System Backup Overcurrent - GE G60 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS

8.2.8 System backup overcurrent

System backup protection is implemented using a phase time overcurrent element with voltage restraint enabled. The
NEUTRL source is to be chosen for this element. Set the pickup of this element at a safe margin above the maximum load
expected on the machine.
The selection of all standard curves (and FlexCurves) is allowed for easy coordination with system relaying. For the
example system, an IEEE extremely inverse curve is used and a setting is chosen such that the operate time for a three-
phase fault on the high side of the transformer is 0.75 seconds. For simplicity, the power system contribution is not
considered.
The equation for an IEEE extremely inverse curve is as follows:
where A = 28.2, B = 0.1217, and p = 2. Solving for TDM, we have:
G60 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
Figure 8-2: Sequential tripping FlexLogic
SETTINGS
Eq. 8-17
Eq. 8-18
Eq. 8-19
Eq. 8-20
Eq. 8-21
Eq. 8-22
8
Eq. 8-23
Eq. 8-24
8-7