@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} } @misc{SchifferEfimovBarabanovetal., author = {Schiffer, Johannes and Efimov, Denis and Barabanov, Nikita and Ortega, Romeo}, title = {Relaxing the Conditions of ISS for Multistable Periodic Systems}, series = {IFAC-PapersOnLine}, volume = {50}, journal = {IFAC-PapersOnLine}, number = {1}, doi = {10.1016/j.ifacol.2017.08.1365}, pages = {7217 -- 7222}, abstract = {The input-to-state stability property of nonlinear dynamical systems with multiple invariant solutions is analyzed under the assumption that the system equations are periodic with respect to certain state variables. It is shown that stability can be concluded via a sign-indefinite function, which explicitly takes the systems' periodicity into account. The presented approach leverages some of the difficulties encountered in the analysis of periodic systems via positive definite Lyapunov functions proposed in Angeli and Efimov (2013, 2015). The new result is established based on the framework of cell structure introduced in Leonov (1974) and illustrated via the global analysis of a nonlinear pendulum with a constant persistent input.}, 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{SchifferOjo, author = {Schiffer, Johannes and Ojo, Yemi}, title = {Towards a Time-Domain Modeling Framework for Small-Signal Analysis of Unbalanced Microgrids}, series = {IEEE PowerTech 2017, Manchester, UK, 2017}, journal = {IEEE PowerTech 2017, Manchester, UK, 2017}, doi = {https://doi.org/10.1109/PTC.2017.7981013}, pages = {6}, abstract = {Small-signal analysis is one of the most frequently used techniques to assess the operating conditions of power systems. Typically, this analysis is conducted by employing a phasor-based model of the power network derived under the assumption of balanced operating conditions. However, distribution networks and, amongst these, microgrids are often unbalanced. Hence, their analysis requires the development of tools and methods valid under such conditions. Motivated by this, we propose a modeling approach for generic nonlinear and unbalanced three-phase microgrids, which allows to derive a small-signal model in a standard fashion. The approach is based on a time-domain decomposition of the electrical waveforms in positive and negative synchronous reference frames. The efficacy of the approach is demonstrated via application to an exemplary unbalanced microgrid.}, language = {en} } @misc{GustavoEscobedoMorenoSchiffer, author = {Gustavo-Escobedo, Juan and Moreno, Jaime A. and Schiffer, Johannes}, title = {Finite-Time Estimation of Time-Varying Frequency Signals in Low-Inertia Power Systems}, series = {8th European Control Conference (ECC)}, journal = {8th European Control Conference (ECC)}, doi = {10.23919/ECC.2019.8795926}, pages = {2108 -- 2114}, abstract = {The ongoing and unprecedented transformation of power systems leads to a reduction in the number of conventional power plants, which are the classical actuators of the grid. In addition, this development results in a decreasing system inertia, which is expected to yield faster frequency dynamics. Therefore power-electronics-interfaced units have to take over system control tasks and, in particular, frequency control. For this purpose, accurate and fast estimation algorithms for time-varying frequency signals are needed. Motivated by this fact, we propose a time-varying parameter estimator and a tuning criterion, which for sufficiently small initial estimation errors allows to reconstruct the time-varying frequency signal of a symmetric three-phase waveform in finite time. The proposed estimator is derived by using a time-varying version of the super twisting algorithm and its performance is illustrated via numerical examples.}, language = {en} } @misc{SchifferEfimovOrtega, author = {Schiffer, Johannes and Efimov, Denis and Ortega, Romeo}, title = {Global synchronization analysis of droop-controlled microgrids—A multivariable cell structure approach}, series = {Automatica}, journal = {Automatica}, number = {109}, issn = {0005-1098}, doi = {10.1016/j.automatica.2019.108550}, pages = {12}, abstract = {The microgrid concept represents a promising approach to facilitate the large-scale integration of renewable energy sources. Motivated by this, the problem of global synchronization in droop-controlled microgrids with radial topology is considered. To this end, at first a necessary and sufficient condition for existence of equilibria is established in terms of the droop gains and the network parameters. Then, the local stability properties of the equilibria are characterized. Subsequently, sufficient conditions for almost global synchronization are derived by means of the multivariable cell structure approach recently proposed in Efimov and Schiffer (2019). The latter is an extension of the powerful cell structure principle developed by Leonov and Noldus to nonlinear systems that are periodic with respect to several state variables and possess multiple invariant solutions. The analysis is illustrated via numerical examples.