scheider electric MiCOM P746, Technical Manual

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Firmware Design P746/EN FD/G31
MiCOM P746 (FD) 9-
17
FD
3.4 Protection and control software
The protection and control software task is responsible for processing all of the protection
elements and measurement functions of the relay. To achieve this it has to communicate
with both the system services software and the platform software as well as organize its own
operations. The protection software has the highest priority of any of the software tasks in
the relay in order to provide the fastest possible protection response. The protection &
control software has a supervisor task that controls the start-up of the task and deals with the
exchange of messages between the task and the platform software.
3.4.1 Overview - protection and control scheduling
The figure 8 shows the parts of Schneider Electric software and their allocation on the
different boards.
The P746 relay contained two global protections, busbar protection and circuit breaker
failure, and additional functions such as overcurrent protection.
3.4.2 Topology processing
Topology algorithm determines dynamically the electric scheme of the substation from the
auxiliary contact of circuit breaker and isolators. At the end of process, the P746 knows the
node of current and zone to trip according to the fault location.
3.4.3 Signal processing
The sampling function provides filtering of the digital input signals from the opto-isolators and
frequency tracking of the analog signals. The digital inputs are checked against their
previous value over a period of half a cycle.
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 24
samples per cycle of the power waveform. The value of the frequency is also stored for use
by the protection & control task.
The protection and control calculates the Fourier components for the analogue signals. The
Fourier components are calculated using a one-cycle, 24-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. The Fourier
components of the input current 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.
3.4.4 Programmable scheme logic
The purpose of the programmable scheme logic (PSL) is to allow the relay user to configure
an individual protection scheme to suit their own particular application. This is achieved
through the use of programmable logic gates and delay timers.
The input to the PSL is any combination of the status of the digital input signals from the
opto-isolators on the input board, the outputs of the protection elements, e.g. protection
starts and trips, control inputs, function keys and the outputs of the fixed protection scheme
logic. The fixed scheme logic provides the relay’s standard protection schemes. The PSL
itself consists of software logic gates and timers. The logic gates can be programmed to
perform a range of different logic functions and can accept any number of inputs. The timers
are used either to create a programmable delay, and/or to condition the logic outputs, e.g. to