Frequency Tracking; Phasors, Transients, And Harmonics; Protection Elements; Logic Inputs - GE 750 Instruction Manual

750/760
Feeder Management Relay

Frequency Tracking

Phasors, Transients,
and Harmonics
Processing of AC
Current Inputs

Protection Elements

Logic Inputs

2–4
Frequency measurement is done by measuring the time between zero crossings of
the Bus VT A and Line VT voltage inputs. Both signals are passed through a 72 Hz
low pass filter to prevent false zero crossings. Frequency readings are discarded if
the rate of change between two successive cycles is greater than 10 Hz/second. This
prevents momentary false frequency readings due to noise, phase reversals, or
faults.
Frequency tracking utilizes the measured frequency to set the sampling rate for
current and voltage which results in better accuracy for the FFT algorithm for off-
nominal frequencies. Also, sampling is synchronized to the Va-x voltage zero
crossing which results in better co-ordination for multiple 750/760 relays on the
same bus. If a stable frequency signal is not available then the sampling rate
defaults to the nominal system frequency.
Current waveforms are processed once every cycle with a DC Offset Filter and a Fast
Fourier Transform (FFT) to yield phasors at the fundamental power system
frequency. The resulting phasors have fault current transients and all harmonics
removed. This results in an overcurrent relay that is extremely secure and reliable
and one that will not overreach. The following diagram illustrates the signal
processing performed on the AC current inputs:
The DC Offset Filter is an infinite impulse response (IIR) digital filter which removes
the DC component from the asymmetrical current present at the moment a fault
occurs. This is done for all current signals used for overcurrent protection; voltage
signals bypass the DC Offset Filter. The filter results in no overreach of the
overcurrent protection; unfortunately, the filter also causes slower overcurrent
response times (0 to 50 ms) for faults marginally over the pickup level.
The Fast Fourier Transform (FFT) uses exactly one cycle of samples to calculate a
phasor quantity which represents the signal at the fundamental frequency only; all
harmonic components are removed. Further explanation of the FFT is beyond the
scope of this discussion but can be found in any text on signal analysis. All
subsequent calculations (e.g. RMS, power, demand, etc.) are based upon the
current and voltage phasors so the resulting values do not have any harmonic
components either.
All protection elements are processed once every cycle to determine if a pickup has
occurred or a timer has expired. The protection elements use RMS current/voltage
based on the magnitude of the phasor; hence, protection is impervious to both
harmonics and DC transients. Timing is not affected by system frequency.
Contact inputs are de-bounced to eliminate false operations due to noise. The inputs
must be in the same state for three consecutive readings spaced evenly over one
power frequency cycle before a new state is recognized.
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Theory of Operation
996709A1.CDR
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