FWAV22 Frequency/RMS/Phase Meter (3-Phase)

Description

Argument Descriptions

Storage

Description

FWAV22 calculates a 3-phase, RMS voltage magnitude, a frequency and a relative phase angle based on a 3-phase input signal.  

NOTE:  The phase angle is a passed through a 'washout' function before it is output, which means that it should only be used to indicate a change in phase angle.  It is not a phase angle measurement!

General

During the opening interval where the simulation time t < 0.1 s, all computations in this subroutine are disabled.  This gives time for any simulation start–up transients to decay, as well as for the function to synchronize itself with the input waveforms A1, A2, and A3.  Once the 0.1 s time is reached, measurements of frequency, RMS voltage and relative phase (i.e. outputs F, W, A and VRMS) are measured and updated at 1 ms intervals until the simulation is finished.  

If at any time, the 3-phase, RMS, L-L voltage magnitude of A1, A2 and A3 drops to less than 80% of the rated voltage VR, then both the frequency and phase angle measurements are suspended and will remain frozen at the last value until the input voltage rises above 80% again.  

RMS Voltage Measurement

The 3-phase, L-L, RMS voltage is calculated in the same manner as that described for the VM3PH2 function.

Frequency Measurement

A narrow pulse is generated at the polarity change of each of the individual phases.  As each phase normally has two zero crossings in a cycle, the resulting pulse train has a frequency of 6 times the fundamental.

Impulses Used to Calculate Frequency of a 3-Phase Waveform

The frequency calculation is 1/6 times the reciprocal of the average interval between pulses over a cycle:

Some smoothing is then applied to f(t) using a first order lag with time constant 3/W0.

NOTE:  Multiple polarity changes (i.e. due to noise or harmonics on the waveform) that occur within a period smaller than 90% of dt = 6/f(t) , f(t) being the most recent frequency measurement, are ignored.

Phase Angle Measurement

The phase angle is calculated by integrating the frequency deviation from rated frequency W0.  This signal is then passed through a washout function as described in DERIV1 with time constant equal to TS.  Thus, assuming that the initial frequency of the 3-phase AC waveform at time t = 0.1 s is W0, then the phase angle output A remains at zero until there is a change in the phase (or frequency) of the input waveform.  The value of A immediately takes on the magnitude of this phase change.  If the phase remains constant thereafter, the output A washes out to a value of zero according to the input time constant TS.  

In a sense, A represents the change of phase of the input using the input’s own initial phase as the reference.  The phase signal A is useful when an indication of a change in phase from the steady-state is required, such as in a phase angle damping controller used in HVDC systems to damp out low frequency AC system oscillations.

Argument Descriptions

      SUBROUTINE FWAV22(A1,A2,A3,VR,WO,TS,F,W,A,VRMS)

Inputs:

Outputs:

Storage

This function requires the following storage allocations per call (see #STORAGE Script Directive and STORx Arrays for more details):

#STORAGE STOR:22