STPM10 datasheet(23/47 Pages) STMICROELECTRONICS

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STPM10

Theory of operation

DocID17728 Rev 5

23/47

7.9.1

Detail operational description

The meter is initially set to normal state, i.e. tamper not detected. In this condition the

primary channel is selected for final integration of energy. In such state the values of both

load currents should not differ more than the accuracy difference of the channels does.

Sixty-four periods of line voltage is used as a tamper checking period. After 24 periods of

line voltage two internal signals MUX and INH are changed in order to enable secondary

current channel and to freeze the last power and RMS values of primary current channel.

The following 16 periods of line frequency are used for tamper detection integration. During

this gap, the final energy calculation does not use the signal from selected channel but the

frozen values. Four line periods after the INH switch, the integration of power from

secondary current channel is started and lasts four periods. Additional four line periods

later MUX signal is switched back to primary current channel and the integration for tamper

detection is started. The timings of MUX and INH signals are shown in the figure below.

Figure 18: Timings of tamper module - primary channel selected

When the secondary channel is selected to be integrated by the final energy integrator, the

MUX and INH signals change according to the figure below.

Figure 19: Timings of tamper module - secondary channel selected

This means that energy of four periods from secondary channel followed by energy of four

periods from primary channel is sampled within the tamper module. From these two

samples, called B and A respectively, the criteria of tamper is calculated and the channel

with higher current is selected, resulting in a new tamper state. If four consecutive new

results of criteria happen, i.e. after elapsed 5.12 s at 50 Hz, the meter enters into tamper

state. Thus, the channel with the higher current is selected for the energy calculation. If

samples of power A and B had different signs, the tamper would be on all the time but, the

channel with bigger power would be still selected for the final integration of energy. If a

tamper status has been detected, the multiplex ratio is 56:8 if the primary channel energy is

greater than the secondary one, otherwise it is 8:56. The detected tamper condition is

stored in the BIT status bit. If BIT = 0 tamper is not detected, if BIT = 1 a tamper condition

has been detected. In standalone mode the BIT flag is also available in the SDATD pin.

7.10

Phase compensation

The STPM10 does not introduce any phase shift between the voltage and current channel.

However, the voltage and current signals come from transducers, which could have

inherent phase errors. For example, a phase error of 0.1 ° to 0.3 ° is not uncommon for a

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