Address: University of Tehran, Technical Faculty, North Kargar St., Tehran

Applying predictive control on parallel compensator based on hybrid multilevel converter

Masoud Shahabadini


 Grid-tied photovoltaic (PV) inverters usually have galvanic isolation. PV systems with transformer have several stages of power conversion which reduce the efficiency, increase the cost of the system. Therefore, removing the transformer can be a suitable option for PV systems. 
However, the leakage current can flow through the system because of no galvanic isolation. This undesirable current leads to more losses, electromagnetic interferences, current harmonic distortion, and safety concerns. 
Nowadays, the utilization of cascaded H-bridge (CHB) converters in PV applications is more attractive. The distinct DC links in CHB inverters provide the independent control on arrays voltages, and therefore, the individual maximum power point tracking (MPPT) algorithms can be applied in each PV string.  Also, the modularity feature of CHB topology makes it possible to reach higher voltage levels readily. Hence, the CHB inverter can process the electric power in one stage and without any extra transformer and boost stage, which results in a higher conversion efficiency. Therefore, the leakage current suppression challenge in the transformerless CHB inverter is investigated in this thesis. 
So, first, the theoretical analysis is presented to model the leakage current; then two new solutions are proposed for suppressing the leakage current which are validated with simulation and experimental results. 
The first solution is a modulation-based method. In the proposed method, the leakage current is significantly reduced and there is no need to extra hardware components.In this method a modified PSPWM based strategy is employed to reduce the leakage current. In contrast to most existing solutions, the proposed method can be easily generalized through the series connection of 5-level CHB blocks.
The second solution presents a hardware-based method and uses high efficient and reliable inverter concept (HERIC) in CHB converters. Using this idea and proper utilization of AC side filters leads to leakage current suppression in individual cells, and thus, the whole system. In contrast to most existing solutions, the proposed technique can be easily extended to CHB inverters with an arbitrary number of voltage levels. Moreover, this solution is applicable in the system’s conditions with unequal parasitic capacitances and unequal DC link voltages of H-bridge cells.

Improving the fault-tolerant of the Cascaded H-bridge Converter when two faults occur

Ehsan Moradi


 The purpose of this thesis is to improve the fault tolerance method based on the hardware of the CHB converter. In this thesis, three new fault tolerance methods for the CHB converter are presented. These three methods produce balanced line-line voltages at the output of the CHB converter without isolating the healthy cells. The first presented method has the capability of compensating a fault in each of phases a, b or c of the CHB converter. Due to the use of a capacitor instead of an independent power source in the dc link of the auxiliary cell used in this proposed method, in terms of the number of parts used, this method has used the least number of parts among the presented methods. The auxiliary cell used in this method is added to the faulty phase after the fault occurs, and its DC link capacitor is charged up to the rated value in 63 milliseconds In this method, a new method has been presented to keep the DC link capacitor voltage of the auxiliary cell balanced. The second and third methods presented in this thesis are proposed for two-faults compensation in the CHB converter. These methods have the ability to replace two faulty cells with two auxiliary healthy cells in case of two faults in each of phases a, b or c of the CHB converter. It should be noted that all three proposed methods preserve the modularity of the converter, which preserves the reliability of the converter after the fault occurs. In this thesis, first, the cascaded H-bridge converter are introduced and the reason of its selection in comparison to similar converters is discussed. Then the challenges of its tolerability against fault and proposed solutions in related articles are observed. Finally, the results of simulation and implementation of the presented methods on the laboratory sample are discussed and reviewed, and based on this, their accuracy and efficiency are confirmed.

Resonant power supply for high voltage capacitive link in electrical discharge application

