202.

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.

Abstract:
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.

201.

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.

Abstract:
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.

200.

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.

Abstract:
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.

199.

X. Qi and J. Holtz, "The Relationship Between Root Locus and Transient Field Components of AC Machines", IEEE Transactions on Industrial Electronics, vol. 67, no. 3, pp. 1836--1843, 2020.

Abstract:
The dynamic analysis of induction motors is supported by well-known theories: the two-axes transformation, and the space vector theory. Yet some inconsistencies with the theory of dynamic systems exist. The machine eigenvalues suggest the existence of two damped oscillators, physically not understandable. The respective eigenfrequencies change with the angular velocity of the reference frame. This contradicts the understanding that eigenfrequencies are inherent system properties. Physically, the dynamics depend on the continuous distribution of magnetic energy and its spatial displacement during transient processes. Information on the system dynamics is lost when dividing the continuum of magnetic energy into discrete portions. Complex state variables associate the dynamics to the propagation in space of distributed magnetic fields. The dynamic analysis reveals the existence single-complex eigenvalues. These define a novel class of system identifiers. They are characterized by having only one imaginary part instead of a conjugate complex pair. The use of complex state variables conveys insight and physical understanding of the dynamic processes within the machine. The approach constitutes an extension to the theory of dynamic systems.

198.

D. Maune, B. Krüger, P. Sahm and S. Soter, "Speed Control for Lifting Devices with Conical Cable Drum through Indirect Position Determination" in 2020 IEEE International Conference on Industrial Technology (ICIT), 2020, pp. 401--405.

Abstract:
Lifting devices are often provided with a spring to compensate their constant weights. But because the force of a spring is depending on its stretch, the compensation ratio during a lifting process is fluctuating. By changing the characteristic of the load with the help of a variable radius drum it is possible to mitigate this drawback. In this paper a torsion spring and a conical cable drum are used for compensation. Because the wound cable length per revolution is changing, the lifting speed also varies. For the introduced speed controller the position of the drum is measured with an absolute, magnetic sensor. The actual lifting speed is then indirectly determined by calculation utilizing the cable drum parameters.

197.

J. O. Krah, J. Holtz and B. Sahan, "Control of a Hybrid IGBT-SiC Inverter" in 2019 2nd International Conference on Smart Grid and Renewable Energy (SGRE), 2019, pp. 1--6.

Abstract:
A novel inverter topology built from a combination of 1.2 kV (standard) IGBTs and SiC-MOSFETs has an attraction for power applications. It consists of twelve controllable semiconductor devices per phase, contained in two identical modules. The complexity of this arrangement requires a coordinated commutation which is the subject of this paper. Concatenated state machines are used for this purpose. Extremely low harmonic content of the output voltage is obtained by interleaved pulsewidth control of two three-level inverter modules per phase. The IGBTs operate uniquely in a soft commutation mode which leads to unprecedented efficiency.

196.

S. Kratz, P. Hanses, B. Krüger, R. Wegener and S. Soter, "Integration of Second Life Batteries into a Smart Overhead Contact System based on SiC-Technology" in 2019 IEEE Transportation Electrification Conference and Expo (ITEC), 2019, pp. 1--5.

Abstract:
This paper presents the starting situation and some interim results of a research project with the aim of a fully electrified short-range public transportation in a German town called Solingen. It gives an insight to the existing trolleybus infrastructure and the benefits of second life battery usage. Following, the design configuration of a refurbished trolleybus battery system for stationary usage in substations is shown and evaluated. Based on this a new silicone carbide power electronic for a direct integration of stationary battery systems are proposed and evaluated. Available semiconductors for the prototype are specified and compared by measurements. The results show that the integration of trolleybus batteries can be done with high system efficiency.

195.

S. Kratz, A. Schmidt, B. Krüger, R. Wegener and S. Soter, "Power Supply of a Short-Range Public Transportation System Based on Photovoltaics - Potential Analysis and Implementation" in 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC), 2019, pp. 3077--3081.

Abstract:
This paper answers the question of if an existing short-range public transportation system in a German town called Solingen could be exclusively supplied by photovoltaics and presents a fitted device for energy conversion. Based on the results of the proposed method for a photovoltaic potential analysis, the above question is answered and the main benefits of the direct integration of photovoltaic arrays are shown. The presented measurements of a 10kW prototype show that a high conversion efficiency can be achieved.

194.

J. O. Krah, T. Schmidt and J. Holtz, "Predictive Current Control with Synchronous Optimal Pulse Patterns" in 2019 2nd International Conference on Smart Grid and Renewable Energy (SGRE), 2019, pp. 1--6.

Abstract:
A method is described that generates synchronous optimal pulse patterns for inverter control in real time. A smart technique, which predicts the current trajectory, closes the current loop with implicit generation of optimized pulse patterns while eliminating all subharmonic components. The algorithm requires very little computational effort, which makes it applicable for low power systems operating at high switching frequency.

193.

T. Schneider, S. Kratz, R. Wegener and S. Soter, "Symmetrical Bidirectional CLLC-Converter with Simplified Synchronous Rectification for EV-Charging in Isolated DC Power Grids" in 2019 IEEE 28th International Symposium on Industrial Electronics (ISIE), 2019, pp. 990--995.

Abstract:
This paper presents a bidirectional CLLC resonant converter for the use in electric vehicle fast charging applications, fed from a trolley bus DC power grid. Bidirectional converters generally introduce an increase in design complexity as well as a decreased power density and conversion efficiency, compared to their unidirectional equivalents. The presented converter, because of its symmetrical layout, is able to concentrate all necessary resonant inductances in one passive component and uses a simplified sensorless synchronous rectification technique in order to mitigate these drawbacks. A 12.5kW CLLC prototype for input voltages between 600-800V and output voltages between 300-400V is used for validation. The achieved maximum efficiency equals 96.6{%}.