The need for Power Line Carrier (PLC) phase couplers is primarily limited to North America and is related to the electrical distribution system used there. A excellent explanation of this distribution system is given in this "HowStuffWorks" article.
The drawing below illustrates the residential 3-wire system which was inherited from Edison's early DC distribution networks.¹ The primary side of the Distribution Transformer is connected between ground and one of the 2400V, 7200V or 13,200V² phases of the utility company's 3-phase distribution network. The secondary of the Distribution Transformer has a grounded center tap and is wound in a manner that supplies two 120V AC phases which are 180° out of phase with each other. In urban areas, one Distribution Transformer typically supplies 1-6 residences.
The phase relationships are shown in the oscilloscope traces.
120V loads are connected between either phase and neutral, while 240V loads are connected between both phases. If a PLC transmitter is connected to PHASE A, the carrier's only path to PHASE B is either through a 240V load (e.g. an oven, dryer, or AC unit) or through the secondary windings of the Distribution Transformer.
There may or may not be any 240V loads and, if present, they are not always on. The distance to the Distribution Transformer plus the impedance it presents to the carrier may be too great for enough signal to get through to PHASE B for reliable control.
In these cases, adding a coupling device which presents a low impedance path for the carrier between PHASE A and PHASE B can improve control. Moving the transmitter as close as possible to the breaker panel can also help.
The coupling device can be as simple as a 0.1µF/600WVDC capacitor or it can be a bandpass filter tuned to the carrier frequency or it can be a more elaborate coupler/repeater which amplifies and repeats the signals to both phases.
While PHASE A and PHASE B are 180° out of phase with each other, they each cross zero at the same times so no special timing is required to couple zero-crossing referenced data from one phase to another. This is not the case with 3-phase systems where the zero crossings occur at 120° intervals.
Phil Kingery of ACT (Advanced Control Technologies, Inc) has treated this topic with far more detail in his X-10 Technical Series written for HomeToys.
120/240v Residential Coupling
Complex Residential Coupling with Considerations for Dim/Bright
Dim/Bright Commands and Coupler-Repeaters
There are areas of the world where 3-phase power is used. Some of the commercial couplers also work with 3-phase wiring. Most expect the PLC transmitters to supply the carrier bursts at 120° intervals to coincide with the 3-phase zero crossings. The actual time intervals between the zero crossings also depends on the frequency of the line voltage which is 60Hz in North America but is 50Hz in Europe and other areas.
While many people insist on calling the 3-wire system single phase, it should be clear from the above that while only one of the three 7200V phases feeds the Distribution Transformer primary, there are two distinct 120V phases coming from the secondary. In effect, the Distribution Transformer is the same as two 7200V:120V stepdown transformers connected so that they are 180° out of phase. Since the two distinct secondary phases are distributed to several residences, calling them a single phase is somewhat ridiculous. Others call it split-phase but this is also a misnomer. In fact, the two 120V phases are combined to create one phase for powering 240V loads. There is no way to derive the two 120V waveforms from the 220V waveform by splitting, slicing, dicing, hacking, whacking, whatever. It can be done with a 220V transformer that had a center-tapped secondary with the center tap grounded so that it outputs two 120V phases with one phase inverted, but that has already been done by the Distribution Transformer. On the other hand, the 240V waveform is easily derived by combining the two 120V waveforms.
¹ In 1882, Thomas Edison wired the town of Sunbury, Pennsylvania using a shared-common three-wire DC distribution system. The cost of copper wire was an important factor, so Edison's engineers devised a way (copied from an earlier English patent) to distribute two circuits using only three wires. With DC, as the amperage increases linearly, the wire diameter increases geometrically. Edison's 3-wire system had a positive DC line, a grounded center line, and a negative DC line. Each hot wire carried only half the total amperage. With balanced loads, the algebraic sum of the currents in the ground leg was zero.
Many of Edison's inventions were just that. The incandescent lamp is an excellent example.
² The actual voltage on the High Voltage side of the Distribution Transformer varies from region to region (or utility to utility). In some places it is as low as 2400V and in others it is as high as 13200V.
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