GE P642, Technical Manual, Page: 95 / 592

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 DIRECT USE OF SAMPLE VALUES
Most of the IED’s protection functionality uses the Fourier components calculated by the device’s signal processing
software. However RMS measurements and some special protection algorithms available in some products use
the sampled values directly.
The disturbance recorder also uses the samples from the input module, in an unprocessed form. This is for
waveform recording and the calculation of true RMS values of current, voltage and power for metering purposes.
In the case of special protection algorithms, using the sampled values directly provides exceptionally fast response
because you do not have to wait for the signal processing task to calculate the fundamental. You can act on the
sampled values immediately.
5.4 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.
P64x Chapter 4 - Software Design
P64x-TM-EN-1.3 71