}, 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{AlghamdiSmithAristidouetal., author = {Alghamdi, Sultan and Smith, Nathan and Aristidou, Petros and Schiffer, Johannes}, title = {Delay-Robust Distributed Secondary Frequency Control: A Case Study}, series = {IEEE PES PowerTech Conference 2019, Milano, Italy}, journal = {IEEE PES PowerTech Conference 2019, Milano, Italy}, address = {Milano}, isbn = {978-1-5386-4722-6}, doi = {10.1109/PTC.2019.8810821}, pages = {6}, abstract = {With the purpose of enabling a low-carbon future, power systems worldwide are undergoing major transformations. These developments require new advanced control and operation approaches to ensure a stable and efficient system operation. Distributed consensus-based algorithms are a promising option to provide the necessary flexibility and scalability to cope with these challenges and have, thus, been widely investigated in the literature. Yet, most available results are limited to scenarios with reduced-order models and ideal communication. Motivated by this, we perform a case study using a detailed dynamic model of the well-known Nordic test system equipped with a consensus-based distributed secondary frequency controller. Our main objectives are to analyse the robustness of the closed-loop system with respect to unmodelled (voltage and higher-order generator) dynamics as well as communication delays. To facilitate the later property, we employ robust-stability conditions in the control design. Then, the performance of the proposed controller is assessed through detailed dynamic simulations covering several disturbances leading to large frequency and voltage excursions.}, language = {en} } @misc{EfimovSchiffer, author = {Efimov, Denis and Schiffer, Johannes}, title = {On boundedness of solutions of state periodic systems: a multivariable cell structure approach}, series = {IEEE Transactions on Automatic Control}, volume = {64}, journal = {IEEE Transactions on Automatic Control}, number = {10}, issn = {0018-9286}, doi = {10.1109/TAC.2019.2892395}, pages = {4094 -- 4104}, abstract = {Many dynamical systems are periodic with respect to several state variables. This periodicity typically leads to the coexistence of multiple invariant solutions (equilibria or limit cycles). As a consequence, while there are many classical techniques for analysis of boundedness and stability of such systems, most of these only permit to establish local properties. Motivated by this, a new sufficient criterion for global boundedness of solutions of such a class of nonlinear systems is presented. The proposed method is inspired by the cell structure approach developed by Leonov and Noldus and characterized by two main advances. First, the conventional cell structure framework is extended to the case of dynamics, which are periodic with respect to multiple states. Second, by introducing the notion of a Leonov function, the usual definiteness requirements of standard Lyapunov functions are relaxed to sign-indefinite functions. Furthermore, it is shown that under (mild) additional conditions the existence of a Leonov function also ensures input-to-state stability, i.e., robustness with respect to exogenous perturbations. The performance of the proposed approach is demonstrated via the global analysis of boundedness of trajectories for a nonlinear system.}, 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} } @misc{SchifferAristidouOrtega, author = {Schiffer, Johannes and Aristidou, Petros and Ortega, Romeo}, title = {Online Estimation of Power System Inertia Using Dynamic Regressor Extension and Mixing}, series = {IEEE Transactions on Power Systems}, volume = {34}, journal = {IEEE Transactions on Power Systems}, number = {6}, issn = {1558-0679}, doi = {10.1109/TPWRS.2019.2915249}, pages = {4993 -- 5001}, abstract = {The increasing penetration of power-electronic-interfaced devices is expected to have a significant effect on the overall system inertia and a crucial impact on the system dynamics. In future, the reduction of inertia will have drastic consequences on protection and real-time control and will play a crucial role in the system operation. Therefore, in a highly deregulated and uncertain environment, it is necessary for transmission system operators to be able to monitor the system inertia in real time. We address this problem by developing and validating an online inertia estimation algorithm. The estimator is derived using the recently proposed dynamic regressor and mixing procedure. The performance of the estimator is demonstrated via several test cases using the 1013-machine ENTSO-E dynamic model.}, language = {en} } @misc{AlghamdiMarkovicStanojevetal., author = {Alghamdi, Sultan and Markovic, Uros and Stanojev, Ognjen and Schiffer, Johannes and Hug, Gabriela and Aristidou, Petros}, title = {Wide-area oscillation damping in low-inertia grids under time-varying communication delays}, series = {ScienceDirect}, volume = {189}, journal = {ScienceDirect}, issn = {0378-7796}, doi = {10.1016/j.epsr.2020.106629}, pages = {9}, abstract = {Wide-Area Control (WAC) can be efficiently used for oscillation damping in power systems. However, to implement a WAC, a communication network is required to transmit signals between the generation units and the control center. In turn, this makes WAC vulnerable to time-varying communication delays that, if not appropriately considered in the control design, can destabilize the system. Moreover, with the increasing integration of renewable energy resources into the grid, usually interfaced via power electronics, power system dynamics are becoming drastically faster and making WAC more vulnerable to communication delays. In this paper, we propose a design procedure for a delay-robust wide-area oscillation damping controller for low-inertia systems. Its performance is illustrated on the well-known Kundur two-area system. The results indicate that the obtained WAC successfully improves the oscillation damping while ensuring robustness against time-varying communication delays.