Marco Stieneker

Comparison of the Modular Multilevel DC Converter and the Dual-Active Bridge Converter for Power Conversion in HVDC and MVDC Grids

It is expected that in the near future the use of high-voltage dc (HVDC) transmission and medium-voltage dc (MVDC) distribution technology will expand. This development is driven by the growing share of electrical power generation by renewable energy sources that are located far from load centers and the increased use of distributed power generators in the distribution grid. Power converters that transfer the electric energy between voltage levels and control the power flow in dc grids will be key components in these systems. The recently presented modular multilevel dc converter (M2DC) and the three-phase dual-active bridge converter (DAB) are benchmarked for this task. Three scenarios are examined: a 15 MW converter for power conversion from an HVDC grid to an MVDC grid of a university campus, a gigawatt converter for feeding the energy from an MVDC collector grid of a wind farm into the HVDC grid, and a converter that acts as a power controller between two HVDC grids with the same nominal voltage level. The operation and degrees of freedom of the M2DC are investigated in detail aiming for an optimal design of this converter. The M2DC and the DAB converter are thoroughly compared for the given scenarios in terms of efficiency, amount of semiconductor devices, and expense on capacitive storage and magnetic components.

Ensuring soft-switching operation of a three-phase dual-active bridge DC-DC converter applying an auxiliary resonant-commutated pole

An auxiliary resonant-commutated pole (ARCP) ensures soft switching in the entire operation range of a three-phase dual-active bridge dc-dc converter. This work evaluates the design and the resulting boost in system efficiency for different semiconductor materials and devices. Afterwards, a full-scale medium-voltage prototype of an ARCP is constructed. The subsequent measurements are presented within this work, before the economic feasibility is discussed.

System efficiency estimation of redundant cascaded-cell converters in applications with high-power battery energy storage systems

Battery energy storage systems (BESS) become increasingly more important due to the impact of volatile energy sources on the power supply and present a good solution for buffering the power imbalances in the electric grid. Particularly with regard to a future scenario with electrical energy, means have to be taken to ensure a stable and reliable power supply. Redundant BESS allow normal operation, even if parts of the storage system are out of service. The nine-level cascaded cell converter is a promising topology for connecting batteries to the medium-voltage grid. In this paper, the extension of this topology to a redundant system is discussed.

Medium-voltage DC distribution grids in urban areas

The potential of medium-voltage dc (MVDC) grids in various applications has been under investigation by different researchers. In several works, the advantages of implementing MVDC technology into collector grids of wind and photovoltaic (PV) parks are pointed out. The main driver is that the grid-side inverter of the distributed generators as well as lossy grid filters and bulky 50 Hz transformer become obsolete. However, MVDC grids are a promising alternative to established ac grids not only for power generation. Within this work, the advantages regarding system efficiency and investment costs of using MVDC for the distribution of electrical energy is presented. Conventional low-voltage ac-grids and possible medium-voltage ac grids are also analyzed to compare the different implementation approaches.

Development of a Modular High-Power Converter System for Battery Energy Storage Systems

The integration of storage systems into the grid is becoming increasingly important due to the growing amount of volatile power sources. This paper shows how to design a modular battery energy storage system (BESS) for medium voltage grids. Typically, this system is scalable in power rated from 5 MW up to 100 MW with a storage capacity of several hours. Using power electronic building blocks (PEBBs) a converter for dc grids and ac grids can be built. In this paper, the chosen topology for the ac solution is the cascaded cell converter. The focus is to determine the optimum number of levels, the modulation technique to avoid microcycles of the batteries and to present the efficiency. A formula to calculate the parasitic capacitance of lead-acid batteries is shown and verified by measurements, which is important for the design of such a converter system. Moreover, a new charging strategy for LiFePO4 — the chosen battery technology for the proposed storage system — is introduced, which prolongs the batterys life, reduces the charging time and decreases the life cycle cost. The proposed charging strategy and battery technology compared to lead-acid batteries is also economically evaluated.