Alireza Bagheri


 Underwater arc discharge is one of the most effective water treatment methods because of the effects of intense shockwaves and high ultraviolet radiations. In this thesis, a high-voltage underwater arc discharge system is proposed to study the inactivation of Escherichia coli bacteria, which has an arc-robust feature. It consists of a high-voltage capacitor charger and a discharge circuit. This thesis includes three goals as described below.
First, the high-voltage capacitor charger is developed by a novel LCCL resonant converter. Generally, the performance of the resonant converter is highly affected by the parasitic elements of the high voltage/high power transformer. Also, short circuit condition is a frequent event in high voltage and arc discharge applications, so that the resonant converter should be robust against the short circuit condition and unwanted arc occurrence. This thesis presents a novel LCCL resonant converter with current source scheme used as a high voltage capacitor charger. The proposed resonant converter eliminates the undesirable effect of the stray capacitance in high voltage devices and is robust against the short circuit condition inherently. Furthermore, a frequency-sweep control method is applied, which has advantages in reducing the resonant current’s peak value and the resonant network’s size. Simulation results show a satisfactory performance of the proposed resonant converter. Moreover, experimental results confirm the effectiveness of the proposed resonant converter in the application of capacitor charging.
Secondly, the discharge circuit is developed by a two-step spark gap scheme using an ultrafast high-voltage trigger circuit. It prevents unwanted arc discharges during the charging. Also, it provides a system operation independent of the water conductivity.
Thirdly, the proposed underwater arc discharge system has been applied to 500ml of pollutant water to study the system’s performance on the inactivation of E. Coli bacteria. The proper microbiological tests have been carried out, showing the effectiveness of the proposed underwater arc discharge system. The complete inactivation of E. Coli bacteria is achieved in an extremely short treatment time. The proposed system inactivates the bacteria completely with 13.2J/ml energy consumption and 0.04Sec./ml treatment time. Also, a 4.5 log reduction of the bacteria is achieved with 7J/ml energy efficiency. The treatment time is 20 seconds, and the solution conductivity is 14.5mS/cm. Moreover, the experimental results are presented, which validate the desired performance of the proposed underwater arc discharge system.

Control of Nested Neutral Point Clamped (NNPC) Converter in Low-Frequency Applications of Electrical Drives

Kourosh Khalaj Monfared


  Using cost-effective high-power (HP) converters in industrial processes has become an important issue in recent years. Based on this, developing multilevel topologies as one of the research fields with high priority has always been considered. Multilevel topologies have gained considerable acceptance due to attractive features such as the possibility of using low voltage devices at medium voltage levels, increasing the output power quality, reducing dv/dt, and removing the output filter. One attractive research area in multilevel converters is achieving topologies with fewer devices and, as a result, lower costs. Based on this, in recent years, four-level Nested Neutral Point Clamped (NNPC) converter has been introduced and received attention in HP electrical drive applications. Despite the advantages of 4L-NNPC over similar topologies, industrial applications of this topology, especially in the motor drive application, face technical challenges. The main technical challenge in the 4L-NNPC converter is controlling flying capacitors (FCs), especially in low-frequency operations. Few types of research have been done to solve this challenge. These presented researches still have defects, such as functional limitations in different operating points of an electric drive (i.e., torque boost) conditions, high switching frequency, and high common-mode voltage (CMV).
In this thesis, firstly, the structure of 4L-NNPC is introduced, and the superiority of this structure is compared to similar structures. Then, the challenge of low-frequency operation in 4L-NNPC and the presented research solutions are reviewed. The simplified space vector modulation (SSVM) and finite control set model predictive control (FCS-MPC) methods are considered super among the existing solutions. These two methods have been investigated and evaluated from the functional point of view, switching frequency, power losses, and CMV. Finally, two methods based on model predictive control are introduced to solve the challenge of low-frequency operation of 4L-NNPC converters and provide lower switching frequency and CMV compared to existing methods. The first proposed method is Improved Finite Control-SetModel Predictive Control (IFCS-MPC) to reduce the switching frequency and switching power losses. The second method is based on Optimal Switching Sequence MPC (OSS-MPC) for providing low CMV. The performance of the suggested methods is verified on a 4LNNPC in MATLAB/SIMULINK. Also, the experimental results are carried out based on a scaled-down hardware prototype validating the theoretical claims. 

Control of islanded parallel inverters with accurate power sharing in presence of harmonic and inter-harmonic loads

