@misc{SchifferAlghamdiFridman, author = {Schiffer, Johannes and Alghamdi, Sultan and Fridman, Emilia}, title = {Distributed Secondary Frequency Control Design for Microgrids: Trading off L2-Gain Performance and Communication Efforts under Time-Varying Delays}, 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}, pages = {6}, language = {en} } @misc{SchifferEfimov, author = {Schiffer, Johannes and Efimov, Denis}, title = {A new criterion for boundedness of solutions for a class of periodic systems}, series = {17th European Control Conference (ECC), Limassol, Cyprus, 2018}, journal = {17th European Control Conference (ECC), Limassol, Cyprus, 2018}, pages = {6}, language = {en} } @misc{ZonettiOrtegaSchiffer, author = {Zonetti, Daniele and Ortega, Romeo and Schiffer, Johannes}, title = {A Tool for Stability and Power Sharing Analysis of a Generalized Class of Droop Controllers for High-Voltage Direct-Current Transmission Systems}, series = {IEEE Transactions on Control of Network Systems}, volume = {5}, journal = {IEEE Transactions on Control of Network Systems}, number = {3}, issn = {2325-5870}, doi = {10.1109/TCNS.2017.2687080}, pages = {1110 -- 1119}, abstract = {The problem of primary control of high-voltage direct-current transmission systems is addressed in this paper, which contains four main contributions. First, a new nonlinear, more realistic model for the system suitable for primary control design, which takes into account nonlinearities introduced by conventional inner controllers, is proposed. Second, necessary conditions-dependent on some free controller tuning parameters-for the existence of equilibria is determined. Third, additional (necessary) conditions are formulated for these equilibria to satisfy the power-sharing constraints. Fourth, establish conditions for the stability of a given equilibrium point. The usefulness of the theoretical results is illustrated via numerical calculations on a four-terminal example.}, language = {de} } @misc{SchifferEfimovOrtega, author = {Schiffer, Johannes and Efimov, Denis and Ortega, Romeo}, title = {Almost Global Synchronization in Multi-Machine Power Systems}, series = {57th IEEE Conference on Decision and Control, Miami Beach, FL, USA, December 17-19, 2018}, journal = {57th IEEE Conference on Decision and Control, Miami Beach, FL, USA, December 17-19, 2018}, publisher = {IEEE}, address = {Piscataway, NJ}, isbn = {978-1-5386-1395-5}, doi = {10.1109/CDC.2018.8619788}, pages = {800 -- 805}, 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{AlghamdiSchifferFridman, author = {Alghamdi, Sultan and Schiffer, Johannes and Fridman, Emilia}, title = {Conditions for Delay-Robust Consensus-Based Frequency Control in Power Systems with Second-Order Turbine-Governor Dynamics}, series = {57th IEEE Conference on Decision and Control, Miami Beach, FL, USA, December 17-19, 2018}, journal = {57th IEEE Conference on Decision and Control, Miami Beach, FL, USA, December 17-19, 2018}, publisher = {IEEE}, address = {Piscataway, NJ}, isbn = {978-1-5386-1395-5}, pages = {786 -- 793}, abstract = {Consensus-based distributed secondary frequency control schemes have the potential to simultaneously ensure real-time frequency restoration and economic dispatch in future power systems with large shares of renewable energy sources. Yet, due to their distributed nature these control schemes critically depend on communication between units and, thus, robustness with respect to communication uncertainties is crucial for their reliable operation. Furthermore, when applied in bulk power systems the control design and analysis should take higher-order turbine-governor dynamics of the generation units explicitly into account. Both aspects have not been addressed jointly in the existing literature. Motivated by this, we derive conditions for robust stability of a consensus-based distributed frequency control scheme applied to a power system model with second-order turbine-governor dynamics in the presence of heterogeneous time-varying communication delays and dynamic communication topology. The result is established by a novel coordinate transformation and reduction to eliminate the invariant subspace in the closed-loop dynamics and by constructing a strict common Lyapunov-Krasovskii functional.