TY - GEN A1 - Bleicher, Achim A1 - Schauer, Thomas A1 - Valtin, M. A1 - Raisch, Jörg A1 - Schlaich, Mike T1 - Active vibration control of a light and flexible stress ribbon footbridge using pneumatic muscles T2 - IFAC Proceedings Volumes Y1 - 2011 U6 - https://doi.org/10.3182/20110828-6-IT-1002.02781 N1 - Proceedings of the 18th World Congress, The International Federation of Automatic Control Milano (Italy) August 28 - September 2, 2011 VL - 44 IS - 1 SP - 911 EP - 916 ER - TY - GEN A1 - Schiffer, Johannes A1 - Krishna, Ajay A1 - Raisch, Jörg T1 - A Consensus-Based Control Law for Accurate Frequency Restoration and Power Sharing in Microgrids in the Presence of Clock Drifts T2 - 2018 European Control Conference, (EEC) 12-15 Jun 2018, Limassol, Cyprus KW - Microgrids Y1 - 2018 SN - 978-3-9524-2698-2 U6 - https://doi.org/10.23919/ECC.2018.8550191 PB - IEEE CY - Piscataway, NJ ER - TY - GEN A1 - Schiffer, Johannes A1 - Hans, Christian A. A1 - Kral, Thomas A1 - Ortega, Romeo A1 - Raisch, Jörg T1 - Modelling, Analysis and Experimental Validation of Clock Drift Effects in Low-Inertia Power Systems T2 - IEEE Transactions on Industrial Electronics N2 - Clock drift in digital controllers is of great relevance in many applications. Since almost all real clocks exhibit drifts, this applies in particular to networks composed of several individual units, each of which being operated with its individual clock. In the present paper, we demonstrate via extensive experiments on a microgrid in the megawatt range that clock drifts may impair frequency synchronization in low-inertia power systems. The experiments also show that-in the absence of a common clock-the standard model of an inverter as an ideal voltage source does not capture this phenomenon. As a consequence, we derive a suitably modified model of an inverter-interfaced unit that incorporates the phenomenon of clock drifts. By using the derived model, we investigate the effects of clock drifts on the performance of droop-controlled grid-forming inverters with regard to frequency synchronization and active power sharing. The modeling and analysis is validated via extensive experiments on a microgrid in the megawatt range. Y1 - 2017 UR - https://ieeexplore.ieee.org/document/7781662 U6 - https://doi.org/10.1109/TIE.2016.2638805 SN - 1557-9948 SN - 0278-0046 VL - 64 IS - 7 SP - 5942 EP - 5951 ER - TY - GEN A1 - Schiffer, Johannes A1 - Krishna, Ajay A1 - Hans, Christian A. A1 - Raisch, Jörg A1 - Kral, Thomas T1 - Steady State Evaluation of Distributed Secondary Frequency Control Strategies for Microgrids in the Presence of Clock Drifts T2 - 25th Mediterranean Conference on Control and Automation N2 - Secondary frequency control, i.e., the task of restoring the network frequency to its nominal value following a disturbance, is an important control objective in microgrids. In the present paper, we compare distributed secondary control strategies with regard to their behaviour under the explicit consideration of clock drifts. In particular we show that, if not considered in the tuning procedure, the presence of clock drifts may impair an accurate frequency restoration and power sharing. As a consequence, we derive tuning criteria such that zero steady state frequency deviation and power sharing is achieved even in the presence of clock drifts. Furthermore, the effects of clock drifts of the individual inverters on the different control strategies are discussed analytically and in a numerical case study. Y1 - 2017 UR - http://eprints.whiterose.ac.uk/115991/ U6 - https://doi.org/10.1109/MED.2017.7984168 SN - 2473-3504 SP - 508 EP - 515 ER - TY - GEN A1 - Krishna, Ajay A1 - Schiffer, Johannes A1 - Monshizadeh, Nima A1 - Raisch, Jörg T1 - A consensus-based voltage control for reactive power sharing and PCC voltage regulation in microgrids with parallel-connected inverters T2 - European Control Conference 2019, Naples, Italy N2 - We consider small-scale power systems consisting of several inverter-interfaced units connected in parallel to a common bus, the point of common coupling (PCC), and sharing a joint load. This is a frequently encountered configuration in microgrid applications. In such a setting, two important control objectives are reactive power sharing and voltage regulation at the PCC. In this paper, we first show that the nonlinear equilibrium equations corresponding to the aforementioned objectives admit a unique positive solution. Then, we propose a consensus-based distributed voltage controller which renders this desired unique solution locally asymptotically stable. Finally, control performance is illustrated via a simulation example. Y1 - 2019 SN - 978-3-907144-00-8 SN - 978-3-907144-01-5 SN - 978-1-7281-1314-2 U6 - https://doi.org/10.23919/ECC.2019.8795924 CY - Neapel ER - TY - GEN A1 - Krishna, Ajay A1 - Schiffer, Johannes A1 - Raisch, Jörg T1 - Distributed Secondary Frequency Control in Microgrids: Robustness and Steady-State Performance in the Presence of Clock Drifts T2 - European Journal of Control N2 - Microgrids are distributed systems with high share of inverter-interfaced renewable energy sources where stable and reliable system operation is realized by suitably controlling the inverters. In this work, we focus on secondary frequency control, which is an important ancillary service provided by the inverters. In the literature on secondary frequency control, the effect of clock drifts has often been neglected. However, clock drifts are practically unavoidable parameter uncertainties in inverter-based microgrids and we show that the most commonly employed distributed secondary frequency controllers exhibit performance deteriorations when taking clock drifts explicitly into consideration. Motivated by this, we propose a novel alternative control law called generalized distributed averaging integral (GDAI) control, which achieves the secondary control objectives of steady-state accurate frequency restoration and proportional power sharing in the presence of clock drifts. In addition, we derive a sufficient tuning criterion in the form of a set of linear matrix inequalities (LMIs) which guarantees robust stability of the closed-loop equilibrium point in the presence of uncertain clock drifts. Finally, our analysis is validated extensively via simulation with comprehensive comparisons to other related distributed control approaches. Y1 - 2020 U6 - https://doi.org/10.1016/j.ejcon.2019.08.003 SN - 0947-3580 VL - 51 SP - 135 EP - 145 ER -