CHAPTER 4: SETPOINTS
PROTECTION
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
4–117
NOTE
NOTE:
The values of the setpoints for minimum pickup, Break 1 and Break 2 are expressed in
terms of CT reference. Please see the definition of CT reference which is described in the
second paragraph of this chapter.
DC Saturation Detection:
D.C. components of both J1-CT and K1-CT bank currents, in phases A, B and C must be
first calculated.
Eq. 3
In other words, the d.c. component is an absolute value of the average of current
samples over one last cycle of data.
Eq. 4
Operation 2 (Eq. 6) is performed three times for each phase separately.
Eq. 5
D
1
is a factory constant of 1/2. A new component of the saturation condition is derived
as in the figure below:
Saturation Detection
External faults near rotating machines typically result in very large time constants of DC
components in the fault currents. Also, when energizing a step-up transformer, the
inrush current being limited only by the machine impedance can be significant and can
last for a very long time. In order to provide additional security against improper
operations during these events, the 889 incorporates saturation detection logic. When
saturation is detected the element makes an additional check on the angle between the
terminal side CT and differential CT currents. If this angle indicates an internal fault then
tripping is permitted. The latter applies only when the Internal summation method is
used.
The saturation detector is implemented as a state machine (see next figure). "NORMAL"
is the initial state of the machine. When in "NORMAL" state, the saturation flag is not set
(SAT = 0). The algorithm calculates the saturation condition, SC. If SC = 1 while the state
machine is "NORMAL", the saturation detector goes into the "EXTERNAL FAULT" state and
sets the saturation flag (SAT = 1). The algorithm returns to the "NORMAL" state if the
differential current is below the first slope, S1, for more than 200 ms. When in the
"EXTERNAL FAULT" state, the algorithm goes into the "EXTERNAL FAULT & CT
SATURATION" state if the differential flag is set (DIF = 1). When in the "EXTERNAL FAULT &
CT SATURATION" state, the algorithm keeps the saturation flag set (SAT = 1). The state
machine returns to the "EXTERNAL FAULT" state if the differential flag is reset (DIF = 0) for
100 ms.
There are three SAT flags (one per phase). The corresponding FlexLogic operands are:
•
Percent Diff Sat A
•
Percent Diff Sat B
•
Percent Diff Sat C
