GE 750, Instruction Manual

Page: 33 / 504

GE Power Management 750/760 Feeder Management Relay 1-
17
1 PRODUCT OVERVIEW 1.3 THEORY OF OPERATION
1
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 rep-
resents the signal at the fundamental frequency only; all harmonic components are removed. Further explana-
tion of the FFT is beyond the scope of this document 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.
1.3.5 PROTECTION ELEMENTS
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, pro-
tection is impervious to both harmonics and DC transients. Timing is not affected by system frequency.
1.3.6 LOGIC INPUTS
Contact inputs are debounced 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 recog-
nized.