TY - GEN A1 - Rueda-Escobedo, Juan G. A1 - Tang, Shiqing A1 - Schiffer, Johannes T1 - A Performance Comparison of PLL Algorithms in Low-Inertia Power Systems Using an Observer-Based Framework T2 - 21st IFAC World Congress, Berlin, Germany, Jul. 11-17, 2020 N2 - 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. KW - Phase-locked loop implementations; Frequency estimation;High-gain observers; Electric power systems; Low-inertia systems; Inverter-based applications Y1 - 2020 UR - https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwj-3Nvqy6zuAhVCi1wKHSFhAWcQFjAAegQIBhAC&url=https%3A%2F%2Fwww-docs.b-tu.de%2Ffg-regelungssysteme%2Fpublic%2FPublications%2FA%2520performance%2520comparison%2520of%2520PLL%2520implementations%2520in%2520low%2520lnertia%2520power%2520systems%2520using%2520an%2520observer-based%2520framework.pdf&usg=AOvVaw0N4TiSh9cu9DeyVoOhNZpf ER - TY - GEN A1 - Machado, Juan Eduardo A1 - Schiffer, Johannes T1 - A Passivity-Inspired Design of Power-Voltage Droop for DC Microgrids with Electrical Network Dynamics T2 - 59th IEEE Conference on Decision and Control (CDC), Jeju Island, Republic of Korea, Dec. 14-18, 2020 N2 - 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. Y1 - 2020 UR - https://www-docs.b-tu.de/fg-regelungssysteme/public/Publications/2020%20A%20Passivity-Inspired%20Design%20of%20Power-Voltage%20Droop%20for%20DC%20Microgrids%20with%20Electrical%20Network%20Dynamics.pdf ER - TY - GEN A1 - Rueda-Escobedo, Juan G. A1 - Schiffer, Johannes T1 - Data-Driven Internal Model Control of Second-Order Discrete Volterra Systems T2 - 59th IEEE Conference on Decision and Control (CDC), Jeju Island, Republic of Korea, Dec. 14-18, 2020 N2 - 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 Y1 - 2020 UR - https://www-docs.b-tu.de/fg-regelungssysteme/public/Publications/2020%20RuedaSchiffer%20Data-Driven%20Internal%20Model%20Control%20of%20Second-Order%20Discrete%20Volterra%20Systems.pdf ER - TY - GEN A1 - Rojas, Michael A1 - Rueda-Escobedo, Juan G. A1 - Espinosa-Pérez, Gerardo A1 - Schiffer, Johannes T1 - Observer-Based Excitation Control for Transient Stabilization of the Single Machine Infinite Bus System T2 - 59th IEEE Conference on Decision and Control (CDC), Jeju Island, Republic of Korea, Dec. 14-18, 2020 N2 - 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. Y1 - 2020 UR - https://www-docs.b-tu.de/fg-regelungssysteme/public/Publications/2020%20RojasRuedaEspinosaSchiffer%20Observer-Based%20Excitation%20Control%20for%20Transient%20Stabilization%20of%20the%20Single%20Machine%20Infinite%20Bus%20System.pdf ER - TY - GEN A1 - Lorenz-Meyer, Nicolai A1 - Bobtsov, Alexey A1 - Ortega, Romeo A1 - Nikolaev, Nikolay A1 - Schiffer, Johannes T1 - PMU-Based Decentralized Mixed Algebraic and Dynamic State Observation in Multi-Machine Power Systems T2 - IET Generation, Transmission & Distribution N2 - 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. Y1 - 2020 UR - https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/iet-gtd.2020.1275 U6 - https://doi.org/10.1049/iet-gtd.2020.1275 SN - 1751-8695 SN - 1751-8687 VL - 14 IS - 25 SP - 6267 EP - 6275 ER - TY - GEN A1 - Metzkow, Renè A1 - Rueda-Escobedo, Juan G. A1 - Döring, Daniela A1 - Schiffer, Johannes T1 - An Internal Model Approach to Robust Current Control of IPMSM Drives with Respect to Unknown and Varying Inductances T1 - Ein interner Modellansatz zur robusten Stromregelung von IPMSM-Antrieben unter Berücksichtigung von unbekannten und variierenden Induktivitäten T2 - IFAC PapersOnLine N2 - 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. Y1 - 2020 UR - https://www.sciencedirect.com/science/article/pii/S2405896320314154 U6 - https://doi.org/10.1016/j.ifacol.2020.12.1046 SN - 2405-8963 VL - 53 IS - 2 SP - 14185 EP - 