Thermal Stress Based Model Predictive Control of Power Electronic Converters in Electric Drives Applications

Power electronics is used increasingly in a wide range of application fields such as variable speed drives, electric vehicles and renewable energy systems. It has become a crucial component for the further development of emerging application fields such as lighting, more-electric aircrafts and medical systems. The reliable operation over the designed lifetime is essential for any power electronic system, particularly because the reliability of power electronics is becoming a prerequisite for the system safety in several key areas like energy, medicine and transportation. The thermal stress of power electronic components is one of the most important causes of their failure. Proper thermal management plays an important role for more reliable and cost effective energy conversion. As one of the most vulnerable and expensive components, power semiconductors, are the focus of this thesis. Active thermal control is a possibility to control the junction temperatures of power semiconductors in order to reduce the thermal stress. For this purpose the finite control-set model predictive control (FCS-MPC) is chosen. In FCS-MPC the switching vector is selected using a multi-parameter optimization that can include non-linear electric and thermal stress related models. This switching vector is directly applied to the physical system. This allows the direct control of the switching-state and the current through each semiconductor at each time instant. For cost-effective control of the thermal stress a measure for the degradation of the semiconductor's lifetime is necessary. Existing lifetime models in literature are based on the thermal cycling amplitudes and maximum values of recorded junction temperature profiles. For online estimation of the degradation, a method to detect the junction temperatures of the semiconductors during operation is designed and validated. An existing and proven lifetime model is adapted for online estimation of the thermal stress. An algorithm for the FCS-MPC is written that utilizes this model to drive the inverter with reduced stress and equalize the degradation of the semiconductors in a power module. The algorithm is demonstrated in simulation and validated in experiment. A technique to find the optimal trade-off between reduction of the thermal stress and allowing additional losses in the system is given. The effect of rotor flux variation of the machine on the junction temperatures of the driving inverter is investigated. It can be used as another parameter to control the junction temperature. This allows increasing the maximal thermal cycling amplitude that can be compensated by an active thermal controller. A suitable controller is proposed and validated in experiment. The integration of this technique into the FCS-MPC is presented

Lifetime prediction for power converters

Renewable energy is developing rapidly and gaining more and more commercial viability. High reliability of the generation system is essential to maximize the output power. The power inverter is an important unit in this system and is believed to be one of the most unreliable parts. In the case of wind power generation, especially in off-shore wind, when the system reliability requirement is high, a technique to predict the inverter lifetime is invaluable as it would help the inverter designer optimize his design for minimal maintenance. Previous researchers studying inverter lifetime prediction, focus either at device level such as device fatigue damage models, or at system level which require experimental data for their selected device. This work presents a new method to estimate the inverter lifetime from a given mission profile within a reasonable simulation time. Such model can be used as a converter design tool or an on-line lifetime estimation tool after being configured to a real converter system. The key contribution of this work is to link the physics of the power devices to a large scale system simulation within a reasonable framework of time. With this technique, the system down time can be reduced and therefore more power can be generated. Also, the failure damage to the system is avoided which reduces the maintenance cost. A power cycling test is designed to gather the lifetime data of a selected IGBT module. Die-attach solder fatigue is found out to be the dominant failure mode of this IGBT module. The accuracy of widely accepted Miner’s rule, which accumulates damage linearly, is discussed and a nonlinear accumulation method is promoted to predict the lifetime of power inverters

Power Loss Analysis of Solar Photovoltaic Integrated Model Predictive Control Based On-Grid Inverter

This paper presents a finite control-set model predictive control (FCS-MPC) based technique to reduce the switching loss and frequency of the on-grid PV inverter by incorporating a switching frequency term in the cost function of the model predictive control (MPC). In the proposed MPC, the control objectives (current and switching frequency) select an optimal switching state for the inverter by minimizing a predefined cost function. The two control objectives are combined with a weighting factor. A trade-off between the switching frequency (average) and total harmonic distortion (THD) of the current was utilized to determine the value of the weighting factor. The switching, conduction, and harmonic losses were determined at the selected value of the weighting factor for both the proposed and conventional FCS-MPC and compared. The system was simulated in MATLAB/Simulink, and a small-scale hardware prototype was built to realize the system and verify the proposal. Considering only 0.25% more current THD, the switching frequency and loss per phase were reduced by 20.62% and 19.78%, respectively. The instantaneous overall power loss was also reduced by 2% due to the addition of a switching frequency term in the cost function which ensures a satisfactory empirical result for an on-grid PV inverter

Centralized Thermal Stress Oriented Dispatch Strategy for Paralleled Grid-Connected Inverters Considering Mission Profiles

One of the major failure causes in the power modules comes from the severe thermal stress in power semiconductor devices. Recently, some local control level methods have been developed to balance the power loss, dealing with the harsh mission profile, in order to reduce the thermal stress. However, there is not any specific system level strategy to leverage these local control level methods responding to the multiple inverters situation. Besides, the impacts of these methods on the thermal cycle and lifetime of the power modules in the long-term time scale have not been evaluated and compared yet. Hence, in this article, a centralized thermal stress oriented dispatch (TSOD) strategy is proposed to take full advantage of these local control level methods, including the switching frequency variation and the reactive power injection, to reduce the thermal stresses for multiple inverters. In addition to the PI controller, the finite control set model predictive control (FCS-MPC) is also explored to synergize with the proposed strategy. The results from the real-time model-in-the-loop testing on a four-paralleled-inverters platform, the reliability assessment, and the experiments all validate the effectiveness of the proposed centralized TSOD strategy on the thermal stress reduction

Real-time model-based loss minimisation control for electric vehicle drives

PhD ThesisEnvironmental concern and the opportunity for commercial gain are two factors driving the expansion of the electric vehicle (EV) market. Due to the limitations of current battery technology, the efficiency of the traction drive, which includes the electric motor and power electronic converter, is of prime importance. Whilst electric machines utilising permanent magnets (PMs) are popular due to their high energy density, industry concerns about the security of supply have led to interest in magnet-free solutions. Induction machines (IMs) offer such an option. Control of IMs is a mature but complex field. Many techniques for optimising the efficiency of the drive system have been proposed. The vast majority of these methods involve an analytical study of the system to reveal relationships between the controlled variable and efficiency, allowing the latter to be optimised. This inevitably involves simplifications of the problem to arrive at a practically-implementable control scheme. What has not been investigated is real-time calculation of the system losses in order to optimise the efficiency, and the work presented in this thesis attempts to achieve this. The conventional control scheme is examined and a new structure implemented where a model of the system loss is able to directly influence the switching action of the inverter, thus reducing loss. The need to maintain performance alongside loss minimisation is recognised and a cost function-based solution proposed. The validation of this structure is performed both in simulation and on a practical test platform. A model of the principle losses in the drive system is derived, taking into account the processing power typically available for this application, and implemented in the structure outlined. The effect of the new control scheme on efficiency is investigated and results show gains of up to 3%-points are achievable under certain conditions