Julian Kaiser

Overview of different topologies and control strategies for DC micro grids

Overview, comparison and evaluation of common DC micro grid design considerations. The focus of this paper is to explore the main differences and advantages/disadvantages of various topologies and control strategies for DC micro grids. The requirements of various application areas can strongly influence the individual system design. Control strategies, single- or two-phase designs, earthing concepts, system voltages and power levels are discussed as well.

Modelling and measuring complex impedances of power electronic converters for stability assessment of low-voltage DC-grids

Interconnecting power converters within a low voltage DC grid can be a challenging task since these devices are rarely tested under final operating conditions during their development process in conjunction with converters from different manufacturers, different kinds of loads and appropriate grid impedances. In the worst case, stationary oscillations might occur in the grid setup for which the actual reason is hard to determine and solving the problem will require time-consuming trial and error means. Therefore, theoretical knowledge about how power converters react on grid-side disturbances is crucial for a preliminary analysis of the static and dynamic performance of low-voltage DC grids before plugging the devices together. Based on these considerations general guidelines for the dimensioning of control loops and grid-side capacitors of power converters can be derived. The following outlines describe the fundamentals of power converter behavior when exposed to disturbances occurring on the output current, e. g. from load steps. Another focus lies on shaping grid-side impedances of converters to avoid stability issues. Furthermore, a method to measure the impedances of interest is thoroughly described. All obtained results are verified in a brief case study.

Energy distribution with DC microgrids in commercial buildings with power electronics

This paper describes the application of a distributed DC microgrid in a commercial environment as well as the current state of the art and standardization efforts. The introduced grid features various sources and loads being interconnected with a 380 VDC bus. Here, focus lies on the implementation of a DC fast charge station for electro mobility into a DC grid as well as to elaborate the advantages compared to charging from the AC grid. Additionally, the application of DC nanogrids in workplace environments and their combination with a superordinate DC microgrid is presented. The benefits offered by nanogrids compared to conventional AC power supply in an office are discussed as well. Finally, the hardware to realize a DC microgrid within one electrical cabinet is introduced. Its versatility to fulfill a wide range of functions in the grid is shown as well.

An advanced voltage droop control concept for grid-tied and autonomous DC microgrids

Droop Control has been a well-established control technique both in AC and DC power distribution grids for many years, because it provides a simple way to equally distribute the load current between remote power sources. With the increasing demand for low voltage DC microgrids supplying high-reliability equipment, like servers in data centers, to work both grid-tied and autonomously without a connection to the AC mains and fueled only by local renewable and conventional power sources, voltage droop control is facing new challenges. With power equipment being delivered from several manufacturers the demand for a communication less control scheme that only uses the voltage at the terminal point or the converter output current as an indicator how the control set point should be changed in order to satisfy the energy demand of the loads arises. In consequence, the commissioning time of the DC microgrid is greatly reduced since all components can be simply plugged together without the need for adaptions. An outline for such an inherently autonomous voltage droop control scheme to keep the system voltage within a narrow band of ± 10 % of its 380 VDC nominal value is given in the following paper by describing voltage droop control modelling basics and the selection of characteristic droop curves for different kinds of power sources as well as by giving simulative results from a small-scale DC microgrid.

Model-Based fault current estimation for low fault-energy 380VDC distribution systems

380 VDC distribution grids have reached maturity for the power supply of data centers and central offices over recent years. New developments in this field are tending towards integrating more distributed energy resources like photovoltaics and wind turbines. Also, high-capacity battery storage systems based on lithium-ion cells are on the rise to increase self-reliance and reduce operating cost. With every grid component being connected to the 380 VDC supply bus via a DC/DC or AC/DC converter, the dynamic system behavior will be entirely dominated by the control loops and operational limits of the power electronic components. In consequence, a re-evaluation of the fault current propagation for various fault types is necessary to dimension safety devices correctly. This paper describes a modelling approach using linearized converter models to analyze the system behavior. The presented models are verified with a laboratory test grid. Finally, guidelines to properly select safety elements and setting up self-protecting mechanisms for power converters in next generation 380VDC distribution grids are outlined.

Resonant electric arcs in DC microgrids with low system impedance in the VLF-band

Various fault scenarios have been analyzed by running a number of differently combined 380 VDC microgrid tests. These tests represented a common grid topology with low system impedance at grid resonance points within the single- or lower double-digit kHz range. At serial arc faults, self-excited resonant modes of the arc plasma column have been observed. They lead to an increased arc column stability compared to non-resonant arcs with colored noise behavior. These characteristics require a special focus on pattern recognition methods for arc fault sensors along with extended suitability tests for mechanical and hybrid switchgear concerning the altered stability of switching arcs. The use of small-signal models for system components such as source and load converters as well as for arcs with regard to large-signal DC operating points and converter control modes is helpful in order to describe the reaction of the system in the event of a malfunction. This is essential for the development of suitable protective components and algorithms.

Grid behavior under fault situations in ±380 V DC distribution systems

DC microgrids have become widely adapted in recent years, with 380 V being the most common voltage choice in applications like data centers and central offices around the world. For future developments with an increasing power demand, adding a second line conductor to a DC microgrid can provide means to increase overall energy efficiency. This paper evaluates fault situations in such a bipolar ±380 V DC microgrid containing only power electronic devices, thereby providing only low fault energy. Two basic grounding methods for bipolar grids and their consequences on grid safety and fault behavior are examined. Initially, exemplary fault situations are simulated. The results are then discussed and verified by setting up a test grid in the laboratory.