GE 889 Instruction Manual page 268

PROTECTION
4–138
The IEEE C37.102-2006 (IEEE Guide for AC Generator Protection) provides typical practices
to set this element.
The following is the guideline used to derive the setting of this element.
The inner circle (Circle 1) diameter is set to machine base impedance (i.e. 1 pu), which
considers the loss of field during full loading to medium loading of the machine. An offset is
one half of the direct axis transient reactance (X'd) - both impedances referring to the relay
side. The corresponding time delay for the inner circle needs to be higher than the worst
case power swing scenario, and hence this value is determined from stability studies
(typically, this value may be in the range of 0.2 s to 0.5 s).
On the other hand, the outer circle (e.g. Circle 2) diameter is set to the synchronous
reactance of the machine (Xd) and an offset equal to one half of the direct axis transient
reactance (X'd) - both impedances referring to the relay side. This allows the machine to be
protected during light load conditions or with reduced field excitation. The corresponding
time delay for the outer circle should be high enough to prevent mis-operations due to
power swings, and hence this value is determined from stability studies (typically, this
value may be in the range of 0.5 s to 2 s).
Path:
Setpoints > Protection > Group 1(6) > Generator > Loss of Excitation
SIGNAL INPUT
Range: Positive Impedance 1, Positive Impedance 2
Default: Positive Impedance 1
The signal input selection to apply this element at the terminal or neutral side CT. The CT
and VT sources used by Positive impedance 1 and 2 are specified in "Metering >
Impedance > Pos Seq Impedance.
CIRCLE 1 FUNCTION
Range: Disabled, Trip, Alarm, Latched Alarm, Configurable
Default: Disabled
CIRCLE 1 DIAMETER
Range: 0.1 to 300.0 ohms in steps of 0.1 ohms
Default: 25.0 ohms
This setting is the diameter of the Circle 1 characteristic in ohms, referring to the CT/VT
secondary (relay).
CIRCLE 1 OFFSET
Range: 0.1 to 300.0 ohms in steps of 0.1 ohms
Default: 2.5 ohms
This setting is the offset of the Circle 1 characteristic in ohms, referring to the CT/VT
secondary (relay).
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS