Multilevel converters are growing fast, whereas industry needs particular tools to evaluate efficiency and performance of such converters. This study focuses on the analysis and practice of power losses in a three-level neutral-point clamped (NPC) inverter. First, a precise mathematical model for three-level inverters is introduced to be used for simulating power losses of the switches, providing AC voltage and current of each phase, voltage and current of the switches as well as both the conduction and switching losses. Further, an NPC inverter was developed to validate the analytical work by comparing experimental results with those of simulations. Additionally, the thermal modelling of semiconductors is obtained using datasheet parameters. Then, the temperature rise is further modelled using the power losses along with the thermal model in the form of RC ladder. Finally, the lifetime of semiconductors is predicted using the heating curves in line with the power-cycling concept. The measured power losses and temperatures in comparison with the presented model suggest this kind of research as an applicable replacement for expensive measuring devices.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-pel.2012.0711

Efficiency of three-level neutral-point clamped converters: analysis and experimental validation of power losses, thermal modelling and lifetime prediction

A software package for extracting parameters to be used in advanced IGBT models is presented. In addition, new model equations and extraction procedures are introduced that more accurately describe a wide range of IGBT types including the Warp-Speed IGBTs. The parameter extraction software package consists of five programs that extract the 20 physical and structural parameters needed in the most recent version of the Hefner IGBT model. Each program has a graphical user interface and uses the IEEE 488 bus to control the measurement instruments. Various algorithms are employed for fitting the IGBT model equations to measured data. The new software package enables users of the simulation software products to extract model parameters themselves and thus permits the simulation of new IGBT part numbers as soon as they are introduced.

/pdf/automated-parameter-extraction-software-for-advanced-igbt-bnq7yhnnox.pdf

Automated Parameter Extraction Software for Advanced IGBT Modeling | NIST

A hybrid modular multilevel converter, comprising a mixture of full-bridge and half-bridge submodules, provides tolerance to dc faults without compromising the efficiency of the converter to a large extent. The inclusion of full bridges creates a new freedom over the choice of ratio of ac-to-dc voltage at which the converter is operated, with resulting impact on the converter's internal voltage, current, and energy deviation waveforms, all of which impact the design of the converter. A design method accounting for this and allowing the required level of derating of nominal submodule voltage and uprating of stack voltage capability to ensure correct operation at the extremes of the operating envelope is presented. A mechanism is identified for balancing the peak voltage that the full-bridge and half-bridge submodules experience over a cycle. Comparisons are made between converters designed to block dc-side faults and converters that also add STATCOM capability. Results indicate that operating at a modulation index of 1.2 gives a good compromise between reduced power losses and additional required submodules and semiconductor devices in the converter. The design method is verified against simulation results, and the operation of the converter at the proposed modulation index is demonstrated at the laboratory scale.

/pdf/dimensioning-and-modulation-index-selection-for-the-hybrid-2heh6tpu06.pdf

Dimensioning and Modulation Index Selection for the Hybrid Modular Multilevel Converter

Power losses in high power HVDC converters are dominated by those that occur within the power electronic devices. This power loss is dissipated as heat at the junction of semiconductor devices. The cooling system ensures that the generated heat is evacuated outside the converter station but temperature management remains critical for the lifetime of the semiconductor devices. This paper presents the results of a study on the temperature profile of the different switches inside a multilevel converter. The steady state junction temperatures are observed through the simulation of a 20 MW Alternate Arm Converter using 1.2kA 3.3 kV IGBT modules. A comparison of the Alternate Arm Converter is made against the case of both the half-bridge and full-bridge Modular Multilevel Converter topologies. Furthermore, the concept of varying the duty-cycle of the two alternative zero-voltage states of the H-bridge modules is introduced. Simulation results demonstrate that it can change the balance of electrical and thermal stress between the two top switches and the two bottom switches of a full-bridge cell.

/pdf/power-loss-and-thermal-characterization-of-igbt-modules-in-51srv6dem9.pdf

Power loss and thermal characterization of IGBT modules in the Alternate Arm converter

Achieving dc fault tolerance in modular multilevel converters requires the use of a significant number of submodules (SMs) that are capable of generating a negative voltage. This results in an increase in the number of semiconductor devices in the current path, increasing converter conduction losses. This paper introduces a thyristor augmented multilevel structure called a Power-Group (PG), which replaces the stacks of SMs in modular converters. Each PG is formed out of a series stack of SMs with a parallel force-commutated thyristor branch, which is used during normal operation as a low loss bypass path in order to achieve significant reduction in overall losses. The PG also offers negative voltage capability and so can be used to construct high-efficiency dc fault tolerant converters. Methods of achieving the turn- on and turn- off of the thyristors by using voltages generated by the parallel stack of SMs within each PG are presented, while keeping both the required size of the commutation inductor, and the thyristor turn- off losses low. Efficiency estimates indicate that this concept could result in converter topologies with power losses as low as 0.3% rated power while retaining high quality current waveforms and achieving tolerance to both ac and dc faults.

/pdf/thyristor-bypassed-submodule-power-groups-for-achieving-high-3uuiir9n9q.pdf

Thyristor-Bypassed Submodule Power-Groups for Achieving High-Efficiency, DC Fault Tolerant Multilevel VSCs

As the power density and switching frequency increase, thermal analysis of power electronics system becomes imperative. The analysis provides valuable information on the semiconductor rating, long-term reliability and efficient heat-sink design. The aim of this thesis is to build a comprehensive thermal model for the power IGBT modules used in three-phase inverters in order to predict the dynamic junction temperature rise under real operating conditions. The thermal model is developed in two steps: first, the losses are calculated and then the junction temperature is estimated. The real-time simulation environment dictates the requirements for the models: easy implementation on the software platform SIMULINK and fast calculation time. The power losses model, which is based on the look-up table method for calculating the conduction and switching losses, are successfully built and implemented in the real time simulation environment. The power losses equations are derived from the experimental data. Several algorithms are developed to catch every switching event and to solve the synchronization problem in a real-time system. An equivalent RC network model is built to perform the thermal analysis. The parameters of the thermal network are extracted from the junction to case and case to ambient dynamic thermal impedance curves. Two separate approaches are followed in deriving these thermal characteristic curves. The first approach uses the experimental data available while the second approach uses the commercial software package ANSYS. The 3-D simulation results using ANSYS agree well with the experimental data and therefore can be relied upon to extract the parameters of the thermal RC network. The latter is successfully implemented using the transfer function method, and is then built in SIMULINK environment for the use in a real time simulator. The power losses and thermal RC network models are extensively tested in a real time simulator environment. The accuracy of the models is confirmed by comparing their predictions with the experimental data.

Zhaohui Luo

Papers

A thermal model for igbt modules and its implementation in a real time simulator