Fault Tolerant Multilevel Inverter Topologies with Energy Balancing Capability: Photovoltaic Application

The continuous increase in energy demand and depletion of conventional resources motivates the research towards the environment friendly renewable energy sources like solar and wind energy. These sources are best suitable for rural, urban and offshore locations, because of easy installation, less running cost and ample resources (sun light and wind). The remote locations are mostly islanded in nature and far away from technical expertise in case of troubleshooting. This motivates the research on development of fault tolerant converters. These fault tolerant converters increases the reliability, which provides the continuous power supply to critical loads. From the last few decades, the integration of multilevel inverters with renewable energy systems is also increasing because of advantages like, improved power quality, total harmonic distortion (THD) and reduced output filter size requirement. Employing conventional multilevel inverters for increasing the number of voltage levels increases the device count and isolated DC sources. As a result probability of semiconductor switch failure is more and energy balancing issue between sources, which in-turn degrades the reliability and performance of the inverter. The majority of conventional multilevel inverter topologies cannot address energy balancing issues between multiple photovoltaic (PV) sources, which may need because of partial shading, hotspots, uneven charging and discharging of associated batteries etc. If energy sharing not addressed effectively, the batteries which are connected to the shaded or faulty PV system will discharge faster which may cause total system shutdown and leads to under-utilization of healthier part of the system. To address these issues, fault tolerant multilevel inverter topologies with energy balancing capability are presented in this thesis. The major contributions of the proposed work are Single phase and three phase fault tolerant multilevel inverter topologies. viii Energy balancing between sources and dc off set minimization (or batteries) due to uneven charging and discharging of batteries for five-level inverter. Extending the fault tolerance and energy balancing for higher number of voltage levels. The first work of this thesis is focused to develop fault tolerant single phase and three phase multilevel inverter topologies for grid independent photovoltaic systems. The topologies are formed by using three-level and two-level half bridge inverters. The topology fed with multiple voltage sources formed by separate PV strings with MPPT charge controllers and associated batteries. Here the topologies are analyzed for different switch open circuit and/or source failures. The switching redundancy of the proposed inverters is utilized during fault condition for supplying power with lower voltage level so that critical loads are not affected. In general, the power generation in the individual PV systems may not be same at all the times, because of partial shading, local hotspots, wrong maximum power point tracking, dirt accumulation, aging etc. To address this issue energy balancing between individual sources is taken care with the help of redundant switching combinations of proposed five-level inverter carried out in second work. Because of partial shading the associated batteries with these panels will charge and discharge unevenly, which results voltage difference between terminal voltages of sources because of SOC difference. The energy balance between batteries is achieved for all operating conditions by selecting appropriate switching combination. For example during partial shading the associated battery with low SOC is discharged at slower rate than the battery with more SOC until both SOC’s are equal. This also helps in minimization of DC offset into the ac side output voltage. The mathematical analysis is presented for possible percentage of energy shared to load by both the sources during each voltage level. The third work provides single phase multilevel inverter with improved fault tolerance in terms of switch open circuit failures and energy balancing between sources. Generally multilevel inverters for photovoltaic (PV) applications are fed ix with multiple voltage sources. For majority of the multilevel inverters the load shared to individual voltage sources is not equal due to inverter structure and switching combination. This leads to under-utilization of the voltage sources. To address this issue optimal PV module distribution for multilevel inverters is proposed. Mathematical analysis is carried out for optimal sharing of PV resources for each voltage source. The proposed source distribution strategy ensures better utilization of each voltage source, as well as minimizes the control complexity for energy balancing issues. This topology requires four isolated DC-sources with a voltage magnitude of Vdc/4 (where Vdc is the voltage requirement for the conventional NPC multilevel inverter). These isolated DC voltage sources are realized with multiple PV strings. The operation of proposed multilevel single phase inverter is analyzed for different switch open-circuit failures. All the presented topologies are simulated using MATLAB/Simulink and the results are verified with laboratory prototyp

Moisture Stress Assessment through NDVI and Climate Tools for Crop Management at Anantpur District, AP

