PLC: data transfer over power lines

In electrical networks, the voltage typically alternates at a frequency of 50 or 60 Hz or is direct current. However, power cables can also carry signals at higher frequencies, even up to tens of MHz. This means the same cable can transmit power and high-speed data in both directions.

This technology is called Power Line Communication, or PLC for short. The early version of PLC was used for dispatcher communication over power cables, operating from 20 kHz to 1 MHz, mainly for the energy sector. This technology emerged in the mid-20th century. In this range, carrier frequencies are defined with a step of 4 kHz, allowing voice signals to be transmitted via amplitude modulation. This setup enables two-way communication and even multiple channels over a single wire.

The digital version of PLC technology started to gain traction in the late 1990s during the internet boom. At that time, energy companies hoped to compete with telecom providers by offering internet access to homeowners. Back then, people had access to two types of internet connections: dialup at speeds up to 56 kbps and ISDN at up to 64 kbps. It’s hard to imagine now, but end users agreed with these speeds. Setting up data transmission between a house and a transformer substation using PLC was easy at these speeds. The equipment for internet access was installed at the substation, which could be a couple miles from the house.

However, internet speeds quickly soared to tens and even hundreds of Mbps. PLC can only handle such speeds within a single apartment or house. The idea of providing internet access via the power grid never made it beyond local experiments, leaving fiber optics and twisted-pair cables to dominate. Still, PLC carved out a niche among telecom technologies.

Combining power and data transmission in one cable

Special couplers are used at both ends of the line to connect PLC equipment. These filters separate the PLC frequency range from the frequency of the power current or direct current. These devices prevent the network voltage from reaching the modem's input and output.

Couplers work based on different principles: antenna, capacitive, inductive, resistive, and optoelectronic.

In antenna couplers, a short piece of wire parallel to the power cable acts as an antenna for sending and receiving signals. The power cable then reradiates the signals received or transmitted by the antenna. This technology is outdated and no longer in use.

The two most common capacitive coupler circuits

These are the most common type and consist of filters made from coupling transformers, chokes, and capacitors.

Inductive couplers come in two types. The first type connects a coupling transformer’s winding to the ground break of the neutral, with the other winding connected to the transceiver. The second type involves Rogowski coils placed around the power lines, which are then connected to the transceivers. Inductive couplers are used in transmission and distribution networks.

Resistive Couplers are simple voltage dividers made from resistors. They are compact and inexpensive but don’t provide galvanic solid isolation.

In optoelectronic couplers, signals are transmitted and received through optocouplers, semiconductor devices containing a light-emitting diode and a photodiode. This setup provides the best galvanic isolation. However, the technology is limited by cost and the nonlinear characteristics of optocouplers, which can distort the signal.

Modern applications of PLC

At the time of writing, PLC technology is widely used for:

- Data transmission from "smart" electricity meters, including remote functions like disconnecting or limiting power supply for the customers that are late on payments
- Controlling street lighting systems
- Automation and dispatching at power facilities
- Monitoring and control in distributed generation systems (e.g., solar power plants)
- Smart home systems
- High-speed data transmission within an apartment or house.

These applications typically involve data transmission from tens of kbps to a few Mbps over distances up to 6 miles.

PLC is used for monitoring solar power generation 

Pros and cons compared to other communication technologies

Compared to fiber optics, PLC does not need an additional communication cable alongside the power line. Moreover, fiber optic cables can’t be bent beyond a certain radius (around 3 inches) and require specialized equipment and skilled technicians for splicing.

PLC also offers benefits compared to wireless technologies. Radio signals can sometimes struggle to pass through obstacles, and the crowded 2.4 GHz band can experience interference. However, PLC offers a more reliable connection than wireless technologies.

The main downside of PLC is that it transmits data over a network originally designed for power delivery. This means there are components where the PLC signal can’t pass. In AC networks, transformers are always a barrier. Additionally, some random devices in unexpected spots of the power network can block the PLC signal, making installation time-consuming and complex as it involves troubleshooting.

Power cables aren’t shielded from interference; they emit the PLC signal, potentially causing radio interference. Special modulation types are used to combat induced interference. To prevent cables from creating interference, the signal spectrum is capped at 500 kHz in the USA and 148.5 kHz in Europe.

PLC-G3 Standard

Another challenge is the lack of standardization in data transmission technology. Often, equipment is incompatible due to different protocols. However, for applications like data collection from electricity meters or solar energy management systems, where speeds of a few tens of kbps are sufficient, there’s an international standard called PLC-G3, formalized in the ITU-T G.9903 (08/17) by the International Telecommunication Union. Additionally, a significant advantage of PLC-G3 is its compatibility with IoT systems and the ability to set up IPv6 networks over PLC-G3 channels.

PLC-G3 uses OFDM modulation, known for its resilience against signal fading and reflections, ensuring high reliability. Data transfer rates for commercially available equipment reach up to 45 kbps (with a theoretical limit of 234 kbps), and a network can include up to 1,000 stations.

In Europe, PLC-G3 operates in the frequency ranges of 35.9–90.6 kHz (GENELEC A) and 98.4–121.9 kHz (GENELEC B). In the USA, it uses the 154.7–487.5 kHz range (FCC); in Asia, it operates between 154.7–403.1 kHz (ARIB). These bands experience low interference in the electrical network, above the frequencies of fluorescent lamp ballasts. Additionally, these bands are not used for broadcasting or public address systems in these regions.

Standardization and excellent electromagnetic compatibility with other equipment have made PLC-G3 the go-to solution for digitalizing power systems. The standard is suitable for AC and DC networks, which opens up its use in solar energy, where solar panels generate DC power.