Sharif Digital Repository

Today, with the development of power electronics and the emergence of various multilevel converters, possibility of operation at the voltage and powers higher than the rated value of components is provided. The reliability of these converters due to their high power is important. In this project, design of multilevel inverter with cascaded H-bridge structure have been studied for a medium voltage and high power 3.3 KV, 1MVA application, due to its advantages over other structures. This converter is composed of several cascaded low voltage full-bridge cells. Therefore, the converter is composed of many semiconductor elements that the reliability of them depends on the physical stresses such... 

One of the challenges that always exists in the field of Testing the performance of converters, uninterruptible power supplies (UPSs) , etc. is to provide various types of loads to be applied to these equipments and test the quality of their performance in various load conditions. Leading or Lagging Power Factor loads, harmonic loads and non-linear loads are mainly required in this investigation. The use of inductors, resistors, capacitors and other devices for these tests is an old solution. Their disadvantages include high price, large volume and very high power loss, which was the initial idea of making programmable loads. A current source converter with the ability to Sink and source the... 

The annual degradation in power quality causes significant losses to electricity consumers, particularly in sensitive industries. One of the most critical challenges associated with power quality is the voltage sags and swells in the grid. voltage sags and swells in the grid pose a major challenge, potentially damaging equipment and disrupting production, which can increase the costs of the final product. Moreover, the AC nature of grid voltage highlights the importance of developing AC-AC converters to mitigate issues arising from grid voltage sags and swells. The primary goal of the employed converter is to compensate for voltage amplitude, without concern for frequency control. Therefore,... 

In this work, design and control of three-phase four-switch rectifier with input power factor correction and operation under unbalance input voltage is investigated. To control of this converter, a modified space vector modulation (SVM) method is proposed able to omit effect of capacitors voltage ripple on output voltage and input current. A small signal model and required transfer function are calculated to control converter. Then appropriate controller is designed in dq space to achieve desired condition. Another controller is added to system to compensate capacitors dc voltage difference. A useful method to decomposition of positive and negative sequence of dq component is introduced.... 

The objective of this project is to design and control and optimize a single-stage three-phase three-switch unity power factor telecom rectifier. For this purpose, the topology of three-phase three-switch converter has been investigated for both isolated and un-isolated converters. Following a comprehensive investigation on the converter structure, the power dissipation factors have been detected and the quantity of the power dissipation has been calculated by analytical methods and it confirmed by simulation results. To avoid extra power losses some novel soft-switched structures for three-phase three-switched converters have been introduced. the proper operation of the soft-switched... 

In this project, the Line-Modulated Fixed Off-Time method is used as the current control strategy for a Power Factor Corrector (PFC) pre-regulator. A zero voltage switching PWM (ZVS-PWM) auxiliary circuit is configured to perform ZVS in the switches. To reduce conduction losses, the rectifier circuit and the PFC are combined to create bridgeless PFC and a new zero-voltage-switching is introduced for reduse switching loss. Soft commutation of the main switch with zero voltage is achieved without additional current stress. When main switch is turned off ,the voltage across the switch rises very slowly. when the current of the switch decays to zero the voltage across the switch is very low.A... 

Given the high rate of energy dissipation in electric urban rail lines, methods of saving energy in these systems have become a topic of interest. The use of energy saving equipment to save the energy wasted in braking (Because of the diode rectifier, thermal energy is usually wasted and is not reversible) can play an important role in reducing the energy loss. The role of energy storage equipment, from other aspects such as voltage regulation and increasing the system reliability is also remarkable. In this thesis, the various possible methods for saving energy in the Metro system are expressed.
The various types of storage devices with increased reuse of braking energy which is the... 

In urban rail transport systems such as subway, Trains are constantly accelerate and brake, so the braking system is of great importance in energy consumption and constitute about 30 to 40 percent of total energy consumption trains, thus retrieval braking energy will lead to significant savings in urban rail transport systems. In this paper, in the beginning, different storage equipments for storage braking energy have been studied and compared and finally Supercapacitors due to technical and economic features have been selected for the storge of equipment. These systems can be implemented in both stationary and onboard. For precise evaluation and select the appropriate storage system, by... 

In recent years Photovoltaic Panels (PV) are very wide spread. Due to AC working mode of many consumers produce DC voltage of PV must be converted to AC voltage by inverters. Nowdays one of the most popular inverter topologies are grid connected transformerless inverters. The main task of transformer is galvanic isolation of PV from grid. These transformers are commonly work with grid frequency i.e 50 Hz, and consequently are bulky, heavy and expensive. Transformer elimination results in common mode leakage current from parasitic capacitors of PV cells and its grounded frame to the grid ground, that will cause in EMI and malfunction of earth fault relays.
In this thesis the factors of... 

Recent developments in improving lighting efficiency and cost reduction of LED’s have caused them to be a suitable alternative for the current lighting systems. In this thesis a novel off-line structure is proposed to drive LED’s. The proposed circuit has a high power factor and high efficiency and a long lifetime. To increase the life time of the converter, the proposed circuit doesn’t use any electrolytic capacitors in the power stage. The proposed circuit consists of a transition mode (TM) flyback converter in order to have power factor correction (PFC), a buck converter is added to the third winding of the flyback transformer in order to create two paths for the electrical power. DC... 

