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Due to an increasing number of distributed energy generation systems connected to low voltage grids, further investigations are required to ensure high power quality and power system stability. One important power system parameter is the frequency depending grid impedance at the points of connection referred to as grid impedance. This impedance considerably affects power quality and system stability especially if remarkable resonances occur. In this paper, the grid impedance composition of different public low voltage distribution systems is measured and analyzed. The primary objective is to reveal the contribution of load and solar power generation to the grid impedance and especially to resonances. This case study shows that grid impedance resonance strongly depends on loads and inductance of the upstream network in low voltage power systems. However, if the load impedance is very high or no load is present, solar power plants lead to remarkable grid impedance resonances especially during nighttime.

Development and research for power converters is focused on high power density, high modularity, low costs as well as high reliability and efficiency. The use of multilevel converters, like Neutral Point Clamped (NPC) Converters, in high power applications in the range of 1.5 MW up to 6MW can be favorable because of the filter size reduction and therefore reduction of weight and total volume of the system. Moreover higher efficiency can be achieved. In this paper an approach, corresponding to the electrical method, is taken into account for evaluating at the same time two different inverters with an apparent power of approximately 1 MVA in one single operation.

The most stressful operation point for the inverter of an electric vehicle drive is at low speed. In this operation point, one power semiconductor is mainly stressed because of a high loss concentration. The possibilities of distributing the losses among the power semiconductors through a new modulation is studied for the three-level NPC and the T-Type inverter. The effect of the proposed modulation on the sizing of the power semiconductors is shown by application of the power semiconductor rating. For both inverter topologies, the presented modulation strategy is verified experimentally.

In this paper the design and measurement results of a grid analysis and grid impedance measurement system are presented. The system is provided to measure the frequency dependent grid impedance in different PoCs to detect resonances and improve the power quality especially of grids with high amount of DERs. Moreover Active Filtering (AF) and reactive power feed-in are possible. It comprises also measurement of the negative and zero sequence as well as of subharmonics. The design of the converter is discussed in detail, the chosen method of impedance measurement is explained briefly, calculations for the selection of the converter topology and the selection of power semiconductor devices are made. A special hysteresis current control for 3-level Active Neutral Point Clamped (ANPC) converters is presented. Finally different measurement results are shown. These measurements of the grid impedance include subsynchronous frequencies in the range of 0.1 Hz to 30 Hz and frequencies in the range of 70 Hz to 10 kHz as well as measurements of the negative sequence and the zero sequence impedance of different low voltage grids.

We outline Maxwell's five decisive papers on his equations governing electrodynamics (1862–1868). We study the metamorphoses of these equations and find essentially twelve different versions of them. We express our preference for the so-called premetric version of the Maxwell equations, which is particularly useful in understanding the structure of electrodynamics. Some selected applications are discussed.

Power semiconductor losses of hysteresis current controlled grid connected 2-level and 3-level ANPC inverters will be presented in this paper. A special focus will be on the calculations for currents consisting of two components, one current with the frequency of the grid and one with a harmonic frequency. These currents are typical for inverters in grid impedance measurement and inverters with active filter functionality.

This paper presents a method for the calculation of the power semiconductor rating based on data sheet parameters of the three-level Neutral-Point-Clamped (NPC) inverter. Beside the basic equations for calculating the turn-on and turn-off losses for Insulated Gate Bipolar Transistors (IGBT) and diodes, the electrical inverter topology, the different current paths for the commutations and the critical operation points are shown. The power semiconductor rating is investigated for different operation conditions and shows the influence of the reverse-recovery performance of the commutation diode on the IGBT behaviour. The method works in this way, that ‘virtual’ (interpolated) power semiconductors are selected in an optimization program to reach in one of possible operation points the maximum junction temperature. The design calculations and analysis are exemplarily carried out for a low power 20 kW electrical drive inverter for an electric vehicle and a high power 1 MVA wind inverter system. This method can well be applied to accurately compare the design rating of power semiconductor topologies.