}, language = {en} } @misc{MatveevMachadoOrtegaetal., author = {Matveev, Alexey S. and Machado, Juan Eduardo and Ortega, Romeo and Schiffer, Johannes and Pyrkin, Anton}, title = {A Tool for Analysis of Existence of Equilibria and Voltage Stability in Power Systems with Constant Power Loads}, series = {IEEE Transactions on Automatic Control}, volume = {65}, journal = {IEEE Transactions on Automatic Control}, number = {11}, issn = {0018-9286}, doi = {10.1109/TAC.2020.2965028}, pages = {4726 -- 4740}, abstract = {It is well known that constant power loads in power systems have a destabilizing effect. Their growing presence in modern installations significantly aggravates this issue, hence, motivating the development of new methods to analyze their effect in ac and dc power systems. Formally, this problem can be cast as the analysis of solutions of a set of nonlinear algebraic equations of the form f(x) = 0, where f : R n → R n , to which we associate the differential equation x = f(x). By invoking advanced concepts of dynamical systems theory and effectively exploiting its monotonicity, the following properties are established: First, prove that, if there are equilibria, there is a distinguished one that is stable and attractive, and give conditions such that it is unique. Second, give a simple online procedure to decide whether equilibria exist or not and to compute the distinguished one. Third, prove that the method is also applicable for the case when the parameters of the system are not exactly known. It is shown how the proposed tool can be applied to the analysis of long-term voltage stability in ac power systems, and to the study of existence of equilibria of multiterminal high-voltage dc systems and dc microgrids.}, language = {en} } @misc{AlghamdiMarkovicStanojevetal., author = {Alghamdi, Sultan and Markovic, Uros and Stanojev, Ognjen and Schiffer, Johannes and Hug, Gabriela and Aristidou, Petros}, title = {Wide-Area Oscillation Damping in Low-Inertia Grids under Time-Varying Communication Delays}, series = {21st Power Systems Computation Conference, Porto, Portugal — June 29 - July 3, 2020}, journal = {21st Power Systems Computation Conference, Porto, Portugal — June 29 - July 3, 2020}, pages = {8}, abstract = {Wide-Area Control (WAC) can be efficiently used for oscillation damping in power systems. However, to implement a WAC, a communication network is required to transmit signals between the generation units and the control center. In turn, this makes WAC vulnerable to time-varying communication delays that, if not appropriately considered in the control design, can destabilize the system. Moreover, with the increasing integration of renewable energy resources into the grid, usually interfaced via power electronics, the power system dynamics becoming drastically faster and making WAC more vulnerable to communication delays. In this paper, we propose a design procedure for a delay-robust wide-area oscillation damping controller for low-inertia systems. Its performance is illustrated on the well-known Kundur two-area system. The results indicate that the obtained WAC successfully improves the oscillation damping while ensuring robustness against time-varying communication delays.}, language = {de} } @misc{AlghamdiSchifferFridman, author = {Alghamdi, Sultan and Schiffer, Johannes and Fridman, Emilia}, title = {Synthesizing Sparse and Delay-Robust Distributed Secondary Frequency Controllers for Microgrids}, series = {IEEE Transactions on Control Systems Technology}, volume = {29}, journal = {IEEE Transactions on Control Systems Technology}, number = {2}, issn = {1558-0865}, doi = {10.1109/TCST.2020.2977300}, pages = {691 -- 703}, abstract = {Consensus-based control schemes experience increasing popularity in the context of secondary frequency control in microgrids. Fundamental aspects in their practical implementation are the design of the communication topology as well as robustness with respect to both time-varying communication delays and exogenous disturbances. Motivated by this, we propose a design procedure for a consensus-based secondary frequency controller that ensures robustness with respect to heterogeneous fast-varying communication delays and simultaneously provides the option to trade off the L₂-gain performance against the number of required communication links. Our design criterion is equilibrium-independent and based on the Lyapunov-Krasovskii method for interval time-varying delays together with the descriptor method. The efficacy of the proposed approach is demonstrated by using numerical experiments on the CIGRE benchmark medium-voltage distribution network.}, language = {en} } @misc{BobtsovOrtegaNikolaevetal., author = {Bobtsov, Alexey and Ortega, Romeo and Nikolaev, Nikolay and Schiffer, Johannes}, title = {A Globally Convergent State Observer for Multimachine Power Systems with Lossy Lines}, series = {21st IFAC World Congress, Berlin, Germany, Jul. 11-17, 2020}, journal = {21st IFAC World Congress, Berlin, Germany, Jul. 11-17, 2020}, pages = {6}, abstract = {We present the first solution to the problem of estimation of the state of multimachinepower systems with lossy transmission lines. We consider the classical three-dimensional "flux-decay" model of the power system and assume that the active and reactive power as wellas the rotor angle and excitation voltage at each generator is available for measurement—ascenario that is feasible with current technology. The design of the observer relies on two recentdevelopments proposed by the authors: a parameter estimation based approach to the problemof state estimation and the use of the dynamic regressor extension and mixing technique toestimate these parameters. Thanks to the combination of these techniques it is possible toovercome the problem of lack of persistent excitation that stymies the application of standardobserver designs. Simulation results illustrate the performance of the proposed observer.