@misc{BleicherSchauerValtinetal., author = {Bleicher, Achim and Schauer, Thomas and Valtin, M. and Raisch, J{\"o}rg and Schlaich, Mike}, title = {Active vibration control of a light and flexible stress ribbon footbridge using pneumatic muscles}, series = {IFAC Proceedings Volumes}, volume = {44}, journal = {IFAC Proceedings Volumes}, number = {1}, doi = {10.3182/20110828-6-IT-1002.02781}, pages = {911 -- 916}, language = {en} } @misc{SchifferKrishnaRaisch, author = {Schiffer, Johannes and Krishna, Ajay and Raisch, J{\"o}rg}, title = {A Consensus-Based Control Law for Accurate Frequency Restoration and Power Sharing in Microgrids in the Presence of Clock Drifts}, series = {2018 European Control Conference, (EEC) 12-15 Jun 2018, Limassol, Cyprus}, journal = {2018 European Control Conference, (EEC) 12-15 Jun 2018, Limassol, Cyprus}, publisher = {IEEE}, address = {Piscataway, NJ}, isbn = {978-3-9524-2698-2}, doi = {10.23919/ECC.2018.8550191}, language = {en} } @misc{SchifferHansKraletal., author = {Schiffer, Johannes and Hans, Christian A. and Kral, Thomas and Ortega, Romeo and Raisch, J{\"o}rg}, title = {Modelling, Analysis and Experimental Validation of Clock Drift Effects in Low-Inertia Power Systems}, series = {IEEE Transactions on Industrial Electronics}, volume = {64}, journal = {IEEE Transactions on Industrial Electronics}, number = {7}, issn = {1557-9948}, doi = {10.1109/TIE.2016.2638805}, pages = {5942 -- 5951}, abstract = {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.}, language = {en} } @misc{SchifferKrishnaHansetal., author = {Schiffer, Johannes and Krishna, Ajay and Hans, Christian A. and Raisch, J{\"o}rg and Kral, Thomas}, title = {Steady State Evaluation of Distributed Secondary Frequency Control Strategies for Microgrids in the Presence of Clock Drifts}, series = {25th Mediterranean Conference on Control and Automation}, journal = {25th Mediterranean Conference on Control and Automation}, issn = {2473-3504}, doi = {10.1109/MED.2017.7984168}, pages = {508 -- 515}, abstract = {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.}, language = {en} } @misc{KrishnaSchifferMonshizadehetal., author = {Krishna, Ajay and Schiffer, Johannes and Monshizadeh, Nima and Raisch, J{\"o}rg}, title = {A consensus-based voltage control for reactive power sharing and PCC voltage regulation in microgrids with parallel-connected inverters}, series = {European Control Conference 2019, Naples, Italy}, journal = {European Control Conference 2019, Naples, Italy}, address = {Neapel}, isbn = {978-3-907144-00-8}, doi = {10.23919/ECC.2019.8795924}, pages = {6}, abstract = {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.}, language = {en} } @misc{KrishnaSchifferRaisch, author = {Krishna, Ajay and Schiffer, Johannes and Raisch, J{\"o}rg}, title = {Distributed Secondary Frequency Control in Microgrids: Robustness and Steady-State Performance in the Presence of Clock Drifts}, series = {European Journal of Control}, volume = {51}, journal = {European Journal of Control}, issn = {0947-3580}, doi = {10.1016/j.ejcon.2019.08.003}, pages = {135 -- 145}, abstract = {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.}, language = {en} }