Cross-phase signal coupling in powerline communication

Bogdan Baraboi -November 14, 2014

In the emerging Internet of Things (IoT) market, powerline communication (PLC) is one of the preferred connectivity solutions for transferring data between devices. Using the omnipresent power line system as a communication medium, PLC allows interconnecting a large variety of appliances by transmitting data through the same wires that provide electrical energy. 

The PLC technologies are usually classified today in two categories: broadband over power lines (BPL), for high-speed data communication (computer networking, HDTV, etc.) and narrowband PLC, for low-speed monitoring and control applications (smart building automation, advanced metering, outdoor lighting control, etc.). The narrowband PLC technologies typically use the frequency band between 3kHz and 500kHz, providing data rates of tens to hundreds of kbps. Broadband PLC uses a much wider frequency band, usually from 2MHz up to 85MHz, and enables data rates of hundreds of Mbps.
Regardless of the PLC technology, the low-voltage AC power line systems are generally a challenging environment for communication. Severe signal attenuation and considerable noise are the main factors that can affect the communication performance.

Electrical devices connected to low-voltage power lines are a significant cause of signal attenuation, since they typically represent low impedance loads for high-frequency PLC signals. This is particularly problematic for BPL technologies, whose higher communication frequency makes the parallel load impedance much lower than for narrowband PLC.  

Another possible cause of signal attenuation is the cross-phase communication. In poly-phase systems, PLC transceivers can be connected on different phases, making the signals travel from one phase to the other. While some "natural" signal coupling between phases exists thanks to wiring and loads, it is usually not consistent and does not guarantee reliable communication between devices connected on different phases, particularly in narrowband PLC systems. In order to improve the PLC signal transfer between phases, coupling devices are usually required to be installed in poly-phase power systems.

Low voltage power systems
Different wiring topologies are used throughout the world for the mains power systems. In many countries, residential houses, apartment units, and small commercial buildings are typically supplied with single-phase power, comprising a phase (live) wire and a neutral (return) wire, with a separate safety ground (earth) wire, usually tied to the neutral at the main panel. The voltage carried between live and neutral differs from country to country. In most of Europe, Asia, Africa, and South America the single-phase mains voltage is between 220V and 240V, with 230V being the nominal voltage in the majority of the places. North America, Japan, Taiwan and some parts of northern South America use a mains voltage between 100V and 127V.

Another type of mains power system, split-phase, includes two live conductors and a neutral, plus the protective ground. The two live wires are derived from the same phase of the distribution system, via a center-tapped stepped-down transformer (see Figure 1). While not a real two-phase system, this configuration provides two equal voltages between each live wire and neutral. Since they are 180° out of phase, the voltage between the two live wires is the double of the voltage measured between each live and neutral. This system is commonly found in homes and light commercial buildings in Canada and USA, where the line-neutral voltage is 120V and is used to supply lights and small appliances. The voltage between the two lines is 240V and serves to supply larger appliances, like electric stoves, dryers, hot-water tanks, central air conditioning units, etc.

Figure 1. Diagram of split-phase power system

Large multiple-unit apartment blocks, as well as commercial and industrial buildings, can be supplied with three-phase power. Such configurations usually include three live wires plus one grounded return wire, and they can provide two voltage systems: phase-to-neutral and phase-to-phase. Various phase-to-neutral / phase-to-phase voltage systems exist around the world, e.g. 120V/208V, 220V/380V, 230V/400V, 277V/480V, 347V/600V, etc. These power lines can supply typical single-phase loads connected phase-to-neutral or phase-to-phase, as well as large three-phase loads, like motors, rectifiers, arc furnaces, etc.

Phase-to-neutral signal coupling
Except for single-phase systems, where all devices are connected between one phase and neutral, in all other power line topologies described above the issue of cross-phase PLC signal attenuation cannot be overlooked. In split-phase and three-phase systems, PLC devices may be installed between any phase and neutral. Typically, in indoor power systems, lighting and outlets are fed by separate circuits; sometimes, outlets very close one to each other may be connected to different phases.

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. 

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