Knowledge of the frequency depending grid impedance is necessary for adequate grid integration, including setting of standards, of renewable energy sources. It is also essential for the design of power converters for renewable energy sources. In this paper wide frequency range measurements of the positive sequence grid impedance in public low voltage grids with high proportion of renewable energy generation systems are presented and analyzed. This case study shows, that the impedance of a point of connection strongly depends on the low voltage grid including its sources and loads. Moreover, the impedance varies strongly during the day but shows similar recurring pattern from day to day. The used measuring system is briefly described.

A neutral point (NP) balancing modulation for a three-level inverter with cosϕ > 0.9 is presented. In the low modulation range, standard space vector modulation (SVM) is applied using the redundancy of the small space vectors (SV) for the NP balancing. In the high modulation range, the switching sequence is adapted so that the neutral-point voltage can be also balanced. The proposed method is compared with the modulation concept of the virtual SV concept. Experimental validation of the modulation scheme is presented.

A comparison of the two-level voltage-source inverter, the three-level Neutral-Point-Clamped inverter and the T-Type inverter based on the power semiconductor rating is presented for drive inverters of electric vehicles. The comparison of the power semiconductor design rating is done for four different drive train configurations: normal operation with a constant DC-link voltage, an overload capability, a varying DC-link due to the state-of-charge of the battery and a variable DC-link voltage which can be generated by an additional DC-DC converter. The design is based on operation performance and power loss calculation. The power losses of the two three-level topologies are evaluated experimentally.

In this contribution the optimal power electronics topology in SS-compensated IPT systems for Electric Vehicles (EVs) is investigated in terms of required rated semiconductor power and efficiency. The novelty of this paper is the focus on high power (> 1 kW) and low output voltage (

Due to the changing structure of the electric power system, stability analysis in the distribution grid are increasingly important. In this paper an impedance based method to determine the stability and the robustness of a grid connected converter under various grid conditions is presented. Therefore, the inverter output impedance is modeled in a modular way based on state space systems. Additionally, grid impedance measurements in a public low voltage grid are carried out. Based on these impedances the stability of the system is evaluated by applying the Nyquist stability criterion. Further, the paper illustrates under which circumstances it is necessary to consider grid resonances to achieve stable operation and it points out how emerging grid impedance resonances and the inverter parameters influence the system stability. The output impedance model is validated by simulation.

Due to the possibility to drastically reduce the Solid State Transformer (SST) volume and weight, its use is becoming a reality in traction and wind power plant applications, while, in the electric distribution system, it is still considered futuristic. A SST, with managerial role in the electric distribution grid, is generally called Smart Transformer (ST). Unfortunately the low efficiency, the low reliability and the high cost still act as barriers for its widespread use in the real world. This paper focuses on the impact of a modular design, by benchmarking different ST topologies. Moreover, the paper provides guidelines on how to choose the semiconductor modules and assessments on how the choice affects the efficiency of the ST.

Fault-tolerant operation could be an efficient way to increase the reliability of the power electronic converter. Although different fault-tolerant topologies have been proposed in the past, there are only few investigations clarifying the benefit for particular applications. This paper aims to answer this question in regard to wind turbine system application using three-level neutral-point-clamped back-to-back converters. Therefore related fault-tolerant converter topologies are studied, compared and evaluated with focus on reliability and cost issues.

The capability of harmonic compensation gains more importance in future grids. As local density of installed distributed power generator systems is increasing, the examination of the control behavior of parallel operating inverters is crucial. In this paper the performance of different, commonly discussed harmonic compensation algorithms for parallel acting inverters in an isolated microgrid is investigated and compared. Similar and different harmonic compensation algorithms in each autonomously controlled inverter are evaluated. Dynamic and stationary performances are considered based on different scenarios for harmonic compensation.

This paper describes a novel way to implement soft switching in DC/DC converters for high current low voltage applications in the automotive field. Adding resonant elements to a bidirectional buck boost converter enables resonant behaviour. In simulation and laboratory it will be shown that this behaviour reduces the switch-off losses strongly.

Power loss determination is done in industry and research institutions commonly by means of three basic methods, the electrical measurement of input and output power, the calorimetric measurement and the single component power loss determination based on data sheets or measurement. All of these methods are characterized by different measurement accuracy and by a different effort for the measuring systems. To achieve a required minimum measurement accuracy with limited effort method and measuring systems have to be carefully chosen. These methods are presented and characterised, their limitations are shown and compared. By this, support for the choice of suitable method, depending on the application, is given.

