STPM11ATR datasheet(16/43 Pages) STMICROELECTRONICS

16 / 43 page

Theory of operation

STPM11/12/13/14

16/43

The BFR flag is also set if the RMS voltage across VIP-VIN drops below a threshold value

calculated with the following formula:

(CT/Shunt)

(Rogowsky)

Where KV is the voltage calibrator value ranging from 0.875 to 1.000.

The BFR flag is cleared when the VIRMS value goes above twice VIRMS-BFR. When the BFR

error is set, the computation of power is suspended and MOP, MON and LED will be held

low.

7.5

Single wire meter mode (STPM13/14 with Rogowsky coil

sensor)

STPM1X supports the Single Wire Meter (SWM) operation when working with Rogowsky

Coil current sensors. In SWM mode there is no available voltage information in the voltage

channel. It is possible that someone has disconnected one wire (live or neutral) of the meter

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 STPM1X must be configured with PST values of 0 or

1. In this way, if the BFR error is detected, STPM1X 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 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 will be the product between IRMS (effectively measured) and an equivalent

VRMS that can be calculated as follows:

VRMS=VPK*KNOM, where VPK represents the maximum line voltage reading of the

STPM1X and KNOM is a coefficient that changes according to Table 10.:

BFR

IRMS

⋅

−

6703

BFR

IRMS

⋅

−

6687

Table 9.

RMS voltage check

BFR-on

BFR-off

Rogowsky

0.009571/Kv

0.019142/Kv

CT-Shunt

0.0078/Kv

0.0156/Kv

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