At the end of the 19th century, alternating current (AC) technology prevailed over direct current (DC) technology in the so-called electricity war. A decisive factor in the success of AC was the use of the dynamo-electric principle discovered by Wernher von Siemens in the 1860s: the steam produced by heating drives turbines, which in turn generate electrical energy from a rotary motion using alternating current generators. This principle is still used in coal or nuclear power plants, for example. Another advantage of AC results from the simple transformation to high voltages in order to transmit the energy over long distances with low losses.

No lossy AC/DC conversion

Nevertheless, direct current is currently becoming increasingly important. This is because the technological development of power electronics is now opening up the possibility of making highly efficient direct current usable at low cost. For example, essential electronic components such as the transistors used in computers and control systems can only be operated with DC. In industry, many consumers operate on a DC link by using frequency converters. In a DC network, the lossy AC/DC conversion can be dispensed with for these consumers. In addition, a sustainable power supply from renewable energy sources - such as photovoltaics, wind energy or fuel cells - always supplies direct current.

Figure 1: Example of an industrial DC grid.

© Phoenix Contact

Using the generated electrical energy without rectification from AC to DC increases efficiency and saves the operator costs for raw materials. This eliminates the need for transformers. Furthermore, the amount of copper required to manufacture cables is reduced, as smaller cable cross-sections are required for the transmitted power. All of this proves to be an advantage that makes direct current economical in industrial production facilities. With its "Empowering the All Electric Society" strategy, Phoenix Contact is driving forward the increasing importance of DC technology with corresponding products and system solutions. In this way, the company supports its customers in implementing a sustainable energy supply for their production facilities as a reliable partner and supplier of system solutions.

Control of the standing arc

Switching processes pose a particular challenge when supplying direct current to industrial DC networks. Whereas with AC, a possible arc goes out automatically after 10 ms at the latest when the current crosses zero, with DC a standing arc develops, particularly when switching off, which must be controlled. The 'Contactron ELR HDC' DC circuit-breaker combines modern semiconductor technology with proven DC relays and switches without arcing. The relays in the power path provide galvanic isolation at both poles when switched off. The Contactron ELR HDC is used in DC networks wherever an energy exchange between sources, loads and the network itself is required - on so-called DC feeders(Figure 1).

Figure 2: The multifunctional Contactron ELR HDC switching device combines the functions of switching, monitoring and protection in a compact housing

© Phoenix Contact

In addition to switching up to 55 A at up to 810 V, the device performs other tasks that are necessary for the operation of a DC network. For example, the circuit-breaker monitors the most important parameters during operation and also makes them available for operating data acquisition. The device also monitors voltage and current during operation and has a three-stage switch-off function in the event of overcurrent, including very fast switch-off times in the event of a short circuit. In the event of an undervoltage or overvoltage in the input, the device also switches off.

In addition, the circuit-breaker precharges capacitance in the output to the input voltage so that current peaks are avoided during switch-on(Fig. 2).