}, language = {en} } @misc{RuedaEscobedoTangSchiffer, author = {Rueda-Escobedo, Juan G. and Tang, Shiqing and Schiffer, Johannes}, title = {A Performance Comparison of PLL Algorithms in Low-Inertia Power Systems Using an Observer-Based Framework}, series = {21st IFAC World Congress, Berlin, Germany, Jul. 11-17, 2020}, journal = {21st IFAC World Congress, Berlin, Germany, Jul. 11-17, 2020}, pages = {7}, abstract = {Phase-locked loop (PLL) implementations are critical components for the control and operation of grid-connected converters. Hence, they have to exhibit a highly reliable behavior under a wide range of operating conditions. Available implementations and performance analyses mainly focus on the impact of unbalances and harmonics. However, in converter-dominated low-inertia power systems an additional important type of perturbation will arise from fast variations in the grid frequency. Motivated by this, we show that the structure of several popular PLL implementations is closely related to that of high-gain observers and, by using this framework, provide a tuning criterion for the PLL gains that mitigates the impact of the rate of change of the frequency (RoCoF) on the estimation performance. This criterion is then used to conduct a numerical comparison of four popular PLL implementations under three distorted conditions: unbalances, harmonics and frequency variations.}, language = {en} } @misc{MachadoSchiffer, author = {Machado, Juan Eduardo and Schiffer, Johannes}, title = {A Passivity-Inspired Design of Power-Voltage Droop for DC Microgrids with Electrical Network Dynamics}, series = {59th IEEE Conference on Decision and Control (CDC), Jeju Island, Republic of Korea, Dec. 14-18, 2020}, journal = {59th IEEE Conference on Decision and Control (CDC), Jeju Island, Republic of Korea, Dec. 14-18, 2020}, pages = {6}, abstract = {We propose a design procedure for a power-voltage droop controller in structure-preserving DC microgrids under explicit consideration of the electrical network dynamics. Differently from most related literature, the system's controlled output is taken as the power—not the current—injection at each generation unit, yielding a nonlinear closed-loop system. This makes the output regulation problem non-trivial, yet far more appealing in a practical setting than the usual linear current-voltage droop control. Our approach is inspired by passivity-based control design in the sense that we exploit the natural port-Hamiltonian representation of the system dynamics and its associated shifted Hamiltonian to derive a control law together with sufficient conditions on the tuning gains that guarantee global asymptotic stability. The analysis is illustrated via detailed simulations, where accurate power sharing is manifested among the distributed generation units in the presence of load variations.}, language = {en} } @misc{RuedaEscobedoSchiffer, author = {Rueda-Escobedo, Juan G. and Schiffer, Johannes}, title = {Data-Driven Internal Model Control of Second-Order Discrete Volterra Systems}, series = {59th IEEE Conference on Decision and Control (CDC), Jeju Island, Republic of Korea, Dec. 14-18, 2020}, journal = {59th IEEE Conference on Decision and Control (CDC), Jeju Island, Republic of Korea, Dec. 14-18, 2020}, pages = {8}, abstract = {The increase in system complexity paired with a growing availability of operational data has motivated a change in the traditional control design paradigm. Instead of modeling the system by first principles and then proceeding with a (model-based) control design, the data-driven control paradigm proposes to directly characterize the controller from data. By exploiting a fundamental result of Willems and collaborators, this approach has been successfully applied to linear systems, yielding data-based formulas for many classical linear controllers. In the present paper, the data-driven approach is extended to a class of nonlinear systems, namely second-order discrete Volterra systems. Two main contributions are made for this class of systems. At first, we show that - under a necessary and sufficient condition on the input data excitation - a data-based system representation can be derived from input-output data and used to replace an explicit system model. That is, the fundamental result of Willems et al.is extended to this class of systems. Subsequently a data-driven internal model control formula for output-tracking is derived. The approach is illustrated via two simulation examples}, language = {en} } @misc{RojasRuedaEscobedoEspinosaPerezetal., author = {Rojas, Michael and Rueda-Escobedo, Juan G. and Espinosa-P{\´e}rez, Gerardo and Schiffer, Johannes}, title = {Observer-Based Excitation Control for Transient Stabilization of the Single Machine Infinite Bus System}, series = {59th IEEE Conference on Decision and Control (CDC), Jeju Island, Republic of Korea, Dec. 14-18, 2020}, journal = {59th IEEE Conference on Decision and Control (CDC), Jeju Island, Republic of Korea, Dec. 14-18, 2020}, pages = {6}, abstract = {The availability of excitation controllers to enhance transient stability has regained significant relevance in recent years, due to the unprecedented ongoing changes in power systems. Yet, the practical deployment of many reported control schemes is hampered by the fact that their implementation requires the measurement of the full state vector. Our main contribution is to address this fundamental obstacle by proposing an observer-based excitation controller using modern phasor measurement technology. For this purpose, a linear time-varying observer scheme for the generator frequency and the internal voltage is derived. This observer is then combined with a classical passivity-based excitation controller. Stability of the resulting nonlinear observer-based closed-loop system is shown by deriving an ISS-based separation principle. The performance of the proposed approach is demonstrated via simulation example.