Using PLC on DC power lines

Modern energy systems are circling back to direct current but with a fresh, tech-savvy approach. Solar power generation, for example, is all about producing DC. Increasingly, industries are adopting internal DC power networks to streamline uninterruptible power supplies by eliminating the need to convert battery voltage to alternating current (AC).

Solar energy production made safe

A single solar panel typically produces 12 to 20 volts, depending on the model. These panels are connected in series to create chains that output around 400 volts of DC power, and these chains are connected in parallel. Initially, the panels in each chain were permanently connected, so breaking the circuit required disconnecting the connectors manually.

Experience with solar power plants has shown that a variety of situations can occur, including fires. If a fire breaks out during the day, the solar panels continue generating power, and the voltage within the solar arrays can reach up to 400 volts. This poses a significant risk of electrical shock to firefighters.

To address this issue, the U.S. National Electrical Code (NEC) was updated in 2017 and 2020 to mandate that, in emergencies, the voltage within solar arrays must drop to 30 volts or less within 30 seconds.

This requirement can only be met by remotely disconnecting the panels in each chain. PLC is the perfect solution here because the cables already connecting the solar panels to each other and the inverter can also be used to send control signals to contactors, which will disconnect the panels in the chain. In this case, PLC is more reliable than wireless systems and is also cheaper, which is a significant advantage given the large number of solar panels installed at a power plant.

Data transmission can also go the other way. Sensors can be installed on the solar panels to transmit information via PLC about the condition of the generating devices. This can enhance the overall safety of the system.

Here’s just one example of using sensors on solar panels: by collecting data on the electricity output of each panel and transmitting it through PLC, then comparing the sum of these readings with the total output of the power plant, you can quickly detect any current leaks, such as those caused by insulation damage.

MPPT System

The output voltage of a solar panel is unstable and depends on various factors, including the amount of sunlight, temperature, and the panel's age. However, at any given moment, there’s an optimal voltage for each panel to charge the battery efficiently at a solar power plant. To solve this, the MPPT (Maximum Power Point Tracking) system was developed. This system can boost electricity production by up to 30% from the same array of panels.

The MPPT system involves installing a DC-DC converter on each solar panel. This converter receives information about the current state of the battery and adjusts to maintain the optimal output voltage. Thus, there’s a need to transmit information to each DC-DC converter. Additionally, monitoring the converters' health by transmitting information back is also desirable. PLC handles these tasks efficiently.

Industry 4.0

Manufacturing is returning to developed countries, but now on a new technological foundation. Digital technologies are widely used, enabling remote digital control signals and extensive use of sensors that transmit information digitally as well. Fewer people are needed compared to traditional production, but those who are needed must be highly skilled. This concept is known as "Industry 4.0."

Wireless data transmission in manufacturing faces many challenges. First, factory buildings typically include many metal components that block signals. Second, industrial equipment may use microwave radiation in unlicensed bands, where wireless communication systems operate (especially problematic at 2.4 GHz). Third, sensitive sensors in manufacturing can be disrupted by wireless device emissions.

Given these challenges, using PLC in an industrial environment is very promising. Particularly, employing PLC on DC lines, increasingly used to power factory equipment, can be highly beneficial. The same cable can provide power, send control commands, and receive data about the equipment’s status.

This concept can also be implemented using Power Over Ethernet (PoE), where a single cable, though consisting of eight wires, can transmit both data and power. However, PoE has limitations that can be critical in manufacturing:

• Power supply voltage is only 48 V
• Power is limited to 71.3 W
• Ethernet cables may not meet specific requirements, such as mechanical strength or resistance to certain chemicals

Therefore, PLC can often provide a better solution for combining power and data transmission in a single cable where PoE falls short.

Conclusion

Using PLC on DC power lines is highly effective in solar power generation and modern high-tech manufacturing. The lack of a need for additional cabling, combined with high reliability and low cost, makes PLC on DC lines a competitive alternative to wireless solutions.

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