GE 845, Instruction Manual

Page: 12 / 552

1–2 845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
DESCRIPTION OF THE 845 TRANSFORMER PROTECTION RELAY CHAPTER 1: INTRODUCTION
Description of the 845 Transformer Protection Relay
CPU
Relay functions are controlled by two processors: a Freescale MPC5125 32-bit
microprocessor that measures all analog signals and digital inputs and controls all output
relays, and a Freescale MPC8358 32-bit microprocessor that controls all the advanced
Ethernet communication protocols.
Analog Input and Waveform Capture
Magnetic transformers are used to scale-down the incoming analog signals from the
source instrument transformers. The analog signals are then passed through a 11.5 kHz
low pass analog anti-aliasing filter. All signals are then simultaneously captured by sample
and hold buffers to ensure there are no phase shifts. The signals are converted to digital
values by a 16-bit A/D converter before finally being passed on to the CPU for analysis.
The 'raw' samples are scaled in software, then placed into the waveform capture buffer,
thus emulating a fault recorder. The waveforms can be retrieved from the relay via the
EnerVista 8 Series Setup software for display and diagnostics.
Frequency
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 bus voltages. The frequency
tracking is switched automatically by an algorithm to the alternative reference source, i.e.,
three-phase currents signal or line voltage for the configuration of tie-breaker, if the
frequency detected from the three-phase voltage inputs is declared invalid. The switching
will not be performed if the frequency from the alternative reference signal is detected
invalid. Upon detecting valid frequency on the main source, the tracking will be switched
back to the main source. If a stable frequency signal is not available from all sources, then
the tracking frequency defaults to the nominal system frequency.
Phasors, Transients, and Harmonics
All waveforms are processed eight times every cycle with a DC decaying removal filter and
a Discrete Fourier Transform (DFT). 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.
Processing of AC Current Inputs
The DC Decaying Removal Filter is a short window digital filter, which removes the DC
decaying 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
use the same DC Decaying Removal Filter. This filter ensures no overreach of the
overcurrent protection.
The Discrete Fourier Transform (DFT) uses exactly one cycle of samples to calculate a
phasor quantity which represents the signal at the fundamental frequency; all harmonic
components are removed. All subsequent calculations (e.g. power, etc.) are based upon the
current and voltage phasors, such that the resulting values have no harmonic
components. RMS (root mean square) values are calculated from one cycle of samples
prior to filtering.
Protection Elements
All voltage, current and frequency protection elements are processed eight times every
cycle to determine if a pickup has occurred or a timer has expired. The voltage and current
protection elements use RMS current/voltage, or the magnitude of the phasor.