}, language = {en} } @misc{LorenzMeyerBobtsovOrtegaetal., author = {Lorenz-Meyer, Nicolai and Bobtsov, Alexey and Ortega, Romeo and Nikolaev, Nikolay and Schiffer, Johannes}, title = {PMU-Based Decentralized Mixed Algebraic and Dynamic State Observation in Multi-Machine Power Systems}, series = {IET Generation, Transmission \& Distribution}, volume = {14}, journal = {IET Generation, Transmission \& Distribution}, number = {25}, issn = {1751-8695}, doi = {10.1049/iet-gtd.2020.1275}, pages = {6267 -- 6275}, abstract = {We propose a novel decentralized mixed algebraic and dynamic state observation method for multi-machine power systems with unknown inputs and equipped with Phasor Measurement Units (PMUs). More specifically, we prove that for the third-order flux-decay model of a synchronous generator, the local PMU measurements give enough information to reconstruct algebraically the load angle and the quadrature-axis internal voltage. Due to the algebraic structure a high numerical efficiency is achieved, which makes the method applicable to large scale power systems. Also, we prove that the relative shaft speed can be globally estimated combining a classical Immersion and Invariance (I\&I) observer with—the recently introduced—dynamic regressor and mixing (DREM) parameter estimator. This adaptive observer ensures global convergence under weak excitation assumptions that are verified in applications. The proposed method does not require the measurement of exogenous inputs signals such as the field voltage and the mechanical torque nor the knowledge of mechanical subsystem parameters.}, language = {en} } @misc{MetzkowRuedaEscobedoDoeringetal., author = {Metzkow, Ren{\`e} and Rueda-Escobedo, Juan G. and D{\"o}ring, Daniela and Schiffer, Johannes}, title = {An Internal Model Approach to Robust Current Control of IPMSM Drives with Respect to Unknown and Varying Inductances}, series = {IFAC PapersOnLine}, volume = {53}, journal = {IFAC PapersOnLine}, number = {2}, issn = {2405-8963}, doi = {10.1016/j.ifacol.2020.12.1046}, pages = {14185 -- 14191}, abstract = {Interior permanent magnet synchronous machines (IPMSMs) are well-suited for high-performance applications, such as traction drives in hybrid and electric vehicles. Yet a major challenge to fully exploit their potential is the fact that their self and cross-coupling inductances vary significantly across the operation range. In addition, this variation is difficult to characterize and complicates the design of provably stabilizing and robust controls. Motivated by this, by using an IPMSM model with current dependant inductances together with the internal model principle, a nonlinear current control scheme is derived that renders the equilibrium point of the closed-loop system exponentially stable. Both the control and the stability result only require the knowledge of an upper bound of the gradient of the inductances as well as lower and upper bounds on the inductance values themselves, while their actual evolution can be completely unknown. This is a major advantage compared to existing (PI-based) current control approaches, as it makes costly practices to determine the inductance variations unnecessary. The efficacy of the proposed control scheme is demonstrated in a simulation example.}, language = {en} } @misc{BaiZhangCaietal., author = {Bai, Handong and Zhang, Hongwai and Cai, He and Schiffer, Johannes}, title = {Voltage regulation and current sharing for multi-bus DC microgrids: A compromised design approach}, series = {Automatica}, volume = {142}, journal = {Automatica}, issn = {0005-1098}, doi = {10.1016/j.automatica.2022.110340}, pages = {7}, abstract = {It is well known that accurate current sharing and voltage regulation are both important, yet conflicting control objectives in multi-bus DC microgrids. In this paper a distributed control scheme is proposed, which simultaneously considers these two control objectives via a trade-off factor. This factor permits to adjust the degree of compromise between accurate voltage regulation and current sharing. At the same time, the voltage of a critical node can be precisely regulated. A sufficient condition for closed-loop stability is given and it is shown that the control parameters can always be chosen, such that stability is guaranteed. In addition, the steady state voltage and current deviations relative to their rated values are quantified via suitable metrics. For a given topology and settings of a DC microgrid, a sufficient condition for the existence of the trade-off factor is provided. The results are illustrated by simulation examples.}, language = {en} } @misc{BobtsovOrtegaNikolaevetal., author = {Bobtsov, Alexey and Ortega, Romeo and Nikolaev, Nikolay and Lorenz-Meyer, Nicolai and Schiffer, Johannes}, title = {State Observation of Power Systems Equipped with Phasor Measurement Units: The Case of Fourth Order Flux-Decay Model}, series = {IEEE Transactions on Automatic Control}, volume = {67}, journal = {IEEE Transactions on Automatic Control}, number = {4}, issn = {0018-9286}, doi = {10.1109/TAC.2021.3073887}, pages = {2123 -- 2130}, abstract = {The problem of effective use of phasor measurement units (PMUs) to enhance power systems awareness and security is a topic of key interest. The central question to solve is how to use these new measurements to reconstruct the state of the system. In this article, we provide the first solution to the problem of (globally convergent) state estimation of multimachine power systems equipped with PMUs and described by the fourth-order flux-decay model. This article is a significant extension of our previous result, where this problem was solved for the simpler third-order model, for which it is possible to recover algebraically part of the unknown state. Unfortunately, this property is lost in the more accurate fourth-order model, and we are confronted with the problem of estimating the full state vector. The design of the observer relies on two recent developments proposed by the authors, a parameter estimation based approach to the problem of state estimation and the use of the dynamic regressor extension and mixing (DREM) technique to estimate these parameters. The use of DREM allows us to overcome the problem of lack of persistent excitation that stymies the application of standard parameter estimation designs. Simulation results illustrate the latter fact and show the improved performance of the proposed observer with respect to a locally stable gradient-descent-based observer.