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1760
Users Manual
3-14
• The 10/12 cycle values are aggregated from 2.048 samples synchronized to the
power frequency
• The 3s (150/180 cycles)-intervals are derived from 15 consecutive 10/12 cycle
intervals
• The 10 minute and free interval values are based on the synchronized 10/12 cycle
values
• 2 hours interval is aggregated from 12 – ten minute intervals
• The 10 minute values are synchronized to the absolute time (e.g. via GPS time
sync option)
Power Frequency
For 10 s frequency values, the sample data are filtered by a 2
nd
order IIR filter (the 3 dB
cut-off frequency is 50 Hz for 50 Hz nominal frequency and 60 Hz for 60 Hz nominal
frequency). Based on the filtered signal whole periods within 10 s intervals (taken from
the internal real time clock) are counted by detecting the zero crossings. The frequency is
calculated by dividing the number of whole periods by the duration of this number of
whole periods. The time interval is derived from the timestamps generated by the
hardware of the first and the last sample within the block of whole periods. A special
calculation method is used for the frequency measurement related to the 10 ms and 20 ms
(half/full cycle) rms values. The measured frequency is the sync frequency of the PLL
which is refreshed every 200 ms (based on the FFT evaluation).
Voltage, Current rms Values, Min-/Max-Values
Half cycle rms is synchronized with the zero crossings of the fundamental component.
The fundamental component zero crossing is calculated from 200 ms FFT. Half cycle rms
is available as real half cycle rms and/or as full cycle rms, updated every half cycle.
The extreme values (Min-, Max-values) are derived from the half cycle rms values or full
cycle rms values updated every half cycle (whatever is configured in the “Nominal and
Limit Values” settings panel in PQ Analyze.
The interval values are averaged squared over the respective time interval.
FFT – Fast Fourier Transformation
FFT is calculated using an algorithm which is optimized for real input and complex
output with 2.048 points. As long as the PLL controlling the sampling frequency is
locked, no window function is applied. If locking cannot be established, a Hanning
window is used. The FFT is caclulated over 200 ms intervals and therefore results in a
frequency spectrum of 1024 5 Hz bins (DC to 5115 Hz).
Calculation of Power Parameters
Two different sets of power parameter calculations are used. One for aggregations of
200ms and higher the other one for half and full cycle aggregations.
Aggregations 200 ms and higher
Real power
Real power
P is derived from the results of the FFT calculations of voltage and current.
∑
=
⋅⋅=
1023
0
cos
i
iii
IUP
ϕ
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