Design of series-connected dual-active bridges for integration of wind park cluster into MVDC grids

The transmission of electrical energy with direct current (dc) is a promising alternative to alternating-current (ac) systems. The collection of energy with medium-voltage dc (MVDC) grids also increases the efficiency. In this paper, a dual-active bridge (DAB) dc-dc converter system for integrating wind turbines (WT) into MVDC grids is presented. Within this approach, the secondary-side bridges are series connected whereas the primary-side bridges are not connected with each other. This provides galvanic isolation between the WT.

Does transmission technology influence acceptance of overhead power lines? An empirical study

For the transmission of electricity across long distances, high voltage direct current (DC) transmission is discussed in Germany as an alternative to the currently used alternating current (AC) as it is more efficient for these distances. Changes in energy infrastructure are known to raise public awareness. However, little is known whether differences in transmission technology are relevant for the public and if so, to what extent. Two consecutive empirical studies were run in which acceptance towards transmission lines operated with DC in contrast to AC was explored. AC and DC power lines were not evaluated differently, yielding overall quite neutral ratings (Study 1) which might be due to a low information level in the public. A closer look (Study 2) showed that giving information on technical and design parameters of the transmission lines used for either AC or DC technology also did not change attitudes substantially. It is therefore concluded that transmission technology alone did not influence acceptance of power lines for the investigated sample. In addition, a need for more information on DC for high voltage transmission was identified. Further research is required on the influence of different power line layout of AC and DC on acceptance.

Grid emulator requirements for a multi-megawatt wind turbine test-bench

Full-scale nacelle test-benches accelerate the development process of new multi-megawatt wind turbines. The device-under-test (DUT) is connected to an artificial grid that is emulated by a power-electronic converter system. Since the desired sinusoidal waveform of the voltage can only be approximated, filters have to be applied to suppress voltage harmonics sufficiently. But still, the output voltage is not purely sinusoidal due to a remaining share of harmonics. For the verification of the total harmonic distortion (THD) caused by the DUT, the influence of the grid emulator on the measurements of voltage and current has to be known. Otherwise the source of the harmonics cannot be identified clearly and the DUT might be characterized incorrectly. Within this work the requirements for grid emulators to allow the sufficiently high measurement of the THD caused by the DUT are derived analytically and validated with simulation models. Furthermore, the dynamic of voltage dips that have to be emulated to test the low-voltage ride-through (LVRT) capability of the DUT has to fulfill the established standards (IEC 61400-21) for emulating a faulty grid realistically. The consequently derived requirements for the power electronic converters are presented within this paper.

Dual-active bridge dc-dc converter systems for medium-voltage DC distribution grids

Electrical energy distribution with dc grids reduces the investment costs of the system and increases the overall efficiency. For interconnecting different dc distribution grids as well as linkage to transmission networks and integration of renewable energy sources, the dual-active bridge (DAB) dc-dc converter system is a promising topology. Modularity and scalability improve production and reduce maintenance costs. Moreover, redundancy can be implemented easily. Furthermore, high efficiency and high-dynamic controllability ensure a good performance within grid applications. This paper presents the modular DAB converter system and its application for different voltage and power levels. The implementation of redundancy and the transient system behavior during failure will be discussed in addition.

Analysis of wind turbines connected to medium-voltage DC grids

The transmission and distribution of electrical energy with direct current (dc) is a promising alternative to established alternating-current (ac) systems. Higher efficiencies, fewer lossy pulse-width modulated converters and smaller transformers reduce operation and investment costs. Moreover, dc collector grids within wind parks leads to benefits for harvesting electrical energy. In this paper, a modular dual-active bridge (DAB) dc-dc converter system for integrating renewable energy sources, especially wind turbines (WT), into medium-voltage dc (MVDC) grids is presented and analyzed. Also, the design and efficiency of the machine-side power converter of the WT for dc grids are discussed.