Technical description
4 Measurement functions
4.1 Measurement accuracy
VM50.EN004
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137
4.
Measurement functions
All the direct measurements are based on fundamental
frequency values. The exceptions are frequency and
instantaneous current for arc protection. Most protection
functions are also based on the fundamental frequency values.
The device calculates the active (P), reactive (Q), apparent
power (S) and energy measures (E+, Eq+, E-, Eq-) from voltage
and current measurements when voltage measurement mode is
set to 1LL (line-to-line voltage) or 1LN (phase-to-neutral
voltage).
The figure shows a current waveform and the corresponding
fundamental frequency component f1, second harmonic f2 and
rms value in a special case, when the current deviates
significantly from a pure sine wave.
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Time (s)
Curr
ent
(PU)
-10
-5
0
5
10
rms
IL2
f1
f2
Load = 0%
InrushCurrentLoad0
Relative
2nd
harmonic
f2/f1
(%)
0
50
100
f2/f1 (%)
Figure 4-1 Example of various current values of a transformer inrush
current.
4.1.
Measurement accuracy
Phase current inputs I
L1
, I
L2
, I
L3
Measuring range
0 – 250 A
Inaccuracy I
7.5 A
0.5 % of value or 15 mA
I > 7.5 A
3 % of value
The specified frequency range is 45 Hz – 65 Hz.