}, language = {en} } @misc{SchifferEfimovBarabanovetal., author = {Schiffer, Johannes and Efimov, Denis and Barabanov, Nikita and Ortega, Romeo}, title = {A Relaxed Characterization of ISS for Periodic Systems with Multiple Invariant Sets}, series = {European Journal of Control}, volume = {Vol. 37}, journal = {European Journal of Control}, issn = {1435-5671}, doi = {10.1016/j.ejcon.2017.04.002}, pages = {1 -- 7}, abstract = {A necessary and sufficient criterion to establish input-to-state stability (ISS) of nonlinear dynamical systems, the dynamics of which are periodic with respect to certain state variables and which possess multiple invariant solutions (equilibria, limit cycles, etc.), is provided. Unlike standard Lyapunov approaches, the condition is relaxed and formulated via a sign-indefinite function with sign-definite derivative, and by taking the system's periodicity explicitly into account. The new result is established by using the framework of cell structure and it complements the ISS theory of multistable dynamics for periodic systems. The efficiency of the proposed approach is illustrated via the global analysis of a nonlinear pendulum with constant persistent input.}, language = {en} } @misc{SchifferBarabanovOrtegaetal., author = {Schiffer, Johannes and Barabanov, Nikita and Ortega, Romeo and Efimov, Denis}, title = {Conditions for Almost Global Attractivity of a Synchronous Generator Connected to an Infinite Bus}, series = {Transactions on Automatic Control}, volume = {62}, journal = {Transactions on Automatic Control}, number = {10}, issn = {1558-2523}, doi = {10.1109/TAC.2017.2671026}, pages = {4905 -- 4916}, abstract = {Conditions for existence and global attractivity of the equilibria of a realistic model of a synchronous generator with constant field current connected to an infinite bus are derived. First, necessary and sufficient conditions for existence and uniqueness of equilibrium points are provided. Then, sufficient conditions for local asymptotic stability and almost global attractivity of one of these equilibria are given. The analysis is carried out by employing a new Lyapunov-like function to establish convergence of bounded trajectories, while the latter is proven using the powerful theoretical framework of cell structures pioneered by Leonov and Noldus. The efficiency of the derived sufficient conditions is illustrated via extensive numerical experiments based on two benchmark examples taken from the literature.}, language = {en} } @misc{SchifferEfimovOrtegaetal., author = {Schiffer, Johannes and Efimov, Denis and Ortega, Romeo and Barabanov, Nikita}, title = {An Input-to-State Stability Approach to Verify Almost Global Stability of a Synchronous-Machine-Infinite-Bus System}, series = {Philosophical transactions of the Royal Society. Series A, Mathematical, physical and engineering sciences}, volume = {375}, journal = {Philosophical transactions of the Royal Society. Series A, Mathematical, physical and engineering sciences}, number = {2100}, issn = {1364-503X}, doi = {10.1098/rsta.2016.0304}, abstract = {Conditions for almost global stability of an operating point of a realistic model of a synchronous generator with constant field current connected to an infinite bus are derived. The analysis is conducted by employing the recently proposed concept of input-to-state stability (ISS)-Leonov functions, which is an extension of the powerful cell structure principle developed by Leonov and Noldus to the ISS framework. Compared with the original ideas of Leonov and Noldus, the ISS-Leonov approach has the advantage of providing additional robustness guarantees. The efficiency of the derived sufficient conditions is illustrated via numerical experiments.}, language = {en} } @misc{SchifferDoerflerFridman, author = {Schiffer, Johannes and D{\"o}rfler, Florian and Fridman, Emilia}, title = {Robustness of Distributed Averaging Control in Power Systems: Time Delays \& Dynamic Communication Topology}, series = {Automatica}, volume = {80}, journal = {Automatica}, issn = {0005-1098}, doi = {10.1016/j.automatica.2017.02.040}, pages = {261 -- 271}, abstract = {Distributed averaging-based integral (DAI) controllers are becoming increasingly popular in power system applications. The literature has thus far primarily focused on disturbance rejection, steady-state optimality and adaption to complex physical system models without considering uncertainties on the cyber and communication layer nor their effect on robustness and performance. In this paper, we derive sufficient delay-dependent conditions for robust stability of a secondary-frequency-DAI-controlled power system with respect to heterogeneous communication delays, link failures and packet losses. Our analysis takes into account both constant as well as fast-varying delays, and it is based on a common strictly decreasing Lyapunov-Krasovskii functional. The conditions illustrate an inherent trade-off between robustness and performance of DAI controllers. The effectiveness and tightness of our stability certificates are illustrated via a numerical example based on Kundur's four-machine-two-area test system.}, 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} }