Measurement Computing WavePort 312P rev.1.0 - Manual (Page 76)

10-2 Frequency Following PowerVista/312 User’s Manual
10.2.1 Frequency Following Process Time
Frequency Following is an additional algorithm, and will take additional process time. The algorithms have
been optimized to minimize floating point calculations and excess looping and function call overhead. If
Frequency Following is enabled, and processing of information cannot be performed adequately without FIFO
overruns, try disabling Frequency Following. This should not be necessary unless an under powered PC is being
utilized. Tests performed using a P150 notebook PC show adequate response when monitoring a 400 Hz system
with a 200 Hz base frequency.
10.2.2 Behavior Without Frequency Following
Without Frequency Following, wave data sampled at a fixed rate is sent directly to demand processing.
EasyPower Measure will set the fixed sample rate at 128 times the Base Frequency. Thus, for a 60 Hz
system, the fixed sample rate will be 7680 Hz. As long as the input wave data has a frequency very near 60 Hz
(which is typical for the U.S. power grid), there will be no noticeable differences in demand quantities from
those calculated with Frequency Following. In fact, fixed sample rate calculation is completely acceptable for
most measurements. However, if the input frequency begins to drift significantly from the Base Frequency, then
the calculated demand results will begin to exhibit a beating modulation. The modulation will have a frequency
equal to the slip or difference between the two frequencies.
There will be some data purists who will choose to disable Frequency Following so that wave data is processed
directly via demand calculations. Others might disable following to minimize computations on an under
powered PC. For most systems however, Frequency Following will give additional data (frequency), and allow
demand information to be calculated correctly under frequency excursions. For isolated power grids or systems
with poor frequency regulation, Frequency Following should always be employed.
10.2.3 Frequency Following Limitations
The only significant limitation in EasyPower Measure's Frequency Following, is the need for cleanliness of
the synchronizing phase. If Phase1 Voltage (the synchronizing phase) has excessive noise or distortion, errors
in frequency and thus demand quantities could result. Version 2.0 of EasyPower Measure will include an
additional digitally filtered front end for the synchronizing phase.