This work introduces the control concept of variable sampling time finite control-set model predictive control (FCS-MPC). The new control concept is introduced in theory based on a review of the conventional FCS-MPC concepts performed with a constant sampling time. Based on the partitioning of the sampling instant in multiple smaller sampling instants it is possible to optimize, besides the switching states, the switching states turn-on times. Therefore, the proposed variable sampling time FCS-MPC sets both: the switching state and the related turn-on times. To utilize the available calculation power for longer sampling instants an adaptation of the control- and prediction horizon to the sampling time is proposed. The theoretical control concepts are applied to the control of a grid connected two-level voltage-source converter where a simple L-type line-filter is used to demonstrate the control performance of the variable sampling time FCS-MPC algorithms in the laboratory environment.

The growing amount of electric energy generated from distributed energy resources (DER), mainly of renewables, requires their appropriate integration into the electrical grid. These distributed renewable power sources are mainly feeding-in the power via converters and can be controlled well. Investigation of their grid integration means investigating their control and hardware concept in the dedicated grid environment. Measurements at downscaled systems in a laboratory environment are a time- and cost-efficient way to investigate this. The properties of the electrical grid, voltage amplitude, frequency and phase as well as the grid impedance can vary substantially by time, the grid impedance moreover is frequency dependent. These properties have to be emulated for the measurements of the grid integration. A dedicated research laboratory test-bench has been developed. The laboratory comprises special developed systems for grid voltage emulation, sag generation, grid impedance emulation, grid voltage correction and grid impedance analysis. Here, the design and realization of these systems in the laboratory test-bench for investigations on grid integration of distributed renewable energy resources are presented. Representative measurements for each system prove their functionality.

Real-time estimation of the equivalent grid impedance and the equivalent grid voltage seen from a power converter connected to the public electric distribution network by means of Extended Kalman-Filter is addressed. The theoretical background of the Extended Kalman-Filter used for equivalent grid impedance estimation is introduced. Practical aspects like the use of the filter in an environment with highly distorted voltage waveforms, the tuning of the noise covariance matrices and the implementation on a laboratory system are discussed. The theoretical analysis is verified on a 22 kW test-bench where a grid impedance emulator is used to simulate grid impedance steps in the laboratory environment. The proposed Extended Kalman-Filter is designed to utilize the noise that is already present at the connection point of the power converter to overcome the need of active disturbance injection to estimate the equivalent grid impedance.

In the book by Robert Black: The History of Electric Wires and Cables. Peregrinus London 1983, 290 pages, the author Black deals with the individual screening of the three cores in a belted power cable, beginning on p. 109 and high-lightening on p. 111: “In the opinion of the writer this development, the introduction of the Hochstadter screen, was the most important single contribution to the art of cable making during the first quarter of the twentieth century.” Martin Hochstadter (1883–1973) — in original German spelling with two umlauts — left a type-written English manuscript of 7 pages, titled as in the headline and signed by him in 1967 when he was 84 years of age. Hochstadter's text is published here with comments by the editor, because after 50 years MH did not correctly remember the facts. A short biography is included with a portrait photo and an explanatory drawing of three-phase cable cross sections.

The increasing penetration of wind power to the electrical grid leads to the need of a stronger contribution of wind turbines to grid control, concerning quality and stability. For the purpose of controlling the active power injection, depending on the needs of the grid and not depending on the power generation, energy storage systems can be used. In this paper this kind of system with a bidirectional interleaved DC/DC-converter and electric double-layer capacitors as storage technology is investigated. The dimensioning of the converter's main elements and its performance for three- and four-phase interleaved configurations is studied and compared. Simulations are carried out in MATLAB/Simulink/Plecs.

