Addressing control and capacitor voltage regulation challenges in multilevel power electronic converters

Multilevel power electronic converters are the current industry solutions for applications that demand medium voltage, reasonable efficiency, and high power quality. The proper operation of these types of power converters requires special control, modulation methods, and capacitor voltage regulation techniques. Both developing capacitor voltage regulation methods and addressing their associated issues with such fall within the primary focus of this dissertation. In this dissertation an investigation was conducted on the capacitor voltage regulation constraints in cascaded H-bridge multilevel converters with a staircase output voltage waveform. In the proposed method, the harmonic elimination technique is used to determine the switching angles. A constraint was then derived to identify modulation those indices that lead to voltage regulation of the capacitor. This constraint can be used in optimization problems for harmonic minimization to guarantee capacitor voltage regulation in these types of converters. Furthermore, a capacitor voltage regulation method was developed using redundant state selection for a flying capacitor active rectifier. This method reduces the number of switching instances by using both online and offline state selection procedure. Additionally, a start-up procedure is proposed that pre-charges the all of capacitors in the rectifier to both avoid overstressing the switches and obtain a smoother start-up. Finally, a flexible capacitor voltage regulation method is proposed that provides the ability to control the voltage of the capacitors in both cascaded H-bridge and hybrid multilevel converters. In this method, the capacitor voltage in each individual H-bridge cell is independently regulated by controlling the active power of each cell

Impacts of COVID-19 Pandemic on Micro and Small Enterprises: Evidence From Rural Areas of Iran.

peer reviewedSince 2020, the outbreak of the COVID-19 crisis has caused a great deal of social and economic damages to micro and small-scale enterprises (MSEs). This research examined the most common damages of this crisis in active and inactive rural MSEs and also assessed different kind of responses the managers and owners of theses MSEs have received dealing with these damages. The sample population of this study consisted of all managers of 72 active and 38 closed rural MSEs in the Dastjerd village, Hamedan, Iran. These MSEs were mainly garment small factories. This research utilized a mixed approach (quantitative-qualitative) to study the research objectives in depth. First, in qualitative part, semi-constructed interviews and field visits were done. Then, using quantitative, results of the qualitative section, previous studies and the existing literature, a researcher-made questionnaire was created. Based on qualitative part information through interviews, damages of rural MSEs during COVID-19 pandemic were categorized into three classes, including damages related to production, and financial and marketing issues. Also, two categories of managers' responses that could be labeled as passive and adaptive behavior were identified. Findings showed that active rural MSEs have taken more adaptive measures and tried to find appropriate ways to reduce or overcome damages. Active MSEs were mainly owned and managed collaboratively by more literate and experienced managers. Also results revealed that rural MSEs' managers reacted to different kinds of damages based on their ability, knowledge, and experience. Based on research results, managers' knowledge and skills can help them find more adaptive solutions to keep the firms stable and overcome damages. It can be concluded that COVID-19 pandemic has a great impact on rural MSEs and they need more financial support and managerial advice to overcome this kind of crisis situation

Investigation on Capacitor Voltage Regulation in Cascaded H-Bridge Multilevel Converters with Fundamental Frequency Switching

Multilevel power electronic converters have gained popularity in high-power applications due to their lower switch voltage stress and modularity. Cascaded H-bridge converters are a promising breed of multilevel converters which generally require several separate dc voltage sources. By utilizing the redundant switching states, it is possible to replace the separate dc voltage sources with capacitors and keep only the one with the highest voltage level. Redundancy in the charge and discharge modes of the capacitors is assumed to be adequate for their voltage regulation. However, the effects of the output current of the converter as well as the time duration of the redundant switching states have been neglected. In this paper, the impacts of the connected load to the cascaded H-bridge converter as well as the switching angles on the voltage regulation of the capacitors are studied. This paper proves that voltage regulation is only attainable in a much limited operating conditions that it was originally reported. In addition, based on the analysis of the converter, a simplified formula is found which can be used to find those modulation indices that regulate the voltage of the capacitor. This formula can be used in harmonic minimization problems while capacitor voltage regulation is ensured. Simulation and laboratory results are provided to confirm the analysis

