Theory Of Operation; Description; Current And Voltage Waveform Capture; Frequency Tracking - GE 750 Instruction Manual

1.3 THEORY OF OPERATION

1 PRODUCT OVERVIEW
1.3 THEORY OF OPERATION

1.3.1 DESCRIPTION

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Relay functions are controlled by two processors. A Motorola 68332 32-bit microprocessor measures all ana-
log signals and logic inputs, outputs all analog signals, and controls all output relays. An Intel 80C186 16-bit
microprocessor reads all user input including communications, and outputs to the faceplate display and LEDs.
The processors pass information to each other via an RS485 serial communications channel. Consult Figure
1–3: HARDWARE BLOCK DIAGRAM on page 1–18 for the role of each processor. The remainder of this sec-
tion describes the algorithms and operations that are critical to protection elements.

1.3.2 CURRENT AND VOLTAGE WAVEFORM CAPTURE

Current transformers (CTs) and voltage transformers (VTs) are used to scale-down the incoming current and
voltage signals from the source instrument transformers. Both current and voltage signals are then passed
through a 400 Hz low pass anti-aliasing filter. All signals are then simultaneously captured by sample and hold
buffers to ensure there are no phase shifts. The signals are then converted to digital values by a 12 bit A/D
converter before finally being passed on to the 68332 CPU for analysis.
Both current and voltage are sampled sixteen times per power frequency cycle with frequency tracking control.
These 'raw' samples are calibrated in software and then placed into the waveform capture buffer thus emulat-
ing a fault recorder. The waveforms can be retrieved from the relay via the 750/760 PC Program for display
and diagnostics.

1.3.3 FREQUENCY TRACKING

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. Fre-
quency readings are discarded if the rate of change between two successive cycles is greater than 10 Hz/s.
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 fre-
quency.

1.3.4 PHASORS, TRANSIENTS, AND HARMONICS

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 tran-
sients 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 cur-
rent inputs:
Figure 1–2: SIGNAL PROCESSING OF AC CURRENT INPUTS
1- 750/760 Feeder Management Relay GE Power Management
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Courtesy of NationalSwitchgear.com