Raimo Juntunen

Variable-Frequency Phase Shift Modulation of a Dual Active Bridge Converter

In this paper, a variable-frequency modulation method for a dual active bridge converter is introduced. The proposed method ensures zero-voltage switching over a wide power range with a minimal circulating current. Unlike previously presented modulation schemes, this modulation method can also be used for half-bridge variants of a traditional dual active bridge converter. The modulation method is given in a closed form, which makes it easy to apply in practice. Further, the phase drift phenomenon is discussed, and a simple phase drift compensation scheme is presented. Finally, a detailed analysis of the proposed modulation method is provided and its feasibility is verified by measurements.

Stray inductance estimation with detailed model of the IGBT module

Stray inductances cause various problems in a power inverter and should be appraised and minimized at an early design stage to avoid later required countermeasures and redesign after the device prototype has been built. The stray inductances of the commutation loops of an Active Neutral Point Clamped (ANPC) inverter are estimated in this paper. Detailed model of the Insulated Gate Bipolar Transistor (IGBT) module is created to improve the accuracy of the stray inductance calculation. The influence of the coupling between the IGBT modules and between the IGBT modules and the busbars on the commutation loop inductance is considered. The partial inductances of the components of the commutation loops are estimated by AnSYS Q3D parasitic extractor to calculate the total stray inductance. It is found that the commutation loop inductances of the studied ANPC inverter are overestimated by neglecting the couplings between the IGBT modules and between the IGBT modules and the busbars.

Modelling of low inductive busbars for medium voltage three-level NPC inverter

Design of low inductive busbars for medium voltage Neutral Point Clamped (NPC) inverter is studied. In a hard-switched inverter the stray inductance causes various problems, such as over voltages, current unbalance, and possible resonance with DC-link capacitors. These problems lead to improper operation of the inverter and an increased level of electromagnetic interference (EMI). The laminated busbars are known to be a good solution for minimizing the stray inductance. It is important to estimate and minimize the stray inductance of the busbars in an early design stage. FLUX3D is used to compare three physical layouts and analyze the influence of the placement of the main circuit components on the inductance value.

Thermal Analysis of the Laminated Busbar System of a Multilevel Converter

Laminated busbar systems are commonly used in power electronic converters because of their low stray inductance. While the electromagnetic analysis of a busbar system is widely presented in the literature, there is a lack of accurate thermal modeling. In this paper, the thermal analysis of the busbar system is presented. An analytical lumped parameter thermal model (LPTM) of the busbar system is developed. The LPTM is applied to the fast estimation of the mean temperature and temperature-dependent power losses of the busbars by the proposed algorithm. Joule losses produced by nonsinusoidal currents flowing through the busbars in the converter are estimated. The skin and proximity effects, which have a strong influence on the ac resistance of the busbars, are considered in the loss estimation. Thus, a comprehensive electrothermal model of the busbar system is developed, which is of practical use in the converter design. It allows optimizing the stray inductance, material consumption, and cost of the busbar system as long as the specified temperature limits are not exceeded. The finite-element method thermal modeling validates the developed LPTM. Laboratory measurements in two operating points of the converter have been performed, and they show good correlation with the simulation results.

Improvement of IGBT model characterization with experimental tests

The dynamic IGBT model is characterized with a circuit simulator characterization tool by utilizing datasheet information and after that the model is tuned by using data from experimental measurements. The circuit simulator simulation results from both models are compared with the experimental results and the possible accuracy improvements are reported. The main research question of this work is how well these models can describe the semiconductor loss behaviour and the different voltage and current overshoots in IGBT switching event and what is the usability of the models. These issues are mainly observed and compared near the nominal collector current, but some example results with higher and smaller current are presented as well. A commercial software tool Simplorer 11 is used for the simulations and the component that is used in the comparison is FZ400R17KE4. Experimental test measurements are performed with an IGBT double pulse tester system. The comparison between the simulation models and the experimental data is reported and conclusions are made. The simulation results show that the characterized IGBT model is usable for voltage behaviour investigation of the IGBT in the hard switched operation. In addition, the measurement data characterized model achieves an acceptable accuracy considering the switching losses.

Identification of resonances in parallel connected grid inverters with LC- and LCL-filters

Modern high power grid inverters use usually LC-or LCL-filter at grid interface. Simpler L-filter does not prove sufficient harmonic attenuation without resulting in to very large and bulky filter components. Higher order filters realize better attenuation with smaller components. However, these filters introduce additional resonances to the system, which may cause instabilities without proper care. Connecting more than one inverter with individual LC- or LCL-filters in parallel will result in shifted resonances. The identification of the resonances in parallel connected LC- and LCL-filter system is essential for proper operation of the grid-connected inverter. For instance the efficiency of active damping is highly depended on the accurate knowledge of the filter resonances. In this paper the transfer functions for parallel-connected filter systems are derived and resonances analyzed. A method for resonance identification is presented.

Comparative analysis of LCL-filter designs for paralleled inverters

This paper presents LCL-filter configurations for parallel connected grid inverters. Three different configurations can be used in an LCL-filter implementation. All of these can be dimensioned to produce the same harmonic attenuation. However, the filter resonance frequencies and how they vary with respect to the number of inverters differs between the configurations. This paper presents and analyses three filter configurations and a comparative study is made. All three configurations are designed to produce the same attenuation. The amount of filter components and their relative dimensions are compared and differences in the effectiveness of the passive damping are discussed.

Design, Fabrication, and Testing of a Low AC-Loss Conduction-Cooled Cryostat for Magnetization Loss Measurement Apparatus

Conduction cooling has become a viable alternative for cooling superconducting devices. However, the thermal pathways of a conduction-cooled cryostat can be problematic for applications where time-varying magnetic fields are present. Such alternating magnetic fields are present, e.g., in a magnetization ac-loss measurement system. The losses in the thermal pathways are unwanted as they increase the heat load into the cryostat and interfere with the measurement. To solve this challenge, a conduction-cooled cryostat with special attention in limiting eddycurrent losses in the cryostat structures was constructed. The design process is illustrated in detail starting from the specifications and proceeding through the fabrication of individual components. The loss dissipated in the cryostat is experimentally examined, and the finished conduction-cooled magnetization loss measurement system is demonstrated by characterizing a multifilamentary MgB2 conductor.