User manual

5 Power quality - a guide
89
Averaging with times less than 10 seconds is somewhat different. Although, they are all ex-
pressed in time units (200 ms, 1 s, 3 s, 5 s), in reality they are measured in multiples of the mains
period. For example, selecting 3-second averaging period means averaging in the time equal to
150/180 mains cycles (fifteen 10/12-cycle measurements).
Fig. 41. Determining the averaging intervals shorter than 10 seconds (with the 3-second
averaging).
The method of determining average values for such periods is shown in Fig. 41. Also in this
case the resynchronization of 10/12-cycle intervals, but it is always done using clock time of 10
minutes. When the clock counts another integer multiple of the 10-min. period, another aggregation
interval is resynchronized and the next interval is started; the aggregation interval (x) is terminated
normally, until the specified number of 10/12-cycle windows are gathered (e.g. for 3-second aver-
aging, always 15 intervals are gathered). The re-synchronization results in generating Overlap 2
(see Fig. 41), where data from two aggregation intervals are simultaneously processed ( (x)-interval
ends, (x+1)-interval starts). The size of the overlap depends on fluctuations in the mains frequency.
The time stamp corresponds to the end of the aggregation interval.
Averaging of measurement results leads to the loss of extreme values (smoothing of results).
In cases when the information about limit values of the measured parameter is important, the user
may use the option of measuring the minimum, maximum values in the averaging period. If a given
parameter is measured in the 10/12-cycle time, the minimum and maximum value is respectively
the smallest and the largest 10/12-cycle value measured in a given averaging interval. On the other
hand, the instantaneous value is the last 10/12-cycle value in this averaging interval.
In case of RMS current and voltage, the method of searching for minimum and maximum values
is more flexible and it is controlled by Min/Max calculation period parameter. The user may choose
from the following options: half period, 200 ms, 1 s, 3 s and 5 s. If the half-period option is selected,
the minimum and maximum values will be searched for with the highest sensitivity – up to U
RMS (1/2)
.
As this time is increasing, additional smoothing is being introduced; for example, with 5 seconds,
first a 5-second average value is calculated which is then used to search for the minimum and
maximum values. This gives a lower sensitivity to instantaneous changes of the measured value.
Note: similarly to the averaging times shorter than 10 seconds, the 200 ms, 1 s, 3 s and 5 s times
are actually the multiples of the mains period - 10/12, 50/60, 150/180 and 250/300 mains cycles,
respectively.