Datasheet

Theory of operation STPMC1
26/77 Doc ID 15728 Rev 7
Each ACx register contains a 20-bit accumulator of the relative phase current I
X
[Ah] and an
8-bit register carrying the information about phase delay between voltage channels.
The SWM mode is indicated by status bit
NAH =0:
● Bit NAH =0 (SWM on) happens when BFR =1 and RMS value of current signal is I
X
>
I
Xmax
/4096 = 16 (I
Xmax
= 2
16
). In this case frequency is out of limits and RMS current I
X
is big enough, so it is accumulated in the corresponding ACx phase register.
● Bit NAH =1 (SWM off) happens if BFR =1 and RMS value of current signal is I
X
<
I
Xmax
/8192 = 8, or BFR =0. In this case either voltage frequency is out of limits but RMS
current I
X
is too small to enter SWM mode, or voltage frequency is in the correct range.
When bit
BFR is set, for a certain phase, its energy registers (active, reactive, fundamental)
are blocked. Then, if RMS value of current signal is big enough, bit
NAH is cleared (0) and a
SWM operation is entered. In this case the RMS value of current signal is accumulated in
ACx register and the value of voltage RMS U
X
is set to zero.
Example 1: Single wire operation with SYS = 0
SYS = 0 (3-phase system) is set and in the R-phase the voltage signal is too low
(status bits of phase R
BFR = 1 and LOW = 1).
Because of the too low voltage signal the frequency can't be calculated and energy
registers related to the R-phase are blocked.
If RMS value of current signal is big enough, the device enters SWM and clears
NAH of
R phase. The ACR register is incremented by adding I
R
, the RMS value of current
signal.
Example 2: Single wire operation with SYS = 0 and TCS = 1
SYS = 0 (3-phase system) and TCS = 1 (CT sensor selection) are set and in all phases
(R, S and T) the voltage signal is too low (status bits
BFR = 1 and LOW = 1 for all
phases).
Because of the too low voltage signal the frequency could not be calculated and all
energy registers are blocked.
Since when TCS
= 1, a frequency value is needed to calculate the RMS value of the
current signal, the default value of 50 Hz or 60 Hz (if bit FRS
=1) is taken. If the RMS
value of current signal is big enough, the device enters SWM and clears
NAH of all
phases and ACR, ACS and ACT registers is fed with the correspondent I
X
.
The accumulators ACx can be read by means of SPI.
To retrieve energy information, RMS value of current signal accumulated in registers ACx
can be multiplied by a constant representing the value of RMS voltage. This operation must
be executed by a microcontroller.
Usually the supply voltage for the electronic meter is taken from the line voltage. In SWM,
since the line voltage is not present anymore, some other power source must be used in
order to provide the necessary supply to STPMC1 and the other electronic components of
the meter.
9.8 Load monitoring (status bit BIL , configuration bit LTCH)
The STPMC1 includes in each phase a no-load condition detection circuit with adjustable
threshold. This circuit monitors the voltage and the current channels and, when the
measured voltage is below the set threshold, an internal signal
BIL becomes high. The