Power Electronics and Energy Systems (PEES) Laboratory was founded in 2012 to provide a training and research environment for young researchers and applied engineers. In this research lab, the following fields are under study and research:

• Design of power electronics systems such as power supplies, inverters and active rectifiers
• High power converters and multilevel converters
• High voltage and HVDC systems based on power electronics
• Energy conversion systems (e.g., photovoltaic converters)
• Power quality and FACTS devices

Recent advances in the field of power electronics have enabled achieving effective wireless power transmission in a wide range of power. Attractive applications such as charging medical implants, charging small, large, and autonomous vehicles in daily life or harsh environments such as underwater or space had an imposing impact on the fast development of wireless power transfer systems, especially in the last ten years.

In this thesis, the process of design, simulation, and implementation of a grid-connected 1 kW wireless power transfer system is presented. The mentioned system is supposed to charge the Li-ion battery located in an electric vehicle.

The proposed system for wireless power transmission is divided into three stages. In the first stage, an interleaved boost PFC (power factor correction) is employed to provide high voltage DC-link as well as improving the quality of input power drawn from the grid. The second stage consists of a resonant converter, transmitter, and receiver coils. This stage is designed so that the received voltage in the secondary side remains constant regardless of changes in the load. Furthermore, the efficiency of the second stage is regulated in accordance with the first and last stages to assure the highest overall efficiency over a wide range of output power changes during charging li-ion batteries.

In the final stage, with regards to the high output current, an interleaved buck converter is used to satisfy the charging characteristics of the li-ion battery. An independent control algorithm is applied to this stage to provide constant current and constant voltage modes according to the battery state of the charge. This control scheme, in line with the LCC compensation topology in the transmitter side, provides safety and high reliability and also eliminates the need for a communicational link between the two sides.

Finally, the designed wireless power transfer system is tested under various load conditions, and its validity is confirmed.

The parameters identification of electric motors to implement vector control method is essential. Lower error in detecting the induction motor electrical parameters in the start-up, have a large influence on the operation of the induction machine. These parameters can obtain with no-load and locked-rotor tests while the two tests require a lot of time and even in some cases not possible to carry out the experiment. In this study, parameters identification provided for electric motor drives with vector control in the start-up (motor is at standstill). Proposed parameter detection method is offline in the start-up of induction motor that five equivalent circuit parameters of induction motors include, stator and rotor resistance, rotor and stator leakage inductance and, magnetizing inductance automatically in a short time and at the beginning are diagnosed, to be added to the vector control. The proposed methods are considered to be able to implemente on medium speed microcontrollers available in the market. The time and number of detected parameters are also compared with one of the advanced industrial electric drives. Simulation and experimental results have been presented to demonstrate the validity of the proposed method.

With the growth of distributed generation resources in recent years, the use of parallel inverters has expanded significantly. In addition to the desired benefits, parallel inverters also pose various fundamental challenges. In this regards, power sharing control methods in parallel inverters can be classified into two main categories: Communication-based control and droop-based control. Recent researches have tried to use hybrid control to minimize the challenges of both methods. However, there is still a need to improve on some issues, such as harmonic and inter-harmonic load sharing, simple control and power quality. In this project, the proposed power sharing control strategies, which has been gradually improved for various structures, are presented. To confirm the feasibility and efficiency of the proposed methods, single-phase and three-phase parallel inverters were used in the laboratory implementation.

