AN3213 datasheet(50/64 Pages) STMICROELECTRONICS

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Application ideas

AN3213

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Doc ID 17429 Rev 1

Application ideas

8.1

Three-phase architecture

The ST7570 modem can be used to communicate on a three-phase network. This is

especially required for low-voltage substation nodes (concentrators), collecting data from

several energy meters all along the three phases of the distribution network.

In the example scheme of Figure 47, the line coupling circuit allows the signal to divide into

the three phases via capacitive coupling. This structure has been designed to keep a similar

impedance on each phase, therefore optimizing the signal distribution between the phases.

A critical point regarding this solution may be the total impedance the ST7570 power

amplifier is required to drive, which is the result of the three phases in parallel. For

concentrator nodes, however, the impedance per phase is likely to be considerably above

the driving limit of the power amplifier, as all the electrical devices supplied by the power line

are placed at a certain distance from the substation.

In the switched coupling scheme of Figure 48, a more complex circuit is shown, being the

coupling to each phase selectable via opto-switches.

Only one phase at a time can be used to transmit. An incoming signal, however, can be

received either from one phase at a time (J1 closed, J2 open) or from any phase at the

same time (J1 open, J2 closed). Both solutions can work well: the first solution has the

advantage of reducing crosstalk between the three phases, while the second allows

listening to the whole network at the same time. The choice depends on electrical and

performance tests as well as on specific protocol requirements.

For the zero crossing coupling, only one phase shall be used as reference, therefore

keeping the same coupling circuit as in the ST7570 single-phase demonstration board.

Figure 47.

Scheme of principle for non-switched three-phase architecture

AN3213 Datasheet(PDF) 50 Page - STMicroelectronics