GE P741, Technical Manual

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P74x/EN FD/Na7 Firmware Design
(FD) 9-
22
MiCOM P741, P742, P743
FD
3.5.1 Overview - protection and control scheduling
Figure 2 shows parts of the GE software and their allocation on the different boards of the
peripheral and central units.
The P74x relays contained two global protections, busbar protection and circuit breaker
failure, and one local function, overcurrent protection. Overcurrent protection is implemented
on peripheral unit and is totally independent of the central unit. On the contrary, busbar
protection and circuit breaker failure are distributed between central unit and peripheral units.
Local functions such as saturation detection algorithm, logic of circuit breaker failure and
local confirmation threshold are performed on each peripheral unit. Sum of current, logic of
differential protection and circuit breaker failure are processed on central unit.
3.5.2 Topology software
Topology algorithm determines dynamically the electric scheme of the substation from the
auxiliary contact of circuit breaker and isolators. The results of local topology performed on
peripheral unit are sending to central unit which determines global topology of the substation.
At the end of process, central unit know the node of current and zone to trip according to the
fault location.
3.5.3 Signal processing
The sampling frequency of analogue signal is fixed to 2400Hz apart from the electric network
frequency.
To ensure that the frequency is identical on each PU, analogue acquisition is based on
interruption signal from communication software. Central unit send frames on optical fibres in
diffusion towards all peripheral units. So they received data at the same instant, this
reception signal starts the acquisition of analogue signal.
The main signal processing algorithms are:
•
Flux calculation and prediction algorithm to detect CT saturation
•
Zero sequence supervision
•
Detection of signal variation
•
Local threshold to block busbar protection on external fault
All this information are transmitted to central unit with the sample of current, they represent
signal quality. The sum of current is processed in central unit each 1200Hz but the signal
processing is executed at 2400Hz on peripheral unit.
The protection and control 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. 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.