Multilevel converters have made many advances in high power applications and renewable energies due to their ability to generate multiple voltage levels. One of the most widely used multilevel converters is the Cascaded H-Bridge Converter (CHB). This converter is made of serializing several H-bridge cells in each phase that produce the desired voltage, and serializing these bridges has made it possible to use redundancy in this converter. The large number of semiconductor switches used in these converters increases the probability of fault in these converters. To ensure that the converter continues to operate in the event of short-circuit and open-circuit faults and to prevent their spread, it is essential to find a suitable solution for the converter to continue operating. In this essay, a method will be presented to improve a cascaded H-bridge converter’s performance after a fault in the application of photovoltaics. This method is based on injecting more active and reactive power into the grid and tries to achieve this goal based on the characteristics of solar panels and by increasing the total voltage generated by the panels compared to the complete elimination of the defective cell, the system’s ability to inject reactive power to the grid increases. In this method, after the fault occurs, only the defective leg is left out, and the other leg continues to work. Also, in order not to produce DC voltage at the voltage generated by the cells, the complementary leg of the defective leg from another healthy cell treats like the defective leg to create a symmetrical voltage at the output. This increases the faulty cell voltage and injects more active power into the grid than completely eliminating the faulty cell. Also, to increase the active power injected into the grid, the modulation coefficient of the defective cell is increased as much as possible, which also increases the active injecting power but reduces the output voltage of the panels. The modulation used is a combination of LSPWM and PSPWM modulations, which has led to the use of both positive features for this system.

Modular Multi-level Converters(MMCs) are widely used in medium and high power applications due to their advantages such as modularity, higher output voltage quality, lower power semiconductors loss and the need for one DC link. Due to the use of a large number of semiconductor devices in the structure of this converter, the probability of fault in this converter increases. Due to the irreparable damage caused by interrupting the operation of the converter after the fault, especially in sensitive and high power applications, it is necessary to detect the occurrence of the fault and locate the defective semiconductor in order to take necessary measures and maintain the the converter operation after the fault.

This thesis provides a method for detecting and locating switch open circuit faults in modular multilevel converters. In the proposed method, only one voltage sensor in each arm utilized and all cell voltage sensors have been removed. The proposed method uses arm voltage to detect and locate the fault. In practice, in order for the converter to operate properly and to control the voltage of the capacitors, it is necessary to balance the capacitor voltage of the cells. Therefore, due to the removal of capacitor voltage sensors, a method is proposed to estimate and balance the voltage of the capacitors. By eliminating the voltage sensors in the conventional MMC structure, the complexity and total cost of the system is significantly reduced.

Maintaining the stability of solar systems is among the most important technical issues in this field. Conventional control methods cannot maintain grid-connected photovoltaic system’s stability under unbalanced conditions in grid-connected inverters. This thesis proposes a novel control and modulation method for a new structure based on cascaded h-bridge converters for grid-connected photovoltaic systems under asymmetric operating conditions with reserve batteries. The partial shading is considered as an asymmetric condition in this work. In this method, the power losses and the number of switches are reduced and the output voltage levels are increased. The control method used to maintain the system’s stability and maximum power point tracking is the predictive control method which is so compatible with power electronic devices. The simulation and experimental results are proposed as well. This method can control the system in 50% partial shading condition.

Maximum power point tracking (MPPT) is an essential concern to photovoltaic systems. Power-voltage (P-V) characteristic of photovoltaic arrays is nonlinear and displays one or several peaks which vary with solar irradiance level and temperature. MPPT of a PV array in any environmental condition is significantly important to improve the overall PV system efficiency. In this study a new two stage MPPT method that combines artificial neural networks and Hill Climbing is proposed. The validity of the proposed method is investigated by simulations and experimental results. This investigations confirm that the proposed method is able to track the GMPP under uniform insolation and partially shaded conditions and provides a quick tracking. Moreover, there is no need for irradiation or temperature sensor or other extra component. The configuration of the neural network is simple with a low computational burden, then it is possible to implement it on low cost processors.

In this project, in order to reducing high frequency circulating current stress in the arms of flying capacitor modular multilevel converter (FC-MMC), which is generated to reduce voltage ripple in sub-module capacitors, the control system of FC-MMC is modified based on mathematical equations. So the specific control on voltage ripple and choosing the right value of this can lead to reducing the high frequency circulating current stress. The feasibility of the proposed control scheme was confirmed by the simulation results carrying out in the MATLAB/Simulink and experiments.

