FUNCTION CHARACTERISTICS
24
NA011 - Manual - 05 - 2022
4.3 I/O DESCRIPTION
Metering inputs
The following input are provided:
• Three phase current inputs for traditional CTs or LPCTs (Low Power Current Transformers).
• One residual current input available on NA011#xxx2 and NA011#xxx3 versions.
• The nominal currents are independently adjustable at 1 A or 5 A through DIP-switches (CTs inputs).
The input circuits are dimensioned in order to withstand the currents which arise when a fault oc-
curs, both in transient and steady state condition.
Signal processing
Various processing levels are involved:
• Acquisition (base level).
• Direct measures of physical channels (first level).
• Calculated measures (second level).
• Derived (third level).
The measures concerning a level are based on data worked out in the previous level.
For each level the required resources concerning the priority for tasks (conditioning circuits, DSP)
are on hand.
ACQUISITION (base level)
The input signals are sampled 24 times per power cycle
-
i
L1
...
i
L3
phase currents instantaneous value
-
i
E
residual 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
• Residual current
I
E[1]
Note 1 The residual current input is available on NA011#xxx2 and NA011#xxx3 versions; when input is unavailable (NA011#xxx1 version), the residual
current is calculated by the vector sum of the three phase currents, measured by three 1A or 5A CTs or by three LPCT type sensors.
sensori.ai
NA011
3 phase CT current inputs
3 phase LPCT current inputs
Residual current input
acquisition.ai
ACQUISITION
INSTANTANEOUS VALUES
≈
IL1.ai
(
I
n
)
ACQUISITION
i
L1
,
i
L2
,
i
L3
DFT
≈
I
L1
,
I
L2
,
I
L3
3 phase CT current inputs
3 phase LPCT current inputs
IE.ai
(
I
En
)
ACQUISITION
i
E
DFT
CT
≈
I
E