Back-to-back Converter Control of Grid-connected Wind Turbine to Mitigate Voltage Drop Caused by Faults

Power electronic converters enable wind turbines, operating at variable speed, to generate electricity more efficiently. Among variable speed operating turbine generators, permanent magnetic synchronous generator (PMSG) has got more attentions due to low cost and maintenance requirements. In addition, the converter in a wind turbine with PMSG decouples the turbine from the power grid, which favors them for grid codes. In this paper, the performance of back-to-back (B2B) converter control of a wind turbine system with PMSG is investigated on a faulty grid. The switching strategy of the grid side converter is designed to improve voltage drop caused by the fault in the grid while the maximum available active power of wind turbine system is injected to the grid and the DC link voltage in the converter is regulated. The methodology of the converter control is elaborated in details and its performance on a sample faulty grid is assessed through simulation

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

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

Power quality issues of 3MW direct-driven PMSG wind turbine

This paper presents power quality issues of a grid connected wind generation system with a MW-class direct-driven permanent magnet synchronous generator (PMSG). A variable speed wind turbine model was simulated and developed with the simulation tool of PSCAD/EMTDC. The model includes a wind turbine with one mass-model drive train model, a PMSG model and a full-scale voltage source back to back PWM converter. The converter controller model is employed in the dq-synchronous rotating reference frame and applied to both generator and grid sides. To achieve maximum power point tracking, a tip speed ratio method is applied in machine side, whereas DC voltage control is applied in grid side to achieve constant DC voltage. Due to wind fluctuation and power oscillation as a result of wind shear and tower shadow effects (3p), there will be a fluctuation in the output power and voltage. The concerned power quality issues in this work are Harmonics, power fluctuation and flicker emission. The measurements will be carried out under different wind speed and circumstances

Multilevel Converters: An Enabling Technology for High-Power Applications

| Multilevel converters are considered today as the state-of-the-art power-conversion systems for high-power and power-quality demanding applications. This paper presents a tutorial on this technology, covering the operating principle and the different power circuit topologies, modulation methods, technical issues and industry applications. Special attention is given to established technology already found in industry with more in-depth and self-contained information, while recent advances and state-of-the-art contributions are addressed with useful references. This paper serves as an introduction to the subject for the not-familiarized reader, as well as an update or reference for academics and practicing engineers working in the field of industrial and power electronics.Ministerio de Ciencia y Tecnología DPI2001-3089Ministerio de Eduación y Ciencia d TEC2006-0386

Full- & Reduced-Order State-Space Modeling of Wind Turbine Systems with Permanent-Magnet Synchronous Generator

Wind energy is an integral part of nowadays energy supply and one of the fastest growing sources of electricity in the world today. Accurate models for wind energy conversion systems (WECSs) are of key interest for the analysis and control design of present and future energy systems. Existing control-oriented WECSs models are subject to unstructured simplifications, which have not been discussed in literature so far. Thus, this technical note presents are thorough derivation of a physical state-space model for permanent magnet synchronous generator WECSs. The physical model considers all dynamic effects that significantly influence the system's power output, including the switching of the power electronics. Alternatively, the model is formulated in the $(a,b,c)$- and $(d,q)$-reference frame. Secondly, a complete control and operation management system for the wind regimes II and III and the transition between the regimes is presented. The control takes practical effects such as input saturation and integral windup into account. Thirdly, by a structured model reduction procedure, two state-space models of WECS with reduced complexity are derived: a non-switching model and a non-switching reduced-order model. The validity of the models is illustrated and compared through a numerical simulation study.Comment: 23 pages, 11 figure

Grid Voltage Unbalance and The Integration of DFIG’s

Double-fed induction generators (DFIG’s) became the predominant generator installed for wind generation applications in the mid 1990’s. Issues pertaining to the operation and control of DFIG’s subsequently became apparent, particularly in weak areas of the grid network. Ironically weak areas of the grid tend to be where the average wind speed is high and the usual location of wind farms. One of the issues that emerged was the quality of the voltage in the network at the point of common coupling (PCC) with the DFIG’s. An important issue is the question of voltage unbalance at the PCC. As part of this work, research was undertaken into the issue of voltage unbalance in a distribution network. Investigative studies were undertaken on a small wind farm connected to the Irish distribution network. The results obtained were then analysed and conclusions drawn, with recording of daily, weekly and seasonal variation of voltage unbalance. The behaviour of DFIG’s to varying levels of network voltage unbalance at the wind farm was analysed, and it was observed that the DFIG’s had difficulty remaining connected to the distribution network when voltage unbalance exceeded certain threshold levels. The behaviour of DFIG’s to the effects of grid network voltage unbalance is further investigated in this work. A literature review was undertaken of the effects that utility network voltage unbalance has on DFIG’s. Emerging from this research, the suitability of appropriate control schemes to alleviate the problems caused by grid voltage unbalance were investigated. Control techniques to improve performance of a DFIG during conditions of asymmetrical grid voltage including measures to control the rotorside and grid-side converters in a DFIG, were designed and then implemented in Matlab/Simulink and results showed improved behaviour. A synchronous generator system was similarly investigated and improvements shown. This research also includes development of a laboratory based DFIG test system. A DSP based digital microcontroller and interfacing hardware has been developed for a 5kVA DFIG laboratory based system. The system comprises of a machine set; a dc machine with common shaft coupling to a three-phase wound rotor induction machine. The dc machine emulates a wind turbine, and drives the induction machine in response to required speed. A converter has been constructed to control the rotor power of the induction machine. Interfacing schemes for the required feedback signals including voltage and current transducers and speed measurement were designed to enable control of both the rotor-side and grid-side converters of the DFIG. Grid/stator voltage oriented control is implemented to control both the rotor side and grid side converters respectively. An additional feature is the implementation of a single DSP controller, configured to control both the rotor side and grid side converters simultaneously. Initially the DFIG test rig was tested as a standalone system, with a load bank connected to the stator terminals of the induction machine. Testing of the DFIG was also conducted with the test rig connected directly to the grid, and the system operated in subsynchronous and super-synchronous modes of operation. Hardware and software solutions were implemented to reasonable success. The laboratory based test rig has been designed for operation as a rotor converter for a DFIG; however the converter can also be configured to operate as a system for a synchronous generator, or for operation as a machine drive. Further research may allow the rig to be used as a DFIG/UPQC (unified power quality controller) test bed