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Cross-phase signal coupling in PLC

01 Dec 2014  | Bogdan Baraboi

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A PLC signal transmitted between one phase and neutral will travel freely on that pair of wires, being only attenuated by the series line inductance and by the parallel capacitive and resistive loads connected between the same lines. However, to reach transceivers connected on other phase(s), the PLC signal has to pass from one line to the other. In the absence of any phase-coupling device, some "natural" coupling is provided by the low phase-to-phase impedances created at PLC frequencies due to appliances connected between phases, closely positioned conductors, and inter-winding capacitance of the supply transformer secondary. These "natural" bridges are more efficient in the MHz range, as the phase-to-phase coupling impedance is lower. However, the "natural" coupling is not consistent and the installation of coupling devices is more often than not recommended in poly-phase systems.

The common solution is the use of a capacitive coupler, typically installed close to the main electrical panel. A passive capacitive coupler basically consists of one or more capacitors connected between phases. The capacitors have to be suited for the power line voltage, and their value is chosen to provide very high impedance at 50Hz/60Hz and low impedance at high frequency. This is so that they block the power line voltage and pass the PLC signals from one phase to the other. Passive couplers can also include signal transformers that provide galvanic isolation and improve the high-pass filter response.

A passive coupler can be combined with a PLC transceiver to create an active coupler (or coupler-repeater). This device does not only allow PLC signals to pass from one phase to another; it detects the original signal upon one phase, and then retransmits it at increased amplitude onto all phases. Active couplers are particularly practical in large PLC networks, mainly in three-phase power systems.

Phase-to-phase signal coupling
While conventional capacitive couplers are intended to pass PLC signals to each phase relative to neutral, they are not suitable for phase-to-phase signals.

When controlling an appliance supplied from two (or three) phases, without neutral wire available, a phase-to-phase PLC receiver must be used. If the original PLC signals are transmitted between one phase and the neutral, a receiver that is connected phase-to-phase will not get the signals referenced to neutral. Moreover, conventional capacitive couplers are more harmful than useful in this situation, since the coupling capacitors connected between phases create very low impedance at high frequency, so they completely cancel out phase-to-phase signals.

Below is an example of a universal coupling circuit that was developed by Ariane Controls to transfer the PLC signals on all phases with respect to the neutral and to each other. Figure 2 shows the simplified schematic of such a passive coupler for use in split-phase power systems.

 Universal passive coupler

Figure 2: Simplified schematic of universal passive coupler for split-phase power system.

The circuit is composed of two coupling capacitors and one 1:1 high-frequency signal transformer (for simplicity, protection components are not included in the schematic). The transformer and capacitors act as a high-pass filter that greatly attenuates the 50Hz/60Hz power line voltage, while passing the high-frequency PLC signals between each pair of wires. The polarity of primary and secondary windings is important when connecting the transformer, as it allows adding the PLC signals so that the phase-to-phase signal will be the sum of the phase-to-neutral signals.

This method can be extended for use in three-phase power lines, by including an extra capacitor and transformer. The passive coupling circuit can also be connected to a PLC transceiver via another 1:1 signal transformer to become an active coupler.

Cross-phase communication is one of the main PLC challenges in split-phase and three-phase power systems. SInce PLC transceivers can be connected between any phase and neutral or even between phases, the signals, typically transmitted between one phase and neutral, have to be coupled onto all wires. Despite some "natural" coupling that exists between phases, the installation of coupling devices is usually required, particularly in narrowband PLC systems. Traditional signal couplers work great to pass PLC signals to all phases relative to neutral, but do not help for phase-to-phase signals. The article described a different coupling solution recommended by Ariane Controls for enabling the PLC signals between all pairs of wires in a poly-phase system.

About the author
Bogdan Baraboi is hardware designer and application engineer at Ariane Controls, a leading developer of narrowband powerline communication solutions. He has a Master's degree in Electrical Engineering from Université Laval (Québec, Canada) and over 10 years of experience in electronics, analogue signal processing, machine learning, power systems, and powerline communications.

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