NEW APPROACHES FOR VERY SHORT-TERM STEADY-STATE ANALYSIS OF AN ELECTRICAL DISTRIBUTION SYSTEM WITH WIND FARMS

Distribution networks are undergoing radical changes due to the high level of penetration of dispersed generation. Dispersed generation systems require particular attention due to their incorporation of uncertain energy sources, such as wind farms, and due to the impacts that such sources have on the planning and operation of distribution networks. In particular, the foreseeable, extensive use of wind turbine generator units in the future requires that distribution system engineers properly account for their impacts on the system. Many new technical considerations must be addressed, including protection coordination, steady-state analysis, and power quality issues. This paper deals with the very short-term, steady-state analysis of a distribution system with wind farms, for which the time horizon of interest ranges from one hour to a few hours ahead. Several wind-forecasting methods are presented in order to obtain reliable input data for the steady-state analysis. Both deterministic and probabilistic methods were considered and used in performing deterministic and probabilistic load-flow analyses. Numerical applications on a 17-bus, medium-voltage, electrical distribution system with various wind farms connected at different busbars are presented and discusse

Stochastic RUL calculation enhanced with TDNN-based IGBT failure modeling

Power electronics are widely used in the transport and energy sectors. Hence, the reliability of these power electronic components is critical to reducing the maintenance cost of these assets. It is vital that the health of these components is monitored for increasing the safety and availability of a system. The aim of this paper is to develop a prognostic technique for estimating the remaining useful life (RUL) of power electronic components. There is a need for an efficient prognostic algorithm that is embeddable and able to support on-board real-time decision-making. A time delay neural network (TDNN) is used in the development of failure modes for an insulated gate bipolar transistor (IGBT). Initially, the time delay neural network is constructed from training IGBTs' ageing samples. A stochastic process is performed for the estimation results to compute the probability of the health state during the degradation process. The proposed TDNN fusion with a statistical approach benefits the probability distribution function by improving the accuracy of the results of the TDDN in RUL prediction. The RUL (i.e., mean and confidence bounds) is then calculated from the simulation of the estimated degradation states. The prognostic results are evaluated using root mean square error (RMSE) and relative accuracy (RA) prognostic evaluation metrics

Prognostics and health management of power electronics

Prognostics and health management (PHM) is a major tool enabling systems to evaluate their reliability in real-time operation. Despite ground-breaking advances in most engineering and scientific disciplines during the past decades, reliability engineering has not seen significant breakthroughs or noticeable advances. Therefore, self-awareness of the embedded system is also often required in the sense that the system should be able to assess its own health state and failure records, and those of its main components, and take action appropriately. This thesis presents a radically new prognostics approach to reliable system design that will revolutionise complex power electronic systems with robust prognostics capability enhanced Insulated Gate Bipolar Transistors (IGBT) in applications where reliability is significantly challenging and critical. The IGBT is considered as one of the components that is mainly damaged in converters and experiences a number of failure mechanisms, such as bond wire lift off, die attached solder crack, loose gate control voltage, etc. The resulting effects mentioned are complex. For instance, solder crack growth results in increasing the IGBT’s thermal junction which becomes a source of heat turns to wire bond lift off. As a result, the indication of this failure can be seen often in increasing on-state resistance relating to the voltage drop between on-state collector-emitter. On the other hand, hot carrier injection is increased due to electrical stress. Additionally, IGBTs are components that mainly work under high stress, temperature and power consumptions due to the higher range of load that these devices need to switch. This accelerates the degradation mechanism in the power switches in discrete fashion till reaches failure state which fail after several hundred cycles. To this end, exploiting failure mechanism knowledge of IGBTs and identifying failure parameter indication are background information of developing failure model and prognostics algorithm to calculate remaining useful life (RUL) along with ±10% confidence bounds. A number of various prognostics models have been developed for forecasting time to failure of IGBTs and the performance of the presented estimation models has been evaluated based on two different evaluation metrics. The results show significant improvement in health monitoring capability for power switches.Furthermore, the reliability of the power switch was calculated and conducted to fully describe health state of the converter and reconfigure the control parameter using adaptive algorithm under degradation and load mission limitation. As a result, the life expectancy of devices has been increased. These all allow condition-monitoring facilities to minimise stress levels and predict future failure which greatly reduces the likelihood of power switch failures in the first place

Development of reliability methodology for systems engineering. Volume I - Methodology - Analysis techniques and procedures Final report

