Health Condition Monitoring and Fault-Tolerant Operation of Adjustable Speed Drives

Adjustable speed drives (ASDs) have been extensively used in industrial applications over the past few decades because of their benefits of energy saving and control flexibilities. However, the wider penetration of ASD systems into industrial applications is hindered by the lack of health monitoring and fault-tolerant operation techniques, especially in safety-critical applications. In this dissertation, a comprehensive portfolio of health condition monitoring and fault-tolerant operation strategies is developed and implemented for multilevel neutral-point-clamped (NPC) power converters in ASDs. Simulations and experiments show that these techniques can improve power cycling lifetime of power transistors, on-line diagnosis of switch faults, and fault-tolerant capabilities.The first contribution of this dissertation is the development of a lifetime improvement Pulse Width Modulation (PWM) method which can significantly extend the power cycling lifetime of Insulated Gate Bipolar Transistors (IGBTs) in NPC inverters operating at low frequencies. This PWM method is achieved by injecting a zero-sequence signal with a frequency higher than that of the IGBT junction-to-case thermal time constants. This, in turn, lowers IGBT junction temperatures at low output frequencies. Thermal models, simulation and experimental verifications are carried out to confirm the effectiveness of this PWM method. As a second contribution of this dissertation, a novel on-line diagnostic method is developed for electronic switch faults in power converters. Targeted at three-level NPC converters, this diagnostic method can diagnose any IGBT faults by utilizing the information on the dc-bus neutral-point current and switching states. This diagnostic method only requires one additional current sensor for sensing the neutral-point current. Simulation and experimental results verified the efficacy of this diagnostic method.The third contribution consists of the development and implementation of a fault-tolerant topology for T-Type NPC power converters. In this fault-tolerant topology, one additional phase leg is added to the original T-Type NPC converter. In addition to providing a fault-tolerant solution to certain switch faults in the converter, this fault-tolerant topology can share the overload current with the original phase legs, thus increasing the overload capabilities of the power converters. A lab-scale 30-kVA ASD based on this proposed topology is implemented and the experimental results verified its benefits

A Fault-Tolerant T-Type Multilevel Inverter Topology with Soft-Switching Capability Based on Si and SiC Hybrid Phase Legs

The performance of a novel three-phase four-leg fault-tolerant T-Type inverter topology is presented in this paper, which significantly improves the inverter\u27s fault-tolerant capability regarding device switch faults. In this new modular inverter topology, only the redundant leg is composed of Silicon Carbide (SiC) power devices and all other phase legs are constituted by Silicon (Si) devices. The addition of the redundant leg, not only provides fault-tolerant solution to switch faults that could occur in the T-Type inverter, but also can share load current with other phase legs. Moreover, quasi zero-voltage switching (ZVS) and zero-current switching (ZCS) in the Si Insulated-Gate Bipolar Transistors (IGBTs) of the main phase legs can be achieved with the assistance of SiC Metal-Oxide Semiconductor Field-Effect Transistors (MOSFETs) in the redundant leg. Simulation and experimental results are given to verify the efficacy and merits of this high-performance fault-tolerant inverter topology

A Fault-Tolerant Control Strategy for Three-level Neutral-Point Clamped (NPC) Inverter

When the open-circuit fault occurs on neutral-point clamped (NPC) inverters, it can cause the distortion of output currents, excessively large fluctuation of output voltages, and unbalanced neutral-point potential. Currently, several existing fault diagnosis methods are able to identify the fault types, but they are unable to identify the switching devices where the open-circuit faults occur. Furthermore, even if the open-circuit faults can be identified, the implications caused by open-circuit faults need to be further addressed. This thesis is focused on diagnosing the open-circuit faults and mitigating the problems caused by these faults. An effective fault diagnosis method is proposed in his study to identify the switching devices where the open-circuit faults occur. The characteristics of different distorted currents are firstly analyzed. By normalizing the sampling currents, calculating the average currents and comparing them with thresholds, different faulty switching devices can all be identified. To address the issues caused by these faults, a fault-tolerant control strategy is proposed. By replacing the impossible space vectors caused by these faults with other vectors which can generate the same output voltages, the problems of distortion of output currents and excessively large fluctuation of output voltages can be solved. But the excessively large difference of neutral-point potential still exists. To address the above issue, a model predictive control strategy is proposed to solve the problem of excessively large difference of neutral-point potential. A cost function is built to track the reference vector and suppress the difference of neutral-point potential. The Lagrange function is used to solve the action times of different space vectors. Furthermore, it is shown that when the neutral-point potential is initially unbalanced, the developed strategy is still able to make the potential balanced and greatly reduce its fluctuation

A Fault-Tolerant T-Type Multilevel Inverter Topology With Increased Overload Capability and Soft-Switching Characteristics

he performance of a novel three-phase four-leg fault-tolerant T-type inverter topology is introduced in this paper. This inverter topology provides a fault-tolerant solution to any open-circuit and certain short-circuit switching faults in the power devices. During any of the fault-tolerant operation modes for these device faults, there is no derating required in the inverter output voltage or output power. In addition, overload capability is increased in this new T-type inverter compared to that in the conventional three-level T-type inverter. Such increase in inverter overload capability is due to the utilization of the redundant leg for overload current sharing with other main phase legs under healthy condition. Moreover, if the redundant phase leg is composed of silicon carbide metal-oxide-semiconductor field-effect transistors, quasi-zero-voltage switching, and zero-current switching of the silicon insulated-gate bipolar transistors (IGBTs) in the conventional main phase legs can be achieved at certain switching states, which can significantly relieve the thermal stress on the outer IGBTs and improve the whole inverter efficiency. Simulation and experimental results are given to verify the efficacy and merits of this high-performance fault-tolerant T-type inverter topology