 Due to increased number of switches in multilevel converters the possibility of failure is more. To avoid further damages the uninterruptable operation of converter, fault detection and localization and fault-tolerant operation are vital. In this project, a fault detection and localization method, and a method for fault-tolerant operation of CHB converters are proposed.
The fault occurrence will be recognized by observing difference between the reference voltage and the output phase voltage. Hence, one voltage sensor and one current sensor are needed per phase which are the least possible number of sensors. The switching method will be changed during the fault localization to generate reference voltage in the output in addition to guarantee identifying the fault location as quickly as possible, independent from modulation technique and modulation index. The validation of proposed method has been proved by simulation and experimental results.
A fault-tolerant operation method is presented, which generates the maximum line to line voltage after the fault, same as the normal condition. In this method an auxiliary module, which is made by six semiconductor switches and one capacitor, is added to converter. Compared to other methods that need additional devices, the proposed method requires less devices and in consequence less cost while the maximum output voltage is same as normal condition. By implementation of this method, both in simulation and experiments, the validity of method have been confirmed.

In this project, the proposed inverter is based on inverting buck-boost DC/DC converter which has been modified to meet the requirements for three phase application. Besides, a novel switching pattern has been used. The overall strategy is that based on the amplitude and phase of three phase grid voltages and the amount of power considered to be injected into the grid, the amplitude and phase of three phase currents are determined. Then, the inverter and the switching pattern are employed so that the desired three phase currents are programed at the output.

With the proper selection of the topology and the novel switching method, it is possible to inject power into the grid regardless of amount of DC voltage with respect to grid voltage. Using non-electrolytic capacitors has improved the reliability of the inverter and by employing an inductor to transfer power from DC side to grid, soft switching has been realized in most of the switching transitions. Also, with the proposed switching pattern, there is no leakage current related to parasitic capacitor of the PV panel.

CHB based STATCOM improves power factor, transmission capability and voltage stability of power networks by exchanging reactive power with the grid. Scaled down three-phase 5-level prototype of CHB STATCOM which is built in PEES Lab is demonstrated in the following figure.

In this prototype, in the first step, the closed loop hierarchical control system of CHB-STATCOM is implemented via DSP by which reactive power is injected to the grid and also, active power is absorbed from the grid to keep the voltage of dc-side capacitors in charge state. Also, the distribution of absorbed active power among the converter cells are controlled to keep all the capacitor voltages balanced. In the next step, operation of the system after fault occurrence in semiconductor power switches of the converter is investigated. A Fault tolerant method is implemented which insures the continuous and uninterrupted post-fault operation of CHB-STATCOM. 

Modular Multilevel Converter, due to its interesting features and distinctive advantages compared to other multi-level converters, has gained a lot of attention. This type of converter is especially interesting for ac to dc and dc to ac conversions at high voltage levels. Voltage balancing, control and reduction of voltage ripple in floating capacitors are the main challenges of this converter topology. The following figure shows a single phase MMC with four cells in each leg which has been made in PEESLab to study about the control and other challenging issues of MMC structure.

Growth in high voltage power conversion applications in recent years shows the importance and requirement of high voltage/high power converters in power electronic applications. The major limitation in this industry is the maximum voltage blocking capability of semiconductor switches. To overcome this restriction, the power switches are connected in series to build a high voltage compact switch. This project proposes a modified circuit based on quasi-active gate control (QAGC) which provides the capability of connecting a desired number of IGBTs in series. One of the important advantages of the proposed circuit is to use a single PWM pulse for switching process. Simulation results show an excellent voltage balancing between four and eight series-connected IGBTs. Also, experimental results are carried out to verify the performance of the proposed circuit.

Applications:

All high voltage power converters

Proposed hybrid converter is a series convection of the full-bridge NPC and the CHB converter. Hybrid converter can make equal number of output voltage levels with less number of power switches compared to basic multilevel converters.

