Improved efficiency and availability with DC grids
Today, energy distribution across the world is based on AC grids. In most AC grids, the polarity changes 50 times per second. This has several advantages: transformers can transform voltages relatively easily and with low losses, and switch arcs disappear at the current zero crossing. However, there are drawbacks: when power transmission over very long distances is necessary, or if a different frequency must be generated by a double conversion of the grid frequency. When alternating voltages reach the limits — for example, during transmission from offshore wind farms to the load centres — high-voltage direct current (HVDC) transmission could be a solution.
Driven by the energy transition, high-voltage direct current transmission at up to 1 million volts is already possible for the transmission of large energy quantities over long distances. In addition, there are many DC applications for light rail systems in the low-voltage range of up to 1,000 V. There are relatively few applications, however, in the range of medium voltage between 3 kV and 30 kV. The decision for a DC system is relatively easy when it comes to isolated grids without their own connection to the interconnected network. One such example is ship grids that are only connected to the interconnected network when they are docked in port.
Although AC solutions predominate here, there are low-voltage ship grids that are almost exclusively DC-based. Advantages: diesel-powered generators can, depending on their power, run at their optimum operating points and must no longer follow a predefined speed. As a result, energy can be saved especially at partial load. However, the currents increase significantly if the output exceeds approximately five megawatts. The solution is a higher voltage — at the same output, a lower current is required. The logical consequence is that DC grids on ships are being considered for the medium voltage range. In addition to ship grids, there are other applications in which DC solutions on the medium-voltage level can be feasible: wind farms or high-performance drivetrains in the industry, such as those found in rolling mills, are merely two such examples.
New components for DC applications
With these solutions, energy and investment costs can be saved when multiple conversions are avoided. At the beginning of the project, the researchers initially sought suitable applications that would benefit from DC. In addition, components were identified that need to be redeveloped for a DC grid, and the specifications they would have to meet were worked out. The most important components worth mentioning are suitable converters, DC switches, DC-capable current measuring methods and fuses. For the converters, the topology and the control concept are important, in addition to the use of novel semiconductors. RWTH Aachen are responsible for researching the reliable operation of a DC grid with multiple sources and different consumer types.
By autumn 2016, a novel DC switch is to be built and then tested under laboratory conditions. If possible, it will be fitted with semiconductor modules relying on an innovative assembly and connector technology, which is Siemens' responsibility, as are the DC switches and the current measurement. To protect the components, a pyrotechnical safety switch from pyroglobe will be designed with an extremely short turn-off time.
05/2015 – 04/2018