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Author: Welck, F. ×

Publications Search Results

Now showing 1 - 6 of 6
  • Publication
    Design and laboratory test of black-start control mode for wind turbines
    ( 2020)
    Shan, M.
    ;
    Shan, W.
    ;
    Welck, F.
    ;
    Duckwitz, D.
    In this article, an operational strategy and control concept for wind turbines (WTs) are described, which would allow them to actively contribute to black‐start events after major power system outages. The approach is based on (a) a new generator/converter control strategy implementing a so called virtual synchronous machine (VSM) and (b) a number of modifications to the superimposed WT controller allowing for operation in black‐start conditions. In order to operate stably even at very low active power levels and to cope with sudden changes in active power, due to switching of loads in the recovered grid, the rotor speed/pitch controller had to be redesigned. The extension of the operational range of the WT towards negative power, ie, power consumption, is discussed, which would allow the turbine to temporarily provide a controllable minimum load to conventional power plants until a sufficient number of consumers has been reconnected. The control system has been implemented and verified using two experimental power converters, each linked to a hardware‐in‐the‐loop (HiL) simulator of a WT and connected to a real medium‐voltage laboratory grid.
  • Publication
    Experimental short-circuit testing of grid-forming inverters in microgrid and interconnected mode
    ( 2019)
    Duckwitz, D.
    ;
    Knobloch, A.
    ;
    Welck, F.
    ;
    Becker, T.
    ;
    Glöckler, C.
    ;
    Bülo, T.
    Power systems with increasing renewable generation are subject to intermittent, at times very high, penetration rates of power electronic generation plants. Grid-forming control schemes are potentially required to guarantee stability at high penetration rates. These schemes introduce voltage-source behavior and replicate the high fault currents of synchronous machines. Adapted control schemes have been developed to reduce fault currents of grid-forming inverters. This contribution presents experimental results on the short-circuit behavior of two grid-forming inverters, one commercial prototype and one experimental device. Two different operation modes, grid-connected and islanded, have been investigated and the different requirements are discussed along the results. The grid-forming schemes are potentially able to follow the same short-circuit requirements as current-controlled plants, but provide a fast inherent reaction to faults.
  • Publication
    NETZ:KRAFT. Netzwiederaufbau unter Berücksichtigung zukünftiger Kraftwerksstrukturen. Öffentlicher Abschlussbericht
    (Fraunhofer IEE, 2019)
    Heckmann, W.
    ;
    Becker, H.
    ;
    Hachmann, C.
    ;
    Spanel, U.
    ;
    Bernhart, A.
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    Müller-Mienack, M.
    ;
    Shan, W.
    ;
    Welck, F.
    ;
    Wecker, M.
    ;
    Nuschke, M.
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    Hau, D.
    ;
    Wunderlich, M.
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    Akbulut, A.
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    Pabon, L.
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    Liebehentze, S.
    ;
    Görig, B.
    ;
    Lafferte, D.
    ;
    Klingmann, A.
    ;
    Valov, M.
    ;
    Fetzer, D.
    ;
    Fischbach, K.
    ;
    Paschedag, T.
    ;
    Lammert, G.
    ;
    Hof, M.
    ;
    Bülo, T.
    ;
    Hardt, C.
    ;
    Mahr, F.
    ;
    Jaworski, M.
    ;
    Schäfer, N.
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    Nölle, C.
    ;
    Lohmeier, D.
    ;
    Rittger, C.
    ;
    Brandl, R.
  • Publication
    Influence of virtual impedance on short circuit performance of virtual synchronous machines in the 9-bus system
    ( 2018)
    Welck, F.
    ;
    Duckwitz, D.
    ;
    Glöckler, C.
    Due to decreasing shares of synchronous generators in power systems, power system stability has increasingly to be ensured through power electronic generators. Grid forming controls like the virtual synchronous machine (VSM) are a suitable approach for the stable operation of power systems with very high shares of inverters. High short-circuit currents and rotor angle oscillations are an inherent property of VSMs - similar to real synchronous machines (SM). These are driving semiconductor cost to multiples of conventional inverter designs. In this paper the behavior of VSM control during grid faults is analyzed with the use of the IEEE 9-Bus system. With the introduction of the virtual circuit (VC) the current during short-circuit faults is reduced to about 50%, while the active power set-point limitation reduces angle displacements and thus improves transient stability.
  • Publication
    Virtual synchronous machine control with virtual resistor for enhanced short circuit capability
    ( 2018)
    Glöckler, C.
    ;
    Duckwitz, D.
    ;
    Welck, F.
    Increasing shares of power electronic converters in renewable generation challenge power system stability. For very high shares the state of the art current-control approach should be replaced by grid forming control schemes. The virtual synchronous machine (VSM) approach is grid forming and fully compatible to conventional synchronous-machine-based power plants. The high fault currents of synchronous machines are also replicated and lead to excessive current ratings of the semiconductors. Current control could be applied again during faults, however the grid-forming property is lost when it is most needed. We propose a VSM with a virtual resistor that reduces fault currents while the grid forming functionality is maintained. For the most severe fault the peak current amplitude is reduced to 2.5 times the nominal current. Swing mode and high-frequency stability is investigated in detail. The results have been confirmed by experiments. The over-current requirements and thus component cost can be reduced drastically through the application of a virtual resistor. Nonetheless, an increased current rating of the hardware is required to handle severe faults. System studies should be performed to confirm the method on system level.
  • Publication
    The Future of Power System Restoration: Using Distributed Energy Resources as a Force to Get Back Online
    ( 2018)
    Braun, M.
    ;
    Brombach, J.
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    Hachmann, C.
    ;
    Lafferte, D.
    ;
    Klingmann, A.
    ;
    Heckmann, W.
    ;
    Welck, F.
    ;
    Lohmeier, D.
    ;
    Becker, H.
    One of the most critical risks for modern societies is a largescale power system blackout. Critical infrastructure has emergency power supplies (e.g., nuclear power plants, hospitals, or communication infrastructure) to confront power outage situations. However, after about 8 h of a blackout, fuel supplies and battery capacities normally run out. Thus, it is of utmost importance to restore the power system as robustly and quickly as possible. This responsibility lies with the transmission system operators (TS Os), and it is in their best interest to rapidly accomplish it.