Anantapur is the driest district of Andhra Pradesh and hence, agriculture conditions are very often precarious. Groundnut grows where other crops fail and thus it is the predominant crop of Anantapur district. Groundnut is grown in about 7.5 lakh ha in Anantapur district; however the average yields are low af 500 kg ha-1. Among various available vegetation indices, normalised difference vegetation index (ND VI) is widely used for all reasons, which is a single numerical indicator of presence and condition of green vegetation. ND VI mapping at a regional scale helps to assess the spatial changes in the vigour of green vegetation and thus occurrence of any moisture stress. Climate tOQls have a great role in understanding the crop performance and estimating the yields. This study was taken up by using freely available MODIS data to understand ND VI in terms of abiotic stresses over Anantapur district and linking with the actual rainfall conditions, groundnut crop acreage and production. Results indicate that low groundnut yields in general,could be related to ND VI-based stress measurements and rainfall quantum and distribution in the area however, with a few exceptions. It is hoped that by combining improved practices through IWM with climate-adapted crop varieties, rainfed farmers of Anantapur district can sustain their crop production under present climate variability and become resilient to future climate chang

Cause of Death and Predictors of All-Cause Mortality in Anticoagulated Patients With Nonvalvular Atrial Fibrillation : Data From ROCKET AF

M. Kaste on työryhmän ROCKET AF Steering Comm jäsen.Background-Atrial fibrillation is associated with higher mortality. Identification of causes of death and contemporary risk factors for all-cause mortality may guide interventions. Methods and Results-In the Rivaroxaban Once Daily Oral Direct Factor Xa Inhibition Compared with Vitamin K Antagonism for Prevention of Stroke and Embolism Trial in Atrial Fibrillation (ROCKET AF) study, patients with nonvalvular atrial fibrillation were randomized to rivaroxaban or dose-adjusted warfarin. Cox proportional hazards regression with backward elimination identified factors at randomization that were independently associated with all-cause mortality in the 14 171 participants in the intention-to-treat population. The median age was 73 years, and the mean CHADS(2) score was 3.5. Over 1.9 years of median follow-up, 1214 (8.6%) patients died. Kaplan-Meier mortality rates were 4.2% at 1 year and 8.9% at 2 years. The majority of classified deaths (1081) were cardiovascular (72%), whereas only 6% were nonhemorrhagic stroke or systemic embolism. No significant difference in all-cause mortality was observed between the rivaroxaban and warfarin arms (P=0.15). Heart failure (hazard ratio 1.51, 95% CI 1.33-1.70, P= 75 years (hazard ratio 1.69, 95% CI 1.51-1.90, P Conclusions-In a large population of patients anticoagulated for nonvalvular atrial fibrillation, approximate to 7 in 10 deaths were cardiovascular, whereasPeer reviewe

Building a tuberculosis-free world: The Lancet Commission on tuberculosis

___Key messages___ The Commission recommends five priority investments to achieve a tuberculosis-free world within a generation. These investments are designed to fulfil the mandate of the UN High Level Meeting on tuberculosis. In addition, they answer

A Fault-Tolerant Single-Phase Five-Level Inverter for Grid-Independent PV Systems

In this paper, a fault-tolerant single-phase five-level inverter configuration is proposed for photovoltaic (PV) generation systems. Conventional two-level inverters are popularly used in PV applications, but these inverters provide the output voltage with considerable harmonic content. One of the efficient ways to improve the power quality of PV generation systems is to replace a two-level inverter with a multilevel inverter. Conventional multilevel inverters reduce total harmonic distortion and filter requirements effectively, but it has limitations in terms of reliability due to increased device count and capacitor voltage balancing issues. Therefore, a fault-tolerant single-phase five-level inverter is presented, which is constructed by using a half-bridge two-level inverter, a three-level diode clamp inverter, and a bidirectional switch. The proposed inverter topology can tolerate the system faults due to failure of the source and/or switching devices with least modification in the switching combinations. It has less number of switching devices compared to conventional five-level inverters. The topology also has the energy-balancing capability between sources which helps in reducing uneven charge of batteries in case of partial shading or hotspots on one side of the PV panels. The proposed system under normal and faulty condition is simulated in MATLAB/Simulink environment, and results are verified with a laboratory prototype

A Three phase Five-Level Inverter with Fault Tolerant and Energy Balancing Capability for Photovoltaic Applications