Applications:

Power Quality

Drive

Selective harmonic elimination (SHE) technique is introduced for a cascaded H-Bridge based STATCOM (CHB-STATCOM) with unequal capacitive dc-link voltages. The asymmetric topology allows increasing the number of voltage levels while keeping the converter losses low. Moreover, reactive power control is realized by using the decoupled current control and the voltage of individual capacitors is controlled by the help of SHE modulation.  Mathematical equations are derived for synthesizing the multilevel ac waveform and regulation of dc-link voltages for a CHB inverter with an arbitrary number of H-bridge cells. Then, to verify the theoretical claims and experimental results are provided for a three-phase CHB inverter with two H-bridge cells in each phase.

Reliable and unceasing exploitation of power electronic converters plays a major part in every application. Concerns of manufacturers about guaranty time period as well as the maintenance cost and its time period encourage researchers to evaluate reliability of converters with an acceptable accuracy. My thesis concentrates on a new opened-up reliability assessment framework and demonstrates the feasibility of using sensitivity analysis for a much more accurate estimation. It deals with a conventional boost DC-DC converter as a case study in two thermal structures (coupled and decoupled). It presents that significant reciprocal effects of components can thoroughly impress the reliability assessment in the thermal coupled structure. It is shown that while IGBT or Diode aging due to the either fatigue or creep (See following image) leads to an increase in the Diode and IGBT junction temperatures, electrical operating point maintains constant even as a case of capacitor degradation. It is while that in the thermal decoupled structures, there are not significant reciprocal effects either in electrical or thermal operating points. The results reveal the importance of reciprocal aging and self-aging effects on the reliability assessment. 

Photovoltaic micro-grids have recently attracted considerable attention for their higher reliability, longer lifespan, modularity, reduction of power transfer costs, greater returns, and economization on financial and environmental expenses. There have been proposed various power sharing methods for micro-grids so far. Among these methods, droop method has attracted widespread interest due to the fact that in this method, there is no critical need to telecommunication links.
Considering the defects and limitations of the traditional droop control method in reactive power sharing and tracking the power peak, there has been proposed a new method based on the traditional method. In this methods, new control formulae are entered to inverters connected to batteries, and a telecommunication link with a narrow bandwidth has been employed for the reactive power sharing in order to equalize the current amplitude in inverters. Later on, a number of methods are suggested to prevent instability under certain conditions and offset the voltage droop at the point of common coupling (PCC).
The proposed method was simulated for a network including three inverters and its performance was approved. The needs for telecommunication links were also covered in the present study. Finally, the proposed method was tested in practice in three different scenarios using two micro-inverters linked to solar panels, designed and operationalized in a laboratory. Results showed that the proposed method could share the active and reactive power well, equalize the current amplitude properly, and simultaneously, keep the stability in the grid.

Nowadays, the growing trend towards qualified foods from the perspective of having the nutritional value for the replacement of thermal treatments with new methods is discussed. Thermic processing is considered an important technology, which is commonly used to increase durability and safe maintenance with low cost in the food industry. In recent years, several technologies have been investigated, which have the ability to deactivate microorganisms at low temperatures. Among all emerging processes, ozone generation is a representative of microbial inactivation (fluids and air, etc.) that is not harmful to the environment. In this paper, a high-voltage and high-frequency power supply for ozone generation is introduced. This circuit uses a single switch and works in the resonant mode with the help of transformer magnetizing inductor and leakage inductor without adding a distinct resonant inductor, a separate resonant capacitor, and a step up transformer to generate high voltage output. The zero current switching (ZCS) condition for the switch is obtained. The simulation results demonstrate that with a 250 V input and 1.66 us pulse width, a 7.5 kV pulse is generated at the output. A hardware prototype with 250 V-DC input and 7.5 kV output pulse has been developed. The pulse repetition frequency can go up to 15 kHz. Both the simulation and experimental results are presented to validate the proposed converter and its operating principle.