Datasheet

Theory of operation STPM01
22/60 Doc ID 10853 Rev 8
for tampering purposes or in case the line voltage is very stable, it is possible to use a
predefined value for computing the energy without sensing it.
In order to enable the SWM mode, the STPM01 must be configured with PST values of 4 or
5, (tamper enabled-Rogowsky coils). In this way, if the BFR error is detected, STPM01
enters in SWM. If BFR is cleared the energy calculation is performed normally, when BFR is
set (no voltage information is available) the energy computation is carried out using a
nominal voltage value according to the NOM configuration bits.
Since there is no more information on the phase shift between voltage and current, the
apparent rather than active power is used for tamper and energy computation. The
calculated apparent energy is the product between I
RMS
(effectively measured) and an
equivalent V
RMS
that can be calculated as follows:
V
RMS
= V
PK
*K
NOM
,
where V
PK
represents the maximum line voltage reading of the STPM01 and K
NOM
is a
coefficient that changes according to the following table:
For example, if a R
1
= 783 kΩ and R
2
= 475 Ω are used as resistor divider when the line
voltage is present, the positive voltage present at the input of the voltage channel of
STPM01 is:
since the maximum voltage value applicable to the voltage channel input of STPM01 is +0.3
V, the equivalent maximum line voltage applicable is:
V
PK
= R
1
+R
2
/R
2
• 0.3 = 494.82
considering the case of NOM=2, the correspondent RMS values used for energy
computation are:
V
RMS
= V
PK
• 0.4219 = 208.76 [V]
Usually the supply voltage for the electronic meter is taken from the line voltage, in SWM,
since the line voltage is not present any more, some other power source must be used in
order to provide the necessary supply to STPM01 and the other electronic components of
the meter.
8.7 Power supply
The main STPM01 supply pin is the V
CC
pin. From the V
CC
pin two linear regulators provide
the necessary voltage for the analog part V
DDA
(3 V) and for the digital part V
DDD
(1.5 V).
The V
SS
pin represents the reference point for all the internal signals. 100 nF low ESR
Table 10. Nominal voltage values
NOM K
NOM
00.3594
10.3906
20.4219
30.4531
2V
RR
R
VI
RMS
21
2
⋅
+
=