Cultural Bases of Jess Castle and Kenesht Majosan in Islamic Early Centuries

With entering Islam to Iran, the pre-Islamic culture and history was preserved by some Iranian groups that loved knowledge and that were lived in west of Kohgilouyeh and east of Arjan around Jess castle. The purpose of this study is to investigate the scientific and cultural status of Jess castle. It had an important role in maintaining history of pre-Islamic kings, Iranian costoms and Zoroaster’s science, and is one of most important occupied castles by Nezari Ismaeilies in 5th century. In this study, it has been tried to answer two fundamental questions. Firstly, what is the exact situation of Jess castle and Kenesht Majosans of Arjan? And secondly, what was the situation of Jess castle in cultural and scientific history of Iran? The method of investigation was historical-analytical and the conclusions of the study were drawn by field investigation and librarian study and with the use of archeological, linguistical data and ancient written sources. Jess castle, which is called "dez castle" by residents of the region, had been the place of maintenance and teaching of historical and scientific books of Iran (especially kings of pre-Islamic period). Strategical situation of this region such as occurance on the road of Arjan to Isfahan, and existence of Zoroaster’s groups around the castle are most important reasons of being occupied by Esmaeilies. Around this castle, there is Kenesht Majosan that its place is buildings called "se gonbadan", and these two places can be mentioned as "cultural and scientific bases"

Fault Recovery Strategy for Hybrid Cascaded H-Bridge Multi-Level Inverters

In the configuration of the multilevel inverter which is used in this study, two cascaded H-bridge cells are connected in series with each phase of a three-phase three-level neutralpoint clamped (NPC) inverter. The NPC inverter is fed by a single DC source; whereas, all of the cascaded H-bridge cells are supplied by capacitors. In this paper, in order to regulate the voltage across the H-bridge capacitors, a small fundamental harmonic is added to or subtracted from the original PWM reference of each H-bridge cell. In addition, the operation of the inverter during a fault in the cascaded H-bridge cells is studied and a method for recovery from fault and compensation of fault using the remaining H-bridge in the corresponding phase is proposed. This method allows acceptable operation of the inverter even when one of H-bridge cells is not functioning. Verification of the method using simulation shows the proper operation of the voltage capacitor regulation and strategy of fault recovery

A Seven-Level Converter Using a Combination of Staircase and PWM Switching Methods

In this paper, a new method is proposed for switching a multilevel converter. In this converter two H-Bridge cells are connected in series with each phase of a three-phase diode-clamped converter. Operation of main converter and one of H-Bridge cells is based on the staircase switching method. Switching angles of the three phase converter and one of the H-Bridges are selected in a way to generate the fundamental harmonic of the phase and eliminate the fifth and seventh harmonics. However, the switching pattern of the second H-Bridge cell is based on PWM method. This cell generates the remaining parts of the desired sinusoidal waveform. Combination of these converters and their switching methods results in an output waveform with low harmonics. Simulation and experimental results agree with each other and show the proper operation of the proposed method

Power Factor Correction Using Projected Cross Point Control (PCPC)

Projected cross point control (PCPC) is a new current-mode control approach which has been recently introduced. The PCPC method benefits from the advantages of traditional fixed-frequency and variable-frequency current-mode control methods. For instance, while PCPC is a fixed-frequency approach, it is stable for the entire range of the duty cycle similar to the variable-frequency hysteresis current-mode control (HCMC) scheme. In this paper, PCPC is used as the control strategy in a power factor correction (PFC) application. The results are compared with PFC methods based on the peak current-mode control (PCMC) and average current-mode control (ACMC) techniques

Fault Detection and Mitigation in Multilevel Converter STATCOMs

Many static synchronous compensators (STATCOMs) utilize multilevel converters due to the following: 1) lower harmonic injection into the power system; 2) decreased stress on the electronic components due to decreased voltages; and 3) lower switching losses. One disadvantage, however, is the increased likelihood of a switch failure due to the increased number of switches in a multilevel converter. A single switch failure, however, does not necessarily force an (2n + 1)-level STATCOM offline. Even with a reduced number of switches, a STATCOM can still provide a significant range of control by removing the module of the faulted switch and continuing with (2n - 1) levels. This paper introduces an approach to detect the existence of the faulted switch, identify which switch is faulty, and reconfigure the STATCOM. This approach is illustrated on an eleven-level STATCOM and the effect on the dynamic performance and the total harmonic distortion (THD) is analyzed