Power Sensing - GE 845 Instruction Manual

CHAPTER 4: SETPOINTS

Power Sensing

NOTE:
FASTPATH:
Power System
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
The power computation in the 845 relay is performed using the voltage and current inputs
from the card inserted in slot J. In cases when the connected VTs and CTs have opposite
polarity, the power sensing menu provides for inverting the power measurement.
Path:
Setpoints > System > Power Sensing > Power 1
PHASE CT&VT POLARITY
Range: Same, Inverse
Default: Same
When "Inverse" is selected, this setpoint inverts (multiplies phase currents by "-1" ) the CT
polarity for the phase currents from CT bank J1, with respect to the phase voltages from
the VT bank J2.
The setpoint for inversion of the power metering will be useful to avoid the physical
inversion of the CT connections on the relay. As the power metering will affect the power
directional elements, the user must determine the correct forward and reverse direction of
the power, before setup.
The selection of CT&VT polartiy for slot "K" is for future products.
Path:
Setpoints > System > Power System
NOMINAL FREQUENCY
Range: 60 Hz, 50 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.
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.
SYSTEM
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