Reliability analyses for equipment in design stag

Optimisation of High Reliability Integrated Motor Drives

The development of integrated motor drives (IMDs) with high volumetric power density and reliability are crucial for the continued development and adoption of electric vehicles (EV). The development of the wide bandgap (WBG) devices, especially Silicon Carbide (SiC) MOSFETs, enables new possibilities for traction drive systems. However, to maximise the benefits of SiC, the IMD design process, including passive component selection, control and thermal management should be optimised. This thesis goes through the initial major design steps in SiC power system design, from SiC device analysis and modelling to circuit design and electrothermal simulation of an IMD system. A novel approach to discrete SiC MOSFET selection, using a method of calculating performance based on experimental data, is described. Dynamic behaviour of a family of 1200 V MOSFETs is studied at temperatures up to 175 °C using a double pulse test to show the combined effect of the differences in internal design between MOSFETs with different current ratings. It is observed that the 30 mΩ MOSFET had a 24 % higher switching loss than a 140 mΩ at a 30 A load current. The study then goes on to compare the effect of switching frequency, paralleling of MOSFETs and the device type used to demonstrate the inverter design with the lowest power losses, which will equate to low temperatures and high lifetime. The novel methodology can find the optimal choice of MOSFET from the family, and number required through paralleling, for a circuit when given the load current, temperature and switching. Understanding the device interdependencies in a single family is utilised to also predict the relative performance between SiC MOSFETs from different manufacturers. An axial-flux permanent magnet synchronous motor (PMSM) driven by a three-phase SiC inverter is simulated in PLECS using experimentally validated MOSFET models chosen by the device selection methodology. Electrothermal analysis shows the influence of switching frequency, temperature, MOSFETs paralleling and DC-link capacitance on voltage ripple, total harmonic distortion, efficiency and MOSFET loss and temperature profiles. With a 60 % decrease in THD and 50 % increase in maximum MOSFET junction temperature when switching frequency is increased from 10 to 100 kHz. The high-temperature stress on the semiconductors due to close proximity with the ma- chine stator means reliability is an important consideration that is yet to be fully investigated in IMD optimisations. This study uses a lifetime model specific to the transistor package TO-247 in reliability optimisation for IMD for the first time. It requires detailed MOSFET simulation outputs to provide a highly accurate lifetime for discrete SiC MOSFETs. Both single and multi-objective optimisations of the volume and lifetime of the three- phase inverter are presented. The single objective optimisation demonstrates the minimum volume and the corresponding switching frequency and lifetime when between three and six MOSFETs are paralleled at a temperature range between 50 and 150 °C. Design constraints were set limiting the feasible switching frequency range to between 13 kHz because of THD and 118 kHz because of efficiency limits, corresponding to required DC-link capacitors of 520 and 55 μF respectively. Increases in temperature were found to further limit the maximum switching frequency and therefore increase the minimum volume of the inverter. A Pareto front identifies a range of possible solutions for the volume and lifetime of an inverter with six paralleled MOSFETs through the multi-objective objective procedure. Further analysis of these possible solutions identified a single optimal solution for the system, using a DC-link capacitance of 190 μF at 45 kHz, giving a combined volume of the capacitor and MOSFETs of 440 cm3 and a lifetime of 12,000 hours. Finally, the electrothermal analysis of a dual inverter driving a symmetric six-phase PMSM is presented with the benefits of modular multi-phase systems in IMDs summarised. Effect on performance of lower per-phase current, interleaving strategies and fault tolerance are analysed and compared to equivalent three-phase systems, for 60 kW and 120 kW operation. A novel method for lifetime prediction of systems with paralleled MOSFETs or fault tolerance capabilities considering incremental damage is developed based on TO-247 lifetime calculations from PLECS simulation, and component-level reliability profiles using Monte Carlo analysis. The dual inverter is used to model the system and implements control schemes for both single-phase and single inverter failure while maintaining the 4000 rpm and 140 Nm speed and torque requirements. A twofold increase in B10 lifetime of is observed when the effect of paralleled SiC MOSFETs prevents immediate system failure in a three-phase inverter. A computational fluid dynamics (CFD) and 3D finite element thermal model are designed to study the inverter behaviour based on the thermal analysis of its shared cooling plate with a 300 mm diameter axial flux PMSM. Concentric layout designs minimise the variation of junction temperatures to 5 °C and the effect of the flow rate and temperature of the coolant in the PMSM cold plate is presented between 5 and 30 l/min. The multi-objective optimisation procedure used to compare the dual inverter demonstrated it outperformed the three-phase inverter with 15 % smaller required DC-link capacitance, higher efficiency and increased lifetime in part due to its fault-tolerant nature. The optimal dual inverter considering the design constraints consists of four 40 μF KEMET film capacitors operating with a switching frequency of 46 kHz giving an inverter volume of 300 cm3 and a lifetime of 16.3 years, assuming 1000 hours of operation annually