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

FPGA-based implementation of the back-EMF symmetric-threshold-tracking sensorless commutation method for brushless DC-machines

The operation of brushless DC permanent-magnet machines requires information of the rotor position to steer the semiconductor switches of the power-supply module which is commonly referred to as Brushless Commutation. Different sensorless techniques have been proposed to estimate the rotor position using current and voltage measurements of the machine. Detection of the back-electromotive force (EMF) zero-crossing moments is one of the methods most used to achieve sensorless control by predicting the commutation moments. Most of the techniques based on this phenomenon have the inherit disadvantage of an indirect detection of commutation moments. This is the result of the commutation moment occurring 30 electrical degrees after the zero-crossing of the induced back-emf in the unexcited phase. Often, the time difference between the zero crossing of the back-emf and the optimal current commutation is assumed constant. This assumption can be valid for steady-state operation, however a varying time difference should be taken into account during transient operation of the BLDC machine. This uncertainty degrades the performance of the drive during transients. To overcome this problem which improves the performance while keeping the simplicity of the back-emf zero-crossing detection method an enhancement is proposed. The proposed sensorless method operates parameterless in a way it uses none of the brushless dc-machine parameters. In this paper different aspects of experimental implementation of the new method as well as various aspects of the FPGA programming are discussed. Proposed control method is implemented within a Xilinx Spartan 3E XC3S500E board

Power Quality Enhancement in Electricity Grids with Wind Energy Using Multicell Converters and Energy Storage

In recent years, the wind power industry is experiencing a rapid growth and more wind farms with larger size wind turbines are being connected to the power system. While this contributes to the overall security of electricity supply, large-scale deployment of wind energy into the grid also presents many technical challenges. Most of these challenges are one way or another, related to the variability and intermittent nature of wind and affect the power quality of the distribution grid. Power quality relates to factors that cause variations in the voltage level and frequency as well as distortion in the voltage and current waveforms due to wind variability which produces both harmonics and inter-harmonics. The main motivation behind work is to propose a new topology of the static AC/DC/AC multicell converter to improve the power quality in grid-connected wind energy conversion systems. Serial switching cells have the ability to achieve a high power with lower-size components and improve the voltage waveforms at the input and output of the converter by increasing the number of cells. Furthermore, a battery energy storage system is included and a power management strategy is designed to ensure the continuity of power supply and consequently the autonomy of the proposed system. The simulation results are presented for a 149.2 kW wind turbine induction generator system and the results obtained demonstrate the reduced harmonics, improved transient response, and reference tracking of the voltage output of the wind energy conversion system.Peer reviewedFinal Accepted Versio

A survey on modeling of microgrids - from fundamental physics to phasors and voltage sources

Microgrids have been identified as key components of modern electrical systems to facilitate the integration of renewable distributed generation units. Their analysis and controller design requires the development of advanced (typically model-based) techniques naturally posing an interesting challenge to the control community. Although there are widely accepted reduced order models to describe the dynamic behavior of microgrids, they are typically presented without details about the reduction procedure---hampering the understanding of the physical phenomena behind them. Preceded by an introduction to basic notions and definitions in power systems, the present survey reviews key characteristics and main components of a microgrid. We introduce the reader to the basic functionality of DC/AC inverters, as well as to standard operating modes and control schemes of inverter-interfaced power sources in microgrid applications. Based on this exposition and starting from fundamental physics, we present detailed dynamical models of the main microgrid components. Furthermore, we clearly state the underlying assumptions which lead to the standard reduced model with inverters represented by controllable voltage sources, as well as static network and load representations, hence, providing a complete modular model derivation of a three-phase inverter-based microgrid

Direct control strategy for a four-level three-phase flying-capacitor inverter

A direct predictive control strategy is proposed for a three-phase four-level flying-capacitor (FC) inverter in this paper. The balancing of the FC voltages, a challenge in applications with small capacitors and low switching frequencies, is done without any modulation, simply using tables calculated offline. These allow the realization of fast-dynamics output currents with reduced dv/dt in the output voltages and reduced switching frequencies. Moreover, no interharmonics are created when operating at low switching frequencies and with reference currents containing multiple harmonic components, which is a key feature for active power filters. Simulations and experimental results are presented to demonstrate the excellent performance of the direct control strategy in comparison with a conventional pulsewidth-modulation control technique, mostly for operation at low switching frequencies

