Direct torque control using cascaded h-bridge multilevel inverter for induction motora

This thesis proposes methods to improve the performance of a Direct Torque Control (DTC) of induction motor drives. The basic principle and theoretical aspects of the DTC using a conventional inverter (DTC-Conv) and the DTC using a 5-level Cascaded H-Bridge Multilevel Inverter (DTC-CHMI) are reviewed with emphasis on two major problems: high torque and flux ripple and variable switching frequency. Based on the basic principle of the DTC, torque and flux are directly controlled by selecting appropriate voltage vectors. A DTC-Conv offers eight voltage vectors to increase (or decrease) both torque and flux. Regardless of the torque’s demand, for the DTC-Conv, the application of voltage vector is limited to these eight voltage vectors. This will give a high torque and flux ripple because of the possible voltage vector selected is not optimal for the condition. Based on the investigation, by proposing the DTC-CHMI, a smaller torque and flux ripple can be achieved. Moreover this method offers a good torque response. This is due to the capability of the DTC-CHMI to offer 61 voltage vectors which give more options to choose the most optimum vector for any circumstances. In addition, less switching burden on the switching devices for the DTC-CHMI compared to DTC-Conv, which results in a lower power rating device to be used. It is well known that the implementation of the DTC-Conv consists of a hysteresis-based controller which results in a variable switching frequency in the switching devices. This undesirable condition will affect the inverter design since it is related to the rate of change of the torque which varies with various operating conditions. Therefore, this thesis proposes the proportional-integral controller constant switching frequency together with the DTC-CHMI to replace the DTC-Conv with a hysteresis-based controller. The proposed torque controller consists of three pairs of triangular carrier signals with three pairs of comparators. With this proposed controller, the variation of switching frequency can be narrowed and fixed at the carrier frequency. Furthermore, it minimizes the torque ripples. Design of the proposed controller is thoroughly discussed in this thesis. To verify the enhancement made by the proposed method, simulation and experiment, as well as the comparison with the DTC-Conv were carried out. Results prove that by using the proposed system, torque and flux ripple are reduced by 38.5% and 7.76% respectively. Apart from that, the switching frequency is fixed at 1.667 kHz and a less distorted sinusoidal phase current is obtained

New asymmetrical modular multilevel inverter topology with reduced number of switches

In this article, a new single-phase multilevel inverter is introduced with a reduced number of power switches and reduced voltage stress on power switches. The proposed topology consists of four input dc sources and nine semiconductor switches (eight unidirectional and one bidirectional switch). The topology can be used for asymmetrical voltage source configuration to generate seventeen voltage levels. The extended topology is constructed by a series connection of the topology circuit to produce higher voltage levels with less voltage stress on the switches without modifying the existing structure. Comparison is made with traditional and recently introduced topologies based on the number of power switches, dc sources, total blocking voltage of switches, and gate driver circuits, to prove the proposed topology's superiority. A simple nearest level modulation has been deployed as the switching scheme. Validation on the viability of the proposed topology has been carried out through simulation and hardware experimental setup

Direct torque control of multilevel inverter-fed induction machines - a survey

Direct Torque Control with Multilevel Inverter (DTC-MLI) has emerged recently in high dynamics AC drives fields for induction machines or permanent magnet machines application. In this paper, a review on a variety of techniques and concepts of direct torque control of multilevel inverter-fed induction machines is presented. The techniques and concept involved are classified as follows: Look-up table hysteresis based DTC-MLI, DTC-MLI with space vector modulation, predictive control strategy of DTC-MLI, hybrid modulation and hybrid inverter strategy of DTC-MLI and DTC-MLI with fuzzy logic controller. From this review, the properties of the discussed controller techniques together with advantages and disadvantages are presented

Direct torque control of induction machines utilizing 3-level cascaded H-bridge multilevel inverter and fuzzy logic