A fault tolerant three phase five-level inverter configuration for the application of islanded photovoltaic (PV) systems is proposed. The fault tolerant five-level inverter configuration is achieved by stacking a two-level inverter, a three-level neutral point clamped inverter along with three bidirectional switches. This configuration is fed with two DC sources. The proposed configuration shows fault tolerant behavior towards switch open circuit fault in one or more inverter legs. In case of source failure it is able to utilize the healthier source for supplying uninterrupted power to essential loads. During fault the proposed fault tolerant inverter operates as three-level inverter with half of the rated power. The configuration also has the provision of energy balancing among two sources during partial shading or malfunctioning of PV system. The operation of the proposed inverter during different failure modes and energy balancing among two sources are discussed and verified using MATLAB/SIMULINK

A multi-level inverter configuration for 4n pole induction motor drive by using conventional two-level inverters

A multi-level inverter configuration for 4n pole induction motor drive is presented in this paper. This topology is developed using three two-level inverters and two isolated dc sources. Among these three two-level inverters one inverter is switched at fundamental frequency and remaining two inverters are switched at higher frequency. Two isolated dc sources with equal magnitude are used in this topology to block the path for zero sequence currents. Low frequency operated inverter is connected to one dc source and high frequency operated inverters are connected to other dc source. The magnitude of dc source voltage required in this topology is four times lesser than the dc source required in conventional five-level NPC inverter configuration. The first center band harmonics are cancelled by providing 1800 phase shift between the high frequency carrier signals. This topology can be operated during the failure of any one inverter switches or any one dc source which will increase the reliability of the system. The number of switching devices used in this configuration is considerably less compared to conventional five-level inverters. This configuration is best suitable for high power applications where low frequency operated inverter can use thyristors as switching devices. The proposed topology is simulated in MATLAB/Simulink with (5HP) four pole induction motor and results are presented to show the validity of this configuration

Pole-phase modulated multiphase induction motor drive with improved dc link utilization

Multiphase induction motor (MIM) drives are popular in the area of high power traction and ship propulsion for their advantages of higher efficiency, lower torque pulsations and greater fault tolerance. Further, using pole-phase modulation (PPM) for these MIM drives widens the speed-torque ranges effectively. But, using multiphase space vector pulse width modulation (SVPWM) to control these MIM drives yield lesser dc link voltage utilization (DLVU) compared to their three-phase (3-φ) counter parts. This is because the offset value added to the sine references in multiphase SVPWM is always lesser compared to the third harmonic order offset value in case of 3-φ SVPWM. Adding third harmonic order offset value even for multiphase sine references yields extra DLVU same as 3-φ SVPWM. But, this introduces dominant third harmonic order currents into the multiphase phase windings in a star connected multiphase drives. A simple and effective technique to avoid dominant lower order harmonic currents in to the phase windings by grouping them into three 3-φ groups is proposed in this paper. The proposed phase grouping technique is implemented on a pole phase modulated nine-phase induction motor (IM) having a pole ratio of 1∶3. Finite element model of nine-phase PPMIM is developed in ANSYS Maxwell two dimensional (2-D) and is fed from a two-level inverter in Simplorer circuit simulator. The proposed PPMMIM drive is experimentally validated for 5 hp nine-phase IM, fed from a two-level inverter. SPARTAN 6 FPGA board is used to implement the inverter control algorithm coded using VHDL

Multilevel inverter configuration for standalone photovoltaic generation system using three phase two-level inverters

In this paper a multilevel inverter configuration is proposed for standalone photovoltaic (PV) generation system. The multilevel inverter is configured using three two level three phase inverter and three phase transformer having multiple winding in primary side with appropriate turns ratio. The three phase inverters are fed from separate PV arrays with asymmetrical voltage and power rating to distribute the total generated power among load and battery storage units. To supply major load demand one of the PV array with associated inverter (main inverter) rated relatively higher power compared to remaining two inverters. To reduce the initial cost, battery backup are provided only to remaining two arrays. The main inverter is controlled by varying the modulation index in such a way that during excess generation at the associated array in addition to supplying the load the batteries associated with other PV arrays are also charged. The topology also has additional advantage of fault tolerant capability in terms of inverter switch open circuit fault and/or source failure. The configuration is validated by using MATLAB/Simulink and a laboratory prototype for various test results with and without fault conditions are presented