding transformer, and for Delta corner grounded windings.
Figure 4-76: 3 CT wiring for the Restricted Ground (Earth) Fault protection
The 845 RGF protection includes ground current supervision mechanism to provide more
security during external non-ground faults associated with CT saturation, that may result
into spurious neutral current, and may jeopardize the security of the RGF 1 protection.
When the GROUND CURRENT SUPERVISION setting is selected as "Enabled", the algorithm
checks if the ground current measured by the relay satisfies the selected GROUND
CURRENT SUPERV LEVEL, before making operation decision. The ground current
supervision is not active if the GROUND CURRENT SUPERVISION is selected as "Disabled".
The RGF 1 protection works without the supervision as well.
The settings of this function are applied to the ground and all three phase currents to
produce pickup and trip flags. The RGF 1 pickup flag is asserted, when the restrained
ground differential current is above the PKP value. The RGF 1 operate flag is asserted if the
element stays picked up for the time defined in RGF 1 PKP DELAY. The element drops from
Pickup without operation, if the calculated restrained ground differential current drops
below 97-98% of the Pickup value, before the time for operation is reached.The Restricted
Ground (Earth) Fault function can be inhibited by a blocking input.
PROTECTION
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