Distributed control of a fault tolerant modular multilevel inverter for direct-drive wind turbine grid interfacing

Modular generator and converter topologies are being pursued for large offshore wind turbines to achieve fault tolerance and high reliability. A centralized controller presents a single critical point of failure which has prevented a truly modular and fault tolerant system from being obtained. This study analyses the inverter circuit control requirements during normal operation and grid fault ride-through, and proposes a distributed controller design to allow inverter modules to operate independently of each other. All the modules independently estimate the grid voltage magnitude and position, and the modules are synchronised together over a CAN bus. The CAN bus is also used to interleave the PWM switching of the modules and synchronise the ADC sampling. The controller structure and algorithms are tested by laboratory experiments with respect to normal operation, initial synchronization to the grid, module fault tolerance and grid fault ride-through

Active current sharing control schemes for parallel connected AC/DC/AC converters

PhD ThesisThe parallel operation of voltage fed converters can be used in many applications, such as aircraft, aerospace, and wind turbines, to increase the current handling capability, system efficiency, flexibility, and reliability through providing redundancy. Also, the maintenance of low power parallel connected units is lower than one high power unit. Significant performance improvement can be attained with parallel converters employing interleaving techniques where small passive components can be used due to harmonic cancellation. In spite of the advantages offered by parallel connected converters, the circulating current problem is still a major concern. The term circulating current describes the uneven current sharing between the units. This circulating current leads to: current distortion, unbalanced operation, which possibly damages the converters, and a reduction in overall system performance. Therefore, current sharing control methods become necessary to limit the circulating current in a parallel connected converter system. The work in this thesis proposes four active current sharing control schemes for two equally rated, directly paralleled, AC/DC/AC converters. The first scheme is referred to as a “time sharing approach,” and it divides the operation time between the converters. Accordingly, in the scheme inter-module reactors become unnecessary, as these are normally employed at the output of each converter. However, this approach can only be used with a limited number of parallel connected units. To avoid this limitation, three other current sharing control schemes are proposed. Moreover, these three schemes can be adopted with any pulse width modulation (PWM) strategy and can be easily extended to three or more parallel connected units since they employ a modular architecture. The proposed current sharing control methods are employed in two applications: a current controller for three-phase RL load and an open loop V/f speed control for a three-phase induction motor. The performance of the proposed methods is verified in both transient and steady state conditions using numerical simulation and experimental testingMinistry of Higher Education and Scientific Research of Iraq

Survey on Photo-Voltaic Powered Interleaved Converter System

Renewable energy is the best solution to meet the growing demand for energy in the country. The solar energy is considered as the most promising energy by the researchers due to its abundant availability, eco-friendly nature, long lasting nature, wide range of application and above all it is a maintenance free system. The energy absorbed by the earth can satisfy 15000 times of today’s total energy demand and its hundred times more than that our conventional energy like coal and other fossil fuels. Though, there are overwhelming advantages in solar energy, It has few drawbacks as well such as its low conversion ratio, inconsistent supply of energy due to variation in the sun light, less efficiency due to ripples in the converter, time dependent and, above all, high capitation cost. These aforementioned flaws have been addressed by the researchers in order to extract maximum energy and attain hundred percentage benefits of this heavenly resource. So, this chapter presents a comprehensive investigation based on photo voltaic (PV) system requirements with the following constraints such as system efficiency, system gain, dynamic response, switching losses are investigated. The overview exhibits and identifies the requirements of a best PV power generation system

Fractional Order Identification Method and Control: Development of Control for Non-Minimum Phase Fractional Order System

The increasing use of renewable energy has resulted in the need for improved a dc-dc converters. This type of electronic-based equipment is needed to interface the dc voltages normally encountered with solar arrays and battery systems to voltage levels suitable for connecting three phase inverters to distribution level networks. As grid-connected solar power levels continue to increase, there is a corresponding need for improved modeling and control of power electronic converters. In particular, higher levels of boost ratios are needed to connect low voltage circuits (less than 1000 V) to medium voltage levels in the range of 13 kV to 34 kV. With boost ratios now exceeding a factor of 10, the inherent nonlinearities of boost converter circuits become more prominent and thereby lead to stability concerns under variable load conditions. This dissertation presents a new method for analyzing dc-dc converters using fractional order calculus. This provides control systems designers the ability to analyze converter frequency response with Bode plots that have pole-zero contributions other than +/- 20 dB/decade. This dissertation details a systematic method of deriving the optimal frequency-domain fit of nonlinear dc-dc converter operation by use of a modified describing function technique. Results are presented by comparing a conventional linearization technique (i.e., integer-order transfer functions) to the describing-function derived equivalent fractional-order model. The benefits of this approach in achieving improved stability margins with high-ratio dc-dc converters are presented

Chaotic Behavior in High-Gain Interleaved DC-DC Converters

In this paper, chaotic behavior in high gain dc-dc converters with current mode control is explored. The dc-dc converters exhibit some chaotic behavior because they contain switches. Moreover, in power electronics (circuits with more passive elements), the dynamics become rich in nonlinearity and become difficult to capture with linear analytical models. Therefore, studying modeling approaches and analysis methods is required. Most of the high-gain dc-dc boost converters cannot be controlled with only voltage mode control due to the presence of right half plane zero that narrows down the stability region. Therefore, the need of current mode control is necessary to ensure the stability of this type of boost converter. A significant number of the work reported so far has concentrated on explaining the chaos phenomena in the language of the nonlinear dynamics literature. In addition to analyzing and studying chaotic behaviors, this presents some ideas about moving toward gainful utilization of the nonlinear properties of power electronics. Simulation and experimental studies are included to validate the theory, and results will be discussed

A magnetically coupled multi-port, multi-operation-mode micro-grid with a predictive dynamic programming-based energy management for residential applications

© 2018 Elsevier Ltd This paper presents the development of a residential micro-grid topology based on a combination of common magnetic and electrical buses. The magnetic bus interfaces two low voltage dc buses linking a PV and a fuel cell to a high voltage dc bus connected to a grid-tied single-phase bidirectional inverter. A battery is used to store the surplus energy of the system and stabilise the dc voltage of the fuel cell bus. A synchronised bus voltage balance (SBVB) technique is used to reduce the conduction losses and increase the soft switching operation range of the converters. To improve the maximum power point tracking (MPPT) performance and system efficiency, appropriate control techniques and compensation blocks are designed. The proposed micro-grid is able to operate in multiple grid-connected and off-grid operation modes according to a predictive 2D dynamic programming-based energy management. A mode selection and transition strategy is developed to select the appropriate operation mode and smooth the mode transition. A detailed study of the micro-grid including steady-state operation, small signal modelling, controller design, and energy management is presented. A prototype of the system is developed, and experimental tests are conducted for an energy management scenario