CHAPTER 4: SETPOINTS
SYSTEM
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
4–71
Power System
Path:
Setpoints > System > Power System
NOMINAL FREQUENCY
Range: 60 Hz, 50 Hz, 25 Hz
Default: 60 Hz
The power system NOMINAL FREQUENCY is used as a default to set the digital sampling
rate if the system frequency cannot be measured from available AC signals. This may
happen if the signals selected for frequency tracking are not present, or a valid
frequency is not detected. Before reverting to the nominal frequency, the frequency
tracking algorithm holds the last valid frequency measurement for a safe period of time
while waiting for the signals to reappear or for the distortions to decay.
PHASE ROTATION
Range: ABC, ACB
Default: ABC
The selection of the PHASE ROTATION setting must match the power system phase
rotation. The phase sequence setting is required to properly calculate sequence
components and power parameters. Note that this setting informs the relay of the
actual system phase sequence, either ABC or ACB. CT and VT inputs on the relay labeled
as a, b, and c, must be connected to system phases A, B, and C for correct operation.
REVERSE PHASE ROTATION
Range: Off, Any FlexLogic operand
Default: Off
This setting dynamically reverses the phase rotation set in the “Setpoints\System\Power
System\Phase Rotation”. For example, if the nominal phase rotation is ABC but the
condition (FlexLogic operand) defined for reverse phase rotation is true (high), then the
phase rotation will switch to ACB. The reverse phase rotation feature is only intended for
use in special applications such as pumped storage schemes. As soon as the reverse
phase rotation condition (FlexLogic operand) status becomes false (low), the phase
rotation goes back to the nominal value set in “Setpoints\System\Power System\Phase
Rotation”.
FREQUENCY TRACKING
Range: Disabled, Enabled
Default: Enabled
Frequency measurement is accomplished by measuring the time between zero
crossings of the composite signal of three-phase bus voltages, line voltage or three-
phase currents. The signals are passed through a low pass filter to prevent false zero
crossings. Frequency tracking utilizes the measured frequency to set the sampling rate
for current and voltage which results in better accuracy for the Discrete Fourier
Transform (DFT) algorithm for off-nominal frequencies.
The main frequency tracking source uses three-phase voltages. The frequency tracking
is switched automatically to the alternative reference source, i.e., three-phase currents
signal, if the frequency detected from the three-phase voltage inputs is declared invalid.
The switching will not be performed if the frequency from the alternative reference
signal is detected invalid. Upon detecting valid frequency on the main source, the
tracking will be switched back to the main source. If a stable frequency signal is not
available from any source, then the tracking frequency defaults to the nominal system
frequency.
