Even if the AnalogInputType setting of a SMAI block is set to
Current, the MinValFreqMeas setting is still visible. This means that
the minimum level for current amplitude is based on UBase. For
example, if UBase is 20000, the minimum amplitude for current is
20000 * 10% = 2000. This has practical affect only if the current
measuring SMAI is used as a frequency reference for the adaptive
DFT. This is not recommended, see the Setting guidelines.
15.9.6 Operation principle
Every SMAI can receive four analog signals (three phases and one neutral value),
either voltage or current. The AnalogInputType setting should be set according to the
input connected. The signal received by SMAI is processed internally and in total 244
different electrical parameters are obtained for example RMS value, peak-to-peak,
frequency and so on. The activation of BLOCK input resets all outputs to 0.
SMAI_20 does all the calculation based on nominal 20 samples per line frequency
period, this gives a sample frequency of 1 kHz at 50 Hz nominal line frequency and 1.2
kHz at 60 Hz nominal line frequency.
The output signals AI1...AI4 in SMAI_20_x function block are direct outputs of the
connected input signals GRPxL1, GRPxL2, GRPxL3 and GRPxN. GRPxN is always
the neutral current. If GRPxN is not connected, the output AI4 is zero. The AIN output
is the calculated residual quantity, obtained as a sum of inputs GRPxL1, GRPxL2 and
GRPxL3 but is equal to output AI4 if GRPxN is connected. The outputs signal AI1,
AI2, AI3 and AIN are normally connected to the analog disturbance recorder.
The SMAI function block always calculates the residual quantities in
case only the three phases (Ph-N) are connected (GRPxN input not
used).
The output signal AI3P in the SMAI function block is a group output signal
containing all processed electrical information from inputs GRPxL1, GRPxL2,
GRPxL3 and GRPxN. Applications with a few exceptions shall always be connected
to AI3P.
The input signal REVROT is used to reverse the phase order.
A few points need to be ensured for SMAI to process the analog signal correctly.
1MRK 511 287-UEN A Section 15
Basic IED functions
571
Technical manual
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