For devices with semiconductor switches, the design does not always ensure reliable isolation of the circuit. Therefore, all-position electrical isolation is implemented with electromechanical components, in a similar way as for contactors or relays.
Switching, measuring, and monitoring in DC grids Discover the advantages of a DC-based power distribution. Reliable switching, precise measuring, and continuous monitoring of energy flows are particularly important here. Learn more about these crucial functions.
What should be considered when switching, measuring, and monitoring in DC grids?
The main difference between direct current (DC) and alternating current (AC) is the way in which the electrical charge flows through the grid. In the case of direct current, the charge flows constantly in one direction, while it changes direction periodically in the event of alternating current.
When switching in DC grids, the current does not cross zero, as is the case in AC grids. This makes switching direct current potentially more dangerous, because electric arcs can arise that are harder to extinguish. Measuring in DC grids is easier, as the voltage and current are constant. In an AC grid, RMS values are measured because the voltage and current vary following a sine waveform.
Monitoring systems for DC grids must also be designed for constant voltage and constant current. Some types of faults that occur in AC grids, such as a phase failure, might not be detected.
Switching in DC grids Requirements on arc-free switching
As part of the DC INDUSTRY2 project, Phoenix Contact, together with participants from industry and research, researched the topic of DC grids. Within this framework, the following requirements for switching in DC grids were formulated.
The main task of a protective device is to safely shut down within a reasonable time in the event of an overcurrent or short circuit. In the case of DC loads, safe shutdown in the event of overcurrents or short circuits is just as important as for AC loads. For this reason, reliable shutdown is crucial for the stability and safety of these systems.
If the voltages are too high, the surge protection system prevents the risk of damage to loads in particular.
The undervoltage protection system ensures that a load is switched on at a sufficiently high voltage.
At the moment of switching, uncharged capacitive loads (e.g., intermediate circuits) at the output of the switching device generate the state of a short circuit for a short time. To avoid these current peaks, the capacitors are precharged until the voltages of the device output and capacitor are harmonized.
A ground fault carries the risk of causing damage to people and systems. These hazards must be taken into consideration. While short-circuit protection switches off in the event of faults in the device, the ground-fault protection protects against errors between the device and ground.
Measuring in DC grids Accuracy in accordance with the Calibration Act
A significant advantage of DC grids is the ability to feed energy back. However, this application requires precise measuring. Energy meters for both alternating current (AC) and direct current (DC) are subject to the Calibration Act when used for financial billing. In DC grids, direct DC measuring is ideal for increasing accuracy and reducing conversion losses. In contrast to AC grids, DC measuring is easier, as phase shifts do not need to be taken into account.
Monitoring in DC grids Closed-loop control is easier than in the AC grid
Measuring is the prerequisite for monitoring in electricity grids. Monitoring ensures that limit values are observed in the system. For example, the voltage in the DC grid must not exceed 650 V. In the event of setpoint deviations, a monitoring system must respond and initiate countermeasures. On the one hand, this can mean regulating or, in the event of an emergency, switching off the system. In contrast to the AC grid, monitoring in the DC grid is simpler since the voltage “only” has to be kept constant. One example is building 60 from Phoenix Contact. Here, the AIC (Active Infeed Converter) plays a crucial role in monitoring and closed-loop control. If the voltage in the grid drops, the AIC supplies power from the battery storage system or the AC grid. If the voltage in the grid increases, energy is either stored or fed back into the public grid.
Innovative DC circuit breakers CONTACTRON ELR HDC for switching high DC loads
The integration of generators, storage systems, and loads in the DC grid poses special challenges, such as arc-free switching. Safe switching must be ensured at every DC branch. The CONTACTRON ELR HDC is the first multifunctional DC circuit breaker on the market that meets the requirements for use in DC grids.
Products and solutions for a safe DC grid
Research and development We are characterized by innovative strength and future security
The use of DC grids in industry is just starting to take off. Therefore, technical hurdles and a lack of standards are currently still posing some challenges for our customers. In order to counteract this and promote the development of DC grids, Phoenix Contact has been involved in international research projects and committees such as the ODCA, Shift2DC, and HybSchaDC on the topic of direct current for years and is investing heavily in its own product development.