FUNCTION CHARACTERISTICS
21
NA016 - Manual - 05 - 2022
4.3 I/O DESCRIPTION
Metering inputs
The following input are provided:
• Three phase current inputs fo traditional CTs or LPCTs (Low Power Current Transformers).
• One residual current input.
• The nominal currents are independently adjustable at 1 A or 5 A through DIP-switches (CTs inputs).
The input circuits are appropriately dimensioned in order to withstand the currents which arise
when a fault occurs, 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
sensori.ai
NA016
3 phase CT current inputs
3 phase LPCT current inputs
one residual current input
acquisition.ai
ACQUISITION
INSTANTANEOUS VALUES
≈
IL1.ai
(
I
n
)
ACQUISITION
i
L1
,
i
L2
,
i
L3
DFT
TA
≈
I
L1
,
I
L2
,
I
L3
IE.ai
(
I
En
)
ACQUISITION
i
E
DFT
TA
≈
I
E
