FUNCTION CHARACTERISTICS
24
NC020 - Manual - 05 - 2022
4.3
I/O DESCRIPTION
—
Metering inputs
The following input are provided:
• Three phase current inputs for traditional CTs.
• One neutral current input.
• The nominal currents are independently adjustable at 1 A or 5 A through DIP-switches (CTs inputs).
—
Signal processing
ACQUISITION (base level)
The input signals are sampled 48 times per power cycle
-
i
L1
...
i
L3
phase currents instantaneous value
-
i
N
Unbalance neutral current instantaneous value
From the sampled quantities, several measures are computed for protection, monitoring and meter-
ing purposes.
Samples are processed by means DFT (Discrete Fourier Transform) algorithm and the phase and
amplitude of fundamental are computed:
• Phase currents
I
L1
,
I
L2
,
I
L3
• RMS phase currents (including up to twenty-third harmonic)
I
L1RMS
,
I
L2RMS
,
I
L3RMS
• Unbalanced neutral current (RMS value of the fundamental component)
I
N
sensori.ai
Three phase current inputs
Neutral unbalance current input
NC020
52
acquisition.ai
ACQUISITION
INSTANTANEOUS VALUES
≈
IL1.ai
(
I
n
)
ACQUISITION
i
L1
,
i
L2
,
i
L3
DFT
TA
≈
I
L1
,
I
L2
,
I
L3
IL1RMS.ai
(
I
n
)
I
L1RMS
I
L2RMS
I
L3RMS
I
L1
I
L1RMS
= √
(
I
L12
+
I
L1-2nd2
+...
I
L1-2 3rd2
)
I
L 2RMS
= √
(
I
L 2 2
+
I
L 2-2nd2
+...
I
L 2-2 3rd2
)
I
L 3RMS
= √
(
I
L 32
+
I
L 3-2nd2
+...
I
L 3-2 3rd2
)
I
L2
I
L3
IN.ai
(
I
Nn
)
ACQUISITION
i
N
DFT
CT
≈
I
N
I
N
