DC switches separate loads
The grid integration of decentralised energy systems such as photovoltaic systems, battery storage systems or offshore wind farms is necessary for expanding the use of renewable energy. These modular generating systems all use DC circuits. In order to achieve highly efficient use, it is important to establish high system voltages for the respective voltage areas and to minimise conversion losses.
Current concepts envisage the use of high power switches for connecting sub-modules in series within the decentralised power supply systems. The interconnection of the modules means that safety switches with a high switching capacity need to be developed that will involve considerable costs and technical complexity. However, the modular structure of decentralised power supply systems provides an opportunity here for a paradigm shift and for an intelligent solution to the switching task.
The concept behind the Smart Modular Switchgear (SMS) project is based on multiple DC switches that connect the individual modules together. In order to disconnect a fault current in all the sub-modules, several switches must share the switching task. The ability of the individual sub-modules to switch off expands and improves the safety features in an advantageous manner, including in regards to faults between the sub-modules.
The SMS research project is therefore concerned with researching and developing switching and protection concepts for use in DC grids with different voltage levels. The project includes the creation of specifications with the help of measurements and simulations as well as the design of switchgear demonstrators. These will be equipped with innovative measurement and detection technology and investigated both in test fields and measuring stations. The simulations used as a basis will be verified using experimental rigs.
The findings will serve as a basis for developing and optimising switching and protection concepts for use in grid topologies with higher system voltages.
Coordinated switching with short circuits
The individual modules in a system are connected together via DC switches. These DC switches are able to switch off fault currents in a sub-module. However, the switching capacity is not sufficient so that a switch can switch off the fault current in all sub-modules. Here several switches must share the switching task. The ability to specifically activate individual sub-modules expands the safety feature advantageously – even with faults between the sub-modules.
The rapid fault detection and transmission of switching commands require smart metering and protection technology. If a short circuit occurs, the fault detection utilises the change in the impedance in the grid. If an unknown event occurs, this is a short circuit. Here it is sufficient to define limit criteria that are then evaluated for fast short circuit detection. Surge and short-circuit currents are thus detected quickly and the switch is switched off using a coordinated command. The required switching capacity is divided over several switches. The same applies to the division of the energy in the DC circuit, which is absorbed by the switches during the switching off.
The challenges lie in the coordinated switching operation for the distributed switchgear: the rapid detection of fault currents and the instantaneous issuing of the disconnection order, the simultaneous mechanical disconnection movement of the DC switch and the uniform distribution of the switching capacity using, for example, surge arresters parallel to the switchgear.
Milestones for low- and medium-voltage grids at a glance
Milestones for the 24-V and 380-V DC grids
- Definition of the technical constraints
- Determination of the DC source characteristic values and the grid topologies
- Design and commissioning of a model system
- Adaptation of the existing measurement devices
- Creation of new calibration equipment for specific waveforms
- Selection and procurement of the voltage and current sensors
- Calibration of the sensors for the applications
- Design and implementation of the detection numeric
- Installation and commissioning of demonstrators
- Checking the demonstrators in measuring and test facilities
- Testing the demonstrators in model systems
Meilensteine für das Mittelspannungsnetz
- Definition der technischen Randbedingungen
- Planung Erweiterungsmöglichkeit Messeinrichtungen
- Planung neuer MS-Kalibriereinrichtungen
- Beschaffung der Spannungs- und Stromsensoren
- Planung eines Mess- und Prüffeldes
- Schalterplanung und Erforschung
- Entwicklung einer Detektionsnumerik
- Planung der Demonstratoren
The researchers are expecting the first findings at the end of the third quarter of 2015.
More safety with coordinated switching operation
Measuring the faults with an intelligent, coordinated switching operation will distribute the switching task to many switches and achieve a high degree of safety and reliability. In other safety-related sectors, such as for example in automotive engineering, the approach presented can offer considerable potential for innovation when combined with mechanical, electrical and information systems.
The challenge of this project lies in the simultaneous switching and associated communication. Until now, there has been no commercially available means for clearly distinguishing between fault and normal operational conditions in such complex switching and measuring systems. This requires technical measurements of the grid’s signal paths with associated mathematical modelling followed by simulations and statistical pattern recognition.
09/2014 – 08/2017
Institut für Hochspannungstechnik und Elektrische Energieanlagen