Power Quality Enhancement in Hybrid Photovoltaic-Battery System based on three–Level Inverter associated with DC bus Voltage Control

This modest paper presents a study on the energy quality produced by a hybrid system consisting of a Photovoltaic (PV) power source connected to a battery. A three-level inverter was used in the system studied for the purpose of improving the quality of energy injected into the grid and decreasing the Total Harmonic Distortion (THD). A Maximum Power Point Tracking (MPPT) algorithm based on a Fuzzy Logic Controller (FLC) is used for the purpose of ensuring optimal production of photovoltaic energy. In addition, another FLC controller is used to ensure DC bus stabilization. The considered system was implemented in the Matlab /SimPowerSystems environment. The results show the effectiveness of the proposed inverter at three levels in improving the quality of energy injected from the system into the grid.Peer reviewedFinal Published versio

Harmonic current extraction of shunt active power filter based on prediction current technique - Hysteresis PWM

Due to the wide spread of power electronics equipment in modern electrical systems, the increase of the harmonics disturbance in the ac mains currents has became a major concern due to the adverse effects on all analysis and simulation using MATLAB-SIMULINK of a three-phase shunt active equipment. This paper presents the Shunt Active Power Filter (SAPF) to compensate the generated harmonics by 3-phase Rectifier Bridge fed R-L load. The harmonic current extraction is based on prediction current extraction technique -hysteresis PWM generation pattern

A practical approach to switching-loss reduction in a large-capacity static VAr compensator based on voltage-source inverters

This paper presents a simple method for reduction of switching and snubbing losses in a large-capacity static VAr compensator (SVC) consisting of multiple three-phase voltage-source square-wave inverters. The proposed method is characterized by a “commutation capacitor” connected in parallel with each switching device. The commutation capacitor allows the SVC to perform zero-voltage switching, and to reduce switching losses. The electric charge stored in the commutation capacitor is not dissipated, but regenerated to the DC-link capacitor. Moreover, a soft-starting method for the SVC is also presented to avoid forming a short circuit across the commutation capacitor during startup. Experimental results obtained from a 10 kVAr laboratory setup are shown to verify the viability of the operating principle of the commutation capacitor </p

Power factor-corrected transformerless three-phase PWM converter for UPS applications

This thesis describes the research of a new transformerless three phase PWM converter for uninterruptible power supplies (UPS) applications. The removal of the bulky three phase transformer in larger power UPS can provide a significant saving in weight and cost of the overall system. The converter consists of a new four-wire rectifier coupled with a four-wire inverter via a dc bus. The supply and load neutral may be connected together without any neutral current flowing into the utility regardless of the load on the inverter. This allows the load to be at the same potential as the utility. The rectifier, inverter and complete UPS and control system are described in detail and simulation results are used extensively to back up the theory. An experimental prototype of the four-wire rectifier provides further confirmation of the principles. A further proposal to digitize the system is given. This would reduce the size of the required control circuit and simplify the hardware requirements

A SiC-based neutral leg for the three-phase four-wire inverter

SiC-based inverters can operate at high switching frequency with high efficiency, which can reduce the size of passive components and heat sinks to achieve high power density. However, in three-phase four-wire inverters supplying unblanced loads, the second-order ripples in the DC bus need to be mitigated by large DC capacitance, which increases the size of the converter. The conventional neutral leg is widely used in three-phase four-wire inverters to provide neutral currents for the unbalanced loads. In this paper, an improved neutral leg is proposed, which can provide neutral currents and reduce the second-order ripples in the DC bus simultaneously. The DC bus ripples can be reduced without adding any hardware components. Furthermore, the proposed neutral leg can save 50% DC capacitance comparing to the conventional neutral leg. The proposed neutral leg was built with SiC MOSFETs and tested with a three-phase four-wire inverter in the laboratory. The experimental results verified the effectiveness of the proposed neutral leg