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Chapter 4 - Software Design
5
PROTECTION AND CONTROL FUNCTIONS
The protection and control software processes all of the protection elements and measurement functions. To
achieve this it has to communicate with the system services software, the platform software as well as organise its
own operations.
The protection task software has the highest priority of any of the software tasks in the main processor board. This
ensures the fastest possible protection response.
The protection and control software provides a supervisory task, which controls the start-up of the task and deals
with the exchange of messages between the task and the platform software.
5.1
ACQUISITION OF SAMPLES
After initialization, the protection and control task waits until there are enough samples to process. The acquisition
of samples on the main processor board is controlled by a 'sampling function' which is called by the system
services software.
This sampling function takes samples from the input module and stores them in a two-cycle FIFO buffer. The
sample rate is 24 samples per cycle. This results in a nominal sample rate of 1,200 samples per second for a 50 Hz
system and 1,440 samples per second for a 60 Hz system. However the sample rate is not fixed. It tracks the
power system frequency as described in the next section.
5.2
FREQUENCY TRACKING
The device provides a frequency tracking algorithm so that there are always 24 samples per cycle irrespective of
frequency drift within a certain frequency range (see technical specifications). If the frequency falls outside this
range, the sample rate reverts to its default rate of 1200 Hz for 50 Hz or 1440 Hz for 60 Hz.
The frequency tracking of the analog input signals is achieved by a recursive Fourier algorithm which is applied to
one of the input signals. It works by detecting a change in the signal's measured phase angle. The calculated value
of the frequency is used to modify the sample rate being used by the input module, in order to achieve a constant
sample rate per cycle of the power waveform. The value of the tracked frequency is also stored for use by the
protection and control task.
The frequency tracks off any voltage or current in the order VA, VB, VC, IA, IB, IC, down to 10%Vn for voltage and
5%In for current.
5.3
FOURIER SIGNAL PROCESSING
When the protection and control task is re-started by the sampling function, it calculates the Fourier components
for the analog signals. Although some protection algorithms use some Fourier-derived harmonics (e.g. second
harmonic for magnetizing inrush), most protection functions are based on the Fourier-derived fundamental
components of the measured analog signals. 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 Fourier components are calculated using single-cycle Fourier algorithm. This Fourier algorithm always uses
the most recent 24 samples from the 2-cycle buffer.
Most protection algorithms use the fundamental component. In this case, the Fourier algorithm extracts the power
frequency fundamental component from the signal to produce its magnitude and phase angle. This can be
represented in either polar format or rectangular format, depending on the functions and algorithms using it.
The Fourier function acts as a filter, with zero gain at DC and unity gain at the fundamental, but with good
harmonic rejection for all harmonic frequencies up to the nyquist frequency. Frequencies beyond this nyquist
frequency are known as alias frequencies, which are introduced when the sampling frequency becomes less than
twice the frequency component being sampled. However, the Alias frequencies are significantly attenuated by an
anti-aliasing filter (low pass filter), which acts on the analog signals before they are sampled. The ideal cut-off point
of an anti-aliasing low pass filter would be set at:
50
P24xM
P24xM-TM-EN-2.1
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