14191 ER - TY - GEN A1 - Bai, Handong A1 - Zhang, Hongwai A1 - Cai, He A1 - Schiffer, Johannes T1 - Voltage regulation and current sharing for multi-bus DC microgrids: A compromised design approach T2 - Automatica N2 - 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. Y1 - 2022 UR - https://www.sciencedirect.com/science/article/pii/S0005109822001893 U6 - https://doi.org/10.1016/j.automatica.2022.110340 SN - 0005-1098 VL - 142 ER - TY - GEN A1 - Bobtsov, Alexey A1 - Ortega, Romeo A1 - Nikolaev, Nikolay A1 - Lorenz-Meyer, Nicolai A1 - Schiffer, Johannes T1 - State Observation of Power Systems Equipped with Phasor Measurement Units: The Case of Fourth Order Flux-Decay Model T2 - IEEE Transactions on Automatic Control N2 - 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. Y1 - 2022 UR - https://doi.org/10.1109/TAC.2021.3073887 U6 - https://doi.org/10.1109/TAC.2021.3073887 SN - 0018-9286 SN - 1558-2523 VL - 67 IS - 4 SP - 2123 EP - 2130 ER - TY - GEN A1 - Ortega, Romeo A1 - Bobtsov, Alexey A1 - Nikolaev, Nikolay A1 - Schiffer, Johannes A1 - Dochain, Denis T1 - Generalized parameter estimation-based observers: Application to power systems and chemical–biological reactors T2 - Automatica N2 - 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. KW - Estimation parameters KW - Nonlinear systems KW - Observers KW - Time-invariant systems KW - Power systems Y1 - 2021 UR - https://www.sciencedirect.com/science/article/pii/S0005109821001552?via%3Dihub#! U6 - https://doi.org/10.1016/j.automatica.2021.109635 SN - 0005-1098 IS - 129 ER - TY - GEN A1 - Rueda-Escobedo, Juan G. A1 - Fridman, Emilia A1 - Schiffer, Johannes T1 - Data-Driven Control for Linear Discrete-Time Delay Systems T2 - IEEE Transactions on Automatic Control N2 - 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 KW - Data-driven control KW - delay systems KW - robust control KW - sampled data control Y1 - 2022 UR - https://ieeexplore.ieee.org/document/9484756 U6 - https://doi.org/10.1109/TAC.2021.3096896 SN - 1558-2523 VL - 67 IS - 7 SP - 3321 EP - 3336 ER - TY - GEN A1 - Krishna, Ajay A1 - Schiffer, Johannes T1 - A Port-Hamiltonian Approach to Modeling and Control of an Electro-Thermal Microgrid T2 - IFAC-PapersOnLine N2 - 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. KW - Passivity-based control KW - port-Hamiltonian systems KW - multi-energy microgrid KW - district heating systems KW - distributed control Y1 - 2021 UR - https://www.sciencedirect.com/science/article/pii/S2405896321021169 U6 - https://doi.org/10.1016/j.ifacol.2021.11.092 SN - 2405-8963 VL - 54 IS - 19 SP - 287 EP - 293 ER - TY - GEN A1 - Rueda-Escobedo, Juan G. A1 - Moreno, Jaime A. A1 - Schiffer, Johannes T1 - L2-Gain Tuning for the Gradient Descent Algorithm in the Presence of Disturbances T2 - 2022 European Control Conference (ECC) N2 - 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. KW - Upper bound KW - Gain measurement KW - Minimization KW - robustness KW - system identification KW - noise measurement Y1 - 2022 UR - https://ieeexplore.ieee.org/document/9838051 SN - 978-3-9071-4407-7 U6 - https://doi.org/10.23919/ECC55457.2022.9838051 SP - 1610 EP - 1616 PB - IEEE CY - Piscataway ER - TY - GEN A1 - Jaramillo-Cajica, Ismael A1 - Schiffer, Johannes T1 - A Dwell-Time Approach for Grid-Aware Operation of a Distributed Generator in an Islanded DC Microgrid T2 - 2022 European Control Conference (ECC) N2 - 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. KW - Switched systems KW - Low voltage KW - Buck converters KW - Stability criteria KW - Switches KW - Microgrids Y1 - 2022 UR - https://ieeexplore.ieee.org/document/9838354 SN - 978-3-9071-4407-7 U6 - https://doi.org/10.23919/ECC55457.2022.9838354 SP - 1079 EP - 1084 PB - IEEE CY - Piscataway ER -