In the urban railway transportation system, large amount of power is consumed for starting the movement and acceleration. A great part of this energy is transformed to the train kinetic energy which in the case of break is reduced to zero. Thus recovering of train kinetic energy during the brake can help to reduce train energy consumption. One of the possible ways is using energy storage during the brake and using it again during acceleration. Storage can be performed in three kinds, fixed, mobile and hybrid. In this thesis, super capacitors have been investigated as fixed and mobile storages. After reviewing electrical railway transportation systems, elements of these systems have been... 

In this thesis a novel T type scheme for direct ac/ac converters is proposed. In this topology, frequency changing takes place directly (without common DC voltage bus) through three converters that are connected in a T structure. H-bridge converters are implemented in each branches of T to make the proposed converter. Also, multilevel inverters, specially cascaded H-bridge ones can be used in branches of T. this will lead to a multilevel ac/ac converter that is suitable for high power applications such as intertie connections and high power motor drives. Using half-bridge converters as T branch converters, leads to an ac/ac converter with unity power factor by only six semiconductor... 

Multilevel converters are considered recently as one of the industrial solutions for high-power and power quality demanding applications. Casacade H-Bridge (CHB) converter is one of the well known topologies among multilevel converters. CHB converter is characterized by its high modularity allowing its power to be easily increased. Modularity also allows any faulty cell to be isolated, so the load can be fed by the remaining operative cells, which leads to an unbalanced output voltage of the CHB converter. To avoid the unbalanced operation, recently a solution based on modifying the phase and the magnitude of the voltage refereces has been proposed. This thesis presents a method to control... 

Real-time Simulator plays an important role in the test of power electronic converters. Using of this simulators and Hardware-in-the-loop tests, cost and time of experiments will redused, furthermore those experiments which are not operative on the real system, can be achived. However, the accuracy of the used model in simulation as well as fast and easy programming of these simulators is very important. Therefore in this thesis, design and implementation of real-time simulator for power electronic converters have been discussed that in addition to high accuracy of the simulation, implementation of the several tests will be easy. As regards the power electronic switch is the most important... 

Nowadays, multilevel converters are significantly developed in industrial applications as well as energy transmission systems regarding proper quality of output waveforms and fault tolerant operation ability. Among the different topologies of multilevel converters, modular multilevel converter (MMC) due to its modularity feature it more accepted in high voltage direct current (HVDC) transmission systems. Fast fault detection in power electronics converters is necessary to bypass the defected part in order to prevent failure propagation to other Submodules and to provide continues servicing. On this basis, different researches are reported in literature for MMCs fault tolerant operation. In... 

Modular multilevel converters provide higher output voltage levels than switches rated voltages by switching between cascaded modules. Increasing the number of modules might results in less reliability because of an increase in the number of elements. On the other hand, an increase in the number of modules decreases the semiconductors rated voltage, switches power losses and thermal stresses and consequently a lower switches failure rate is expected. Therefore the optimal number of modules, would minimize the failure rate. To investigate it, in this project, first fundamental elements of converter are designed for a MMC with different number of output levels, to drive a medium voltage medium... 

LED Drivers with near 1 power factor, Because of using electrolytic capacitors (to become flicker-free) have lifetime drawbacks. This research will focus on solutions to use magnetic energy storage instead of electrolytic capacitors (used for energy storage) to solve lifetime drawbacks and decrease output light flicker. Calculations show that magnetic energy density using iron powder cores are lower than electrolytic capacitors, also a little lower than ceramic capacitors, but obviously higher than film capacitors. After calculations, conventional isolated and non-isolated LED drivers will be reviewed and new non-isolated off-line 230v with 10w output power LED driver with magnetic energy... 

A quasi-Z-source inverter is a beneficial structure in photovoltaic applications. In a transformerless grid-connected inverter system, the common-mode voltage fluctuations cause the leakage current flow through the ground. The low-frequency harmonics of the common-mode voltage results in the malfunction of the residual current device. On the other hand, the high-frequency harmonics of the common-mode voltage cause EMI problems, additional loss and disturbance in the output currents. In this thesis, some methods are presented to reduce the leakage current in a three-phase quasi-Z-source inverter. First, different shoot-through implementation methods are investigated. By modifying the number... 

Grid simulator is a system that, as a voltage source, can produce various current waveforms in power grid at its output, and also as a load, can draw different currents of different loads from the power grid. Grid simulators may be used for testing power grid connected systems, such as chargers, power supplies, etc. In this thesis the system is consisted of programmable AC current source, dual active bridge converter and back-to-back grid-tied inverter. Dual active bridge can normally operate while the current source is in different modes of function, such as constant current, constant power and constant impedance. Depending on the mode of function, the current at the DC link can behave... 

The reliability of power converters in various applications, including industrial, military, aerospace, and commercial applications, is of great importance. For this reason, the development of fault-tolerant power converters is crucial from the perspective of system availability and the prevention of adverse consequences. Tolerating various types of faults, maintaining full output capacity after a fault, and cost-effectiveness are the primary challenges for many existing solutions. In this thesis, a fault tolerance solution is presented for a five-level active-neutral-point-clamped converter. The proposed approach enhances fault-tolerant control for the older version of this converter by...