Analysis of Five Level Neutral Point Clamped Inverter Fed from 18 Pulse ‎Output Multiphase Rectifier for Grid Tied Applications

 تلعب محولات إلكترونيات الطاقة دورًا رئيسيًا في تطبيقات معالجة الطاقة المختلفة. في كثير من الحالات ، تنشأ الحاجة إلى استخدام محولين لتحقيق معالجة الطاقة أو التكييف أو كليهما. في هذا العمل ، تمت دراسة نظام محولين يعتمد على محولين متعدد الأطوار / متعدد المستويات. الهدف الرئيسي هو معالجة طاقة التيار المتناوب من مصدر يمكن أن يكون مصدر طاقة موزع (DER) عن طريق تحويله إلى تيار مستمر قبل حقنه في نظام تيار متردد آخر. يتم استخدام مقوم متعدد الأطوار ذو 18 نبضة للهدف الأول. يتم تحقيق مهمة المعالجة الثانية ، والتي تتضمن تحويل طاقة التيار المستمر إلى التيار المتناوب من خلال محول محايد ذو خمسة مستويات مثبتة .(NPC)  النتائج المبينة تحقق فوائد المعدل متعدد الأطوار في إنتاج جهد تيار مستمر سلس. ومن جانب أخر ، تتميز NPC ذات المستويات الخمسة بميزة مثيرة للاهتمام وهي ان الجهد الأساسي ذو قيمة عالية.Power electronics converters play a major role in various energy processing applications. In many instances the need arises to use two converters to achieve power processing, conditioning or both. In this work, two converter systems are studied, which is based on two multiphase/multilevel converters. The main objective is to process AC power from a source which can be a distributed energy source (DER) by converting it to DC before injecting it into another AC system. An 18 pulse multiphase rectifier is used for the first objective. The second processing task, which involves converting the DC power back to AC is achieved by five level neutral point clamped converter (NPC). Results verify the benefits of the multiphase rectifier in producing a smooth DC voltage. On the other side, the five level NPC revels an interesting feature of high fundamental voltage

Capacitor current analysis of a three-level neutral point clamped converter under unbalanced loading conditions

A neutral point clamped (NPC) converter is considered a forefront in industrial applications. Supplying a typically balanced stand-alone load is one of those applications. However, the loading may become unbalanced which can impact the capacitors’ current and voltage ripple. In this work, an approach is proposed that analyze the capacitor current under unbalanced loading. The proposed method is based on a combination of two-dimensional Fourier series and symmetrical components. Since, two-dimensional Fourier series sectorize the spectrum into harmonics that are either defined by multiples of, fundamental, carrier or both frequencies, the method derives the Fourier coefficients for each sector, corresponding to a sequence current. Therefore, based on the presented approach, each harmonic amplitude in the spectrum sector is defined by three Fourier coefficients reflecting zero, positive and negative sequence current. The capacitor current spectrum is obtained by vector summing sequence coefficients. The method is tested on unbalanced load conditions with (out) a ground path. Results verify feasibility of the proposed method in deriving Fourier coefficients of capacitor currents that accurately reflects the loading status of the NPC. This is vital for converter design in terms of proper sizing of DC capacitor and can help in avoiding components failure

Investigation of a three-phase forced-commutation series capacitor operating with variable-voltage and variable-frequency systems

PhD ThesisThis thesis investigates the use of a three-phase forced-commutation series capacitor (FCSC) for power factor correction in stand-alone variable-voltage variable-frequency (VVVF) systems. Two environmentally-friendly applications are chosen to represent the VVVF systems. The first application is a renewable energy resource as a direct drive wave energy conversion (WEC) buoys. The second application is the more electric aircraft (MEA). In such systems, permanent magnet PM generators are most commonly used. A generator-set generally consists of a three-phase generator connected directly to a conventional three-phase diode bridge rectifier for simplicity and cost reduction. Due to the high inherited inductance of the PM generators used in such applications, this configuration suffers from a poor power factor as a result of commutation overlap. Several controlled series compensator (CSC) topologies have been employed for decades in power systems where the voltage levels and frequency are fixed. However, in applications such as WEC and MEA, the voltage and frequency vary. Therefore, in this work, a variable switched capacitor is used in order to inject a capacitive reactance and therefore compensate the inductive reactance of the generator, which prevents power factor degradation. In a VVVF system, it is important to inject variable capacitive reactance since the inductive reactance changes with frequency variations. Five commonly used CSC circuits are compared and the FCSC is considered as the most suitable circuit topology which is able to cope with a range of frequency variations. This research mainly investigates the performance of the three-phase FCSC circuit when controlled by novel control strategy, in terms of power factor, output voltage, and output power under various load conditions, including constant and variable load. The harmonic content of the three-phase FCSC is also investigated in order to propose this topology for MEA power system. In an aerospace system, the power quality is required to meet high standards and harmonic distortion should not exceed the limited level set by aerospace industry authorities. Therefore, several types of conventional power factor corrector (PFC) are excluded from aerospace systems, due to the associated distortion levels. Preface Abstract iv In this thesis, a novel symmetrical duty cycle control (SDCC) scheme is proposed in order to qualify the three-phase FCSC converter to be employed in different ranges of frequency variation, including 1-3 Hz for wave energy and 50-500 Hz as part of aircraft frequencies. The approach is simple to implement, with no need for a sophisticated controller design. The switch duty cycle is a function of the supply frequency and this allows the FCSC circuit to cope with frequency variation. The modes of operation for both single and three-phase circuit topologies are presented. The three-phase FCSC circuit is designed and tested in the laboratory environment. The performance of the three-phase FCSC circuit when using SDCC is tested experimentally and assessed by comparison of its performance with that of the conventional three-phase diode bridge rectifier. Experimental and simulation results validate the capability of the three-phase FCSC- rectifier to improve the power factor to approximately unity in addition to increasing the output voltage and power at higher voltage and frequency values. However, only limited improvements are achieved at the lower values of the frequency spectrum.Ministry of Higher Education and Scientific Research of Ira