Lehrstuhl für Elektrische Maschinen und Antriebe


T. Schmidt, J. O. Krah and J. Holtz, "Diverse Redundant Drive Architecture with External Diagnostics Enables Safety-Related Motor Control based on Proven Standard Components at Low Cost" in 2024 4th International Conference on Smart Grid and Renewable Energy (SGRE), 2024, pp. 1--6.

A concept for safety-related motor control for autonomous mobile robots (AMR), automated guided vehicles (AGV), etc. is presented. The drive-internal safety functions are reduced to safe torque off (STO) and safe brake control (SBC) via a safety-related fieldbus for a reliable stopping of the motor. The safe protocol data units (SPDUs) for safe motion are evaluated in a higher-level safety programmable logic controller (PLC). The current and position SPDUs from the drives are transmitted to the safety PLC via a safety-related fieldbus using the gray-channel-principle. Similar to safety-related encoders, the concept is based on the fact that a major part of the diagnostics from the drive is performed externally in the higher-level safety PLC. In combination with a diverse redundant drive architecture, low-cost standard components can be used instead of more expensive safety-certified components. As a result, a fully certified safe logic is not required in the drive.
T. Wilkening, J. Holtz and J. O. Krah, "Mixed-Critical Control Architecture for Industry 5.0" in 2024 4th International Conference on Smart Grid and Renewable Energy (SGRE), 2024, pp. 1--6.

Industry 5.0 achieves productivity gains through human-machine collaboration. Various architectures for safety-related control of cobots, automated guided vehicles (AGV) and autonomous mobile robots (AMR) are presented and compared concerning their safety-related computational performance. From this, an appropriate mixed-critical control architecture is proposed, with an emphasis on functional safety including multi-axis safe motion.
J. O. Krah and J. Holtz, "Fast Current Limiting with Virtual Synchronous Generators" in 2024 4th International Conference on Smart Grid and Renewable Energy (SGRE), 2024, pp. 1--6.

Installations of renewable energy systems like photovoltaic arrays and wind power generators are increasing in number. Problems may occur when such systems get installed on weak grids. Other than in major power distribution grids can sudden load changes endanger a stable operation of such systems. This is due to the high internal impedance of weak grids. The installation of virtual synchronous generators (VSG) stabilizes local grids. It absorbs the transients and existing current harmonics and also improves the power quality by compensating reactive components. Combined with fast current limitation, fault ride through (FRT) events are now possible, even with very fast switching inverters and their low inductance.
J. Holtz, "Predictive Finite-State Control---When to Use and When Not", IEEE Transactions on Power Electronics, vol. 37, no. 4, pp. 4225--4232, 2022.

A multistep predictive algorithm eliminates overshoots of the current vector outside the boundary circle while the processor identifies the next optimal switching state vector following a boundary transition. The predictions of the current vector trajectories refer to a simplified machine model consisting only of the leakage inductance. Nevertheless is good accuracy of the predictions achieved. Operation at low switching frequency minimizes both harmonic distortion and switching losses. Using higher switching frequencies does not lead to noticeable improvement over ordinary carrier modulation. Maximum fundamental output voltage is achieved by predictive overmodulation.
K. Nötzold, N. Brissing, A. Uphues and S. Soter, "Generalized Integrator Based Active Damping" in 2022 IEEE 7th Southern Power Electronics Conference (SPEC), 2022, pp. 1--6.

To actively damp the inherent resonance of a LCL-topology applied as an output filter for a grid-connected inverter without capacitor current measuring capability, the derivative of the capacitor voltage can be utilized and fed back instead. However, digital implementation of this derivative is generally challenging due to noise-sensitivity or an introduced phase lag. In literature a second-order nonideal generalized integrator is proposed to indirectly differentiate the capacitor voltage and successfully tested in combination with a control loop based on grid current feedback. Here, this damping approach is adapted to a control loop based on converter current feedback. Simultaneously, and in contrast to more complex damping functions suggested in literature, the phase lag caused by computational delay is particularly compensated by simple proportional capacitor voltage's derivative feedback. To determine the required feedback constant, the feedback path is transformed into an equivalent impedance, connected in parallel to the filter capacitor. Consequently, the generalized integrator as well as the proportional feedback constant is simply parameterizable regarding the desired overall damping constant. Finally, experimental results with a 625 kW inverter demonstrate the active damping capability across a large frequency range.
M. Weisbach, T. Schneider, D. Maune, H. Fechtner, U. Spaeth, R. Wegener, S. Soter and B. Schmuelling, "Intelligent Multi-Vehicle DC/DC Charging Station Powered by a Trolley Bus Catenary Grid", Energies, vol. 14, no. 24, pp. 8399, 2021.

