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DESCRIPTION OF THE 869 MOTOR PROTECTION SYSTEM
Description of the 869 Motor Protection System
1–2
A summary of the available functions and a single-line diagram of protection and control
features is shown below. For a complete understanding of each feature operation, refer to
Chapter 4: Setpoints. The logic diagrams include a reference to every setpoint related to a
feature and show all logic signals passed between individual features. Information related
to the selection of settings for each setpoint is also provided.
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 k Hz
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 digital 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 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 through 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
CHAPTER 1: INTRODUCTION
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
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