Communication protocols for PLC on DC lines

The main feature of any PLC system, whether on DC or AC lines, is that communication uses cables initially designed for power delivery. This results in the following challenges:

• Cable parameters are not certified for the PLC system frequencies
• No impedance matching between line elements, leading to multiple signal reflections
• No protection from external interference
• No shielding from radio waves emitted by the line
• Interference caused by load switching and, for DC lines, DC-DC conversions
• The signal transmitted via the power line can be intercepted into the surrounding space

The following solutions are known to address these issues:

• Limiting maximum transmission speed to tens of kbps is fine for control systems
• Using modulation types that are resistant to reflections and interference
• Selecting frequency ranges free from strong interference, such as the one from electronic ballasts of fluorescent lamps
• Choosing frequency ranges where radio stations and critical broadcast systems do not operate
• Using appropriate DSP algorithms to combat interference and reflections

Modulation

Previous generations of PLC systems used FSK (Frequency Shift Keying) modulation and its variants, where data is transmitted by abrupt frequency shifts. This modulation type inefficiently uses the spectrum, so data rates in PLC systems using FSK rarely exceed 2.4 kbps.
Most modern PLC systems use OFDM (Orthogonal Frequency-Division Multiplexing), where information is transmitted on several orthogonal subcarriers at equal frequency intervals. Each subcarrier is modulated using quadrature amplitude modulation or amplitude-phase shift keying. OFDM's main advantage is its resistance to multiple reflections. Additionally, OFDM uses the frequency spectrum more efficiently, allowing higher data rates than FSK without expanding bandwidth.

PLC-G3

This protocol is used on both AC and DC lines. The maximum transmission speed for commercial equipment is up to 45 kbps (the theoretical limit is 234 kbps), with a communication range of up to 1300 feet. In Europe, it operates in the 35.9–90.6 kHz (GENELEC A) and 98.4–121.9 kHz (GENELEC B) bands; in the USA, it operates in 154.7–487.5 kHz (FCC), and in Asia, 154.7–403.1 kHz (ARIB).
The advantages of PLC-G3 include its universality and international standardization (ITU-T G.9903), ensuring full equipment compatibility within a region of the same frequency.
It is currently used to control green energy systems, collect data from electricity meters, and send consumption limitation signals to the equipment of non-paying customers. 
Data is encrypted with AES-128.

PLC-Lite

The PLC-Lite protocol is promoted by TI and designed for DC lines with voltages from 18 to 35V, typically 24V. PLC-Lite operates in the 42–90 kHz frequency range with data rates up to 21 kbps.

The main advantage of this protocol is its cost-effectiveness (today, a low-frequency 60 MHz processor is sufficient). TI also provides a ready-to-use set of chips and software, simplifying equipment production. The disadvantages include a communication range limited to 130 feet and dependency on a specific manufacturer, which reduces the selection of available proprietary components.
This protocol is mainly used in factories with robots. It transmits both 24V power and control signals over the same cable.
Since PLC-Lite systems are typically used within a single factory floor, encryption is not provided by default.

IEEE 2847-2021

This protocol supports data transmission over DC power lines at speeds up to 9.6 kbps. The line voltage must not exceed 50V, and the load power can range from 10 to 2000 watts, with a minimum power supply output of 100 watts. The communication range is up to 3300 feet.
The protocol covers nearly all possible PLC use cases for control purposes, such as lighting, motors, energy storage, UPS systems, and solar power generation.
The protocol is based on HPDS-PLC (High Power Differential Signal) technology, which uses differential transmission. Identical signals with a 180-degree phase shift are sent through two wires, and at the receiving end, one signal is subtracted from the other, doubling the transmitted signal. External interference affects both wires equally and is mainly canceled out. This also reduces electromagnetic interference emitted by the PLC system. As a result, the transmitted signal's power can be increased, enhancing resistance to interference from DC-DC converters.
Although IEEE is based in the US, its standards are recognized worldwide. HPDS-PLC is expected to be well-standardized and has good equipment compatibility. The protocol offers the longest communication range among PLC protocols on DC lines. However, the data transmission rate is limited to 9.6 kbps, which is relatively minimal by modern standards, even for electrical equipment control. Additionally, some power line constraints, such as LED lighting, may be excessive. The standard is still very new (introduced in 2022), and no known widespread equipment has yet to support it.

Microwave Systems for Electric Vehicles

A new direction in DC PLC development is data exchange via a DC charging cable between smart EV battery sensors and the charging station. This allows the battery to charge optimally, considering its parameters and wear.

For this, data transmission occurs on freely available frequencies (868 MHz and 2.4 GHz), with the charging cable acting as a waveguide for radio waves. The radio signal cannot pass through the metal body of the electric vehicle. Still, it can travel inside and outside through the wires connected to the charging port. The internal wiring serves as an antenna, and sensors on the battery exchange data wirelessly.
This technology still needs to be standardized, but it can utilize well-established solutions from wireless communication systems.

Conclusions

The main applications of DC PLC are in solar energy and industry. In both cases, long communication ranges (in AC power lines of 6–35 kV, the PLC signal can travel up to 6 miles) and high data rates are not required. More important is cost-effectiveness. If PLC equipment becomes too expensive, installing additional signal cables or wireless systems becomes more viable. Therefore, the communication protocol parameters are a reasonable compromise to achieve an acceptable price point.