Reza Razi


  In recent years, the use of parallel inverters has expanded dramatically significantly due to the distributed generation growth in microgrids, the need to process a high volume of power, reliability increasing and access to the high-speed processors. In addition to the desired benefits, parallel inverters also pose various fundamental challenges. Power sharing control methods in parallel inverters can be classified into two main categories: Communication-based control and droop-based control. The first category has advantages such as high accuracy of power sharing and excellent voltage regulation, but high costs and low reliability are the main disadvantages that will stimulate the use of another way. On the other hand, the droop control techniques are based on the local measurements. However, this method also has many disadvantages such as frequency and voltage deviation, inappropriate sharing of harmonics and etc. 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 load sharing, simple control and power quality. Besides, there is no sharing of inter-harmonic loads in past literature. In this thesis, first, the structure of islanded parallel inverters is introduced and then, its equations are extracted. In the third chapter, the literature review for power sharing in parallel inverters is performed, and the advantages, disadvantages and challenges of each method are analyzed. In the following chapters, the proposed power sharing control strategy, which has been gradually improved for various structures, is discussed. In this regard in Chapter 4, Impedance-Power droop is proposed for accurate power sharing in resistive low-voltage microgrids. The suggested control scheme removes the inherent impedances of inverters and adopts impedance-power algorithm to adjust the equivalent impedances. Accordingly, there is no need to recognize system parameters and the use of conventional droop. Furthermore, slight deviations in voltage and frequency are observed. In Chapter 5, the Impedance-Power droop is modified to achieve power sharing in harmonic and inter-harmonic currents, such that the inherent delays of digital control to remove inherent impedance and virtual inductors to regulate power are considered. Also, two other techniques are proposed to control and use of negative virtual impedances. In Chapter 6, Extended Impedance-Power Algorithm is proposed to obtain accurate power sharing in multi-bus microgrids with complex line impedances. In fact, the resistive and inductive terms of the virtual impedance is adaptively obtained by using nonlinear extended droop. At the end of each chapter, the results of simulation and experimental in different scenarios are presented to confirm the feasibility and accuracy of the proposed methods. 

A Systematic Method for Reliability Evaluation of DC-DC Electronic Converters

Vahid Samavatian


  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.

Improving fault tolerance in the full-bridge series converter in the parallel compensator

Yousef Neyshabouri


  In recent years, due to arising power quality issues in power networks and remarkable developments in power electronic multilevel converters, cascaded H bridge based STATCOM has achieved noticeable attention. Hence, reliability and fault tolerant operation of CHB based STATCOM is brought up as a significant challenge in order to insure continuous and uninterrupted operation of the converter after fault occurrence. In this thesis, fault condition is investigated from two points of view: Internal fault which refers to the failure in power switches of the converter, and external fault which refers to faulty condition in the grid side of CHB-STATCOM. In this thesis, in the first step, the comprehensive power flow equations for the whole converter and within the converter legs are derived and analyzed. Based on this analysis, a new internal fault tolerant strategy is introduced for CHB-STATCOM. In the proposed strategy, after fault detection in a power switch, the faulty leg of the faulty cell is bypassed and the remaining healthy leg is used as a half-bridge cell to generate the maximum attainable line-line voltages. The dc-link voltages are used as another degree of freedom and by an efficient increase at the dc link voltages, the rated capacity of STATCOM is restored. This strategy distributes active power evenly among the converter legs which makes it compatible with STATCOM application. Furthermore, by applying an effective individual voltage balancing scheme with suitable balancing margin, voltage of dc link capacitors are kept balanced in post-fault condition. In the next step, from the power flow equations within the converter legs, the degrees of freedom and limitations of operation range of CHB-STATCOM during grid fault condition are analyzed. Based on this analysis, an external fault tolerant strategy is proposed in which the STATCOM exchanges both positive and negative sequence reactive currents with the grid within itsrated range, during an unbalanced condition. Also, both the injected zero sequence voltage and negative sequence active current to the legs are controlled to achieve 1. Voltage balancing of dc links, and 2. Operation in nominal voltage and current ratings. Both internal and external fault tolerant methods are simulated in PSCAD/EMTDC environment and the experimental results are provided based on scaled down laboratory prototypes which validates the performance and effectiveness of the introduced methods.

Improving wind turbine droop control performance with two-way power supply generator for wind farms in microgrid

Mohammad Saleh Marhaba


  This study presents an innovative control scheme to improve the power sharing among doubly-fed induction generator (DFIG) wind units in a medium-voltage (MV) microgrid. The control objectives of DFIGs in an islanded mode of microgrid operation are to achieve: (i) stabilisation of the microgrid voltage amplitude and frequency, (ii) proper active/reactive power sharing among wind units. To satisfy these requirements, the DFIG control loop based on the traditional droop control is designed. This method, however, cannot satisfactorily operate in a MV microgrid with dominantly resistive line impedances from power sharing point of view. To overcome this problem, a modified control strategy is proposed in this study. The mathematical modelling is developed, and time-domain simulations are presented to verify the novel control scheme in a typical microgrid case study.

Abous Us

Power Electronics and Energy Systems Laboratory was established in 2010 with the aim of training efficient engineers and researchers. In this educational and research environment, specialized fields such as the design of power electronic systems, including all types of power supplies, inverters and rectifiers, high and multi-level power converters, types of high voltage or HV systems, energy conversion systems, etc. are under study and research.

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