}, language = {en} } @misc{OrtegaBobtsovNikolaevetal., author = {Ortega, Romeo and Bobtsov, Alexey and Nikolaev, Nikolay and Schiffer, Johannes and Dochain, Denis}, title = {Generalized parameter estimation-based observers: Application to power systems and chemical-biological reactors}, series = {Automatica}, journal = {Automatica}, number = {129}, issn = {0005-1098}, doi = {10.1016/j.automatica.2021.109635}, abstract = {In this paper we propose a new state observer design technique for nonlinear systems. It consists of an extension of the recently introduced parameter estimation-based observer, which is applicable for systems verifying a particular algebraic constraint. In contrast to the previous observer, the new one avoids the need of implementing an open loop integration that may stymie its practical application. We give two versions of this observer, one that ensures asymptotic convergence and the second one that achieves convergence in finite time. In both cases, the required excitation conditions are strictly weaker than the classical persistent of excitation assumption. It is shown that the proposed technique is applicable to the practically important examples of multimachine power systems and chemical-biological reactors.}, language = {en} } @misc{RuedaEscobedoFridmanSchiffer, author = {Rueda-Escobedo, Juan G. and Fridman, Emilia and Schiffer, Johannes}, title = {Data-Driven Control for Linear Discrete-Time Delay Systems}, series = {IEEE Transactions on Automatic Control}, volume = {67}, journal = {IEEE Transactions on Automatic Control}, number = {7}, issn = {1558-2523}, doi = {10.1109/TAC.2021.3096896}, pages = {3321 -- 3336}, abstract = {The increasing ease of obtaining and processing data together with the growth in system complexity has sparked the interest in moving from conventional model-based control design toward data-driven concepts. Since in many engineering applications time delays naturally arise and are often a source of instability, we contribute to the data-driven control field by introducing data-based formulas for state feedback control design in linear discrete-time time-delay systems with uncertain delays. With the proposed approach, the problems of system stabilization as well as of guaranteed cost and H∞ control design are treated in a unified manner. Extensions to determine the system delays and to ensure robustness in the event of noisy data are also provided}, language = {en} } @misc{KrishnaSchiffer, author = {Krishna, Ajay and Schiffer, Johannes}, title = {A Port-Hamiltonian Approach to Modeling and Control of an Electro-Thermal Microgrid}, series = {IFAC-PapersOnLine}, volume = {54}, journal = {IFAC-PapersOnLine}, number = {19}, issn = {2405-8963}, doi = {10.1016/j.ifacol.2021.11.092}, pages = {287 -- 293}, abstract = {We address the problems of modeling and controlling multi-energy microgrids (meMGs) composed of an electrical and a thermal system, which are connected via heat pumps (HPs). At first, we model the individual subsystems in a port-Hamiltonian (pH) framework. Then, by exploiting the structural properties of pH systems, we interconnect the subsystems in a passive manner and show that the overall meMG is shifted passive with respect to the control input-output mapping. We then use this property to propose a distributed passivity based-control (PBC) that addresses frequency and temperature regulation by utilizing the resources in the meMG in a proportional fashion and renders the closed-loop equilibrium asymptotically stable.}, language = {en} } @misc{RuedaEscobedoMorenoSchiffer, author = {Rueda-Escobedo, Juan G. and Moreno, Jaime A. and Schiffer, Johannes}, title = {L2-Gain Tuning for the Gradient Descent Algorithm in the Presence of Disturbances}, series = {2022 European Control Conference (ECC)}, journal = {2022 European Control Conference (ECC)}, publisher = {IEEE}, address = {Piscataway}, isbn = {978-3-9071-4407-7}, doi = {10.23919/ECC55457.2022.9838051}, pages = {1610 -- 1616}, abstract = {Due to its simplicity and inexpensive computation, the gradient descent algorithm is one of the most used tools in adaptive control and system identification. Although it has been studied for decades, little has been achieved in terms of tuning methods in the presence of disturbances. One of the main difficulties in its analysis is the time-varying nature of the algorithm. In this work, we contribute in such direction by providing LMI tools for tuning the gradient descent algorithm gain such that a guaranteed upper bound on the L2 -gain with respect to parameter variations and measurement noise is achieved. Two academic examples are provided to illustrate the efficient application of the method.