Condition Monitoring of Power Electronic Systems through Data Analysis of Measurement Signals and Control Output Variables

A major disadvantage of existing condition monitoring methods is the need for additional sensors and measuring equipment. In this work, this disadvantage is eliminated by completely avoiding additional hardware. Instead, software-based methods from the field of machine learning are used. Therefore, measurement signals and control output variables are utilized which are acquired and processed in any power electronic system for the purpose of converter control. The publication focuses on two main converter components: power semiconductors and DC link capacitor. For each component, the aging mechanisms that have been studied in the literature are explained. Based on the aging mechanisms, the degradation indicators are identified. Then, a converter model is built that allows the variation of degradation indicators in order to analyze their effects on the available data set. These findings form the basis for mathematical models, which detect future failure mechanisms of this type during converter operation. The test setup must offer the possibility of generating reproducible failure cases in various components with the aid of additional failure equipment. Finally, failure mechanisms are intentionally introduced at the test bench in order to validate the methodology of the developed approach

Improving fault tolerant drives for aerospace applications

D EngThe aerospace industry is moving towards the more electric aeroplane where traditional hydraulic systems are being replaced with electrical systems. Electrical technology offers some strong advantages compared to hydraulic technology including; cost, efficiency, power on demand and relative ease of maintenance. As with most new technologies, a major disadvantage is its limited reliability history. A lot of research in the aerospace field therefore focuses on improving fault tolerant electrical systems. Work done in this thesis builds on an existing fault tolerant drive, developed by Newcastle University and Goodrich Actuation Systems as part of the ELGEAR (Electrical Landing Gear) project. The purpose of this work is to continue improving the drive’s fault tolerant features; especially in areas where the drive is most vulnerable. The first part of this thesis focuses on improving the overall system reliability by monitoring the health of the dc-link capacitors in the fault tolerant drive. The implemented estimation technique makes use of voltage and current sensors which are already in place for protection and control purposes. The novel aspect of the proposed technique relates to monitoring capacitors in real time whilst the motor is operational. No external interferences, such as injected signals or special operation of the drive, are required. The condition monitoring system is independent of torque and speed, and hence independent of a variation in load. The work was validated using analytical methods, simulation, low voltage experimentation and high voltage implementation on the ELGEAR drive. The second part of this thesis focuses on single shorted turn faults, in fault tolerant permanent magnet (PM) motors. Despite the motor being able to withstand a wide range of faults, the single shorted turn fault remains a difficult fault to detect and handle. The problem arises from the magnets on the spinning rotor that cannot be ‘turned off’ at will. This thesis investigates the severity of the faulted current in a shorted turn and how it varies depending on the turn’s location in the stator slot. The severity of the fault is studied using 2D finite element analysis and practical implementation on the ELGEAR rig. Finally, recommendations are proposed for improving the ELGEAR motor for future fault tolerant designs.EPRSC and Goodrich Aerospace (now United Technologies

Modular multilevel converter : submodule dimensioning, testing method, and topology innovation

The modular multilevel converter (MMC) is being developed as a core technology for the next generation of high-power, voltage source converters (VSCs). The focus of this thesis lies in the SM dimensioning, testing method and topology innovation for the MMC. First, the thesis presents a new submodule (SM) capacitor selection method, considering the three main voltage requirements: the maximum capacitor voltage, the voltage ripple and the SM voltage capability. The effect of the arm inductor is included. A quick way to estimate the capacitor ripple current stress is also provided to check the selection. Second, the thesis proposes two model assisted SM testing schemes for the MMC. The prototype SM can be thoroughly tested according to the targeted operating modes without having to build a complete MMC. During the test, the converter arm current can be faithfully achieved, which contains not only the fundamental frequency component, but also dc offset and harmonic circulating current components. One scheme is the uncompensated testing scheme, which uses fewer devices, and has simpler control and faster transient dynamics. The other is the compensated testing scheme, which requires much lower dc supply voltage, smaller coupling inductance, and provides higher current tracking accuracy in steady state. Both testing schemes have been verified through simulation and experiments. Third, the thesis proposes a compact SM topology for the MMC based on stacked switched capacitor (SSC) architecture. Feasibility study shows that the total physical volume of all capacitors in each SM can be reduced by more than 40% without significantly increasing the power loss. Design concept and control principles are presented. Practical considerations for a high-voltage, high-power system are also provided, which are demonstrated through experiments on a scaled down laboratory prototype SM. Finally, this thesis evaluates the offshore 50/3 Hz ac power transmission and the use of back-to-back (B2B) MMC for frequency conversion. The high-level design of a B2B MMC is presented. System performance is briefly evaluated using computer simulation

Thermally-Compensated Modulation Strategies for Modular Power Converters

Power electronics converters are employed in various fields to take the advantages. Especially, a modular structure is getting attention, which exhibits a constitution of multiple basic cells rated for a lower power. Nevertheless, the reliability issue has been intensively argued due to the higher number of power semiconductor devices and capacitors. It has been revealed that the thermal stresses are highly responsible for the major failures of power semiconductor devices. The active thermal control methods have been considered, since they can be simply adopted by a software. However, the existing methods intend to increase the losses for compensating the temperature swing, which implies a limited lifetime improvement and a decreased efficiency. Considering the modular converter consisting of modular inverter and DC/DC converter, two active thermal control methods are proposed: multi-frequency and discontinuous modulations, which aims at controlling a loading power of each module. The DC/DC converters connected in parallel can have a different loading according to their remaining useful lifetime. Remarkably, the discontinuous modulation allows to improve the reliability of both DC/DC converters and inverter. Electrolytic capacitors are degraded due to the thermal stresses, which are determined with ESR and capacitor current. Therefore, the major task is to reduce the capacitor current. The optimal DC-link design approach has been introduced in literature. However, a clear relation between current frequency and thermal stresses has not been investigated. The power converter for deriving the relation is proposed, which injects a designated AC current and DC bias voltage to capacitor under test. The multi-level converter is most suitable with allowing lower THD and lower rated power devices. Also, the proposed control scheme simultaneously regulates two parameters. Finally, the correlation between the current frequency and the thermal stresses is experimentally derived