The growing amount of electric energy generated from distributed or decentralized energy resources (DER), mainly of renewables, requires their appropriate grid integration. Measurements at downscaled systems in a laboratory environment are a time- and cost-efficient way to investigate grid-integration of distributed renewable energy resources. These renewable power sources have special characteristics as they are fluctuating, time-and locally-dependent and most of the feed in is by means of a converter. Moreover the properties of the electrical mains, amplitude and impedance, are also time dependent. These properties have to be emulated for the measurement. A dedicated laboratory test-bench has been developed. Here an overview on the self developed laboratory special systems and some exemplary analysis' of the grid integration of DERs by measurement on this laboratory test-bench are presented. These are the performance of DERs at low voltage, application of FACTs to support this performance, analyzing of PLLs at distorted voltage and grid adaptive control of DERs. A perspective application for a micro grid is presented.

This work presents an extended disturbance observer design for grid voltage disturbance rejection of voltage source PWM converters with LCL filters. The proposed observer design leads to a cascaded extended disturbance observer using either the converter side current or the line side current for feedback. Theoretical aspects including the LCL filter system observability, observer pole placement strategy and practical implementation are discussed to achieve an observer formulation. The analysis presented is based on a fully analytical observer formulation in the discrete time-domain. The discrete state-space modeling of the LCL filter dynamics is discussed and a step-by-step observer formulation is presented. The theoretical analysis is verified on a 22 kW laboratory type adjustable speed drive system where the control algorithms are implemented on a dSpace system. The analysis reveals excellent disturbance rejection capabilities of the proposed cascaded extended disturbance observer design for both feedback cases.

To obtain highest efficiency in electric vehicles and to increase the lifetime of the traction battery, a battery buffer system which consists of a DC/DC converter and electric double layer capacitors can be implemented. To achieve low volume and high efficiency, multiphase interleaved converters are used for the DC/DC converters. The inductors represent a main part concerning the converter volume. Coupled inductors can reduce the volume and increase the efficiency. In case of high coupling, additional filtering is required to reduce the current ripple. There are different options to realize the filtering. In this paper, a characterisation for coupled inductors in a three-phase interleaved converter is presented. The different filter types are introduced and their influence to the system characteristics is analyzed and discussed.

To improve the efficiency and to reduce the volume of dc-dc converters and their filters, interleaved converters are used. For further reduction of the inductor volume, coupled inductors can be applied. In these multiphase converters, appropriate active current sharing is necessary due to the inherent current unbalance caused by the parameter deviations of the components and the duty cycles of the phases. The common method to realize current sharing is to use a single current sensor for each phase whereas the current in each phase is controlled independently. Due to the costs of the current sensors, a reduction of the quantity is desirable. In this contribution, a method is presented which allows the current sharing by means of only one current sensor without extra circuitry and computation effort. This method is analyzed exemplarily for the application in a threephase interleaved converter with coupled inductors in theory and practice. Successful operation can be stated.

Multilevel topologies reduce the required filter effort and therefore the weight and volume of the system. Additionally some topologies have a built in redundance and increase the reliability. The cooling of the power semiconductors and thus the power dissipation is a crucial criterion for the design of a PWM converter. Methods for the calculation of power semiconductor losses in two level inverters and in some three level inverter topologies are well known. Here a complete analytical calculation of power semiconductor losses for different five level inverter topologies is presented. Conduction losses as well as switching losses are included in the calculations. These are based on the power semiconductor datasheet information using a simplified model. The derived equations are applied to the different topologies and the results are compared to each other.

The stability of fixed speed wind turbines can be improved by a StatCom, which is well known and documented in literature for balanced grid voltage dips. Under unbalanced grid voltage dips the negative sequence voltage causes additional generator torque oscillations. In this paper investigations on a fixed speed wind farm with squirrel cage induction generators directly connected to the grid in combination with a StatCom under unbalanced grid voltage fault are given by means of theory and simulations. A StatCom control structure with the capability to control the positive or negative sequence of the voltage is used. The simulation results clarify the effect of positive and negative sequence voltage compensation by a StatCom on the operation of a fixed speed wind farm. The influence of different StatCom control targets is investigated that lead to an increased network voltage stabiliy and lower torque pulsations.