}, language = {en} } @misc{JaramilloCajicaSchiffer, author = {Jaramillo-Cajica, Ismael and Schiffer, Johannes}, title = {A Dwell-Time Approach for Grid-Aware Operation of a Distributed Generator in an Islanded DC Microgrid}, series = {2022 European Control Conference (ECC)}, journal = {2022 European Control Conference (ECC)}, publisher = {IEEE}, address = {Piscataway}, isbn = {978-3-9071-4407-7}, doi = {10.23919/ECC55457.2022.9838354}, pages = {1079 -- 1084}, abstract = {We propose a switched control law for a DC-DC Buck converter that enables a grid-aware operation of a distributed generator (DG) in a low-voltage islanded DC microgrid (MG). The qualifier grid-aware means that the DG adjusts its operation mode in dependency of the MG status. The resulting closed-loop system is a switched system, in which the subsystems possess different equilibrium points. By means of a time- and state-dependent switching logic together with dwell-time stability analysis methods, we derive sufficient stability criteria that ensure the existence of a unique and globally exponentially stable equilibrium point of the resulting closed-loop switched system. The performance of the proposed control is illustrated via a numerical example.}, language = {en} } @misc{MendozaAvilaEfimovMercadoUribeetal., author = {Mendoza-Avila, Jesus and Efimov, Denis and Mercado-Uribe, Angel and Schiffer, Johannes}, title = {On Relaxed Conditions of Integral ISS for Multistable Periodic Systems}, series = {Proceedings of the 2021 61st IEEE Conference on Decision and Control (CDC)}, journal = {Proceedings of the 2021 61st IEEE Conference on Decision and Control (CDC)}, pages = {6}, abstract = {A novel characterization of the integral Inputto- State Stability (iISS) property is introduced for multistable systems whose dynamics are periodic with respect to a part of the state. First, the concepts of iISS-Leonov functions and output smooth dissipativity are introduced, then their equivalence to the properties of bounded-energy-bounded-state and global attractiveness of solutions in the absence of disturbances are proven. The proposed approach permits to relax the usual requirements of positive definiteness and periodicity of the iISSLyapunov functions. Moreover, the usefulness of the theoretical results is illustrated by a robustness analysis of a nonlinear pendulum with a constant bias input and an unbounded statedependent input coefficient.}, language = {en} } @misc{RuedaEscobedoMorenoSchiffer, author = {Rueda-Escobedo, Juan G. and Moreno, Jaime A. and Schiffer, Johannes}, title = {Design and Tuning of the Super-Twisting-Based Synchronous Reference Frame Phase-Locked-Loop}, series = {2022 IEEE 61st Conference on Decision and Control (CDC) ; December 6-9, 2022, Cancun, Mexico}, journal = {2022 IEEE 61st Conference on Decision and Control (CDC) ; December 6-9, 2022, Cancun, Mexico}, abstract = {The worldwide transition to climate-friendly energy systems entails the substitution of conventional energy generation based on synchronous generators by renewable energy sources based on power electronics. As a consequence, the overall inertia of the grid decreases, resulting in high volatility of the frequency and posing new challenges for the estimation of the latter quantity. To address this problematic, in the present paper a phase-locked-loop (PLL) based on the super-twisting algorithm is considered for the estimation of the phase angle and time-varying frequency of a symmetric three-phase signal. A rigorous proof of the algorithm's exact convergence in the presence of a fast-varying frequency together with tuning rules for its gains are derived by means of Lyapunov theory. Additionally, an estimate of the region of attraction is provided. The effectiveness of the proposed tuning method is illustrated in numerical simulations, while comparing its performance against a standard synchronous reference frame PLL.}, language = {en} } @misc{SyedMachadoSchiffer, author = {Syed, Wasif H. and Machado, Juan E. and Schiffer, Johannes}, title = {Distributed Adaptive Control for a DC Power Distribution System of a Series-Hybrid-Electric Propulsion System of a Commuter Aircraft}, series = {2024 American Control Conference (ACC)}, journal = {2024 American Control Conference (ACC)}, publisher = {IEEE}, doi = {10.23919/ACC60939.2024.10644618}, pages = {2598 -- 2603}, language = {en} } @misc{RuedaEscobedoMetzkowSchiffer, author = {Rueda-Escobedo, Juan G. and Metzkow, Ren{\´e} and Schiffer, Johannes}, title = {Robust current control of IPMSM drives under uncertain and varying inductances}, series = {Automatica}, volume = {152}, journal = {Automatica}, issn = {0005-1098}, doi = {10.1016/j.automatica.2023.110998}, abstract = {Interior permanent magnet synchronous machines (IPMSMs) are gaining popularity in e-mobility applications due to their wide constant power speed range when compared with other electric motors. The high-performance of IPMSMs is due to the pronounced degree of magnetic saliency, which originates from embedding their magnets inside the rotor. Yet, this feature complicates their control since it increases nonlinear phenomena such as cross-magnetization and saturation. This results in pronounced variations of the machine inductances, which are highly dependent on the machine current. This behaviour is of particular relevance in e-mobility applications, where a highly dynamic operation of the machine is required. In contrast to most existing current control approaches, where precise inductance knowledge is required, we address this challenge by deriving a controller that exploits the machine dynamics to achieve exponential current tracking in the presence of unknown and varying inductances. By interpreting the flux linkage as a modelled disturbance, we propose a dynamic compensator based on the internal model principle. As a by-product, the method provides an estimate of the flux linkage. The gain in performance compared to the standard PI controller is illustrated in simulation.