P54x/EN FD/La4 Firmware Design
(FD) 9-16
MiCOM P543, P544, P545 & P546
FD
3.4.1 Overview - protection and control scheduling
After initialization at start-up, the protection and control task on the main processor board is
suspended until the co-processor board re-starts via an interrupt. In the case where the co-
processor board has failed, the protection task will automatically start after six analog
samples have been received. In normal operation the task will be re-started by the co-
processor 16 times per cycle. The acquisition of samples on the main processor board is
controlled by a ‘sampling function’ which is called by the system services software and takes
each set of new samples from the input module and stores them in a two-cycle buffer, these
samples are also stored concurrently by the co-processor.
3.4.2 Signal processing
The samplin
g function provides filtering of the digital input signals from the opto-isolators and
frequency tracking of the analog signals.
The frequency tracking of the analog input signals is achieved by a recursive Fourier
algorithm which is applied to one of the input signals, and works by detecting a change in the
measured signal’s phase angle. The calculated value of the frequency is used to modify the
sample rate being used by the input module so as to achieve a constant sample rate of 48
samples per cycle of the power waveform. The value of the frequency is also stored for use
by the protection and control task.
When the protection and control task is re-started by the sampling function, it calculates the
Fourier components for the analogue signals. The Fourier components are calculated using
a one-cycle, 48 sample Discrete Fourier Transform (DFT). The DFT is always calculated
using the last cycle of samples from the 2-cycle buffer, i.e. the most recent data is used. The
DFT used in this way extracts the power frequency fundamental component from the signal
and produces the magnitude and phase angle of the fundamental in rectangular component
format. The DFT provides an accurate measurement of the fundamental frequency
component, and effective filtering of harmonic frequencies and noise. This performance is
achieved in conjunction with the relay input module which provides hardware anti-alias
filtering to attenuate frequencies above the half sample rate, and frequency tracking to
maintain a sample rate of 48 samples per cycle. The Fourier components of the input
current and voltage signals are stored in memory so that they can be accessed by all of the
protection elements’ algorithms. The samples from the input module are also used in an
unprocessed form by the disturbance recorder for waveform recording and to calculate true
rms values of current, voltage and power for metering purposes.
3.4.3 Main protection digital filtering
All of the p
rocessing for the differential and distance protection algorithms is performed on
the co-processor board.
3.4.3.1 Differential protection
The differenti
al protection is based on the relays at the line ends exchanging data messages
four times per cycle. To achieve this the co-processor takes the frequency-tracked samples
at 48 samples per cycle from the input board and converts these to 8 samples per cycle
based on the nominal frequency (i.e. not frequency tracked). The co-processor calculates
the Fourier transform of the fixed rate samples after every sample, using a one-cycle
window. This generates current measurements eight times per cycle which are used for the
differential protection algorithm and transmitted to the remote relay(s) using the HDLC (high-
level data link control) communication protocol.
The co-processor is also responsible for managing intertripping commands via the
communication link, and re-configuration instigated from the remote relay(s).
Data exchange between the co-processor board and the main processor board is achieved
through the use of shared memory on the co-processor board. When the main processor
accesses this memory, the co-processor is temporarily halted. After the co-processor code
has been copied onto the board at initialization, the main traffic between the two boards
consists of setting change information, commands from the main processor, differential
protection measurements and output data.
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