Abstract This article deals with the major challenge of electric vehicle charging infrastructure in urban areas—installing as many fast charging stations as necessary and using them as efficiently as possible, while considering grid level power limitations. A smart fast charging station with four vehicle access points and an intelligent load management algorithm based on the combined charging system interface is presented. The shortcomings of present implementations of the combined charging system communication protocol are identified and discussed. Practical experiments and simulations of different charging scenarios validate the concept and show that the concept can increase the utilization time and the supplied energy by a factor of 2.4 compared to typical charging station installations.
S. Kratz, B. Krüger, R. Wegener and S. Soter, "Expansion of a Trolleybus Infrastructure towards a 100{%} Renewable Energy Usage" in 2021 IEEE 48th Photovoltaic Specialists Conference (PVSC), 2021, pp. 0233--0236.

The german government furthers a research project targeting the full electrification of the largest trolleybus network in Germany in prospect of deriving a pilot scheme for major cities. Based on the good interim results, another near future aim arised: operation of the trolleybus network with 100{%} renewable energy. To achieve this, the large-scale integration of photovoltaic arrays in combination with second life bus batteries into the network is planned. This paper presents the actual status of the research project and the already closed steps towards the aim. It presents measurement results of the developed hardware from the laboratory and the last adaptations for field testing. Overall the measurements show a high efficient integration of photovoltaics and batteries which leads to a positive prediction regarding the aim with an integration and interaction in large-scale.
J. Holtz, "Complex State Variables as Analytical Tool for Control System Design of Medium-Voltage Drives", IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 8, no. 2, pp. 1824--1832, 2020.

Complex state variables are used to analyze the dynamics of medium-voltage drives. These operate at low switching frequency to reduce the dynamic losses of the power semiconductor devices. It results in a low sampling rate of the digital signal processing system, causing signal delays that deteriorate the dynamic behavior. Undesired cross-coupling results between the current components id and iq. The effect is shown to be even more adverse than described by the conventional control theory. The solution is modeling the system by single-complex state variables. These permit designing a current controller of high dynamic performance, which exhibits zero cross-coupling. Improved performance at very low switching frequency is demonstrated by experimental results.
J. Holtz, "Event Driven Control of Voltage and Current Gradients of Medium Voltage IGBTs", IEEE Transactions on Industrial Electronics, vol. 67, no. 8, pp. 6323--6330, 2020.

Medium voltage insulated gate bipolar transistors are fast switching devices that require low gate drive power. They inherently generate high voltage and current gradients during switching transitions. These are generally limited by retarding the changes of the gate charge. Additional gate resistors are usually installed for this purpose. The drawback is high switching losses. A novel method is described in this article that aims at controlling the voltage and current gradients during switching transitions. The gate charge of the input MOS device is controlled by injected gate currents. These follow particular command functions that are predefined and stored in a memory. The method requires reacting within microseconds, which traditional closed-loop control cannot do. Specific time events are therefore defined to trigger the respective command function. These functions depend on the instantaneous values of the collector current or the collector-emitter voltage, variables that are identified without delay, knowing their predefined gradients and counting the time from the respective event to reach their final values. Experimental results show the performance of event driven control. Low current and voltage gradients are enforced while the switching losses are reduced.
X. Qi and J. Holtz, "Modeling and Control of Low Switching Frequency High-Performance Induction Motor Drives", IEEE Transactions on Industrial Electronics, vol. 67, no. 6, pp. 4402--4410, 2020.

Complex state variables are used to study the dynamic behavior of induction motors considering the propagation in space of the distributed magnetic field inside the machine. The objective of this paper is to improve the dynamics of pulsewidth modulation inverters in medium-voltage drive systems. To keep the dynamic losses of the power devices at a tolerable level, the switching frequency must be below 1 kHz. The sampling rate of the digital signal processing system is then low which introduces considerable signal delay. The delay has an adverse influence on the dynamic behavior of the current control system. It introduces undesired cross coupling between the current components id and iq. The degree of cross coupling is described by a cross-frequency transfer function. It is shown that the mechanism of cross coupling is different and more adverse than the conventional theory discloses. A current controller structure having poles and zeroes of the single-complex type is synthesized. Cross coupling is completely eliminated at any low switching frequency. Experimental results demonstrate that high dynamic performance and zero cross coupling is achieved even at very low switching frequency.

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