Multidimensional two-level multiphase space vector PWM algorithm and its comparison with multifrequency space vector PWM method

A multilevel multiphase space vector pulsewidth modulation (SVPWM) algorithm has been introduced recently, in which the reference is separated into an integer part and a fractional part. The fractional part is, in essence, a two-level multiphase space vector algorithm. This paper shows that, with appropriate adaptations, the fractional part of the general space vector multilevel multiphase PWM can be applied as a stand-alone PWM method in conjunction with two-level voltage-source converters with any number of phases. Simulation results of the five- and six-phase cases are shown, and the new algorithm is compared with another recent multifrequency SVPWM algorithm, which follows the standard approach of selecting the switching vectors and calculating their application times using planes. The experimental verification is provided using a five-phase two-motor series-connected induction motor drive, supplied from a custom-designed five-phase voltage-source inverter.Ministerio de Ciencia e Innovación | Ref. DPI2009-0700

Study and comparison of discontinuous modulation for modular multilevel converters in motor drive applications

Discontinuous modulation applied to modular multilevel converters is an effective method for reducing the capacitor voltage ripples. In this paper, the discontinuous modulation is adapted and used in a motor drive application. For proper operation of the converter, a new energy controller is presented, which is suitable for operation with nonsinusoidal reference signals. Experimental results comparing the discontinuous modulation with other techniques operating at low motor speeds are shown. The results demonstrate the effectiveness of the discontinuous modulation on reducing capacitor voltage ripples and power losses.Postprint (published version

Power Quality Enhancement in Electricity Grids with Wind Energy Using Multicell Converters and Energy Storage

In recent years, the wind power industry is experiencing a rapid growth and more wind farms with larger size wind turbines are being connected to the power system. While this contributes to the overall security of electricity supply, large-scale deployment of wind energy into the grid also presents many technical challenges. Most of these challenges are one way or another, related to the variability and intermittent nature of wind and affect the power quality of the distribution grid. Power quality relates to factors that cause variations in the voltage level and frequency as well as distortion in the voltage and current waveforms due to wind variability which produces both harmonics and inter-harmonics. The main motivation behind work is to propose a new topology of the static AC/DC/AC multicell converter to improve the power quality in grid-connected wind energy conversion systems. Serial switching cells have the ability to achieve a high power with lower-size components and improve the voltage waveforms at the input and output of the converter by increasing the number of cells. Furthermore, a battery energy storage system is included and a power management strategy is designed to ensure the continuity of power supply and consequently the autonomy of the proposed system. The simulation results are presented for a 149.2 kW wind turbine induction generator system and the results obtained demonstrate the reduced harmonics, improved transient response, and reference tracking of the voltage output of the wind energy conversion system.Peer reviewedFinal Accepted Versio

Multilevel multiphase space vector PWM algorithm with switching state redundancy

Multilevel multiphase technology combines the benefits of multilevel converters and multiphase machines. Nevertheless, new modulation techniques must be developed to take advantage of multilevel multiphase converters. In this paper, a new space vector pulsewidth modulation algorithm for multilevel multiphase voltage source converters with switching state redundancy is introduced. As in three-phase converters, the switching state redundancy permits to achieve different goals like extending the modulation index and reducing the number of switchings. This new algorithm can be applied to the most usual multilevel topologies; it has low computational complexity, and it is suitable for hardware implementations. Finally, the algorithm was implemented in a field-programmable gate array, and it was tested by using a five-level five-phase inverter feeding a motor.Ministerio de Educación y Ciencia | Ref. ENE2006-0293

Experimental Investigation of Decoupled Discontinuous PWM Strategies in Open-End Winding Induction Motor Supplied by a Common DC-link

© 2023 IEEE. This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1109/JESTPE.2023.3258799Currently, open-end winding induction motors fed by a dual inverter (OEWIM-DI) present an innovative approach to enhance the performance of modern electric drive systems, such as electrical vehicles and electric aircraft applications. However, the DI topology requires a proper switching control strategy to enable the OEWIM drive to fully achieve its performance. This work aims to investigate experimentally the impact of different decoupled discontinuous pulsewidth modulation (DDPWM) control strategies on the performance of the OEWIM-DI supplied by a common dc-link. The criteria performances adopted in this study are: 1) the total harmonic distortion (THD) of the current and voltage; 2) the zero sequence voltage (ZSV); 3) the common mode voltage (CMV); and 4) the DI losses. The various DDPWM control schemes for the 1.5-kW OEWIM-DI motor drive are implemented on a dSPACE 1104 board, and the results are compared with the popular and widely used space-vector PWM (SVPWM) strategy. From the results, it can be concluded that the optimized DDPWM technique gives the best performance. This technique has reduced the CMV by one level and reduces the losses by 50% while having the same THD and ZSV obtained with the SVPWM technique.Peer reviewe