In distributed power generation systems a fast and accurate positive-sequence fundamental grid-voltage frequency and magnitude tracking is required to synchronize grid-connected converter-systems with the mains. In this paper state-of-the-art fundamental grid-voltage detection methods based on the extension of the Synchronous Reference-Frame phase-locked loop with either an additional pre-filter for the phase detection or an extended loop-filter are presented and compared to each other. The performance of these methods is studied in the laboratory environment during harmonic voltage-distortions as well as characteristic grid-faults. Furthermore, a new approach using a combined pre- and extended loop-filter technique to improve the harmonic voltage-distortion rejection abilities is presented and thoroughly verified by measurements. The analysis reveals that the optimal choice of the PLL-methods is a trade-off between the dynamic performance requirements and the accuracy of the grid-voltage tracking during highly distorted grid-conditions. Furthermore, the proposed combined filter approach reveals fast voltage tracking performance with good voltage distortion rejection abilities.

This work presents a control concept for a PWM-converter switching with medium-low frequencies (2.5 kHz) based on grid-impedance measurement to adapt the converters control online to variable grid-conditions and to improve the voltage-quality at the converters connection point. Thus, this work contributes to the actual problem of connecting distributed renewable energy sources to the grid. To improve the voltage-quality, additional active-filter functionality is implemented to mitigate unbalances and harmonics (5th and 7th). To meet these control objectives with medium-low switching-frequencies, and thus with medium-low control-frequencies (5 kHz), the control-design is done using discrete time-domain analysis. The theoretical approaches are verified in experiments on a 22 kVA laboratory setup composed of a voltage-source back-to-back converter connected with an LCL-filter to the mains, an interior permanent-magnet synchronous-machine and a grid-impedance analyzer. The results reveal that stable converter operation is reached over a wide range of grid-impedances and that it is possible to improve the voltage-quality with the additional active-filter functionality adapting the measured grid-impedance for the control.

The design and analysis of a flatness based speed control method for drive systems with elastically coupled loads is presented. Drive systems with elastically coupled loads tend to mechanical vibrations due to a finite stiffness of the drive shafts material. These unwanted vibrations stress the system and can reduce its lifetime. The way out and aim of this analysis is a high performance speed control of drive systems with resonant loads with damping of mechanical oscillations. Conventional control methods like proportional-integral based controllers are not able to damp these oscillations effectively. Model based control methods are more appropriate to handle this problem. Therefore, the rather new model based control concept based on differential flatness is applied to the considered system. High dynamic and suppression of mechanical oscillations can be achieved with flatness based control without the reduction of the disturbance reaction. Furthermore, only measurement of the motor speed is required. Simulation and measurement results confirm these statements. The results are compared to conventional PI-control.

To obtain highest efficiency in electric vehicles and to increase the lifetime of the traction battery, a battery backup system is mandatory. In this paper, the investigation of a system for an electric lift truck propulsion system consisting of Electric Double Layer Capacitors and a DC/DC-converter, is presented. Primarily, the super capacitor voltage range is analyzed comparing super capacitor, inductor and MOSFET rating as well as prospected losses. The most suggestive range is determined. Accordingly, a high efficient interleaved DC/DC-converter is designed and its benefits against single phase converters are specified. The proposed configuration is analyzed and built up in the laboratory. Measurements confirm the theoretical considerations and show the advantages of this addional backup system.

Low Voltage Ride Through is an important feature for wind turbine systems to fulfill grid code requirements. In case of wind turbine technologies using doubly fed induction generators the reaction to grid voltage disturbances is sensitive. Hardware or software protection must be implemented to protect the converter from tripping during severe grid voltage faults. In this paper two methods for low voltage ride through of symmetrical grid voltage dips are investigated. As a basis, an analysis of the rotor voltages during grid fault is given. First, the conventional hardware method using a crowbar is introduced. Then the stator current reference feedback solution is presented. Both methods are investigated and compared by simulation results using 2 MW wind turbine system parameters. Measurement results on a 22 kW laboratory DFIG test bench show the effectiveness of the proposed control technique.

Interconnection and Damping Assignment Passivity-Based Control of grid-connected PWM converters with LCL-filter is designed and analyzed. Dynamic performance, resonance damping and robustness against grid impedance variations are studied by simulation and show good results. Experimental tests on a 22 kW induction motor drive prove the simulation results.