GE 750 Instruction Manual page 44

THEORY OF OPERATION
2.2.4 Phasors, Transients, and Harmonics
2.2.5 Processing of AC Current Inputs
2.2.6 Protection Elements
2 - 6
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.
A novel filtering of the phase and ground currents is employed on the relay to assure fast
and secure Phase and Ground Instantaneous over-current operation. With an integrated
CT saturation detection mechanism, these protections can detect and produce stable
operation even for fault currents heavily distorted by the saturation of the Current
Transformers.
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.
The phase input currents for the Phase over-current protections are estimated at rate of
four times per cycle.
The figure below presents the IOC trip times – from fault inception till closure of a trip rated
output – of the SR750 relay. The response time depends on the ratio between the fault
current and the applied setting and varies from 16ms for high multiples of pickup to 25ms
for multiple of 1.2 times pickup.
CHAPTER 2: INTRODUCTION
750 FEEDER MANAGEMENT RELAY – INSTRUCTION MANUAL
996709A1.CDR