Even if the AnalogInputType setting of a SMAI block is set toCurrent, the MinValFreqMeas setting is still visible. This means thatthe minimum level for current amplitude is based on UBase. Forexample, if UBase is 20000, the minimum amplitude for current is20000 * 10% = 2000. This has practical affect only if the currentmeasuring SMAI is used as a frequency reference for the adaptiveDFT. This is not recommended, see the Setting guidelines.17.9.6 Operation principleEvery SMAI can receive four analog signals (three phases and one neutral value),either voltage or current. The AnalogInputType setting should be set according to theinput connected. The signal received by SMAI is processed internally and in total 244different 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 frequencyperiod, this gives a sample frequency of 1 kHz at 50 Hz nominal line frequency and 1.2kHz at 60 Hz nominal line frequency.The output signals AI1...AI4 in SMAI_20_x function block are direct outputs of theconnected input signals GRPxL1, GRPxL2, GRPxL3 and GRPxN. GRPxN is alwaysthe neutral current. If GRPxN is not connected, the output AI4 is zero. The AIN outputis the calculated residual quantity, obtained as a sum of inputs GRPxL1, GRPxL2 andGRPxL3 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 incase only the three phases (Ph-N) are connected (GRPxN input notused).The output signal AI3P in the SMAI function block is a group output signalcontaining all processed electrical information from inputs GRPxL1, GRPxL2,GRPxL3 and GRPxN. Applications with a few exceptions shall always be connectedto 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 504 135-UEN A Section 17Basic IED functions585Technical manual