This paper proposes the use of a 3-level Cascaded H-Bridge Multilevel Inverter (CHMI) topology which results in further torque ripple minimization compared to the 2-level inverter-based Direct Torque Control (DTC). This is due to the increase in the inverter switching voltage vectors that allows minimization of the torque error. This in turn can reduce the Total Harmonic Distortion (THD) of the output voltage and current as well. This paper also presents two different control methods in selecting the appropriate output voltage vector for reducing the torque and flux error to zero. The first is based on the conventional DTC scheme using a pair of hysteresis comparators and look-up table to select the output voltage vector for controlling the torque and flux. The second is based on a new fuzzy logic controller (FLC) with Sugeno as its inference method to select the output voltage vector by replacing the hysteresis comparators and look-up table in the conventional DTC scheme. The latter has solved the problem of variable switching frequency which is the main characteristic of the former. By using FLC DTC not only the flux ripples reduce significantly but also the THD of the phase current decreases since a more sinusoidal current waveform is achieved. The simulation results have proven that by using the 3-level CHMI, torque ripple reduction is obtained compared to the 2-level inverter-based DTC while fuzzy DTC shows reduction in the stator flux ripples and the THD of the phase current

Virtual inertial support extraction using a super‐capacitor for a wind‐PMSG application

The replacement of a conventional synchronous generator (SG) with higher wind penetration greatly reduces the inertial support available for the system. Therefore, a new consideration – the dynamic frequency changes in a wind energy system using a super‐capacitor (SC) – is proposed here to provide faster, limitless, inertial response during load disturbances. By considering the dynamic frequency change in defining the power exchange between SC and network, this proposed method not only provides inertial and primary frequency response but is also able to avoid a second frequency dip. To avoid deep charge and discharge of SC, a simple energy management system (EMS) which considers the system frequency deviation and the state of charge (SOC) of SC is adopted in this technique. The effectiveness of the proposed technique is modelled in MATLAB/Simulink software. Comparative analyses between the proposed virtual inertial support (VIS)‐based SC and existing VIS control strategies are performed. The simulation results obtained prove that the proposed VIS greatly reduces the frequency nadir and peak frequency of the system during sudden load increase and decrease, respectively

An improved control rod selection algorithm for core power control at TRIGA PUSPATI Reactor

The 1 MWth TRIGA PUSPATI Reactor known as RTP undergoes more than 37 years of operation in Malaysia. The current core power control utilized Feedback Control Algorithm (FCA) and a conventional Control Rod Selection Algorithm (CRSA). However, the current power tracking performance suffers and increase the workload on Control Rod Drive Mechanism (CRDM) if the range between minimum and maximum rod worth value for each control rod has a significant difference. Thus, it is requiring much time to keep the core power stable at the power demand value within the acceptable error bands for the safety requirement of the RTP. In conventional CRSA, regardless of the rod worth value, the lowest position of the control rod is selected for up-movement to regulate the reactor power with 2% chattering error. To improve this method, a new CRSA is introduced named Single Control Absorbing Rod (SCAR). In SCAR, only one rod with highest reactivity worth value will be selected for coast tuning during transient and the lowest reactivity worth value will be selected for fine-tuning rod movement during steady-state. The simulation model of the reactor core is represented based on point kinetics model, thermal-hydraulic models and reactivity model. The conventional CRSA model included with control rod position dynamic model and actual reactivity worth curve data from RTP. The FCA controller is designed based on Proportional-Integral (PI) controller using MATLAB Simulink simulation. The core power control system is represented by the integration of a reactor core model, CRSA model and FCA controller. To manifest the effectiveness of the proposed SCAR algorithm, the results are compared to the conventional CRSA in both simulation and experimentation. Overall, the results shows that the SCAR algorithm offers generally better results than the conventional CRSA with the reduction in rising time up to 44%, workload up to 35%, settling time up to 26% and chattering error up to 18% of the nominal value

Switched-capacitor-based modular t-type inverter with reduced switch count

Switched-capacitor (SC) based inverter topologies are very attractive for renewable energy applications to step-up low dc voltage to higher ac voltage in single-stage without the need of front-end dc-dc converters, thus improving overall system efficiency, reliability and power density. This paper presents a new modular multilevel inverter based on switched-capacitor technique. Herein, series-parallel conversion technique is utilized to effectively self-balance the switched-capacitor voltages. Compared to other latest SC-based MLIs topology, the proposed structure features several advantages such as: low number of conducting semiconductor devices in load current paths and SC Charging paths. In addition, each capacitor is charged for at least half of the fundamental period that reduces voltage ripple and improves the inverter efficiency. Consequently, the presented inverter is suitable for high levels generation to achieve the rated ac voltage directly from a low voltage dc source. The merits and feasibility of the proposed inverter are verified through simulation for seven levels with sinusoidal PWM at different modulation indices and load conditions