}, language = {en} } @misc{SchifferEfimov, author = {Schiffer, Johannes and Efimov, Denis}, title = {Strong and Weak Leonov Functions for Global Boundedness of State Periodic Systems}, series = {IEEE Transactions on Automatic Control}, volume = {68}, journal = {IEEE Transactions on Automatic Control}, number = {12}, issn = {0018-9286}, doi = {10.1109/TAC.2023.3251903}, pages = {7958 -- 7965}, abstract = {We present new results for the analysis of global boundedness of state periodic systems. Thereby, we address both the case of systems, whose dynamics is periodic with respect to a part of the state vector, and the case of systems, whose dynamics is periodic with respect to all state variables. To derive the results, the notion of strong Leonov functions is introduced. The main results are complemented by a number of relaxations based on the concept of weak Leonov functions.}, language = {en} } @misc{MercadoUribeMendozaAvilaEfimovetal., author = {Mercado-Uribe, Angel and Mendoza-{\´A}vila, Jes{\´u}s and Efimov, Denis and Schiffer, Johannes}, title = {A Leonov Function for Almost Global Synchronization Conditions in Acyclic Networks of Heterogeneous Kuramoto Oscillators}, series = {IFAC-PapersOnLine}, volume = {56}, journal = {IFAC-PapersOnLine}, number = {2}, issn = {2405-8963}, doi = {10.1016/j.ifacol.2023.10.248}, pages = {9505 -- 9510}, abstract = {Sufficient conditions for almost global synchronization in acyclic networks of Kuramoto oscillators with heterogeneous coupling strengths and natural frequencies are presented. The result is established by employing the recently developed Leonov function framework for systems whose dynamics are periodic for all state variables. The synchronization property is accompanied by necessary and sufficient conditions to guarantee the existence of equilibria. The implications of these conditions on the network topology, the oscillator's coupling strengths and natural frequencies are discussed. Finally, the results are illustrated via a numerical example.}, language = {en} } @misc{MendozaAvilaMercadoUribeEfimovetal., author = {Mendoza-Avila, Jesus and Mercado-Uribe, Angel and Efimov, Denis and Schiffer, Johannes}, title = {Design of controls for ISS and Integral ISS Stabilization of Multistable State Periodic Systems}, series = {IFAC-PapersOnLine}, volume = {56}, journal = {IFAC-PapersOnLine}, number = {2}, doi = {https://doi.org/10.1016/j.ifacol.2023.10.936}, pages = {4472 -- 4477}, language = {en} } @misc{WuerfelLorenzMeyerSchiffer, author = {W{\"u}rfel, Hans and Lorenz-Meyer, Nicolai and Schiffer, Johannes}, title = {Experimentally Validated Reduced-Order Models for Grid-Connected Inverters Using Balanced Residualization}, series = {2023 8th IEEE Workshop on the Electronic Grid (eGRID), Karlsruhe, Germany, 16-18 October 2023}, journal = {2023 8th IEEE Workshop on the Electronic Grid (eGRID), Karlsruhe, Germany, 16-18 October 2023}, address = {Karlsruhe}, isbn = {979-8-3503-2700-7}, doi = {10.1109/eGrid58358.2023.10380931}, pages = {6}, language = {en} } @misc{GernandtKrenzlinHagemannetal., author = {Gernandt, Hannes and Krenzlin, Franziska and Hagemann, Willem and Schiffer, Johannes}, title = {Data-based modeling for optimizing the operation of seasonal underground storages}, series = {Der Geothermiekongress 2023 : 17.-19. Oktober 2023 in Essen : Tagungsband}, journal = {Der Geothermiekongress 2023 : 17.-19. Oktober 2023 in Essen : Tagungsband}, address = {Essen}, organization = {Geothermische Vereinigung - Bundesverband Geothermie}, pages = {6}, abstract = {Underground thermal energy storages (UTES) are essential for decarbonizing the existing heating networks. To accurately simulate the storage and the connected heat supply system, typically detailed numerical models of the UTES and the heat supply system are required. In a co-simulation these models must be coupled, which is often time-consuming and has therefore not been used to operate systems in real-time. In this contribution, we present a method that leads to a data-based surrogate model of low complexity, which can then be directly integrated in the operational optimization. To illustrate the method, we derive a surrogate model from a Feflow model of a borehole thermal energy storage (BTES) as a particular type of UTES and integrate the surrogate model into a supply system that is optimized for electrical energy consumption using the Python toolbox Pyomo.}, language = {de} } @misc{ZuritaBustamanteRuedaEscobedoSchiffer, author = {Zurita-Bustamante, Erik W. and Rueda-Escobedo, Juan G. and Schiffer, Johannes}, title = {Passivity-Based Conditions for Asymptotic Stability of Speed Control for Three-Phase and Dual Three-Phase Permanent Magnet Synchronous Motors}, series = {62nd IEEE Conference on Decision and Control (CDC), 2023}, journal = {62nd IEEE Conference on Decision and Control (CDC), 2023}, isbn = {979-8-3503-0124-3}, issn = {2576-2370}, doi = {10.1109/CDC49753.2023.10383817}, pages = {6}, language = {en} } @misc{MendozaAvilaEfimovMercadoUribeetal., author = {Mendoza-Avila, Jesus and Efimov, Denis and Mercado-Uribe, Angel and Schiffer, Johannes}, title = {Design of Controls for Boundedness of Trajectories of Multistable State Periodic Systems}, series = {62nd IEEE Conference on Decision and Control (CDC), 2023}, journal = {62nd IEEE Conference on Decision and Control (CDC), 2023}, isbn = {979-8-3503-0124-3}, issn = {2576-2370}, doi = {10.1109/CDC49753.2023.10383528}, pages = {6}, language = {en} } @misc{MercadoUribeMendozaAvilaEfimovetal., author = {Mercado-Uribe, Angel and Mendoza-{\´A}vila, Jes{\´u}s and Efimov, Denis and Schiffer, Johannes}, title = {A Control Leonov Function Guaranteeing Global ISS of Two Coupled Synchronverters}, series = {62nd IEEE Conference on Decision and Control (CDC), 2023}, journal = {62nd IEEE Conference on Decision and Control (CDC), 2023}, isbn = {979-8-3503-0124-3}, issn = {2576-2370}, doi = {10.1109/CDC49753.2023.10383811}, pages = {6}, language = {en} }